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Effect of nitrogen fertilization timing on the demographic processes of wild oat(Avena fatua) in barley (Hordeum vulgare)Author(s) Julio A Scursoni and Roberto Benech ArnoldSource Weed Science 50(5)616-621 2002Published By Weed Science Society of AmericaDOI httpdxdoiorg1016140043-1745(2002)050[0616EONFTO]20CO2URL httpwwwbiooneorgdoifull1016140043-1745282002290505B06163AEONFTO5D20CO3B2

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616 bull Weed Science 50 SeptemberndashOctober 2002

Weed Science 50616ndash621 2002

Effect of nitrogen fertilization timing on the demographicprocesses of wild oat (Avena fatua) in barley (Hordeumvulgare)

Julio A ScursoniCorresponding author Departamento de ProduccionVegetal Facultad de Agronomıa Universidad deBuenos Aires Av San Martın 4453 (1417) BuenosAires Argentina scursoniagroubaar

Roberto Benech ArnoldDepartamento de Produccion Vegetal Facultad deAgronomıa Universidad de Buenos Aires Av SanMartın 4453 (1417) Buenos Aires Argentina

Wild oat is the most important weed in Argentine barley and one of the worst weedsin wheat and barley worldwide During 1997 and 1998 field experiments wereperformed to determine the response of wild oat to N fertilizer at planting and atearly tillering at 55 kg ha21 with and without the use of diclofop-methyl Seedlingemergence and survival rate individual fecundity and preharvest seed dispersal rateof wild oat were assessed The effect on the individual growth of wild oat was alsostudied N fertilizer increased both wild oat seedling survival rate and fecundityespecially when it was applied at early tillering However the effect of N fertilizationwas negligible with diclofop-methyl treatment In addition the application of Nfertilizer reduced the seed dispersal rate immediately before crop harvest Quantifi-cation of the effects of each agronomic practice such as N fertilization is useful todevelop simulation models that predict the population dynamics When N was ap-plied at sowing the annual growth rate of the wild oat population was almost 25lower than when applied at early tillering

Nomenclature Diclofop wild oat Avena fatua L AVEFA barley Hordeum vulgareL wheat Triticum aestivum L

Key words Integrated weed management population dynamics seed dispersalseed production seed survival soil seed bank

Wild oat is the most important weed in Argentine barleyinfesting over 80 of the barley fields (Scursoni 1995)Wild oat control is strongly dependent on herbicides suchas diclofop methyl and they represent an important cost ofcrop production

The use of herbicides and agronomic practices may berequired to keep wild oat infestations at a low level for long-term economical crop production Wild oat populations areregulated by demographic features that take place through-out the crop cycle (ie seedling establishment seedling sur-vival plant fecundity and seed dispersal) and the fallowperiod (ie germination predation and mortality from theseed bank) Scursoni et al (1999) found that demographicprocesses are affected by agronomic practices such as cropchoice and planting rate

Tillage fertilization and herbicide application are amongthe main agronomic practices used in crop production (Ke-gode et al 1999) However the effects of fertilization onthe weed community on weed population dynamics as wellas on weedndashcrop interactions have been studied less thantillage systems crop rotations and herbicide performance(OrsquoDonovan et al 1997) Fertilization is an important ag-ronomic strategy used extensively to increase crop yieldNevertheless although nutrients clearly promote cropgrowth many studies have shown that in some cases fer-tilizers benefit weeds more than crops (DiTomaso 1995)For example Carlson and Hill (1986) found that the ad-dition of N fertilizer to wild oatndashinfested wheat increasedthe density of wild oat panicles without increasing cropyield However Satorre and Snaydon (1992) showed that Nfertilizer reduced the severity of competition experienced bywild oat from six spring cereals The increase in weed com-

petition at higher N rates has been suggested to be relatedto an increase in the efficiency of nutrient accumulation anduse by weeds (DiTomaso 1995) In addition Brandt et al(2000) reported differences in weed response to soil fertilityfor wheat and canola (Brassica napus L) suggesting that thismay be exploited through the development of agronomicsystems that stimulate crop growth over weed growth More-over Knezevic et al (2000) noted that insufficient N canreduce corn (Zea mays L) tolerance to weeds and canlengthen the critical period for weed control

Fertilizer application can have an influence on the com-position of the weed community In this sense Jornsgard etal (1996) demonstrated an interaction between the growthof individual weed species and the level of N fertilizer incereal crops They found that common lambsquarters (Che-nopodium album L) in competition with spring barley hasa lower N optimum than does the crop whereas burningnettle (Urtica urens L) has a higher N optimum than doesthe crop Anderson et al (1998) found that in a springwheatndashwinter wheatndashsunflower (Helianthus annuus L) ro-tation conducted under a no-till system N application in-creased crop competitiveness and reduced weed densityLikewise the increase in soil fertility as a result of the in-troduction of subterranean clover (Trifolium subterraneumL) and the application of superphosphate has favored dif-ferent species of thistles (Cirsium spp) in Australia (Sindel1991) However Anderson and Milberg (1998) in long-term experiments found that the effect of crop species onweed flora was more important than the effect of N appli-cation Moreover N application did not strongly influencethe composition of the weed flora suggesting that the effect

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 617

TABLE 1 Monthly rainfall for 1997 and 1998 at Puan BA

Month

Rainfall

1997 1998

mm

JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember

7860

1468

38116

95123387

10528

193236

387518

875

2368135

12273

Total 806 875

of N fertilization is not direct but depends on the densityand competitiveness of the crop

The method of N application can also have a markedinfluence on the competitive ability of crops and the degreeof interference from weeds (DiTomaso 1995) Green foxtail(Setaria viridis L) emergence and fecundity decreased withincreasing rates of N fertilizer banded 8-cm deep comparedwith broadcast application (OrsquoDonovan et al 1997) In ad-dition Thill et al (1994) found that in spring barley therewere 28 to 60 fewer wild oat plants where N had beenbanded than when it had been broadcast Kirkland and Be-ckie (1998) found that weed density biomass and N uptakemeasured early in the growing season averaged 20 to 40less and grain yield of wheat at maturity was higher whenfertilizer had been banded than when it had been broadcast

Although inorganic fertilizers are extensively used inwheat and barley there is little information to assess theeffect of fertilizer application on wild oat population dynam-ics The knowledge of the effect of N fertilization on weedpopulation dynamics might be useful for recommending themost convenient method and stage of the crop cycle forfertilizer application to both increase the crop yield and re-duce the growth rates of weed populations

The objectives of this study were to determine the effectof N application with and without the use of diclofop atbarley sowing and at early tillering on (1) wild oat seedlingemergence and survival (2) wild oat seed production andbiomass and (3) seed dispersal rate before crop harvest

Materials and Methods

Study Site Experimental Design and Treatments

Demographic studies of wild oat were conducted in PuanBuenos Aires province (378329S 628469W) during 1997 and1998 Rainfall conditions for both years are shown in Table1 The experiments were designed as a randomized completeblock with a split-plot arrangement with diclofop-methylapplication on the main plot and N treatment on the sub-plot Each treatment was replicated three times Barley wassown on July 18 1997 and July 6 1998 with 15-cm rowspacing in tilled soil with a conventional drill Each replicateof the main plot occupied an area of 126 m2 and eachsubplot was allocated to half the area (63 m2) The amounts

of N in the soil at sowing were 70 and 63 kg ha21 in 1997and 1998 respectively Soil type was an Entic Hapludollwith sandy loam texture pH 66 and 28 organic matterIn both years the experimental site was fertilized with 32kg P2O5 ha21 broadcast and incorporated immediately be-fore sowing

The herbicide treatment consisted of no herbicide or di-clofop-methyl sprayed at 511 g ha21 with 100 L water ha21

at 345 kPa when the crop was at the 1423 Zadoks stage(Zadoks et al 1974) 40 and 45 d after emergence in 1997and 1998 The main broadleaf weeds present in the exper-imental area were wild buckwheat (Polygonum convolvulusL) and prostrate knotweed (Polygonum aviculare L) Theywere controlled with bromoxynil at 363 g ha21 N treat-ments were 0 and 55 kg ha21 as granulated urea applied atcrop sowing and when barley was at the (1423) Zadoksstage immediately after diclofop application

Population Dynamics of Wild Oat

Immediately before crop sowing the amount of wild oatseed in the top 4 cm of soil was assessed Samples of thefirst 4 cm of soil were extracted from the experimental areasieved and the wild oat seed counted To assess weed pop-ulation dynamics three 04-m2 areas were delimited withinthe subplots Seedling emergence seedling survival and seedproduction per plant were documented within these areas

Seedling Emergence Seedling Survival Individual Plant SeedProduction and Biomass

Seedling emergence was first assessed 35 d after the crophad emerged Two different cohorts were identified andseedlings from each cohort were ringed with a different-colored plastic-coated wire The first cohort was composedof seedlings that had reached at least the four-leaf stage andthe other less developed seedlings were considered as be-longing to the second cohort Twenty days later a secondassessment was carried out and the third cohort was iden-tified and marked After that no seedling emergence wasdetected

The fate of each seedling was followed until crop matu-rity At crop maturity wild oat plants belonging to the dif-ferent cohorts were counted and classified as dead or aliveWild oat plants were removed by hand before crop harvestand the number of panicles spikelets and seed per spikeletswere counted

Biomass of plants belonging to each cohort was deter-mined after drying at 70 C for 48 h Seedling survival ratewas calculated as the number of plants found at crop ma-turity in relation to the number of plants identified as be-longing to each cohort Plant fecundity was obtained byaveraging the number of seed produced by all the individ-uals from each cohort

Quantification of Seed Rain Before Crop Harvest

Seed rain was quantified from individuals removed justbefore crop harvest to assess individual fecundity and dryweight On average the number of seed per spikelet was25 The number of empty spikelets was multiplied by 25and was considered as the preharvest seed rain per plant

618 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 2 Density of each wild oat (Avena fatua L) cohort that emerged in barley (Hordeum vulgare L) crops with and without diclofopuse and with varying nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt)a

Cohorts

No herbicde

N0 Ns Nt

Herbicide

N0 Ns Nt Averagea

seedlings m22

Cohort 1Cohort 2Cohort 3

212212

29

208306

42

221201

5

233183

37

204287

42

253282

62

222 a245 a44 b

LSD (P 005) NS 58

a Data for each treatment are combined over yearsb Means followed by the same letter are not significantly different (P 5 005)

The percentage of total seed production dispersal before har-vest was considered the seed dispersal rate

Data Analysis

The numbers of individuals (at any of the demographicstages considered) recorded under the various treatmentswere subjected to combined analysis of variance across yearsfor the respective experiments In the tables and figures eachdata point is averaged across years for an experiment exceptin those cases where a significant interaction (P 005)between factors and years was detected Means were com-pared using LSD (P 005) Demographic rates were pre-viously transformed to arcsine values to homogenize the var-iance (Sokal and Rohlf 1969)

Results and Discussion

Seed in Soil and Seedling Emergence

The numbers of seed per square meter buried in the soilimmediately before the sowing were 51 and 123 for the1997 and 1998 experiments respectively Three differentcohorts were identified during the crop cycle The numberof seedlings that emerged per square meter for each cohortwas not affected by N or by diclofop In addition there wasno significant interaction (P 005) between N diclofop-methyl application and seedling cohort size However av-eraging across treatments the number of seedlings belongingto the third cohort was lower than the number of seedlingsfrom the other two cohorts (P 005) (Table 2) Seedlingemergence represented 83 and 49 of the first 4 cm soilseed bank during 1997 and 1998 respectively

Seedling Survival

The effects of N on seedling survival rate were related tothe time of N application N application at crop tilleringincreased seedling survival rate However this effect de-pended on the wild oat cohort Seedling survival rate forindividuals belonging to the first cohort was not affected byN application (P 005) but it was affected significantlyby diclofop-methyl application (P 005) Averaging yearsand fertilizer treatments seedling survival rate was 28 and91 (P 005) in plots with and without herbicide ap-plication respectively (data not shown) For individuals be-longing to the second cohort without herbicide applicationseedling survival rate was higher (P 005) when N wasapplied at tillering than in plots without N application This

effect was not seen when herbicide was applied (interactionherb 3 fert P 005) (Figure 1A) N application at tiller-ing also increased the survival rate of seedlings from thethird cohort during 1997 but as in the case of the secondcohort this effect was not detected when diclofop-methylwas applied (interaction herb 3 fert P 005) (Figure 1B)Seedling survival rate for individuals belonging to the thirdcohort was increased only when N was applied at crop til-lering (P 005) in plots with diclofop-methyl application(interaction herb 3 fert P 005) in 1998 (Figure 1C)Thus two features seemed noteworthy (1) survival of thethird cohort in the absence of herbicide application was un-expectedly higher in 1998 than in 1997 indicating that thebeneficial effect of fertilization at crop tillering on seedlingsurvival was observed during 1997 only and (2) survivalwas also enhanced during 1998 under herbicide treatmentrevealing a beneficial effect of fertilization at crop tilleringon diclofop-treated seedlings that was not evident under the1997 conditions During 1997 early-season rainfall wasgreater than in 1998 (Table 1) It might be that because ofthe differential rainfall conditions between years aggressive-ness of the crop against this cohort of the weed had beenhigher in 1997 than in 1998 Under such conditions onlyfertilization at crop tillering can increase seedling survival

Individual Seed Production and Individual GrowthSimilar to seedling survival rate the effect of N on the

amount of seed produced per plant and individual plantbiomass was related to the time of N application For allthe experiments plant biomass and seed output per plantof wild oat at crop harvest was reduced as emergence timingwas delayed N application at tillering increased the individ-ual plant biomass of wild oat However when diclofop-methyl was applied no effect of N application was detected(interaction herb 3 fert P 005) Without diclofop in-dividual biomass (grams per plant) was 45 47 and 79 forN0 (no N) N at sowing (Ns) and N at tillering (Nt)respectively In the treated plots these figures were 27 42and 29 respectively

Seed production per wild oat plant was greater when Nhad been applied at tillering than without fertilizer Never-theless as in the study of individual plant biomass whendiclofop-methyl was applied there was no effect of N fer-tilization (P 005) (Table 3)

With regard to crop yield there were no significant dif-ferences (P 005) when comparing N application at sow-ing with that at crop tillering (data not shown) Howeverthe higher biomass and fecundity registered in individual

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 619

FIGURE 1 (A) Seedling survival rate for wild oat of the second cohort in barley without (2) and with (1) diclofop-methyl application under varying Nfertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for theherbicide 3 fertilization interaction (B) Seedling survival rate for wild oats of the third cohort in barley without (2) and with (1) diclofop-methylapplication under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) in 1997 Vertical linesindicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (C) Seedling survival rate for wild oats of the third cohort in barley without (2)and with (1) diclofop-methyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering(Nt) in 1998 Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (D) Seed dispersal rate () before harvest of wildoat in barley without (2) and with (1) diclofop-mehtyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction

TABLE 3 Seed production per plant of wild oat (Avena fatua L) for each cohort growing in barley (Hordeum vulgare L) crops with andwithout diclofop use and with varying nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at earlytillering (Nt)a

Cohorts

No herbicde

N0 Ns Nt

Herbicide

N0 Ns Nt

seeds per plantb

Cohort 1Cohort 2Cohort 3

107 bc61 cd42 cd

186 ab72 cd65 cd

258 a83 cd91 cd

59 cd27 cd17 cd

19 cd8 d4 d

61 cd21 cd10 d

LSD (P 005) 93

a Data for each treatment are combined over yearsb Means followed by the same letter are not significantly different (P 5 005)

wild oat plants show that the weed might be more favoredthan the crop by the addition of N at crop tillering Catulloet al (1982) found greater crop yield and seed productionof wild oat in wheat fertilized at tillering than in the absenceof fertilizer These results agree with those of Satorre andSnaydon (1992) who showed that fertilization reduced the

severity of competition experienced by wild oat In additionForcella (1984) showed a competitive advantage of wheatover annual ryegrass (Lolium rigidum Gaud) when N wasapplied before the three-leaf stage of the crop thus suggest-ing that sufficient nitrate availability in the soil after croptillering would stimulate annual ryegrass tillering and its

620 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 4 Annual growth rate of wild oat (Avena fatua L) popu-lation l 5 (NtN0) with and without diclofop use and with vary-ing nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 atsowing (Ns) 55 kg N ha21 at early tillering (Nt)ab

N treatments (2) No herbicide (1) Herbicide

N0NsNt

389 b431 b566 a

092 c022 c036 c

LSD (P 005) 118

a l 5 (NtN0) where Nt and N0 are the sizes of the soil seed banks atthe beginning and the end of the t-year period

b Means followed by the same letter are not significantly different (P 5005)

competitive ability Gonzalez Ponce (1998) studied the rel-ative competitive ability of barley and annual ryegrass withdifferent N availability and concluded that competitionfrom barley was greater than from the weed because of ear-lier tillering and N absorption by barley It appears that theearlier N is available the more beneficial it is for the croprsquoscompetitive ability In contrast if N is available at later cropstages (ie tillering) the weedrsquos growth is favored Henceit would be beneficial to apply N fertilizer at sowing espe-cially in those situations where the density of wild oat islow and it would not be advantageous to apply a herbicideThis means that to avoid the increase in size of the weedpopulation in relation to what might be expected in theabsence of N application and chemical control it would beadvantageous to apply N fertilizer at sowing If N is appliedat tillering then the application of diclofop-methyl or anyother appropriate herbicide is essential

Total Seed Production

Total seed production was estimated considering the dif-ferent demographic rates for all the treatments As expectedthe amount of seed per square meter was higher when Nwas applied at tillering (P 005) and as noted in thestudy of adult fecundity the effect of N was negligible whenherbicide was applied In herbicide-free plots these figureswere 3031 5545 and 7287 seed m22 for N0 Ns andNt respectively (data not shown) On the other hand inthe treated plots these figures were 547 171 and 536 seedm22 for N0 Ns and Nt respectively The first cohort pro-duced 67 63 and 76 of the total seed production in plotswhere N was applied at tillering but not at sowing

Seed Dispersal Before Crop HarvestThe effects of N fertilization at tillering on both seedling

survival rate and individual fecundity are beneficial to wildoat However the seed dispersal rate of wild oat before har-vest was lower when N was applied than it was in controlplots (Figure 1D) This is a compensation for higher wildoat seed production under fertilization at crop tillering inproduction systems where grain is not cleaned by the com-bine and is therefore exported from the field In this caseit would be reasonable to recommend that crop harvestshould not be delayed especially in those situations whereN has been applied at tillering to make use of the effect ofa N-induced lower seed dispersal rate In addition the de-laying effect of N applied at tillering on seed dispersal wasenhanced when diclofop-methyl was applied (interactionherb 3 fert P 005) (Figure 1D)

The lower seed dispersal rate found under N applicationcompared with that recorded in control plots could be as-sociated with a delay in seed maturation This could beconsidered a direct effect of N on weed phenology IndeedFrank and Bauer (1996) reported the effects of N on thedevelopment of wheat Alternatively an indirect effect onseed maturation attributable to different environmental con-ditions for seed development in plots with N applicationcould be considered In this case it might be possible thattemperature conditions for wild oat seed developing belowthe canopy were lower in fertilized plots because of the high-er crop biomass than in plots without fertilizer applicationFor example Adkins et al (1987) reported a delay in the

time of maturation of wild oat seed when the temperatureduring their development was 20 C compared with 25 CRegardless of the processes influenced by N application weconclude that crop harvest should not be delayed to makeuse of the negative effects of N on seed dispersal rate beforeharvest

The Estimated Size of The Soil Seed Bank andThe Annual Population Growth Rate

Averaging both experimental years and regarding all thedemographic processes the amounts of seed per square me-ter that should have returned to the soil in diclofop plotswere 252 49 and 80 for N0 Ns and Nt treatments re-spectively On the other hand in plots without diclofop-methyl the estimated amounts of seed that should have re-turned to the soil were 1303 1445 and 1859 for N0Ns and Nt respectively Previous studies have shown thatseed predation is the most important demographic featurereducing the soil seed bank during the fallow period (Wilson1972 1985 Wilson and Cussans 1975) Moreover studiescarried out under similar conditions in the same area as forthe current study (Scursoni et al 1999) confirmed thatwhen seed were left on the soil surface during fallow pre-dation was the most important process for decreasing thesize of the seed population

To estimate the annual growth rate of the population l5 (NtN0) where Nt and N0 are the size of the soil seedbank at the beginning and the end of the t-year period(Fernandez Quintanilla et al 1986) we considered the pre-dation and emergence rates recorded by Scursoni et al(1999) These figures were 062 and 012 for predation andemergence in control plots and 043 and 018 for the sameprocesses in the treated plots respectively Thus the ratesof increase during the crop cycle and the fallow period withor without diclofop-methyl are shown in Table 4

The effectiveness of predation depends on environmentalconditions the location of seed in the soil tillage predatorsand food availability and the amount and type of posthar-vest residues (Cromar et al 1999) Therefore it would benecessary to focus on the study of this process in order tomake use of predation to diminish the size of the seed bankThe results obtained in this work allowed us to identify theeffects of N application on the population dynamics of wildoat in barley Development of functional relations that in-tegrated these data into population models could be usefuland could allow the prediction of population dynamics In-

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 621

tegrated models would also permit the design of scientifi-cally sound strategies to manage wild oat populations

Acknowledgments

The authors thank Malterıa Pampa SA and especially MarioCattaneo for their assistance in field experiments and Frank Forcellafor his suggestions on the manuscript

Literature Cited

Adkins S W M Lowen and S J Symons 1987 Variation within purelines of wild oats (Avena fatua) in relation to temperature of devel-opment Weed Sci 35169ndash172

Anderson T and P Milberg 1998 Weed flora and the relative importanceof site crop crop rotation and nitrogen Weed Sci 4630ndash38

Anderson R L D L Tanaka A L Black and E Scheizer 1998 Weedcommunity and species response to crop rotation tillage and N fer-tility Weed Technol 12531ndash536

Brandt R H H Janzen and R E Blackshaw 2000 Ranking weed re-sponse to added nitrogen and phosphorous Abstracts of the III In-ternational Weed Science Congress Foz do Iguassu Brazil 6ndash11 June2000 26 p

Carlson H L and J E Hill 1986 Wild oat (Avena fatua) competitionwith spring wheat effects of nitrogen fertilization Weed Sci 3429ndash33

Catullo J M L Rodrıguez and I Colombo 1982 Efecto de la Avenafatua L sobre el cultivo de trigo influencia de la fertilizacion CongrNac Malezas San Miguel de Tucuman Argentina 216ndash21

Cromar H E S D Murphy and C J Swanton 1999 Influence of tillageand crop residue on postdispersal predation of weeds seed Weed Sci47184ndash194

DiTomaso J M 1995 Approaches for improving crop competitivenessthrough the manipulation of fertilization strategies Weed Sci 43491ndash497

Fernandez Quintanilla C L Navarrete J L Gonzalez Andujar A Fer-nandez and M J Sanchez 1986 Seedling recruitment and age-spe-cific survivorship and recruitment in populations of Avena sterilis sppludoviciana J Appl Ecol 23945ndash955

Forcella F 1984 Wheat and ryegrass competition for pulses of mineralnitrogen Aust J Exp Agric Anim Husb 24421ndash425

Frank A B and A Bauer 1996 Temperature nitrogen and carbon di-oxide effects on spring wheat development and spikelet numbers CropSci 36659ndash665

Gonzalez Ponce R 1998 Competition between barley and Lolium rigidumfor nitrate Weed Res 38453ndash460

Jornsgard B K Rasmussen J Hill and J L Christiansen 1996 Influenceof nitrogen on competition between cereals and their natural weedpopulations Weed Res 36461ndash470

Kegode G F Forcella and S Clay 1999 Influence of crop rotationtillage and management inputs on weed seed production Weed Sci47175ndash183

Knezevic S S Evans C Shapiro and J Lindquist 2000 Effect of nitro-gen on critical period of weed control in corn Abstracts of the IIIInternational Weed Science Congress 6ndash11 June 2000 Foz do Iguas-su Brazil 52 p

Kirkland K and H Beckie 1998 Contribution of nitrogen fertiliser place-ment to weed management in spring wheat (Triticum aestivum) WeedTechnol 12507ndash514

OrsquoDonovan J T D W McAndrew and G Thomas 1997 Tillage andnitrogen influence weed population dynamics in barley (Hordeum vul-gare) Weed Technol 11502ndash509

Satorre E H and R W Snaydon 1992 A comparison of root and shootcompetition between spring cereals and Avena fatua L Weed Res 3245ndash55

Scursoni J 1995 Relevamiento de malezas en cultivos de cebada cervecera(Hordeum vulgare L) en la Provincia de Buenos Aires Argentina RevFac Agron La Plata 71235ndash243

Scursoni J R Benech Arnold and H Hirchoren 1999 Demography ofwild oat (Avena fatua L) in barley crops effect of crop crop densityand herbicide treatment Agron J 91478ndash485

Sindel B M 1991 A review of the ecology and control of thistles inAustralia Weed Res 31189ndash201

Sokal R R and F J Rohlf 1969 Biometry The Principles and Practicesof Statistics in Biological Research San Francisco Freeman pp 380ndash387

Thill D C J T OrsquoDonovan and C A Mallory Smith 1994 Integratedweed management strategies for delaying herbicide resistant in wildoats Phytoprotection 75(Suppl)61ndash70

Wilson B J 1972 Studies of the fate of Avena fatua seeds on cereal stubbleas influenced by autumn treatment Pages 242ndash247 in Proceedings ofthe 11th British Weed Control Conference Brighton UK

Wilson B J 1985 Effect of seed age and cultivation on seedling emergenceand seed decline of Avena fatua L in winter barley Weed Res 25213ndash219

Wilson B J and G W Cussans 1975 A study of the population dynamicsof Avena fatua L as influenced by straw burning seed shedding andcultivation Weed Res 15249ndash258

Zadoks J C T Chang and C Konzak 1974 A decimal code for thegrowth stage of cereals Weed Res 14415ndash421

Received July 31 2001 and approved April 4 2002

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 617

TABLE 1 Monthly rainfall for 1997 and 1998 at Puan BA

Month

Rainfall

1997 1998

mm

JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember

7860

1468

38116

95123387

10528

193236

387518

875

2368135

12273

Total 806 875

of N fertilization is not direct but depends on the densityand competitiveness of the crop

The method of N application can also have a markedinfluence on the competitive ability of crops and the degreeof interference from weeds (DiTomaso 1995) Green foxtail(Setaria viridis L) emergence and fecundity decreased withincreasing rates of N fertilizer banded 8-cm deep comparedwith broadcast application (OrsquoDonovan et al 1997) In ad-dition Thill et al (1994) found that in spring barley therewere 28 to 60 fewer wild oat plants where N had beenbanded than when it had been broadcast Kirkland and Be-ckie (1998) found that weed density biomass and N uptakemeasured early in the growing season averaged 20 to 40less and grain yield of wheat at maturity was higher whenfertilizer had been banded than when it had been broadcast

Although inorganic fertilizers are extensively used inwheat and barley there is little information to assess theeffect of fertilizer application on wild oat population dynam-ics The knowledge of the effect of N fertilization on weedpopulation dynamics might be useful for recommending themost convenient method and stage of the crop cycle forfertilizer application to both increase the crop yield and re-duce the growth rates of weed populations

The objectives of this study were to determine the effectof N application with and without the use of diclofop atbarley sowing and at early tillering on (1) wild oat seedlingemergence and survival (2) wild oat seed production andbiomass and (3) seed dispersal rate before crop harvest

Materials and Methods

Study Site Experimental Design and Treatments

Demographic studies of wild oat were conducted in PuanBuenos Aires province (378329S 628469W) during 1997 and1998 Rainfall conditions for both years are shown in Table1 The experiments were designed as a randomized completeblock with a split-plot arrangement with diclofop-methylapplication on the main plot and N treatment on the sub-plot Each treatment was replicated three times Barley wassown on July 18 1997 and July 6 1998 with 15-cm rowspacing in tilled soil with a conventional drill Each replicateof the main plot occupied an area of 126 m2 and eachsubplot was allocated to half the area (63 m2) The amounts

of N in the soil at sowing were 70 and 63 kg ha21 in 1997and 1998 respectively Soil type was an Entic Hapludollwith sandy loam texture pH 66 and 28 organic matterIn both years the experimental site was fertilized with 32kg P2O5 ha21 broadcast and incorporated immediately be-fore sowing

The herbicide treatment consisted of no herbicide or di-clofop-methyl sprayed at 511 g ha21 with 100 L water ha21

at 345 kPa when the crop was at the 1423 Zadoks stage(Zadoks et al 1974) 40 and 45 d after emergence in 1997and 1998 The main broadleaf weeds present in the exper-imental area were wild buckwheat (Polygonum convolvulusL) and prostrate knotweed (Polygonum aviculare L) Theywere controlled with bromoxynil at 363 g ha21 N treat-ments were 0 and 55 kg ha21 as granulated urea applied atcrop sowing and when barley was at the (1423) Zadoksstage immediately after diclofop application

Population Dynamics of Wild Oat

Immediately before crop sowing the amount of wild oatseed in the top 4 cm of soil was assessed Samples of thefirst 4 cm of soil were extracted from the experimental areasieved and the wild oat seed counted To assess weed pop-ulation dynamics three 04-m2 areas were delimited withinthe subplots Seedling emergence seedling survival and seedproduction per plant were documented within these areas

Seedling Emergence Seedling Survival Individual Plant SeedProduction and Biomass

Seedling emergence was first assessed 35 d after the crophad emerged Two different cohorts were identified andseedlings from each cohort were ringed with a different-colored plastic-coated wire The first cohort was composedof seedlings that had reached at least the four-leaf stage andthe other less developed seedlings were considered as be-longing to the second cohort Twenty days later a secondassessment was carried out and the third cohort was iden-tified and marked After that no seedling emergence wasdetected

The fate of each seedling was followed until crop matu-rity At crop maturity wild oat plants belonging to the dif-ferent cohorts were counted and classified as dead or aliveWild oat plants were removed by hand before crop harvestand the number of panicles spikelets and seed per spikeletswere counted

Biomass of plants belonging to each cohort was deter-mined after drying at 70 C for 48 h Seedling survival ratewas calculated as the number of plants found at crop ma-turity in relation to the number of plants identified as be-longing to each cohort Plant fecundity was obtained byaveraging the number of seed produced by all the individ-uals from each cohort

Quantification of Seed Rain Before Crop Harvest

Seed rain was quantified from individuals removed justbefore crop harvest to assess individual fecundity and dryweight On average the number of seed per spikelet was25 The number of empty spikelets was multiplied by 25and was considered as the preharvest seed rain per plant

618 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 2 Density of each wild oat (Avena fatua L) cohort that emerged in barley (Hordeum vulgare L) crops with and without diclofopuse and with varying nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt)a

Cohorts

No herbicde

N0 Ns Nt

Herbicide

N0 Ns Nt Averagea

seedlings m22

Cohort 1Cohort 2Cohort 3

212212

29

208306

42

221201

5

233183

37

204287

42

253282

62

222 a245 a44 b

LSD (P 005) NS 58

a Data for each treatment are combined over yearsb Means followed by the same letter are not significantly different (P 5 005)

The percentage of total seed production dispersal before har-vest was considered the seed dispersal rate

Data Analysis

The numbers of individuals (at any of the demographicstages considered) recorded under the various treatmentswere subjected to combined analysis of variance across yearsfor the respective experiments In the tables and figures eachdata point is averaged across years for an experiment exceptin those cases where a significant interaction (P 005)between factors and years was detected Means were com-pared using LSD (P 005) Demographic rates were pre-viously transformed to arcsine values to homogenize the var-iance (Sokal and Rohlf 1969)

Results and Discussion

Seed in Soil and Seedling Emergence

The numbers of seed per square meter buried in the soilimmediately before the sowing were 51 and 123 for the1997 and 1998 experiments respectively Three differentcohorts were identified during the crop cycle The numberof seedlings that emerged per square meter for each cohortwas not affected by N or by diclofop In addition there wasno significant interaction (P 005) between N diclofop-methyl application and seedling cohort size However av-eraging across treatments the number of seedlings belongingto the third cohort was lower than the number of seedlingsfrom the other two cohorts (P 005) (Table 2) Seedlingemergence represented 83 and 49 of the first 4 cm soilseed bank during 1997 and 1998 respectively

Seedling Survival

The effects of N on seedling survival rate were related tothe time of N application N application at crop tilleringincreased seedling survival rate However this effect de-pended on the wild oat cohort Seedling survival rate forindividuals belonging to the first cohort was not affected byN application (P 005) but it was affected significantlyby diclofop-methyl application (P 005) Averaging yearsand fertilizer treatments seedling survival rate was 28 and91 (P 005) in plots with and without herbicide ap-plication respectively (data not shown) For individuals be-longing to the second cohort without herbicide applicationseedling survival rate was higher (P 005) when N wasapplied at tillering than in plots without N application This

effect was not seen when herbicide was applied (interactionherb 3 fert P 005) (Figure 1A) N application at tiller-ing also increased the survival rate of seedlings from thethird cohort during 1997 but as in the case of the secondcohort this effect was not detected when diclofop-methylwas applied (interaction herb 3 fert P 005) (Figure 1B)Seedling survival rate for individuals belonging to the thirdcohort was increased only when N was applied at crop til-lering (P 005) in plots with diclofop-methyl application(interaction herb 3 fert P 005) in 1998 (Figure 1C)Thus two features seemed noteworthy (1) survival of thethird cohort in the absence of herbicide application was un-expectedly higher in 1998 than in 1997 indicating that thebeneficial effect of fertilization at crop tillering on seedlingsurvival was observed during 1997 only and (2) survivalwas also enhanced during 1998 under herbicide treatmentrevealing a beneficial effect of fertilization at crop tilleringon diclofop-treated seedlings that was not evident under the1997 conditions During 1997 early-season rainfall wasgreater than in 1998 (Table 1) It might be that because ofthe differential rainfall conditions between years aggressive-ness of the crop against this cohort of the weed had beenhigher in 1997 than in 1998 Under such conditions onlyfertilization at crop tillering can increase seedling survival

Individual Seed Production and Individual GrowthSimilar to seedling survival rate the effect of N on the

amount of seed produced per plant and individual plantbiomass was related to the time of N application For allthe experiments plant biomass and seed output per plantof wild oat at crop harvest was reduced as emergence timingwas delayed N application at tillering increased the individ-ual plant biomass of wild oat However when diclofop-methyl was applied no effect of N application was detected(interaction herb 3 fert P 005) Without diclofop in-dividual biomass (grams per plant) was 45 47 and 79 forN0 (no N) N at sowing (Ns) and N at tillering (Nt)respectively In the treated plots these figures were 27 42and 29 respectively

Seed production per wild oat plant was greater when Nhad been applied at tillering than without fertilizer Never-theless as in the study of individual plant biomass whendiclofop-methyl was applied there was no effect of N fer-tilization (P 005) (Table 3)

With regard to crop yield there were no significant dif-ferences (P 005) when comparing N application at sow-ing with that at crop tillering (data not shown) Howeverthe higher biomass and fecundity registered in individual

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 619

FIGURE 1 (A) Seedling survival rate for wild oat of the second cohort in barley without (2) and with (1) diclofop-methyl application under varying Nfertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for theherbicide 3 fertilization interaction (B) Seedling survival rate for wild oats of the third cohort in barley without (2) and with (1) diclofop-methylapplication under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) in 1997 Vertical linesindicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (C) Seedling survival rate for wild oats of the third cohort in barley without (2)and with (1) diclofop-methyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering(Nt) in 1998 Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (D) Seed dispersal rate () before harvest of wildoat in barley without (2) and with (1) diclofop-mehtyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction

TABLE 3 Seed production per plant of wild oat (Avena fatua L) for each cohort growing in barley (Hordeum vulgare L) crops with andwithout diclofop use and with varying nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at earlytillering (Nt)a

Cohorts

No herbicde

N0 Ns Nt

Herbicide

N0 Ns Nt

seeds per plantb

Cohort 1Cohort 2Cohort 3

107 bc61 cd42 cd

186 ab72 cd65 cd

258 a83 cd91 cd

59 cd27 cd17 cd

19 cd8 d4 d

61 cd21 cd10 d

LSD (P 005) 93

a Data for each treatment are combined over yearsb Means followed by the same letter are not significantly different (P 5 005)

wild oat plants show that the weed might be more favoredthan the crop by the addition of N at crop tillering Catulloet al (1982) found greater crop yield and seed productionof wild oat in wheat fertilized at tillering than in the absenceof fertilizer These results agree with those of Satorre andSnaydon (1992) who showed that fertilization reduced the

severity of competition experienced by wild oat In additionForcella (1984) showed a competitive advantage of wheatover annual ryegrass (Lolium rigidum Gaud) when N wasapplied before the three-leaf stage of the crop thus suggest-ing that sufficient nitrate availability in the soil after croptillering would stimulate annual ryegrass tillering and its

620 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 4 Annual growth rate of wild oat (Avena fatua L) popu-lation l 5 (NtN0) with and without diclofop use and with vary-ing nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 atsowing (Ns) 55 kg N ha21 at early tillering (Nt)ab

N treatments (2) No herbicide (1) Herbicide

N0NsNt

389 b431 b566 a

092 c022 c036 c

LSD (P 005) 118

a l 5 (NtN0) where Nt and N0 are the sizes of the soil seed banks atthe beginning and the end of the t-year period

b Means followed by the same letter are not significantly different (P 5005)

competitive ability Gonzalez Ponce (1998) studied the rel-ative competitive ability of barley and annual ryegrass withdifferent N availability and concluded that competitionfrom barley was greater than from the weed because of ear-lier tillering and N absorption by barley It appears that theearlier N is available the more beneficial it is for the croprsquoscompetitive ability In contrast if N is available at later cropstages (ie tillering) the weedrsquos growth is favored Henceit would be beneficial to apply N fertilizer at sowing espe-cially in those situations where the density of wild oat islow and it would not be advantageous to apply a herbicideThis means that to avoid the increase in size of the weedpopulation in relation to what might be expected in theabsence of N application and chemical control it would beadvantageous to apply N fertilizer at sowing If N is appliedat tillering then the application of diclofop-methyl or anyother appropriate herbicide is essential

Total Seed Production

Total seed production was estimated considering the dif-ferent demographic rates for all the treatments As expectedthe amount of seed per square meter was higher when Nwas applied at tillering (P 005) and as noted in thestudy of adult fecundity the effect of N was negligible whenherbicide was applied In herbicide-free plots these figureswere 3031 5545 and 7287 seed m22 for N0 Ns andNt respectively (data not shown) On the other hand inthe treated plots these figures were 547 171 and 536 seedm22 for N0 Ns and Nt respectively The first cohort pro-duced 67 63 and 76 of the total seed production in plotswhere N was applied at tillering but not at sowing

Seed Dispersal Before Crop HarvestThe effects of N fertilization at tillering on both seedling

survival rate and individual fecundity are beneficial to wildoat However the seed dispersal rate of wild oat before har-vest was lower when N was applied than it was in controlplots (Figure 1D) This is a compensation for higher wildoat seed production under fertilization at crop tillering inproduction systems where grain is not cleaned by the com-bine and is therefore exported from the field In this caseit would be reasonable to recommend that crop harvestshould not be delayed especially in those situations whereN has been applied at tillering to make use of the effect ofa N-induced lower seed dispersal rate In addition the de-laying effect of N applied at tillering on seed dispersal wasenhanced when diclofop-methyl was applied (interactionherb 3 fert P 005) (Figure 1D)

The lower seed dispersal rate found under N applicationcompared with that recorded in control plots could be as-sociated with a delay in seed maturation This could beconsidered a direct effect of N on weed phenology IndeedFrank and Bauer (1996) reported the effects of N on thedevelopment of wheat Alternatively an indirect effect onseed maturation attributable to different environmental con-ditions for seed development in plots with N applicationcould be considered In this case it might be possible thattemperature conditions for wild oat seed developing belowthe canopy were lower in fertilized plots because of the high-er crop biomass than in plots without fertilizer applicationFor example Adkins et al (1987) reported a delay in the

time of maturation of wild oat seed when the temperatureduring their development was 20 C compared with 25 CRegardless of the processes influenced by N application weconclude that crop harvest should not be delayed to makeuse of the negative effects of N on seed dispersal rate beforeharvest

The Estimated Size of The Soil Seed Bank andThe Annual Population Growth Rate

Averaging both experimental years and regarding all thedemographic processes the amounts of seed per square me-ter that should have returned to the soil in diclofop plotswere 252 49 and 80 for N0 Ns and Nt treatments re-spectively On the other hand in plots without diclofop-methyl the estimated amounts of seed that should have re-turned to the soil were 1303 1445 and 1859 for N0Ns and Nt respectively Previous studies have shown thatseed predation is the most important demographic featurereducing the soil seed bank during the fallow period (Wilson1972 1985 Wilson and Cussans 1975) Moreover studiescarried out under similar conditions in the same area as forthe current study (Scursoni et al 1999) confirmed thatwhen seed were left on the soil surface during fallow pre-dation was the most important process for decreasing thesize of the seed population

To estimate the annual growth rate of the population l5 (NtN0) where Nt and N0 are the size of the soil seedbank at the beginning and the end of the t-year period(Fernandez Quintanilla et al 1986) we considered the pre-dation and emergence rates recorded by Scursoni et al(1999) These figures were 062 and 012 for predation andemergence in control plots and 043 and 018 for the sameprocesses in the treated plots respectively Thus the ratesof increase during the crop cycle and the fallow period withor without diclofop-methyl are shown in Table 4

The effectiveness of predation depends on environmentalconditions the location of seed in the soil tillage predatorsand food availability and the amount and type of posthar-vest residues (Cromar et al 1999) Therefore it would benecessary to focus on the study of this process in order tomake use of predation to diminish the size of the seed bankThe results obtained in this work allowed us to identify theeffects of N application on the population dynamics of wildoat in barley Development of functional relations that in-tegrated these data into population models could be usefuland could allow the prediction of population dynamics In-

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 621

tegrated models would also permit the design of scientifi-cally sound strategies to manage wild oat populations

Acknowledgments

The authors thank Malterıa Pampa SA and especially MarioCattaneo for their assistance in field experiments and Frank Forcellafor his suggestions on the manuscript

Literature Cited

Adkins S W M Lowen and S J Symons 1987 Variation within purelines of wild oats (Avena fatua) in relation to temperature of devel-opment Weed Sci 35169ndash172

Anderson T and P Milberg 1998 Weed flora and the relative importanceof site crop crop rotation and nitrogen Weed Sci 4630ndash38

Anderson R L D L Tanaka A L Black and E Scheizer 1998 Weedcommunity and species response to crop rotation tillage and N fer-tility Weed Technol 12531ndash536

Brandt R H H Janzen and R E Blackshaw 2000 Ranking weed re-sponse to added nitrogen and phosphorous Abstracts of the III In-ternational Weed Science Congress Foz do Iguassu Brazil 6ndash11 June2000 26 p

Carlson H L and J E Hill 1986 Wild oat (Avena fatua) competitionwith spring wheat effects of nitrogen fertilization Weed Sci 3429ndash33

Catullo J M L Rodrıguez and I Colombo 1982 Efecto de la Avenafatua L sobre el cultivo de trigo influencia de la fertilizacion CongrNac Malezas San Miguel de Tucuman Argentina 216ndash21

Cromar H E S D Murphy and C J Swanton 1999 Influence of tillageand crop residue on postdispersal predation of weeds seed Weed Sci47184ndash194

DiTomaso J M 1995 Approaches for improving crop competitivenessthrough the manipulation of fertilization strategies Weed Sci 43491ndash497

Fernandez Quintanilla C L Navarrete J L Gonzalez Andujar A Fer-nandez and M J Sanchez 1986 Seedling recruitment and age-spe-cific survivorship and recruitment in populations of Avena sterilis sppludoviciana J Appl Ecol 23945ndash955

Forcella F 1984 Wheat and ryegrass competition for pulses of mineralnitrogen Aust J Exp Agric Anim Husb 24421ndash425

Frank A B and A Bauer 1996 Temperature nitrogen and carbon di-oxide effects on spring wheat development and spikelet numbers CropSci 36659ndash665

Gonzalez Ponce R 1998 Competition between barley and Lolium rigidumfor nitrate Weed Res 38453ndash460

Jornsgard B K Rasmussen J Hill and J L Christiansen 1996 Influenceof nitrogen on competition between cereals and their natural weedpopulations Weed Res 36461ndash470

Kegode G F Forcella and S Clay 1999 Influence of crop rotationtillage and management inputs on weed seed production Weed Sci47175ndash183

Knezevic S S Evans C Shapiro and J Lindquist 2000 Effect of nitro-gen on critical period of weed control in corn Abstracts of the IIIInternational Weed Science Congress 6ndash11 June 2000 Foz do Iguas-su Brazil 52 p

Kirkland K and H Beckie 1998 Contribution of nitrogen fertiliser place-ment to weed management in spring wheat (Triticum aestivum) WeedTechnol 12507ndash514

OrsquoDonovan J T D W McAndrew and G Thomas 1997 Tillage andnitrogen influence weed population dynamics in barley (Hordeum vul-gare) Weed Technol 11502ndash509

Satorre E H and R W Snaydon 1992 A comparison of root and shootcompetition between spring cereals and Avena fatua L Weed Res 3245ndash55

Scursoni J 1995 Relevamiento de malezas en cultivos de cebada cervecera(Hordeum vulgare L) en la Provincia de Buenos Aires Argentina RevFac Agron La Plata 71235ndash243

Scursoni J R Benech Arnold and H Hirchoren 1999 Demography ofwild oat (Avena fatua L) in barley crops effect of crop crop densityand herbicide treatment Agron J 91478ndash485

Sindel B M 1991 A review of the ecology and control of thistles inAustralia Weed Res 31189ndash201

Sokal R R and F J Rohlf 1969 Biometry The Principles and Practicesof Statistics in Biological Research San Francisco Freeman pp 380ndash387

Thill D C J T OrsquoDonovan and C A Mallory Smith 1994 Integratedweed management strategies for delaying herbicide resistant in wildoats Phytoprotection 75(Suppl)61ndash70

Wilson B J 1972 Studies of the fate of Avena fatua seeds on cereal stubbleas influenced by autumn treatment Pages 242ndash247 in Proceedings ofthe 11th British Weed Control Conference Brighton UK

Wilson B J 1985 Effect of seed age and cultivation on seedling emergenceand seed decline of Avena fatua L in winter barley Weed Res 25213ndash219

Wilson B J and G W Cussans 1975 A study of the population dynamicsof Avena fatua L as influenced by straw burning seed shedding andcultivation Weed Res 15249ndash258

Zadoks J C T Chang and C Konzak 1974 A decimal code for thegrowth stage of cereals Weed Res 14415ndash421

Received July 31 2001 and approved April 4 2002

618 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 2 Density of each wild oat (Avena fatua L) cohort that emerged in barley (Hordeum vulgare L) crops with and without diclofopuse and with varying nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt)a

Cohorts

No herbicde

N0 Ns Nt

Herbicide

N0 Ns Nt Averagea

seedlings m22

Cohort 1Cohort 2Cohort 3

212212

29

208306

42

221201

5

233183

37

204287

42

253282

62

222 a245 a44 b

LSD (P 005) NS 58

a Data for each treatment are combined over yearsb Means followed by the same letter are not significantly different (P 5 005)

The percentage of total seed production dispersal before har-vest was considered the seed dispersal rate

Data Analysis

The numbers of individuals (at any of the demographicstages considered) recorded under the various treatmentswere subjected to combined analysis of variance across yearsfor the respective experiments In the tables and figures eachdata point is averaged across years for an experiment exceptin those cases where a significant interaction (P 005)between factors and years was detected Means were com-pared using LSD (P 005) Demographic rates were pre-viously transformed to arcsine values to homogenize the var-iance (Sokal and Rohlf 1969)

Results and Discussion

Seed in Soil and Seedling Emergence

The numbers of seed per square meter buried in the soilimmediately before the sowing were 51 and 123 for the1997 and 1998 experiments respectively Three differentcohorts were identified during the crop cycle The numberof seedlings that emerged per square meter for each cohortwas not affected by N or by diclofop In addition there wasno significant interaction (P 005) between N diclofop-methyl application and seedling cohort size However av-eraging across treatments the number of seedlings belongingto the third cohort was lower than the number of seedlingsfrom the other two cohorts (P 005) (Table 2) Seedlingemergence represented 83 and 49 of the first 4 cm soilseed bank during 1997 and 1998 respectively

Seedling Survival

The effects of N on seedling survival rate were related tothe time of N application N application at crop tilleringincreased seedling survival rate However this effect de-pended on the wild oat cohort Seedling survival rate forindividuals belonging to the first cohort was not affected byN application (P 005) but it was affected significantlyby diclofop-methyl application (P 005) Averaging yearsand fertilizer treatments seedling survival rate was 28 and91 (P 005) in plots with and without herbicide ap-plication respectively (data not shown) For individuals be-longing to the second cohort without herbicide applicationseedling survival rate was higher (P 005) when N wasapplied at tillering than in plots without N application This

effect was not seen when herbicide was applied (interactionherb 3 fert P 005) (Figure 1A) N application at tiller-ing also increased the survival rate of seedlings from thethird cohort during 1997 but as in the case of the secondcohort this effect was not detected when diclofop-methylwas applied (interaction herb 3 fert P 005) (Figure 1B)Seedling survival rate for individuals belonging to the thirdcohort was increased only when N was applied at crop til-lering (P 005) in plots with diclofop-methyl application(interaction herb 3 fert P 005) in 1998 (Figure 1C)Thus two features seemed noteworthy (1) survival of thethird cohort in the absence of herbicide application was un-expectedly higher in 1998 than in 1997 indicating that thebeneficial effect of fertilization at crop tillering on seedlingsurvival was observed during 1997 only and (2) survivalwas also enhanced during 1998 under herbicide treatmentrevealing a beneficial effect of fertilization at crop tilleringon diclofop-treated seedlings that was not evident under the1997 conditions During 1997 early-season rainfall wasgreater than in 1998 (Table 1) It might be that because ofthe differential rainfall conditions between years aggressive-ness of the crop against this cohort of the weed had beenhigher in 1997 than in 1998 Under such conditions onlyfertilization at crop tillering can increase seedling survival

Individual Seed Production and Individual GrowthSimilar to seedling survival rate the effect of N on the

amount of seed produced per plant and individual plantbiomass was related to the time of N application For allthe experiments plant biomass and seed output per plantof wild oat at crop harvest was reduced as emergence timingwas delayed N application at tillering increased the individ-ual plant biomass of wild oat However when diclofop-methyl was applied no effect of N application was detected(interaction herb 3 fert P 005) Without diclofop in-dividual biomass (grams per plant) was 45 47 and 79 forN0 (no N) N at sowing (Ns) and N at tillering (Nt)respectively In the treated plots these figures were 27 42and 29 respectively

Seed production per wild oat plant was greater when Nhad been applied at tillering than without fertilizer Never-theless as in the study of individual plant biomass whendiclofop-methyl was applied there was no effect of N fer-tilization (P 005) (Table 3)

With regard to crop yield there were no significant dif-ferences (P 005) when comparing N application at sow-ing with that at crop tillering (data not shown) Howeverthe higher biomass and fecundity registered in individual

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 619

FIGURE 1 (A) Seedling survival rate for wild oat of the second cohort in barley without (2) and with (1) diclofop-methyl application under varying Nfertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for theherbicide 3 fertilization interaction (B) Seedling survival rate for wild oats of the third cohort in barley without (2) and with (1) diclofop-methylapplication under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) in 1997 Vertical linesindicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (C) Seedling survival rate for wild oats of the third cohort in barley without (2)and with (1) diclofop-methyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering(Nt) in 1998 Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (D) Seed dispersal rate () before harvest of wildoat in barley without (2) and with (1) diclofop-mehtyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction

TABLE 3 Seed production per plant of wild oat (Avena fatua L) for each cohort growing in barley (Hordeum vulgare L) crops with andwithout diclofop use and with varying nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at earlytillering (Nt)a

Cohorts

No herbicde

N0 Ns Nt

Herbicide

N0 Ns Nt

seeds per plantb

Cohort 1Cohort 2Cohort 3

107 bc61 cd42 cd

186 ab72 cd65 cd

258 a83 cd91 cd

59 cd27 cd17 cd

19 cd8 d4 d

61 cd21 cd10 d

LSD (P 005) 93

a Data for each treatment are combined over yearsb Means followed by the same letter are not significantly different (P 5 005)

wild oat plants show that the weed might be more favoredthan the crop by the addition of N at crop tillering Catulloet al (1982) found greater crop yield and seed productionof wild oat in wheat fertilized at tillering than in the absenceof fertilizer These results agree with those of Satorre andSnaydon (1992) who showed that fertilization reduced the

severity of competition experienced by wild oat In additionForcella (1984) showed a competitive advantage of wheatover annual ryegrass (Lolium rigidum Gaud) when N wasapplied before the three-leaf stage of the crop thus suggest-ing that sufficient nitrate availability in the soil after croptillering would stimulate annual ryegrass tillering and its

620 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 4 Annual growth rate of wild oat (Avena fatua L) popu-lation l 5 (NtN0) with and without diclofop use and with vary-ing nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 atsowing (Ns) 55 kg N ha21 at early tillering (Nt)ab

N treatments (2) No herbicide (1) Herbicide

N0NsNt

389 b431 b566 a

092 c022 c036 c

LSD (P 005) 118

a l 5 (NtN0) where Nt and N0 are the sizes of the soil seed banks atthe beginning and the end of the t-year period

b Means followed by the same letter are not significantly different (P 5005)

competitive ability Gonzalez Ponce (1998) studied the rel-ative competitive ability of barley and annual ryegrass withdifferent N availability and concluded that competitionfrom barley was greater than from the weed because of ear-lier tillering and N absorption by barley It appears that theearlier N is available the more beneficial it is for the croprsquoscompetitive ability In contrast if N is available at later cropstages (ie tillering) the weedrsquos growth is favored Henceit would be beneficial to apply N fertilizer at sowing espe-cially in those situations where the density of wild oat islow and it would not be advantageous to apply a herbicideThis means that to avoid the increase in size of the weedpopulation in relation to what might be expected in theabsence of N application and chemical control it would beadvantageous to apply N fertilizer at sowing If N is appliedat tillering then the application of diclofop-methyl or anyother appropriate herbicide is essential

Total Seed Production

Total seed production was estimated considering the dif-ferent demographic rates for all the treatments As expectedthe amount of seed per square meter was higher when Nwas applied at tillering (P 005) and as noted in thestudy of adult fecundity the effect of N was negligible whenherbicide was applied In herbicide-free plots these figureswere 3031 5545 and 7287 seed m22 for N0 Ns andNt respectively (data not shown) On the other hand inthe treated plots these figures were 547 171 and 536 seedm22 for N0 Ns and Nt respectively The first cohort pro-duced 67 63 and 76 of the total seed production in plotswhere N was applied at tillering but not at sowing

Seed Dispersal Before Crop HarvestThe effects of N fertilization at tillering on both seedling

survival rate and individual fecundity are beneficial to wildoat However the seed dispersal rate of wild oat before har-vest was lower when N was applied than it was in controlplots (Figure 1D) This is a compensation for higher wildoat seed production under fertilization at crop tillering inproduction systems where grain is not cleaned by the com-bine and is therefore exported from the field In this caseit would be reasonable to recommend that crop harvestshould not be delayed especially in those situations whereN has been applied at tillering to make use of the effect ofa N-induced lower seed dispersal rate In addition the de-laying effect of N applied at tillering on seed dispersal wasenhanced when diclofop-methyl was applied (interactionherb 3 fert P 005) (Figure 1D)

The lower seed dispersal rate found under N applicationcompared with that recorded in control plots could be as-sociated with a delay in seed maturation This could beconsidered a direct effect of N on weed phenology IndeedFrank and Bauer (1996) reported the effects of N on thedevelopment of wheat Alternatively an indirect effect onseed maturation attributable to different environmental con-ditions for seed development in plots with N applicationcould be considered In this case it might be possible thattemperature conditions for wild oat seed developing belowthe canopy were lower in fertilized plots because of the high-er crop biomass than in plots without fertilizer applicationFor example Adkins et al (1987) reported a delay in the

time of maturation of wild oat seed when the temperatureduring their development was 20 C compared with 25 CRegardless of the processes influenced by N application weconclude that crop harvest should not be delayed to makeuse of the negative effects of N on seed dispersal rate beforeharvest

The Estimated Size of The Soil Seed Bank andThe Annual Population Growth Rate

Averaging both experimental years and regarding all thedemographic processes the amounts of seed per square me-ter that should have returned to the soil in diclofop plotswere 252 49 and 80 for N0 Ns and Nt treatments re-spectively On the other hand in plots without diclofop-methyl the estimated amounts of seed that should have re-turned to the soil were 1303 1445 and 1859 for N0Ns and Nt respectively Previous studies have shown thatseed predation is the most important demographic featurereducing the soil seed bank during the fallow period (Wilson1972 1985 Wilson and Cussans 1975) Moreover studiescarried out under similar conditions in the same area as forthe current study (Scursoni et al 1999) confirmed thatwhen seed were left on the soil surface during fallow pre-dation was the most important process for decreasing thesize of the seed population

To estimate the annual growth rate of the population l5 (NtN0) where Nt and N0 are the size of the soil seedbank at the beginning and the end of the t-year period(Fernandez Quintanilla et al 1986) we considered the pre-dation and emergence rates recorded by Scursoni et al(1999) These figures were 062 and 012 for predation andemergence in control plots and 043 and 018 for the sameprocesses in the treated plots respectively Thus the ratesof increase during the crop cycle and the fallow period withor without diclofop-methyl are shown in Table 4

The effectiveness of predation depends on environmentalconditions the location of seed in the soil tillage predatorsand food availability and the amount and type of posthar-vest residues (Cromar et al 1999) Therefore it would benecessary to focus on the study of this process in order tomake use of predation to diminish the size of the seed bankThe results obtained in this work allowed us to identify theeffects of N application on the population dynamics of wildoat in barley Development of functional relations that in-tegrated these data into population models could be usefuland could allow the prediction of population dynamics In-

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 621

tegrated models would also permit the design of scientifi-cally sound strategies to manage wild oat populations

Acknowledgments

The authors thank Malterıa Pampa SA and especially MarioCattaneo for their assistance in field experiments and Frank Forcellafor his suggestions on the manuscript

Literature Cited

Adkins S W M Lowen and S J Symons 1987 Variation within purelines of wild oats (Avena fatua) in relation to temperature of devel-opment Weed Sci 35169ndash172

Anderson T and P Milberg 1998 Weed flora and the relative importanceof site crop crop rotation and nitrogen Weed Sci 4630ndash38

Anderson R L D L Tanaka A L Black and E Scheizer 1998 Weedcommunity and species response to crop rotation tillage and N fer-tility Weed Technol 12531ndash536

Brandt R H H Janzen and R E Blackshaw 2000 Ranking weed re-sponse to added nitrogen and phosphorous Abstracts of the III In-ternational Weed Science Congress Foz do Iguassu Brazil 6ndash11 June2000 26 p

Carlson H L and J E Hill 1986 Wild oat (Avena fatua) competitionwith spring wheat effects of nitrogen fertilization Weed Sci 3429ndash33

Catullo J M L Rodrıguez and I Colombo 1982 Efecto de la Avenafatua L sobre el cultivo de trigo influencia de la fertilizacion CongrNac Malezas San Miguel de Tucuman Argentina 216ndash21

Cromar H E S D Murphy and C J Swanton 1999 Influence of tillageand crop residue on postdispersal predation of weeds seed Weed Sci47184ndash194

DiTomaso J M 1995 Approaches for improving crop competitivenessthrough the manipulation of fertilization strategies Weed Sci 43491ndash497

Fernandez Quintanilla C L Navarrete J L Gonzalez Andujar A Fer-nandez and M J Sanchez 1986 Seedling recruitment and age-spe-cific survivorship and recruitment in populations of Avena sterilis sppludoviciana J Appl Ecol 23945ndash955

Forcella F 1984 Wheat and ryegrass competition for pulses of mineralnitrogen Aust J Exp Agric Anim Husb 24421ndash425

Frank A B and A Bauer 1996 Temperature nitrogen and carbon di-oxide effects on spring wheat development and spikelet numbers CropSci 36659ndash665

Gonzalez Ponce R 1998 Competition between barley and Lolium rigidumfor nitrate Weed Res 38453ndash460

Jornsgard B K Rasmussen J Hill and J L Christiansen 1996 Influenceof nitrogen on competition between cereals and their natural weedpopulations Weed Res 36461ndash470

Kegode G F Forcella and S Clay 1999 Influence of crop rotationtillage and management inputs on weed seed production Weed Sci47175ndash183

Knezevic S S Evans C Shapiro and J Lindquist 2000 Effect of nitro-gen on critical period of weed control in corn Abstracts of the IIIInternational Weed Science Congress 6ndash11 June 2000 Foz do Iguas-su Brazil 52 p

Kirkland K and H Beckie 1998 Contribution of nitrogen fertiliser place-ment to weed management in spring wheat (Triticum aestivum) WeedTechnol 12507ndash514

OrsquoDonovan J T D W McAndrew and G Thomas 1997 Tillage andnitrogen influence weed population dynamics in barley (Hordeum vul-gare) Weed Technol 11502ndash509

Satorre E H and R W Snaydon 1992 A comparison of root and shootcompetition between spring cereals and Avena fatua L Weed Res 3245ndash55

Scursoni J 1995 Relevamiento de malezas en cultivos de cebada cervecera(Hordeum vulgare L) en la Provincia de Buenos Aires Argentina RevFac Agron La Plata 71235ndash243

Scursoni J R Benech Arnold and H Hirchoren 1999 Demography ofwild oat (Avena fatua L) in barley crops effect of crop crop densityand herbicide treatment Agron J 91478ndash485

Sindel B M 1991 A review of the ecology and control of thistles inAustralia Weed Res 31189ndash201

Sokal R R and F J Rohlf 1969 Biometry The Principles and Practicesof Statistics in Biological Research San Francisco Freeman pp 380ndash387

Thill D C J T OrsquoDonovan and C A Mallory Smith 1994 Integratedweed management strategies for delaying herbicide resistant in wildoats Phytoprotection 75(Suppl)61ndash70

Wilson B J 1972 Studies of the fate of Avena fatua seeds on cereal stubbleas influenced by autumn treatment Pages 242ndash247 in Proceedings ofthe 11th British Weed Control Conference Brighton UK

Wilson B J 1985 Effect of seed age and cultivation on seedling emergenceand seed decline of Avena fatua L in winter barley Weed Res 25213ndash219

Wilson B J and G W Cussans 1975 A study of the population dynamicsof Avena fatua L as influenced by straw burning seed shedding andcultivation Weed Res 15249ndash258

Zadoks J C T Chang and C Konzak 1974 A decimal code for thegrowth stage of cereals Weed Res 14415ndash421

Received July 31 2001 and approved April 4 2002

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 619

FIGURE 1 (A) Seedling survival rate for wild oat of the second cohort in barley without (2) and with (1) diclofop-methyl application under varying Nfertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for theherbicide 3 fertilization interaction (B) Seedling survival rate for wild oats of the third cohort in barley without (2) and with (1) diclofop-methylapplication under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering (Nt) in 1997 Vertical linesindicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (C) Seedling survival rate for wild oats of the third cohort in barley without (2)and with (1) diclofop-methyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at early tillering(Nt) in 1998 Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction (D) Seed dispersal rate () before harvest of wildoat in barley without (2) and with (1) diclofop-mehtyl application under varying N fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55kg N ha21 at early tillering (Nt) Vertical lines indicate the LSD (P 5 005) for the herbicide 3 fertilization interaction

TABLE 3 Seed production per plant of wild oat (Avena fatua L) for each cohort growing in barley (Hordeum vulgare L) crops with andwithout diclofop use and with varying nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 at sowing (Ns) 55 kg N ha21 at earlytillering (Nt)a

Cohorts

No herbicde

N0 Ns Nt

Herbicide

N0 Ns Nt

seeds per plantb

Cohort 1Cohort 2Cohort 3

107 bc61 cd42 cd

186 ab72 cd65 cd

258 a83 cd91 cd

59 cd27 cd17 cd

19 cd8 d4 d

61 cd21 cd10 d

LSD (P 005) 93

a Data for each treatment are combined over yearsb Means followed by the same letter are not significantly different (P 5 005)

wild oat plants show that the weed might be more favoredthan the crop by the addition of N at crop tillering Catulloet al (1982) found greater crop yield and seed productionof wild oat in wheat fertilized at tillering than in the absenceof fertilizer These results agree with those of Satorre andSnaydon (1992) who showed that fertilization reduced the

severity of competition experienced by wild oat In additionForcella (1984) showed a competitive advantage of wheatover annual ryegrass (Lolium rigidum Gaud) when N wasapplied before the three-leaf stage of the crop thus suggest-ing that sufficient nitrate availability in the soil after croptillering would stimulate annual ryegrass tillering and its

620 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 4 Annual growth rate of wild oat (Avena fatua L) popu-lation l 5 (NtN0) with and without diclofop use and with vary-ing nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 atsowing (Ns) 55 kg N ha21 at early tillering (Nt)ab

N treatments (2) No herbicide (1) Herbicide

N0NsNt

389 b431 b566 a

092 c022 c036 c

LSD (P 005) 118

a l 5 (NtN0) where Nt and N0 are the sizes of the soil seed banks atthe beginning and the end of the t-year period

b Means followed by the same letter are not significantly different (P 5005)

competitive ability Gonzalez Ponce (1998) studied the rel-ative competitive ability of barley and annual ryegrass withdifferent N availability and concluded that competitionfrom barley was greater than from the weed because of ear-lier tillering and N absorption by barley It appears that theearlier N is available the more beneficial it is for the croprsquoscompetitive ability In contrast if N is available at later cropstages (ie tillering) the weedrsquos growth is favored Henceit would be beneficial to apply N fertilizer at sowing espe-cially in those situations where the density of wild oat islow and it would not be advantageous to apply a herbicideThis means that to avoid the increase in size of the weedpopulation in relation to what might be expected in theabsence of N application and chemical control it would beadvantageous to apply N fertilizer at sowing If N is appliedat tillering then the application of diclofop-methyl or anyother appropriate herbicide is essential

Total Seed Production

Total seed production was estimated considering the dif-ferent demographic rates for all the treatments As expectedthe amount of seed per square meter was higher when Nwas applied at tillering (P 005) and as noted in thestudy of adult fecundity the effect of N was negligible whenherbicide was applied In herbicide-free plots these figureswere 3031 5545 and 7287 seed m22 for N0 Ns andNt respectively (data not shown) On the other hand inthe treated plots these figures were 547 171 and 536 seedm22 for N0 Ns and Nt respectively The first cohort pro-duced 67 63 and 76 of the total seed production in plotswhere N was applied at tillering but not at sowing

Seed Dispersal Before Crop HarvestThe effects of N fertilization at tillering on both seedling

survival rate and individual fecundity are beneficial to wildoat However the seed dispersal rate of wild oat before har-vest was lower when N was applied than it was in controlplots (Figure 1D) This is a compensation for higher wildoat seed production under fertilization at crop tillering inproduction systems where grain is not cleaned by the com-bine and is therefore exported from the field In this caseit would be reasonable to recommend that crop harvestshould not be delayed especially in those situations whereN has been applied at tillering to make use of the effect ofa N-induced lower seed dispersal rate In addition the de-laying effect of N applied at tillering on seed dispersal wasenhanced when diclofop-methyl was applied (interactionherb 3 fert P 005) (Figure 1D)

The lower seed dispersal rate found under N applicationcompared with that recorded in control plots could be as-sociated with a delay in seed maturation This could beconsidered a direct effect of N on weed phenology IndeedFrank and Bauer (1996) reported the effects of N on thedevelopment of wheat Alternatively an indirect effect onseed maturation attributable to different environmental con-ditions for seed development in plots with N applicationcould be considered In this case it might be possible thattemperature conditions for wild oat seed developing belowthe canopy were lower in fertilized plots because of the high-er crop biomass than in plots without fertilizer applicationFor example Adkins et al (1987) reported a delay in the

time of maturation of wild oat seed when the temperatureduring their development was 20 C compared with 25 CRegardless of the processes influenced by N application weconclude that crop harvest should not be delayed to makeuse of the negative effects of N on seed dispersal rate beforeharvest

The Estimated Size of The Soil Seed Bank andThe Annual Population Growth Rate

Averaging both experimental years and regarding all thedemographic processes the amounts of seed per square me-ter that should have returned to the soil in diclofop plotswere 252 49 and 80 for N0 Ns and Nt treatments re-spectively On the other hand in plots without diclofop-methyl the estimated amounts of seed that should have re-turned to the soil were 1303 1445 and 1859 for N0Ns and Nt respectively Previous studies have shown thatseed predation is the most important demographic featurereducing the soil seed bank during the fallow period (Wilson1972 1985 Wilson and Cussans 1975) Moreover studiescarried out under similar conditions in the same area as forthe current study (Scursoni et al 1999) confirmed thatwhen seed were left on the soil surface during fallow pre-dation was the most important process for decreasing thesize of the seed population

To estimate the annual growth rate of the population l5 (NtN0) where Nt and N0 are the size of the soil seedbank at the beginning and the end of the t-year period(Fernandez Quintanilla et al 1986) we considered the pre-dation and emergence rates recorded by Scursoni et al(1999) These figures were 062 and 012 for predation andemergence in control plots and 043 and 018 for the sameprocesses in the treated plots respectively Thus the ratesof increase during the crop cycle and the fallow period withor without diclofop-methyl are shown in Table 4

The effectiveness of predation depends on environmentalconditions the location of seed in the soil tillage predatorsand food availability and the amount and type of posthar-vest residues (Cromar et al 1999) Therefore it would benecessary to focus on the study of this process in order tomake use of predation to diminish the size of the seed bankThe results obtained in this work allowed us to identify theeffects of N application on the population dynamics of wildoat in barley Development of functional relations that in-tegrated these data into population models could be usefuland could allow the prediction of population dynamics In-

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 621

tegrated models would also permit the design of scientifi-cally sound strategies to manage wild oat populations

Acknowledgments

The authors thank Malterıa Pampa SA and especially MarioCattaneo for their assistance in field experiments and Frank Forcellafor his suggestions on the manuscript

Literature Cited

Adkins S W M Lowen and S J Symons 1987 Variation within purelines of wild oats (Avena fatua) in relation to temperature of devel-opment Weed Sci 35169ndash172

Anderson T and P Milberg 1998 Weed flora and the relative importanceof site crop crop rotation and nitrogen Weed Sci 4630ndash38

Anderson R L D L Tanaka A L Black and E Scheizer 1998 Weedcommunity and species response to crop rotation tillage and N fer-tility Weed Technol 12531ndash536

Brandt R H H Janzen and R E Blackshaw 2000 Ranking weed re-sponse to added nitrogen and phosphorous Abstracts of the III In-ternational Weed Science Congress Foz do Iguassu Brazil 6ndash11 June2000 26 p

Carlson H L and J E Hill 1986 Wild oat (Avena fatua) competitionwith spring wheat effects of nitrogen fertilization Weed Sci 3429ndash33

Catullo J M L Rodrıguez and I Colombo 1982 Efecto de la Avenafatua L sobre el cultivo de trigo influencia de la fertilizacion CongrNac Malezas San Miguel de Tucuman Argentina 216ndash21

Cromar H E S D Murphy and C J Swanton 1999 Influence of tillageand crop residue on postdispersal predation of weeds seed Weed Sci47184ndash194

DiTomaso J M 1995 Approaches for improving crop competitivenessthrough the manipulation of fertilization strategies Weed Sci 43491ndash497

Fernandez Quintanilla C L Navarrete J L Gonzalez Andujar A Fer-nandez and M J Sanchez 1986 Seedling recruitment and age-spe-cific survivorship and recruitment in populations of Avena sterilis sppludoviciana J Appl Ecol 23945ndash955

Forcella F 1984 Wheat and ryegrass competition for pulses of mineralnitrogen Aust J Exp Agric Anim Husb 24421ndash425

Frank A B and A Bauer 1996 Temperature nitrogen and carbon di-oxide effects on spring wheat development and spikelet numbers CropSci 36659ndash665

Gonzalez Ponce R 1998 Competition between barley and Lolium rigidumfor nitrate Weed Res 38453ndash460

Jornsgard B K Rasmussen J Hill and J L Christiansen 1996 Influenceof nitrogen on competition between cereals and their natural weedpopulations Weed Res 36461ndash470

Kegode G F Forcella and S Clay 1999 Influence of crop rotationtillage and management inputs on weed seed production Weed Sci47175ndash183

Knezevic S S Evans C Shapiro and J Lindquist 2000 Effect of nitro-gen on critical period of weed control in corn Abstracts of the IIIInternational Weed Science Congress 6ndash11 June 2000 Foz do Iguas-su Brazil 52 p

Kirkland K and H Beckie 1998 Contribution of nitrogen fertiliser place-ment to weed management in spring wheat (Triticum aestivum) WeedTechnol 12507ndash514

OrsquoDonovan J T D W McAndrew and G Thomas 1997 Tillage andnitrogen influence weed population dynamics in barley (Hordeum vul-gare) Weed Technol 11502ndash509

Satorre E H and R W Snaydon 1992 A comparison of root and shootcompetition between spring cereals and Avena fatua L Weed Res 3245ndash55

Scursoni J 1995 Relevamiento de malezas en cultivos de cebada cervecera(Hordeum vulgare L) en la Provincia de Buenos Aires Argentina RevFac Agron La Plata 71235ndash243

Scursoni J R Benech Arnold and H Hirchoren 1999 Demography ofwild oat (Avena fatua L) in barley crops effect of crop crop densityand herbicide treatment Agron J 91478ndash485

Sindel B M 1991 A review of the ecology and control of thistles inAustralia Weed Res 31189ndash201

Sokal R R and F J Rohlf 1969 Biometry The Principles and Practicesof Statistics in Biological Research San Francisco Freeman pp 380ndash387

Thill D C J T OrsquoDonovan and C A Mallory Smith 1994 Integratedweed management strategies for delaying herbicide resistant in wildoats Phytoprotection 75(Suppl)61ndash70

Wilson B J 1972 Studies of the fate of Avena fatua seeds on cereal stubbleas influenced by autumn treatment Pages 242ndash247 in Proceedings ofthe 11th British Weed Control Conference Brighton UK

Wilson B J 1985 Effect of seed age and cultivation on seedling emergenceand seed decline of Avena fatua L in winter barley Weed Res 25213ndash219

Wilson B J and G W Cussans 1975 A study of the population dynamicsof Avena fatua L as influenced by straw burning seed shedding andcultivation Weed Res 15249ndash258

Zadoks J C T Chang and C Konzak 1974 A decimal code for thegrowth stage of cereals Weed Res 14415ndash421

Received July 31 2001 and approved April 4 2002

620 bull Weed Science 50 SeptemberndashOctober 2002

TABLE 4 Annual growth rate of wild oat (Avena fatua L) popu-lation l 5 (NtN0) with and without diclofop use and with vary-ing nitrogen fertilization 0 kg N ha21 (N0) 55 kg N ha21 atsowing (Ns) 55 kg N ha21 at early tillering (Nt)ab

N treatments (2) No herbicide (1) Herbicide

N0NsNt

389 b431 b566 a

092 c022 c036 c

LSD (P 005) 118

a l 5 (NtN0) where Nt and N0 are the sizes of the soil seed banks atthe beginning and the end of the t-year period

b Means followed by the same letter are not significantly different (P 5005)

competitive ability Gonzalez Ponce (1998) studied the rel-ative competitive ability of barley and annual ryegrass withdifferent N availability and concluded that competitionfrom barley was greater than from the weed because of ear-lier tillering and N absorption by barley It appears that theearlier N is available the more beneficial it is for the croprsquoscompetitive ability In contrast if N is available at later cropstages (ie tillering) the weedrsquos growth is favored Henceit would be beneficial to apply N fertilizer at sowing espe-cially in those situations where the density of wild oat islow and it would not be advantageous to apply a herbicideThis means that to avoid the increase in size of the weedpopulation in relation to what might be expected in theabsence of N application and chemical control it would beadvantageous to apply N fertilizer at sowing If N is appliedat tillering then the application of diclofop-methyl or anyother appropriate herbicide is essential

Total Seed Production

Total seed production was estimated considering the dif-ferent demographic rates for all the treatments As expectedthe amount of seed per square meter was higher when Nwas applied at tillering (P 005) and as noted in thestudy of adult fecundity the effect of N was negligible whenherbicide was applied In herbicide-free plots these figureswere 3031 5545 and 7287 seed m22 for N0 Ns andNt respectively (data not shown) On the other hand inthe treated plots these figures were 547 171 and 536 seedm22 for N0 Ns and Nt respectively The first cohort pro-duced 67 63 and 76 of the total seed production in plotswhere N was applied at tillering but not at sowing

Seed Dispersal Before Crop HarvestThe effects of N fertilization at tillering on both seedling

survival rate and individual fecundity are beneficial to wildoat However the seed dispersal rate of wild oat before har-vest was lower when N was applied than it was in controlplots (Figure 1D) This is a compensation for higher wildoat seed production under fertilization at crop tillering inproduction systems where grain is not cleaned by the com-bine and is therefore exported from the field In this caseit would be reasonable to recommend that crop harvestshould not be delayed especially in those situations whereN has been applied at tillering to make use of the effect ofa N-induced lower seed dispersal rate In addition the de-laying effect of N applied at tillering on seed dispersal wasenhanced when diclofop-methyl was applied (interactionherb 3 fert P 005) (Figure 1D)

The lower seed dispersal rate found under N applicationcompared with that recorded in control plots could be as-sociated with a delay in seed maturation This could beconsidered a direct effect of N on weed phenology IndeedFrank and Bauer (1996) reported the effects of N on thedevelopment of wheat Alternatively an indirect effect onseed maturation attributable to different environmental con-ditions for seed development in plots with N applicationcould be considered In this case it might be possible thattemperature conditions for wild oat seed developing belowthe canopy were lower in fertilized plots because of the high-er crop biomass than in plots without fertilizer applicationFor example Adkins et al (1987) reported a delay in the

time of maturation of wild oat seed when the temperatureduring their development was 20 C compared with 25 CRegardless of the processes influenced by N application weconclude that crop harvest should not be delayed to makeuse of the negative effects of N on seed dispersal rate beforeharvest

The Estimated Size of The Soil Seed Bank andThe Annual Population Growth Rate

Averaging both experimental years and regarding all thedemographic processes the amounts of seed per square me-ter that should have returned to the soil in diclofop plotswere 252 49 and 80 for N0 Ns and Nt treatments re-spectively On the other hand in plots without diclofop-methyl the estimated amounts of seed that should have re-turned to the soil were 1303 1445 and 1859 for N0Ns and Nt respectively Previous studies have shown thatseed predation is the most important demographic featurereducing the soil seed bank during the fallow period (Wilson1972 1985 Wilson and Cussans 1975) Moreover studiescarried out under similar conditions in the same area as forthe current study (Scursoni et al 1999) confirmed thatwhen seed were left on the soil surface during fallow pre-dation was the most important process for decreasing thesize of the seed population

To estimate the annual growth rate of the population l5 (NtN0) where Nt and N0 are the size of the soil seedbank at the beginning and the end of the t-year period(Fernandez Quintanilla et al 1986) we considered the pre-dation and emergence rates recorded by Scursoni et al(1999) These figures were 062 and 012 for predation andemergence in control plots and 043 and 018 for the sameprocesses in the treated plots respectively Thus the ratesof increase during the crop cycle and the fallow period withor without diclofop-methyl are shown in Table 4

The effectiveness of predation depends on environmentalconditions the location of seed in the soil tillage predatorsand food availability and the amount and type of posthar-vest residues (Cromar et al 1999) Therefore it would benecessary to focus on the study of this process in order tomake use of predation to diminish the size of the seed bankThe results obtained in this work allowed us to identify theeffects of N application on the population dynamics of wildoat in barley Development of functional relations that in-tegrated these data into population models could be usefuland could allow the prediction of population dynamics In-

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 621

tegrated models would also permit the design of scientifi-cally sound strategies to manage wild oat populations

Acknowledgments

The authors thank Malterıa Pampa SA and especially MarioCattaneo for their assistance in field experiments and Frank Forcellafor his suggestions on the manuscript

Literature Cited

Adkins S W M Lowen and S J Symons 1987 Variation within purelines of wild oats (Avena fatua) in relation to temperature of devel-opment Weed Sci 35169ndash172

Anderson T and P Milberg 1998 Weed flora and the relative importanceof site crop crop rotation and nitrogen Weed Sci 4630ndash38

Anderson R L D L Tanaka A L Black and E Scheizer 1998 Weedcommunity and species response to crop rotation tillage and N fer-tility Weed Technol 12531ndash536

Brandt R H H Janzen and R E Blackshaw 2000 Ranking weed re-sponse to added nitrogen and phosphorous Abstracts of the III In-ternational Weed Science Congress Foz do Iguassu Brazil 6ndash11 June2000 26 p

Carlson H L and J E Hill 1986 Wild oat (Avena fatua) competitionwith spring wheat effects of nitrogen fertilization Weed Sci 3429ndash33

Catullo J M L Rodrıguez and I Colombo 1982 Efecto de la Avenafatua L sobre el cultivo de trigo influencia de la fertilizacion CongrNac Malezas San Miguel de Tucuman Argentina 216ndash21

Cromar H E S D Murphy and C J Swanton 1999 Influence of tillageand crop residue on postdispersal predation of weeds seed Weed Sci47184ndash194

DiTomaso J M 1995 Approaches for improving crop competitivenessthrough the manipulation of fertilization strategies Weed Sci 43491ndash497

Fernandez Quintanilla C L Navarrete J L Gonzalez Andujar A Fer-nandez and M J Sanchez 1986 Seedling recruitment and age-spe-cific survivorship and recruitment in populations of Avena sterilis sppludoviciana J Appl Ecol 23945ndash955

Forcella F 1984 Wheat and ryegrass competition for pulses of mineralnitrogen Aust J Exp Agric Anim Husb 24421ndash425

Frank A B and A Bauer 1996 Temperature nitrogen and carbon di-oxide effects on spring wheat development and spikelet numbers CropSci 36659ndash665

Gonzalez Ponce R 1998 Competition between barley and Lolium rigidumfor nitrate Weed Res 38453ndash460

Jornsgard B K Rasmussen J Hill and J L Christiansen 1996 Influenceof nitrogen on competition between cereals and their natural weedpopulations Weed Res 36461ndash470

Kegode G F Forcella and S Clay 1999 Influence of crop rotationtillage and management inputs on weed seed production Weed Sci47175ndash183

Knezevic S S Evans C Shapiro and J Lindquist 2000 Effect of nitro-gen on critical period of weed control in corn Abstracts of the IIIInternational Weed Science Congress 6ndash11 June 2000 Foz do Iguas-su Brazil 52 p

Kirkland K and H Beckie 1998 Contribution of nitrogen fertiliser place-ment to weed management in spring wheat (Triticum aestivum) WeedTechnol 12507ndash514

OrsquoDonovan J T D W McAndrew and G Thomas 1997 Tillage andnitrogen influence weed population dynamics in barley (Hordeum vul-gare) Weed Technol 11502ndash509

Satorre E H and R W Snaydon 1992 A comparison of root and shootcompetition between spring cereals and Avena fatua L Weed Res 3245ndash55

Scursoni J 1995 Relevamiento de malezas en cultivos de cebada cervecera(Hordeum vulgare L) en la Provincia de Buenos Aires Argentina RevFac Agron La Plata 71235ndash243

Scursoni J R Benech Arnold and H Hirchoren 1999 Demography ofwild oat (Avena fatua L) in barley crops effect of crop crop densityand herbicide treatment Agron J 91478ndash485

Sindel B M 1991 A review of the ecology and control of thistles inAustralia Weed Res 31189ndash201

Sokal R R and F J Rohlf 1969 Biometry The Principles and Practicesof Statistics in Biological Research San Francisco Freeman pp 380ndash387

Thill D C J T OrsquoDonovan and C A Mallory Smith 1994 Integratedweed management strategies for delaying herbicide resistant in wildoats Phytoprotection 75(Suppl)61ndash70

Wilson B J 1972 Studies of the fate of Avena fatua seeds on cereal stubbleas influenced by autumn treatment Pages 242ndash247 in Proceedings ofthe 11th British Weed Control Conference Brighton UK

Wilson B J 1985 Effect of seed age and cultivation on seedling emergenceand seed decline of Avena fatua L in winter barley Weed Res 25213ndash219

Wilson B J and G W Cussans 1975 A study of the population dynamicsof Avena fatua L as influenced by straw burning seed shedding andcultivation Weed Res 15249ndash258

Zadoks J C T Chang and C Konzak 1974 A decimal code for thegrowth stage of cereals Weed Res 14415ndash421

Received July 31 2001 and approved April 4 2002

Scursoni and Benech Arnold Nitrogen fertilization timing on demographic processes bull 621

tegrated models would also permit the design of scientifi-cally sound strategies to manage wild oat populations

Acknowledgments

The authors thank Malterıa Pampa SA and especially MarioCattaneo for their assistance in field experiments and Frank Forcellafor his suggestions on the manuscript

Literature Cited

Adkins S W M Lowen and S J Symons 1987 Variation within purelines of wild oats (Avena fatua) in relation to temperature of devel-opment Weed Sci 35169ndash172

Anderson T and P Milberg 1998 Weed flora and the relative importanceof site crop crop rotation and nitrogen Weed Sci 4630ndash38

Anderson R L D L Tanaka A L Black and E Scheizer 1998 Weedcommunity and species response to crop rotation tillage and N fer-tility Weed Technol 12531ndash536

Brandt R H H Janzen and R E Blackshaw 2000 Ranking weed re-sponse to added nitrogen and phosphorous Abstracts of the III In-ternational Weed Science Congress Foz do Iguassu Brazil 6ndash11 June2000 26 p

Carlson H L and J E Hill 1986 Wild oat (Avena fatua) competitionwith spring wheat effects of nitrogen fertilization Weed Sci 3429ndash33

Catullo J M L Rodrıguez and I Colombo 1982 Efecto de la Avenafatua L sobre el cultivo de trigo influencia de la fertilizacion CongrNac Malezas San Miguel de Tucuman Argentina 216ndash21

Cromar H E S D Murphy and C J Swanton 1999 Influence of tillageand crop residue on postdispersal predation of weeds seed Weed Sci47184ndash194

DiTomaso J M 1995 Approaches for improving crop competitivenessthrough the manipulation of fertilization strategies Weed Sci 43491ndash497

Fernandez Quintanilla C L Navarrete J L Gonzalez Andujar A Fer-nandez and M J Sanchez 1986 Seedling recruitment and age-spe-cific survivorship and recruitment in populations of Avena sterilis sppludoviciana J Appl Ecol 23945ndash955

Forcella F 1984 Wheat and ryegrass competition for pulses of mineralnitrogen Aust J Exp Agric Anim Husb 24421ndash425

Frank A B and A Bauer 1996 Temperature nitrogen and carbon di-oxide effects on spring wheat development and spikelet numbers CropSci 36659ndash665

Gonzalez Ponce R 1998 Competition between barley and Lolium rigidumfor nitrate Weed Res 38453ndash460

Jornsgard B K Rasmussen J Hill and J L Christiansen 1996 Influenceof nitrogen on competition between cereals and their natural weedpopulations Weed Res 36461ndash470

Kegode G F Forcella and S Clay 1999 Influence of crop rotationtillage and management inputs on weed seed production Weed Sci47175ndash183

Knezevic S S Evans C Shapiro and J Lindquist 2000 Effect of nitro-gen on critical period of weed control in corn Abstracts of the IIIInternational Weed Science Congress 6ndash11 June 2000 Foz do Iguas-su Brazil 52 p

Kirkland K and H Beckie 1998 Contribution of nitrogen fertiliser place-ment to weed management in spring wheat (Triticum aestivum) WeedTechnol 12507ndash514

OrsquoDonovan J T D W McAndrew and G Thomas 1997 Tillage andnitrogen influence weed population dynamics in barley (Hordeum vul-gare) Weed Technol 11502ndash509

Satorre E H and R W Snaydon 1992 A comparison of root and shootcompetition between spring cereals and Avena fatua L Weed Res 3245ndash55

Scursoni J 1995 Relevamiento de malezas en cultivos de cebada cervecera(Hordeum vulgare L) en la Provincia de Buenos Aires Argentina RevFac Agron La Plata 71235ndash243

Scursoni J R Benech Arnold and H Hirchoren 1999 Demography ofwild oat (Avena fatua L) in barley crops effect of crop crop densityand herbicide treatment Agron J 91478ndash485

Sindel B M 1991 A review of the ecology and control of thistles inAustralia Weed Res 31189ndash201

Sokal R R and F J Rohlf 1969 Biometry The Principles and Practicesof Statistics in Biological Research San Francisco Freeman pp 380ndash387

Thill D C J T OrsquoDonovan and C A Mallory Smith 1994 Integratedweed management strategies for delaying herbicide resistant in wildoats Phytoprotection 75(Suppl)61ndash70

Wilson B J 1972 Studies of the fate of Avena fatua seeds on cereal stubbleas influenced by autumn treatment Pages 242ndash247 in Proceedings ofthe 11th British Weed Control Conference Brighton UK

Wilson B J 1985 Effect of seed age and cultivation on seedling emergenceand seed decline of Avena fatua L in winter barley Weed Res 25213ndash219

Wilson B J and G W Cussans 1975 A study of the population dynamicsof Avena fatua L as influenced by straw burning seed shedding andcultivation Weed Res 15249ndash258

Zadoks J C T Chang and C Konzak 1974 A decimal code for thegrowth stage of cereals Weed Res 14415ndash421

Received July 31 2001 and approved April 4 2002