Alfalfa Growth

8/10/2019 Alfalfa Growth

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Alfalfa

Germination& Growth

A3681

Dan Undersander

Marvin H. Hall

Paul Vassalotti

Dennis Cosgrove


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Contents

Basics of alfalfa growth 2

The seed 3

Germination & emergence 4

Seedling growth & establishment 6

Crown formation and development 9

Vegetative stage 10

Spring green-up 10

Growth after cutting 11

Winter hardening and winter survival 12

When to cut 12

Flowering process 14

Estimating preharvest alfalfa quality 17

Summary of management tips 17

Glossary 18

Knowing how alfalfa germinates

will help producers create

conditions necessary for good

stand establishment. Knowing how

seedlings develop will be useful in

determining when to control diseases,

insects, and weeds. Understanding

patterns of growthspring green-up

and growth after cuttingwill aid in

identifying winter injury and disease

problems, and in managing weeds.

And being familiar with the mature

stages of alfalfa growthwill be helpful

when making decisions about when

to cut in order to ensure the maximum

yield of forageof the quality required.


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2 A L F A L F A G E R M I N A T I O N A N D G R O W T H

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Basics of alfalfa growth

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lfalfa growth is driven byphotosynthesis, a process thatcaptures the suns energy and

converts it into chemical energy. Glucosesugar is the primary chemical energyproduct of photosynthesis. Glucosecombines with other nutrient elementsto provide all the components neededfor alfalfa growth.

Alfalfa requires nitrogen, phosphorus,potassium, sulfur, calcium, magnesium,iron, boron, manganese, zinc, copper,molybdenum, and a few othermicronutrients for growth. These

nutrients, along with water, are absorbedby the root from the soil. Also, sometrace minerals may be taken up throughthe leaves if they are applied foliarly.Part of the nitrogen needed for growthis obtained from bacterial nitrogenfixation in the root nodules if soil pH issufficiently high (6.5 or more).

Living cells extract energy from glucosethrough a process called respiration. Foralfalfa to have sustained growth, glucoseproduction by photosynthesis must begreater than glucose use by respiration.

Alfalfa cells divide to produce new cellsat the tips of stems and roots in tissueknown as the apical meristem. Belowthe apical meristem of the stem, cells ofthe stems (but not the leaves) developthicker walls which contain more ligninand are less digestible. Consequently,the oldest and least digestible part ofthe alfalfa stem is at the bottom, whilethe youngest and most digestible part is

at the top.

At points where leaf petioles and stembranches attach to the stem are axillarybuds (see figure 1). Axillary buds contain

meristematic (growing) tissue whichcan develop into leaves, stem branches,or new stems. Axillary buds generallyremain inactive until the apical meristembegins producing flower buds or isremoved. Alfalfa cut high may regrowfrom axillary buds rather than fromcrown buds, which generally meansyield will be reduced because of theexcessive forage left in the field as aresult of cutting the alfalfa higher.

Leaflets

Petiol

Axillary buds

Radicle

Hypocotyl

Unifoliate leaf

Apical meristem

Figure 1. The parts of an alfalfa plant.


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Figure 2. Alfalfa seed, external and internal views.

The seed

A

lfalfa seeds are generally kidneyshaped, small (by by inch, with about 225,000 seeds

to a pound), and yellow-brown to olivegreen in color. The seed consists of twocotyledons (embryonic leaves), a radicle(embryonic root), a hypocotyl (the areaof radicle just below the cotyledons),and an epicotyl (embryonic stem). Allof these embryonic plant parts aresurrounded by a protective seed coat(see figure 2). Carbohydrates, proteins,and fats stored in the cotyledonssupply the energy needed duringgermination until the true leaves beginphotosynthesis. The radicle becomesthe primary root and is the initialanchoring and absorption structureduring alfalfa emergence. The epicotylis the growing point (where themeristematic tissue is located) of thefuture stem and is protected betweenthe two cotyledons until the cotyledonsare above ground and separate. Thescar tissue on the seed coat, knownas the hilum, is the previous point of

attachment of the seed to the pod.

Sometimes an alfalfa seed may have acoat not permeable to water, and this isknown as a hard seed. A hard seed can

lie dormant in the soil for several weeksor years before germinating. Hard seedpercentage is noted on the seed tagand is included in the total germinationcalculation. Most hard seed germinateswithin 30 to 60 days of planting. Alfalfahard seed cannot be depended upon tothicken a thin stand.

Several kinds of coatingsfor example,fungicide, Rhizobium bacteria, nutrients,lime, or claymay be applied to the

seed as it is processed and preparedfor marketing. In general, these seedcoatings have no effect on seed qualitybut are placed on the seed to aidgrowth and development after planting.

Management tips Store seed at cool temperatures

and in low humidity.Manyprecautions are taken by the alfalfa

seed industry to ensure that thepurchaser receives very high-quality,weed-free seed. Decline in seedquality on the farm can be slowed bystoring seed in a cool, dry place.

Re-inoculate with Rhizobium

bacteria if stored beyond

expiration date. Alfalfa is generallyinoculated with Rhizobium(a bacteriaresponsible for nitrogen fixation)during seed processing. How long

Rhizobiumwill live depends on

environmental conditions during seedstorage. Preinoculated seed storedbeyond the Rhizobiumexpirationdate (printed on the seed bag label)should be inoculated again with fresh

Rhizobiumbefore planting.

External Internal

Seed coat

Cotyledon

Hilum

Radicle

Epicotyl

Hypocotyl

G R O W I N G A L F A L F A

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Germination & emergence

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lfalfa seeds begin to germinateafter absorbing about 125percent of their weight in water

and swelling to break the seed coat(see figure 3). Alfalfa can germinateat temperatures greater than 37F butoptimum germination temperature isbetween 65 and 77F. As soil warms, therate of germination increases becauseof increased water movement intothe seed and because of increases inthe rate of other metabolic activitiesassociated with germination.

Germination begins with the radicle(young root) emerging through theseed coat near the hilum (see figure 4)

and anchoring itself in the soil as anunbranched tap root (see figure 5).

Supplying the right conditionswill help to assure maximumgermination. Prepare a firm, weed-

free seedbed to minimize competitionfrom weeds and to ensure good seed-

to-soil contact. Avoid overworking thesoil surface, as rainfall following seedingmay crust the surface, preventingseedling emergence.

Management tips Optimize seed-to-soil contact.

Poor seed-to-soil contact restrictswater movement from the soil to theseed and causes poor or sporadicgermination. In conventionally tilled

seedbeds, the best seed-to-soilcontact is achieved when the seedbedis free of clods and when press wheelsor a cultipacker are used to press thesoil onto the seed. Many no-till alfalfaplantings fail because they are donewhen the soil is too wet and the seedfurrow does not close, resulting inpoor seed-to-soil contact.

Plant no deeper than inch.

Seeding alfalfa at depths greaterthan inch makes it difficult or

impossible for the hypocotyl topull the cotyledons above ground,especially in heavy-textured soils.Alfalfa seedlings that do emergefrom deeper than inch usually areweaker because of greater energyexpenditure during emergence. Afirm seedbed is extremely helpful inregulating and maintaining seedingdepth. If seed is drilled and noseeds are visible on the surface afterplanting, it is likely that the seed wasplanted too deep.

Figure 3. The seed on the right is

swollen with water taken up from the

soil. This condition marks the first step

in seed germination. Under favorable

conditions, water uptake begins within

24 to 48 hours after planting.

Figure 4. The first part of the seedling to

emerge is the root radicle. The radicle

anchors the seedling in the soil and

always grows downward, regardless ofthe direction it is pointing when it first

comes out of the seed.



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As the radicle tip grows deeper intothe soil, the hypocotyl elongatesand pulls the cotyledons and

epicotyl (growing point) above thesoil surface. As the cotyledons emerge

above the soil surface, the seed coat fallsoff and cotyledons open to expose theepicotyl (see figure 6).

Management tips Avoid planting outside

recommended planting window.

Planting too early will reduce thegermination rate and increasethe likelihood of disease infectingthe crop. Also, a few hours of air

temperatures of 24

F or lower can killan alfalfa seedling when the secondtrifoliolate leaf has just emerged.

Provide adequate levels of

nutrients. Adequate levels ofnutrients are important for alfalfashoot and root development. Nutrientlevels are especially important duringearly seedling establishment toensure a root system large enough tosupport maximum herbage growthfor the life of the plant.

Minimize risk of soil crusting. Soilsthat crust easily create an additionalproblem during alfalfa emergence.The germinated seed will die ifthe elongating hypocotyl cannot

penetrate this crusted soil and pullthe cotyledons above the soil surface.Excessive tillage can increase thepotential for crusting. Rain, irrigationor light tillage may be required tosoften or break the crusted layer.

Dont plant alfalfa into established

alfalfa fields. Established alfalfaplants release a compound into thesoil that reduces germination andgrowth of new alfalfa seedlings, aphenomenon known as autotoxicity,

or self-toxicity. The extent ofautotoxicity in a field depends on theamount of alfalfa residue in the soil,how long since the previous stand ofalfalfa was killed, soil type, amount ofrain or irrigation prior to the seedingof the new crop, soil temperature, andother factors. At high autotoxicity,alfalfa germination will be inhibited;at somewhat lower autotoxicity, seedwill germinate but plants will bestunted and never yield well. To avoidalfalfa autotoxicity in medium toheavy soils, plow under or chemicallykill an established alfalfa field, thengrow a crop other than alfalfa for oneseason or, preferably, two seasons.This also will help reduce disease andinsect populations that may havedeveloped in the original alfalfa field.

Figure 5. As the radicle grows, the

portion nearest the seed enlarges

and forms a hook. It pushes up

through the soil surface, dragging

the cotyledons and seed coat with

it. Once the seedling emerges aboveground and is exposed to light, it

straightens out. At the same time,

small root hairs are developing on

the lower radicle. The root hairs

absorb water and nutrients from the

soil.


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Seedling growth& establishment

The establishment phase isthe period between seedlingemergence and first harvest.

Cotyledons turn green afteropening (figure 6) but have limitedphotosynthetic activity, for the energystored in the cotelydons is quicklyused up and they soon shrivel, turnbrown, and die. The epicotyl producesthe first leaf, which is a single leaflet(unifoliolate). The meristematic region ofthe epicotyl continues to grow, adding

trifoliate leaves (three-leaflet leaves) ormultifoliolate leaves (more than threeleaflets per leaf) on alternate sides of thedeveloping stem (see figures 7, 8, and 9).

While the epicotyl continues to addstem, leaves, and flowers, the bud inthe axil of the unifoliolate leaf beginsto grow and form a secondary stem.Additional secondary stems developfrom the axillary buds at the cotyledonnodes (the points where cotyledonsattach to the stem). Temperaturesof 68 to 85F during early seedlingdevelopment are optimum for growthand development; however, as theseedling continues to develop, slightly

cooler temperatures (60 to 75F) areideal.

Youngseedlings are particularlyvulnerable to diseases andherbicide damage. As the seedlings

mature they become more resistant toseedling diseases and less susceptible

to herbicide damage. Many herbicideshave application restrictions untila specific growth stage is reached.It is important to be able recognizeeach growth stage so you can applytreatments at the correct time.

Management tips Minimize risks of seedling diseases.

Soil moisture is necessary for nutrientabsorption by the roots and for rapidseedling development, but excessivesoil moisture can stop root growth.Wet soilconditions also can leadto fungal diseases such as Pythium(damping off ), Phytophthora (rootrot),Aphanomyces (damping off), and

Rhizoctonia (stem and root canker).Seed-applied fungicides can reducethe incidence of these diseases, butplanting in well-drained soils can dothe same.

Ensure that there are adequate

amounts of Rhizobium in thesoil.Rhizobium bacteria may notbe present if alfalfa has not beengrown recently in the field or if soilpH is low. Once pH is corrected, usepreinoculated alfalfa seed or inoculatethe seed with live Rhizobium prior toplanting in order to provide adequatebacteria. In general, inoculation isan inexpensive way of ensuring thatadequate Rhizobium bacteriaarepresent.

Optimize nitrogen-fixing potential.

Low soil pH (below 6.5) limitsnodulation and the availability ofmolybdenum, which is essential in thenitrogen fixation process. Apply limeif a soil test indicates low pH.

Figure 6. Cotyledons are usually

the first visible portion of an alfalfa

seedling as it emerges above the soil

surface.

Figure 7. The first true leaf to develop

on an alfalfa seedling has just one

leaflet and is called the unifoliate leaf.



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Control weeds during the first 60

days to prevent stand loss.Weeds inestablishing stands may out-competeand shade young alfalfa plants,thinning the stand. All weeds should

be controlled in new seedings, bothspring and fall. Herbicides shouldgenerally be applied as early asalfalfa will tolerate the herbicide. If,for example, five leaflets are requiredbefore the herbicide should be used,this means five trifoliate leaflets (notcounting the unifoliate leaf) mustbe visible, with each of the leafletsexpanded so they are not touchingeach other (see figure 10).

When to harvest a new seeding.

New plantings should notbe harvested until sufficientcarbohydrates have been stored inthe roots to support rapid regrowth.Depending on growing conditions,this generally occurs around 60days after emergence. Delayingharvesting beyond 60 days generallydoes not improve regrowth and maydramatically reduce forage quality.

Figure 8. The second

leaf to appear has three

leaflets. This is called

a trifoliate leaf. Most

subsequent leaves are

trifoliate, althoughsome varieties produce

leaves with four or

more leaflets per leaf,

and these are called

multifoliate leaves.

Figure 10. Counting

only the fully expanded

trifoliate leaves reveals

that this plant is at

the five-leaf stage.

A leaf is considered

fully expanded when

the outer edges of all

leaflets are no longer

touching.

Figure 9. This plant isat the two-leaf stage.

Note the two trifoliate

leaves in addition to

the unifoliate leaf. At

this stage, the seedling

can manufacture all

of its energy through

photosynthesis.


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The radicle tip continues to grow deeperinto the soil. The hypocotyl and upperportion of the radicle begin contractilegrowth, a process in which these organsenlarge laterally and contract vertically.

The radicle begins to thicken anddevelop into the primary taproot (seefigure 13). At the same time, smallersecondary roots begin to develop onthe radicle as it grows deeper. The

location and extent of secondary rootformation is genetically regulated.Root growth during establishment isgenerally about 80 percent of herbagegrowth. That is, for every ounce ofherbage growth, there is 0.8 ounceof root growth. However, under hottemperatures (greater than 90F), rootgrowth may drop to 40 percent ofherbage growth.

Within four weeks after germination,root hairs on the radicle become

infected with the nitrogen fixingbacteria Rhizobium meliloti, and beginto form the nodules (see figure 11)where atmospheric nitrogen fixationoccurs. The rate of Rhizobium infectiondepends on the soil nitrogen contentand the rate of seedling growth. OnlyRhizobium meliloti bacteria will infectalfalfa root hairs. Rhizobium bacteria thatinfect other legumes (e.g. soybeans)are incapable of infecting alfalfa, andRhizobium meliloti is incapable ofinfecting other legumes. Only about 5percent of the alfalfa root hairs becomeinfected, and only about 30 percentof these infections result in noduleformation, regardless of the quantity of

Rhizobium present.

Management tips Ensure that soil pH and fertility is

optimum for nitrogen fixing. Lowsoil pH (below 6.5) limits nodulation

and the availability of molybdenum,which is essential in the nitrogenfixation process.

Factors reducing nitrogen fixation.

Waterlogged soils inhibit oxygen andnitrogen movement to the roots andnodules. Harvesting alfalfa herbagewill cause nitrogen fixation to declineuntil herbage regrowth is adequateto supply the nodules with sugars(energy) from photosynthesis.

Figure 11. Within four weeks after

germination, if Rhizobiumbacteria

are present, round structures called

nodules will form on alfalfa roots. The

bacteria help the plant use nitrogen

from the air in a process known as

nitrogen fixation.


Nitrogen fixing nodules


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Crown formation anddevelopment

Contractile growth (that pulls thelowermost buds below groundto form a crown) begins as early

as one week after emergence and isusually complete within 16 weeks.Contractile growth pulls the axillarybuds below the soil and forms thecrown (area between the soil surfaceand the cotyledonary nodes). Alfalfavarieties with greater fall dormancy tendto have more pronounced contractilegrowth than those with less dormancy:

their nodes are pulled farther below thesoil surface. Plants with deep crownstend to be more persistent than plantswith shallow crowns because they haveincreased soil protection from extremelycold air temperatures. Axillary buds thatare pulled below the soil surface duringcontractile growth are referred to ascrown buds.

Management tips To allow enough time for crown

development,avoid seeding afterthe recommended late-summer

planting dates. Plants that have notformed a crown will not survivethe winter. Following contractilegrowth, alfalfa stems may be frostedbut the plant can survive freezingair temperatures because thecrown buds are protected belowthe soil surface. Planting laterthan recommended reduces theprobability of having a successfulyield, for the crop will face highertemperatures, more soil drying, and

more weed problems. Summer-seeded alfalfa generally needs aminimum of six weeks of growth afteremergence to survive winter withoutinjury and to produce maximumyields the following year.

Even brief exposures to airtemperatures below 27F will killyoung alfalfa seedlings. Alfalfa plantssurvive cold temperatures by pullingthe lowermost buds below ground toform a crown.

Figure 12. Crown development has not

yet begun on these young plants. The

cotyledons, though brown, are still

present.

Figure 13. The lowermost

buds have been pulled

below ground to form the

crown. Crown development

on this plant is already

sufficient for winter survival.

The crown will continue to

enlarge throughout the life

of the plant.


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Areas wherecrown will develop

Crown area


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Vegetative stage

D

uring early vegetative growth in the spring or after harvest, the alfalfahas insufficient leaf area to produce enough energy, purely throughphotosynthesis, to support growth. Energy for early regrowth comes from

carbohydrates and other nutrients stored in the root and crown. Once vegetativegrowth has attained about eight inches of height, leaf area and photosynthesis haveincreased enough to supply adequate energy for continued growth and to beginreplenishing the root and crown reserves (see figure 15).

Alfalfa growth in the spring is predominantly from crown buds and depends ontemperature (see figure 14).

Springgreen-up

Spring growth ideally comes fromcrown buds formed the previousyear during late summer and fall.

Green-up occurs when the buds locatedon the crown begin to grow in responseto warm spring temperatures. Inaddition to weather, the timing of springgreen-up depends on plant health, thegenetic fall dormancy of the variety, andthe amount of dormancy developed inthe plants during the fall (determinedby fall weather conditions). If plants are

injured or slow to green up, considerplanting more winter-hardy varieties.

Figure 14. This well-developed crown shows healthy crown buds growing to

produce the seasons first cutting.

Crown buds

Developing shoot

Figure 15. Carbohydrate content of alfalfa roots.

growthinitiation

6-8 inchheight

budstage

fullflower

rootcarbohy

drate

cut at full flower

cut atbud stage


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Growth after cutting

R

egrowth after harvest is primarilyfrom crown buds (figure 14) butmay also come from axillary

buds if cutting is high. The number ofstems that develop from either axillaryor crown buds depends on the variety,the developmental stage at the time ofharvest, the health of the crown, and thecutting height.

Environmental conditions play animportant role in determining the rateand amount of vegetative growth. Themaximum number of stems on a plantis set within 14 days after harvest and

declines as the plant matures. Droughtor flooding stress during the first 14days after harvest can greatly reduce thenumber of stems a plant will produce inthat regrowth period. Drought reducesstem growth more than leaf growth,causing shortened plants that are low-yielding but generally high in qualitybecause of increased leaf-to-stem ratio.

Immediately after harvest, alfalfaregrowth is fastest when temperaturesare 85 to 90F. However, as alfalfa

growth continues, maximum growthrate occurs when temperatures arebetween 50 and 80F. Leaf-to-stem ratiodeclines as the plant matures withineach regrowth period. Leaf-to-stemratio is less for growth in spring than inmidsummer. The warmer temperaturesand longer days of summer cause morerapid plant development and greatercell wall lignification than occurs in thecooler temperatures and shorter days ofspring.

Alfalfa yield is determined by thenumber of stems in a given landarea and the weight of each stem.The maximum number of stems perplant and the weight of each stemare determined during the period ofvegetative development. Consequently,it is important that pH, fertility, moisture,and pests be carefully controlled duringthis period.

Management tips The ideal cutting height is at least

two inchesabove the soil surface(three to four inches if a grass is mixed

with the alfalfa). Maximize stem density.Short

cutting intervals (less than 35 days)will reduce the number of axillarybuds. Moisture stress immediatelyafter cutting will reduce the numberof crown and axillary buds andtherefore will reduce stem densityand yield, even if rain occurs later inthe growth cycle.

Figure 16 and 17.Regrowth after cutting arises primarily from

crown buds, and also from axillary buds if forage is cut high.

Axillary buds occur wherever a leaf petiol attaches to the stem.


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Figure 18. The plant on

the left has suffered

winter injury while

the plant on the right

has not. Injured plants

become less vigorous

and are slow to recover.


Winter hardeningand winter survival

Shortening days and decliningtemperatures in the fall can cause

some alfalfa varieties to changetheir vegetative growth pattern. Stemgrowth in the fall serves as the basisfor assigning varieties a relative falldormancy rating. A fall dormancy of1 indicates the greatest fall dormancyand least height of fall growth, whilea rating of 11 indicates the least falldormancy and greatest height of fallgrowth. Usually, the more fall-dormantthe variety, the slower it regrowsafter harvest but the better it survives

adverse winter conditions.

During the fall, dormant varieties altertheir chemical processes to prepare for

winter survival. Some of the starch inthe crown and root is converted intosugars that function as antifreeze andhelp keep the crown, crown buds, androot from freezing at temperatures wellbelow 32F. Crown buds, which willbe the source of growth the followingspring, are formed during the fall.

Dormant alfalfa varieties can survivetemperatures of about 5F after theyhave been prepared for winter survivalby having had at least two weeks oftemperatures alternating between near-freezing nights and 60F days. Planttissue below the soil surface is insulatedfrom cold air temperatures by soil andlayers of snow. When there is no snowcover, extremely cold air temperaturescan cause the soil temperature to dropbelow 5F. This will injure or possibly killthe alfalfa plant.

When to cutManagement tips

When to cut for hay or haylage.Forage yield, quality, and standpersistence are affected by thecutting schedule chosen. Forage yieldincreases until the plant reaches fullflower, while forage quality decreasescontinually during growth. Cuttingfor high quality will reduce totalseason yield, so one must ensurethat the high quality will producea return to offset the yield loss.Early season growth may not flower

normally and quality will decline ifit doesnt. Therefore, using a foragequality stick (available from somestate forage associations and somealfalfa seed marketing companies), ormeasuring forage height and plantstage (as described later), is crucialin determining when to do the firstcutting in order to harvest alfalfa ofthe desired quality. The stage to cutalfalfa for optimum forage qualityfor dairy cattle ranges from the

vegetative to early bud stage on firstcutting and is generally at bud stageon later cuttings. Later stages maybe harvested for animals with lowernutritional requirements.

Watch for pests. Some insect pestsfeed on the alfalfa bud and flower.Excessive numbers of these insectsin an alfalfa field can remove budsas they form, causing the casualobserver to assume that the alfalfa

has not begun to form buds. Ifinitiation of alfalfa harvesting is basedon plant development, then carefulmonitoring of insect populations andplant development is necessary.

Figure 19. If crown buds have been

killed, the plant must form new

buds to replace them. The taller

shoots are from buds that survived

the winter. The lower layer of shoots

are from spring-formed buds.


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Understand the risks of harvesting

during the bud stage. Alfalfavarieties developed in the past 10

years have greater disease resistancethan older varieties. With adequatefertility and pest control, these newervarieties can survive aggressiveharvest schedules better than oldervarieties. In addition, newer alfalfavarieties have been selected for rapidregrowth, which increases their yieldpotential. Faster regrowth is a resultof decreased production of auxin (aplant hormone) during the flower budstage, allowing earlier regrowth fromcrown buds. Harvesting during theflower bud stage provides relativelyhigh forage quality, but continuouslyharvesting at this stage will result inlow root carbohydrate reserves andstand decline. Alfalfa stands that areintended to remain in productionfor more than three years must beallowed to mature beyond the flowerbud stage at least once during theseason to replenish root reserves.

Forage yield, quality, and stand

persistence are considerations

when deciding when to cut.

Forage yield increases until the plantreaches fullflower, while foragequality decreases continually duringgrowth or regrowth (see figure 20).The optimum balance between yieldand quality depends on the cuttingand the forage quality desired. Earlyin the growing season, plants maynot flower normally and quality maydecline even though the plant is notflowering. This can be determinedusing a forage quality stick or bymeasuring plant height. Take latercuttings based on growth stage ortime interval (35 to 45 days).

The stage to cut alfalfa for

optimum yield and forage qualityfor milking dairy cowsranges fromthe vegetative to early bud stageon the first cutting, to 10 percentflower on the second and thirdcuttings, to fullflower on a latefall cutting. For animals with lower

nutritional requirements, later stagesmay be harvested. The pictures onthe following pages depict growthstages commonly referred to whendiscussing cutting management.

forage yield

first-cut forage quality

late-season forage

quality

vegetative bud first-flower full-flower post-flower

Figure 20. Forage yield and quality at various stages of growth.


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Flowering processFlower bud stage

The formation of a flower budmarks a major transition in alfalfagrowth and development. Up

to this point, the alfalfa plant has beenusing energy from photosynthesis tobuild a larger photosynthetic factory(more leaves) and to store energyin the crown and roots. When floraldevelopment begins, the plant shifts alarge portion of its energy into flowerproduction and, ultimately, seedproduction.

Alfalfa has an indeterminate growthpattern, meaning it continues toproduce both leaves and stems along

with reproductive (bud) structures.Because of this, it is possible to have asingle alfalfa stem with newly emergedleaves, flower buds, flowers, andseedpods. Continued vegetative growthmeans the alfalfa crop will increasethrough the flower bud and early flowerstage of development.

The onset of floral development iscontrolled primarily by temperatureand photoperiod. In midsummer, flower

buds may appear on alfalfa regrowththree weeks earlier than in the springwhen temperatures are lower andthe photoperiod shorter. Flower budsbegin to appear at the 6th through14thnode on the stem, depending onenvironmental and genetic factors.Flower bud formation usually beginsat lower nodes in summer (6ththrough8thnodes) than in spring. After the firstflower bud appears, a new bud willappear about every three days until the

stem is harvested or reaches maturity.

Early budFigure 21.The flowering process

begins when small buds form in the

top one or two leaf axils. These buds

are detectable as small swellings in

the axils of the leaves. Forage cut at

this stage will be very high in quality.

However, yield will be reduced. Cutting

several times per season at this stage

will decrease persistence.

Mid budFigure 22. At the mid bud stage, flower

buds are larger and easier to detect.

Some buds are beginning to lengthen.

Additional buds are forming lower on

the stem.

Late budFigure 23.At the late bud stage, flower

buds are large and lengthening rapidly.

Buds begin developing at three or more

leaf axils lower on the stem.


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The appearance of the first flower budusually indicates that forage quality isbeginning to rapidly decline (see figure

20). This decline is partly due to the lossof lower leaves when they are shadedby leaves higher on the stem, andpartly due to continued thickening andlignification of cells in the stem. Someproducers begin harvesting when budsfirst appear in order to avoid the rapiddecline in quality thereafter. However,root carbohydrate reserves used tosupport early vegetative growth havenot been fully replenished at the budstage.

Flower stageThe length of time between whenflower buds form and when flowersopen depends on the environment, but

is generally about five days. The flowerstage is the interval between when thefirst flower opens and when a seedpodis formed. In the fall, when there arefewer than 12 hours of daylight, flowerbuds often wither and fall off withoutdeveloping into flowers.

The alfalfa flower possesses both female(pistil) and male (stamen) structures. Theflower petals (corolla) of alfalfa grown inthe United States are usually some shadeof purple because of Medicago sativa

parentage (see figure 24). Yellow flowersshow falcataparentage which indicatesincreased winter hardiness, thoughwith less yield. However, alfalfa flowercolors can vary from cream to yellow, toshades of blue-green, depending on theparentage of the plant (see figure 25).

Because alfalfa is largely self-incompatible and self-sterile, it istypically cross pollinatedpollen from

one flower fertilizes the ovules of aflower on a different plant. For alfalfato be pollinated the flower must betripped. That is, the stamen and pistilmust be released from the keel petals(see figure 26). This tripping is usuallycaused by nectar- or pollen-collectinginsects such as honeybees, alkali bees,and leaf-cutter bees. Alfalfa pollenis carried by insects, not wind. Aftertripping and pollination, pollen usuallyfertilizes the ovule within 24 to 32 hours.

Each alfalfa flower has between 6 and 18ovules in its ovary, each of which couldbecome a seed. However, only 10 to 12ovules usually develop.

Flowers generally do not progress toseed production in the fall.Flower

Figure 24.Alfalfa flowers grow in

clusters attached to a stem (raceme).

The most common flower color is

purple. Purple flowering plants

come from Mediterranean stock and

have high yield and quick recovery.

Plants with yellow flowers are from

more winter-hardy stock. A small

percentage of flowers may also be

blue, cream, or white.

Figure 26. Flower before tripping

on right. Tripped flower on left.

Figure 25. Most alfalfa has purple

flowers but many other colors are

possible.


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B

O

TO

N

Y

Figure 30. Alfalfa

seedpods take

many shapes,

depending on the

number of seeds

they contain and

the parentage of

the plant, but most

alfalfa varieties

commonly found in

the United States

have seedpods

that are spiral

shaped.

10 percent flowerFigure 27.At this stage, 10 percent of the

stems have open flowers. Allowing one

cutting per year to grow to this stage

will improve stand persistence. Late-

season harvests at this stage may still

provide dairy-quality hay.

50 percent flowerFigure 28.Half of the stems have

open flowers. Roots have the most

carbohydrates stored at this stage. This

is beyond the recommended harvest

stage for quality hay.

100 percent flowerFigure 29. All stems now have open

flowers. Shoots from crown buds for the

next growth cycle may be visible at the

base of the plant.

Seedpod stageAfter the ovules have been fertilized,they begin to develop into seeds andstretch the ovary, which becomes the

pod surrounding the seeds (see figure30). The petals wither and fall off,exposing the green seedpod, whichbecomes brown with maturity. Alfalfaseed must develop for six to eight weeksin the pod before it will become viableseed.

Long days and low humidity favorflowering, pollination, and seeddevelopment. Too much or too littlesoil moisture will decrease seedyield. Minimum temperatures above

68F favor seed production. Lowtemperatures during seed developmenttend to increase the percent of hardseed. In the United States, seed isproduced primarily in California, Idaho,Nevada, Oregon, and Washington,states where humidity is low and rainscarce during the summer while seed ismaturing and being harvested .


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Estimating preharvestalfalfa quality

Choose five representative 2-square-foot areas in the field. (Sample moretimes for fields larger than 30 acres.) In each area, determine the stage of the

most mature stem (see below).Then measure the height of the tallest stem,NOT the highest leaf. Note that the tallest stem may not be the most mature stem.Use the chart to determine relative feed value of the standing alfalfa forage. Thisprocedure does not account for changes in quality due to wilting, harvesting, andstorage. These factors may lower relative feed value by 10 to 20 points.

Management tips foroptimum alfalfa growthand development Select alfalfa varieties with proven

high-yield potential, diseaseresistance, and winter hardiness inyour region.

Get the soil tested, then fertilizeand lime according to the soil testrecommendation.

Plant alfalfa in well-drained fields.

Buy preinoculated seed, or inoculateyour own seed.

Plant less than inch deep at anadequate rate for your area.

Ensure good seed-to-soil contact byfirming soil around seed with presswheels on drill or cultipacker.

Control weeds, especially during first60 days after seeding.

Control insects.

Apply needed nutrients annually,based on soil or tissue tests.

Follow a harvest schedule whichwill provide forage of the required

quality and will lead to the desired lifeexpectancy of the alfalfa stand.

Additional information

The following publications are availablefrom your county extension office.

Alfalfa Management Guide (NCR547)

Alfalfa Stand Assessment: Is This Stand

Good Enough to Keep?(A3620)

Forage Variety Update for Wisconsin(A1525)

latevegetative stem is more than 12 inches tall, no visible buds or flowers

early bud 1 to 2 nodes have visible buds; no flowers or seed pods present

late bud more than 2 nodes have visible buds; no open flowers or seed pods

early flower 1 node with at least 1 open flower

late flower 2 or more nodes have open flowers

stage of most mature stem

height oftallest stem

(inches)

latevegetative early bud late bud

earlyflower late flower

relative feed value

16 234 220 208 196 186

17 229 215 203 192 182

18 223 211 199 188 178

19 218 206 195 184 175

20 213 201 191 181 171

21 209 197 187 177 168

22 204 193 183 173 16523 200 189 179 170 161

24 196 185 175 167 158

25 191 181 172 163 155

26 187 178 169 160 152

27 184 174 165 157 150

28 180 171 162 154 147

29 176 167 159 151 144

30 173 164 156 148 141

31 169 161 153 146 139

32 166 158 150 143 136

33 163 155 147 140 13434 160 152 145 138 132

35 156 149 142 135 129

36 154 146 139 133 127

37 151 144 137 131 125

38 148 141 134 128 123

39 145 138 132 126 121

40 142 136 130 124 118

Source: Derived from equations developed by R.W. Hintz, V.N. Owens, and K.A.Albrecht at the University of Wisconsin-Madison, Department of Agronomy.

S U M M A R Y

17

Dairyqualityforage


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GlossaryApical meristemThe meristem

at the top of a shoot or tip of aroot; here cells are produced that

eventually become shoots, leaves,or buds.

AuxinA plant hormone stimulatinggrowth.

AxilThe upper angle between thepetiole of a leaf and the stem fromwhich it grows.

Axillary budThe bud formed in theaxil of a leaf.

Contractile growthGrowth in which

the root cells contract vertically,pulling the cotyledonary nodes andtheir associated axillary buds belowthe soil surface to form the crown.

CotyledonsStructures in seeds thatcontain concentrated amounts ofproteins, oils, and carbohydrates tobe used as energy sources duringgermination and emergence.

CrownThe area at the base of thestem with tightly packed nodesand internodes, which generatevegetative growth. The formation ofthe crown results from contractilegrowth pulling axillary buds on thefirst branches of a seedling belowthe soil surface. The crown is thearea of meristematic activity.

Crown budThe bud formed at theaxillary site on the crown.

EpicotylThe portion of the plantwhich is just above the cotyledons.

FertilizationThe union between thepollen grain and the ovule.

HilumScar tissue on an alfalfa seedwhere the seed was attached to thepericarp (seedpod).

HypocotylThe portion of the plantwhich is just below the cotyledons.

LeafThe plant organ borneby the stem, responsiblefor photosynthesis and gas

exchange; comprised of thepetiole, leaflets, and stipule.

MeristemThe area of dividingcells; undifferentiated tissuecapable of differentiating intospecialized tissue.

NodeThe location on thestem where the leaf attaches.

NoduleThe site of nitrogenfixation, composed of amass of root cells surrounding theRhizobium bacteria.

OvaryLocated at the base of thepistil, it contains the ovules, and itwill become the seedpod (pericarp)in legumes.

OvuleA rudimentary seed beforefertilization, containing the femaleegg cell.

PedicelA single branchlet whichconnects the flower bud, flower,or seedpod to the peduncle (mainstem).

PericarpWall of the ovary (seedpod)which encloses seeds.

PistilThe female portion of a flower,consisting of the ovary, style, andstigma.

PollinationPollen coming in contactwith the stigma.

RacemeA type of flower where thefloral structures are attached to the

peduncle via a pedicel.RegrowthVegetative bud and shoot

elongation of new shoots eitherafter the existing shoots are cutor after the shoots have begun toflower.

RhizobiumBacteria responsible fornitrogen fixation in legumes.

StamenThe male portion of theflower, consisting of anther andfilament.

TrippingRelease of the pistil andstamen from the fused keel petals,resulting in the rupture of thestigmata cuticle.


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Copyright 2011bythe Board of

Regents of the University of Wisconsin

System doing business as the division of

Cooperative Extension of the University of

Wisconsin-Extension. All rights reserved.

Send copyright inquiries to: Cooperative

Extension Publishing, 432 N. Lake St., Rm.

227, Madison, WI 53706, [email protected].

Authors: Dan Undersander is professor ofagronomy, College of Agricultural and Life

Sciences, University of Wisconsin-Madison

and University of Wisconsin-Extension,

Cooperative Extension. Marvin H. Hall is

professor of forage management, The

Pennsylvania State University, University

Park, Pennsylvania. Paul Vassalotti is a

master technical service representative,

American Cyanamid Co., Cross Plains,

Wisconsin. Dennis Cosgrove is professor

of agronomy, College of Agricultural and

Life Sciences, University of Wisconsin-River

Falls, and University of Wisconsin-Extension,

Cooperative Extension. Cooperative

Extension publications are subject to peer

review.

University of Wisconsin-Extension,Cooperative Extension,in cooperation

with the U.S. Department of Agriculture

and Wisconsin counties, publishes this

information to further the purpose of the

May 8 and June 30, 1914, Acts of Congress.

An EEO/AA employer, the University

of Wisconsin-Extension, Cooperative

Extension provides equal opportunities in

employment and programming, including

Title IX and ADA requirements. If you

need this information in an alternative

format, contact Equal Opportunity and

Diversity Programs, University of Wisconsin-

Extension, 432 N. Lake St., Rm. 501,

Madison, WI 53706, [email protected],

phone: (608) 262-0277, fax:

(608) 262-8404, TTY: 711 Wisconsin Relay.This publication is availablefrom your

county UW-Extension office (www.uwex.

edu/ces/cty) or from Cooperative Extension

Publishing. To order, call toll-free (877)

947-7827 (WIS-PUBS) or visit our website,

learningstore.uwex.edu.

National Alfalfa & Forage Alliance (NAFA)

www.alfalfa.org

Alfalfa Germination and Growth (A3681) R-08-2011

Cooperative Extension
mailto:[email protected]:[email protected]://www.uwex.edu/ces/ctyhttp://www.uwex.edu/ces/ctyhttp://learningstore.uwex.edu/http://learningstore.uwex.edu/http://www.uwex.edu/ces/ctyhttp://www.uwex.edu/ces/ctymailto:[email protected]:[email protected]

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