This chapter deals with the general pattern of …€¦ · Web viewIn the young growing animal,...

4. GrowthJohn Thompson

Learning objectivesAt the end of this topic you should:

Describe the general pattern of growth and the associated changes in carcase composition Understand and describe the efficiency of meat production that occurs as a sheep proceeds

from birth to maturity Employ this knowledge to improve the efficiency of a meat-producing enterprise

4.1 IntroductionThere is currently a trend towards the use of larger mature-size breeds and strains for meat production. In addition, most performance recording schemes for both meat sheep and wool- producing breeds include some selection for growth. In a self-replacing flock, the larger, faster-growing animals have a greater output in terms of lean tissue produced; but, as they also require more food than smaller animals, mature body size has little effect on the biological efficiency of meat production. The increased food required when running animals of larger mature size is mainly a result of the increased food intake required by the larger dams. Consequently a crossbreeding system in which maternal food costs are defrayed, either by the use of a large-mature-size sire mated to a small-mature-size dam or by an increase in fertility, will result in an increase in the efficiency of meat production.

4.2 Growth and development As the sheep eats, it partitions food between the deposition of new body tissues, maintenance of body weight, wool growth and, in breeding ewes, the growth of the foetus and milk production. Only a very small proportion is de- posited in wool fibres (each kg of food produces about 3 grams of wool), the greatest proportion being used for either the deposition of new body tissues, or the maintenance of body weight. The partitioning of food changes as the sheep matures. In the young growing animal, most goes towards the deposition of new tissues, whereas in the mature animal, which has finished growing, virtually all food goes to maintaining body tissues.

The 'mature weight' of an animal can be defined as the final weight that it attains in a particular environment, prior to the decline in body weight associated with old age. Sheep fed a high-quality ration ad libitum would be expected to reach their final size by about 2 -3 years. On a lower level of nutrition this may take longer. Sheep in a more extreme environment where they were continually subjected to stresses, such as poor nutrition or heat, may take longer to reach a lower mature weight. Therefore the mature weight attained by an animal, either in the pen or paddock, is a function of genotype and environment.

The relative differences in mature weights between groups of animals are more important than the absolute figures. In a pen-feeding situation, mature weight may be estimated as the final weight attained once no further gains are made. In a paddock, where environmental and nutritional stresses fluctuate over the year, mature weight is often difficult to measure. One procedure is to average adult body weights over a number of years. Since a large proportion of the variation between adults is due to fat, another approach is to adjust each one to the same fat score. For comparative purposes, estimates of mature weight should be obtained from animals raised in the same environment.

ANPR420/520 Sheepmeat Production and Marketing - 4 - 1©2009 The Australian Wool Education Trust licensee for educational activities University of New England

As the sheep increases in body weight, the proportion of fat increases, the proportion of muscle1:"emains relatively constant and the proportion of bone decreases. From the consumers' viewpoint the ideal carcase has the maximum amount of muscle, the minimum amount of bone and the optimum amount of fat. However, when contemplating changes in carcase composition, it is important to consider the functional requirements of the animal also. Regardless of what is commercially ideal, the sheep still has to be capable of walking, eating, reproducing and withstanding the various nutritional and environmental stresses of the Australian environment, for which it needs some fat and, more importantly, a skeletal framework on which to support the musculature.

Although about 85% of our sheep-meats are consumed on the domestic market, little information is available on the type of lamb that Australian consumers prefer. If producers are to increase -or even maintain - lamb consumption, they need to know this type so that they can modify their production systems to supply it. A recent study, conducted by the Livestock and Meat Authority of Queensland, surveyed attitudes and preferences from 180 households in both Brisbane and Melbourne (Hopkins et al., 1985). It revealed a clear preference for retail cuts from heavier leaner carcases. In general, retail cuts from carcases that had a 'GR' fat depth measurement of 6-10 mm (equivalent to a carcase with 1-3 mm of fat over the eye muscle at the 12/13th rib site) and were heavier than 20 kg. were preferred. The majority of lambs being sold through the present auction system have a fat score of 3, or 4, with an average carcase weight of 17 kg. Therefore present lamb carcases are both too fat and too light to satisfy such a demand. Unless this problem is rectified. lamb consumption can be expected to decline further.

Although the consumer trend is for leaner meat, it is not for a carcase that is totally devoid of fat, but for 1 to 3 mm of subcutaneous fat covering the muscle. Certain flavours are contained in the fat tissue and a small amount of subcutaneous fat is required to stop the meat drying out and also to provide a barrier against microbial contamination. Therefore a light, even covering of fat is desirable, both for the eating quality of the meat and to ensure an adequate shelf life for the retail cuts. A knowledge of the processes of growth and development of the body tissue can help producers reduce lamb carcase fatness.

4.3 The pattern of feeding and growth This section examines the changes in food intake. gross food efficiency, growth rate, body composition and the biological efficiency of meat production that occur as the sheep grows and matures. To illustrate these changes. it uses data from a long-term feeding and growth experiment in which Peppin Merino rams with a mature weight of 70 kg were fed a high-concentrate ration until approximately 2 years of age (see Thompson et al., 1985 a,b). At regular intervals, sheep were slaughtered and detailed compositional studies undertaken.

Food intake When the supply of food was not limiting (that is, ad libitum), food intake in the rams increased rapidly from birth to a maximum at approximately 40 weeks of age. Thereafter food intake declined with age (Figure 4.1). The results from a number of experiments indicate that the rate of decline may vary for different breeds and strains of sheep. This decline in food intake is consistent with a decline in metabolic rate of the animal with age.

4 - 2 ANPR420/520 Sheepmeat Production and Marketing ©2009 The Australian Wool Education Trust licensee for educational activities University of New England

Figure 4.1 The pattern of food intake (MJ of ME per week) as a function of age (wks) from birth to maturity for Merino rams. The concentrate ration contained 10.2 MJ of ME/kg of dry

feed. Source: Thompson et al. (1985a,b).

Gross food efficiency This is defined as the ratio of body-weight gain to food consumed (kg of body-weight gain per MJ or ME food intake). It declined linearly as body weight increased (Figure 11.2a), with animals having the highest efficiency at birth and zero efficiency at maturity.

Figures 4.2 (a) and 4.3 (b) Gross food efficiency of Merino rams (kg of body weight per MJ of ME intake) as a function of: a) bodyweight (kg) and b) age (wks) from birth to maturity.

Source: Thompson et al. (1985a,b).


Gross food efficiency showed an exponential decline as age increased (Figure 4.3). The rate of decline was greatest at young ages. At 2 years when the sheep had reached their mature weight, it had declined to zero.

Growth rate The animal's growth rate is a function of feed intake, and the efficiency with which the animal converts this feed to body weight. Figure 4.1 described the animal lifetime feed intake, while Figure 4.3 describes the conversion of this feed to body weight. If the two are multiplied together, the resultant curve represents the growth rate of the animal (Figure 4.4).

Growth rate was low immediately after birth, but rapidly increased to a maximum at about 18 weeks, then declined at an exponential rate to zero, at which stage the animal had reached its mature weight.

Figure 4.4 Growth rate (kg/wk) as a function of age (wks) from birth to maturity for Merino rams. Source: Thompson et al. (1985a,b).

However, the concept of growth as a rate, in units of kg per week, is difficult to visualise in an animal. A more familiar way of describing growth is as a cumulative measure -that is, body weight in kg. When body weight (calculated as the area under the growth rate curve) was graphed against age, the resultant curve was the S-shaped or sigmoidal growth curve shown in Figure 4.5. Body weight increased from birth to maturity, although the rate of increase was positive only up until 20 weeks and then declined until the animal reached maturity.

Figure 4.5 Body weight (kg) as a function of age (wks) from birth to maturity for Merino rams. Source: Thompson et al. (1985a,b).


The growth curve in Figure 4.5 has been adjusted for environmental stresses. In a grazing situation, where animals are subjected to seasonal variations in food quality and food quantity, disease and climatic variations, it may not be so regular, although the underlying S- shaped curve would still be evident.

Body composition The body can be divided into carcase and offal components. The offal components include the thoracic and abdominal organs, various internal fat depots and the pelt, head and legs. The carcase is the most valuable component of the body and comprises three main tissues: muscle, bone and fat, which vary widely in their economic value, with muscle being the most valuable.

Figure 4.6 shows the proportional changes in carcase composition from birth to maturity. At birth it is necessary that the skeleton of the animal be well developed, both to act as a protective cage around the vital organs and to provide a solid support to muscles. During growth the increase in bone weight was smaller than those for muscle and fat and therefore the proportion of bone decreased as stage of maturity increased. At birth the newborn rams had approximately 9% carcase bone in the body, compared with 6% in mature rams weighing 70 kg. Bone is therefore classed as an early-maturing tissue, or said to have a low-impetus growth pattern.

Figure 4.6 Proportional changes in bone, muscle and fat in the body as a function of stage of maturity of Merino rams. Source: Thompson et al. (1985a,b).

The proportion of muscle in the body remained relatively constant, only increasing from 24% in the newborn lamb to 26% in the mature ram. Carcase muscle is therefore classed as an average-maturing tissue with an average-impetus growth pattern. As bone had a low-impetus growth pattern, the muscle to bone ratio in the carcase increased as the animal matured. Fat is largely a long-term storage site for energy in the body. At birth, fat tissue was the most poorly developed of the three carcase tissues, with dissected fat comprising only about 7% of the body. The proportion of fat in the body at birth will have an influence on lamb survival, with low birth weights being associated with low body fat reserves and reduced survival. As shown in Figure 4.6, the proportion of body fat increased rapidly during postnatal growth. Depending upon the environment, the mature


ram may contain up to 30 or 40% dissectible fat in the body. As the proportion of fat increased as the animal matured, it is classed as a late-maturing tissue with a high-impetus growth pattern. Stage of maturity, or body weight at which the animal is slaughtered, is the main determinant of carcase composition. From Figure 4.6, a ram slaughtered at 35 kg liveweight, or at 50% of its mature weight, comprised 18% carcase fat, 25% muscle and 7.5% bone. However, when grown out to maturity and slaughtered at 70 kg liveweight, the proportion of fat increased to 30%, carcase muscle increased slightly to 25%, and carcase bone decreased to 6%.

Biological efficiency of meat productionThe biological efficiency of a meat-producing enterprise is calculated as the ratio of output (weight of lean tissue) to input (MJ of ME consumed). In a breeding enterprise the output should include both the weight of lambs turned off and the cull weight of the ewe. Similarly, inputs should include the food consumed by both the progeny and the ewe. The latter is often overlooked when calculating efficiency. Depending upon the slaughter weight of the lamb, the cost of maintaining the breeding ewe may comprise 50-60% of the annual food consumed by the enterprise. However,


in a self-replacing flock, when food costs for the enterprise are adjusted for the cost of raising the ewe from birth to 'breeding age, and the removal of one female progeny at birth to be used as the replacement animal, the proportion of food consumed by the ewe increases to over 80% of the total food input for the enterprise.

The biological efficiency of meat production for the ewe/lamb unit over the lifetime of the ewe can be calculated using a simple output/ input function:

Biological efficiency of the ewe/lamb unit

=

lean tissue produced from sale progeny

+ lean tissue from the cull ewe

food consumed by the progeny to slaughter (not including the replacement female)

+ lifetime food consumed by ewe

Such a ratio has major disadvantages: it does not take into account other on-farm costs in a sheep breeding enterprise (such as management and capital costs), and assumes the same cost per unit of food over all seasons (that is, a non- seasonal food supply) and for all classes of stock. It also assumes the same price per unit of lean for both lambs and cull ewes. Although economic efficiency is more relevant to producers when comparing specific options, biological efficiency is a more stable comparison of enterprises across geographical regions and time. Therefore it should provide a reference, which after adjustment for the relative economic weightings can provide a measure of economic efficiency of the enterprise.

The biological efficiency of the ewe/lamb unit was calculated for a Merino strain with mature weights for rams and ewes of 70 and 50 kg respectively, based on the previous figures. Maiden ewes were mated at 19 months of age to first lamb at 2 years of age, and ewes were kept for a total of 6 lambings before culling. An average weaning rate of 120% was assumed for all lambings.

As stage of maturity increased, ewe/lamb efficiency increased to a maximum at 75% of mature weight and then declined (Figure 4.7). As discussed earlier, gross food efficiency of the individual lamb was at a maximum at birth and declined to zero at maturity. As slaughter weight of lambs increased, the fixed cost of maintaining the ewe was defrayed per kg of lamb produced, and the ewe/lamb efficiency increased. The maximum efficiency was attained at a slaughter weight of 75% of mature weight, but the shape of the curve was relatively flat with little change in efficiency if lambs were slaughtered at 45 to 85% of their mature weight.

Most prime lambs are slaughtered commercially at about 50% of their mature weight. If carcases were penalised for over-fatness, the optimum stage of maturity for slaughter in terms of economic efficiency might be much lower than predicted from Figure 4.7, as fatness increases with slaughter weight.

Figure 4.7 Lifetime biological efficiency for the Merino ewe/lamb unit (for example, lean tissue per MJ of ME) as a function of stage of maturity for slaughter progeny. Source:

Thompson et al. (1985a,b).


Changing mature sizeLarge-mature-size sheep grow faster and produce more meat than small-mature-size sheep. If judged solely on meat output per animal, large-mature-size sheep appear superior. However, this increase in meat production is largely offset by an increase in food intake, so there is little change in the efficiency of meat production.

An increase in mature size may be obtained by utilising either between- or within-breed variation in mature size. Breed substitution may be used to replace the present breed or strain with a larger-mature-sized one. For example, Merinos show substantial breed and strain variation in mature size. Alternatively one can select for increased body size within a breed. Body size at any age has a low to moderate heritability and is positively correlated with mature weight. Therefore selection of ram and ewe replacements within a flock for increased weight will result in an increase in weight, both at the age of selection and also at maturity. Whereas breed substitution can have an immediate effect on mature weight, selection for increased weaning or yearling weight will take longer to have an effect.

These methods provide very different means of changing mature size, and their effects on the feeding, growth and compositional characteristics of animals were investigated in two long-term feeding experiments. The first of these compared large rams with small ones from strong-wool and fine-wool Merino strains (see Butterfield et al., 1983; Thompson and Parks, 1983). the second experiment compared rams and ewes between two Peppin Merino flocks, which had been selected for high and low weaning weight over a period of 30 years (Thompson et al., 1985). In both experiments, animals from the various strains and sexes were individually fed a high-concentrate ration to 2 years of age. At various intervals from birth to maturity, animals were slaughtered for detailed compositional studies.

Figure 4.8 Ewes from the high (left) and low(right) weaning weight selection lines. These flocks, established at Trangie Agricultural Research Centre in 1950, were selected for high

and low weaning weight for 30 years. Source: J. Thompson (2006).

In the Merino strain comparison, mature weights for rams from the large and small strains were 116 and 91 kg respectively, which represented a divergence of about 30% in mature size. In the weaning weight experiment, mature weights for the high- and low-weaning-weight selection flocks were 67 and 50 kg. This also represented a divergence of 30% in mature size, but it had been achieved by a markedly different mechanism. The first divergence was due to a strain effect, but the second came from within-flock selection for high and low weaning weight. The following section examines the effect of this divergence in mature size on feeding, growth and compositional characters.


Figures 4.9 (a) and 4.10 (b)The pattern of food intake in MJ of ME as a function of age for: (a) rams from the strong- and fine-wool Merino strains and (b) rams and ewes from the High

and Low weaning weight flocks. Source: Thompson et al. (1985a,b).

Figures 4.9 and 4.10 show the food intake curves in both experiments, revealing a divergence in food intakes, although it was generally less than the divergence in mature size. In addition to the increase in mature feed intake in the high-weight strains compared with the low ones, there was also a small increase in appetite (that is, mature food intake was reached at an earlier age).

The differences in food efficiency between animals of different mature size varied, depending upon how the comparisons were made. At the same body weight the large-mature-size animals were more efficient than the small animals, although when compared at the same stage of maturity of body weight, the groups showed no difference. In other words, differences in food efficiency were only apparent when animals were compared at different physiological stages of growth. However, once this bias was removed and animals were compared at the same physiological stage of development (that is, the same stage of maturity), all animals had a similar food efficiency, regardless of their mature weight. Other studies have also found little difference in food efficiency between other sheep breeds, and even between cattle and sheep (Taylor, 1987).

Growth curves showed the large-mature-size groups from both experiments were heavier and had a faster growth rate at any age. The increase in appetite in the high-weaning-weight flock did have a small effect on the shape of the growth curve, with these animals maturing at a slightly faster rate than expected. In both experiments, animals were slaughtered at various intervals along the growth curve, and at maturity. Composition of mature rams from the Merino strains are shown in Table 4.1. Although the rams from the two strains had different mature liveweights and weights of dissected tissues, they had similar proportions of fat, muscle and bone. Similar results were obtained for the weaning-weight flocks. Within sexes, there was no difference between the strains in mature body composition. Therefore it would appear that animals have a similar body composition at maturity, regardless of the final end- point, or mature weight, they attained.


Table 4.1 Mean carcase tissue weights expressed in kg or as a proportion of shorn full liveweight for mature rams from strong- and fine-wool Merino strains. Source: Butterfield et al (1983).

Weight of tissue (kg) Proportion of liveweightStrong-wool Fine-wool Strong-wool Fine-wool

Carcase fat 26.7 18.8 0.23 0.21Carcase muscle 25.9 20.7 0.22 0.23Carcase bone 6.4 4.9 0.055 0.055Shorn full liveweight (kg) 116.0 91.0

Merino strains did not differ in the rate at which fat, muscle and bone matured relative to the rate at which the total body weight matured. On the other hand, in the weaning-weight flocks, fat matured at a slightly faster rate and bone at a slightly lower rate in the high-weaning-weight flock. However, these differences were not large and were most likely a response to a greater milk intake by lambs in the high-weaning-weight flock.

Therefore, mature body size had little effect on body composition at maturity or on the rate at which fat, muscle and bone matured in the body. Given this, Figure 4.6 can be used to describe changes in percentage body composition relative to stage of maturity in all sheep, regardless of their mature size.

If large- and small-mature-size animals were compared at the same liveweight, the large-mature-size animals would be at an earlier stage of maturity and would therefore have a lower proportion of fat, a similar proportion of muscle and a higher proportion of bone (Figs 4.11 and 4.12). Similarly, if animals were compared at the same age, the large-mature-size ones would be less mature (although the differences would be less extreme than comparison at the same liveweight), and therefore would have a slightly less mature composition (that is, a lower proportion of fat, a similar proportion of muscle and a greater proportion of bone).

Figures 4.11 (a) and 4.12 (b) Bodyweight (kg) as a function of age (wks) for a) rams from the strong- and fine-wool Merino strains and b)rams and ewes from the high and low weaning

weight flocks. Source: Thompson et al. (1985a,b).


Finally, if large- and small-mature-size animals were compared at the same stage of maturity or level of finish, they would have a similar body composition. However, to achieve the same level of finish or body composition, large-mature- size animals have to be kept slightly longer and be turned off at a heavier weight.

The effect of change in mature size on the biological efficiency of meat production for the ewe/lamb unit was modelled using the same inputs for weaning rate, age at first joining and culling age as for Figure 4.7. For a 30% increase in mature size, there was only a 4% increase in lifetime biological efficiency. Obviously the benefit of increased output from larger- mature-size animals is substantially offset by the increased food intake requirements of the larger animals, particularly the increased food requirements to maintain the heavier ewes.

However, given the consumer trend for leaner, heavier retail cuts, and a move towards an objective marketing system that can discriminate between levels of fat and weight 16), it is likely that a price premium will be paid for leaner, heavier carcases. Therefore although increasing mature size may offer little biological advantage, changing it to produce a particular weight/fatness combination may yield an economic advantage.

Crossbreeding

As the food consumed by the dam comprises such a large proportion of total food costs, techniques to reduce these food costs will result in a large increase in the biological efficiency of the enterprise. Crossbreeding, whereby the feed intake characteristics of the sire and dam breeds are used to complement each other, is one such technique. A large-mature-size sire breed mated to a small-mature-size dam breed will effectively lower the proportion of food required to maintain the dam, relative to the progeny, and so increase the biological efficiency of meat production. In addition to complementation of sire and dam effects, crossbreeding also has the advantage of heterosis. A first-cross system can make use of both complementarity of sire and dam effects and heterosis for growth traits of the progeny. In addition to these advantages, a second-cross system can also make use of heterosis for fertility by using first-cross females. This effectively increases the output for the same maternal cost, and so results in an increase in efficiency.

Heterosis has little advantage for mature weight. Estimates of heterosis for growth rate or body weight at a particular age in immature animals are usually 0 to 10%. Increase in growth rate is most likely achieved through an increase in appetite, rather than a change in the efficiency with which food is converted to liveweight. Thus heterosis for growth will have little effect on biological efficiency for meat production. Heterosis for fertility traits is generally 5 to 25%. If a large-mature-size sire is crossed with a small-mature-size dam, the mature weight of the progeny can be estimated as the average of the two parent breeds (that is, there is little heterosis for mature weight). Therefore the output and input costs associated with both the progeny and the dam and the effect of crossbreeding on the biological efficiency of meat production can be calculated.

Figure 4.13 Biological efficiency (g lean tissue per MJ of ME) as a function of stage of maturity of slaughter progeny, for a purebred, a first-cross and a second-cross enterprise.

Source: Thompson et al. (1985a,b).


Biological efficiency was calculated for the following three systems:

a purebred enterprise, in which the ewes had a mature weight of 50 kg. A weaning rate of 120% was assumed, with ewes lambing at 2 years of age and being culled after 6 lambings.

a first-cross enterprise, using base ewes with a mature weight of 50 kg, mated to a sire breed with 30% greater mature size. The crossbred progeny also exhibited 10% heterosis for growth (which occurred through an increase in appetite of the crossbred progeny). Other parameters were as above.

a second-cross enterprise, using crossbred ewes with a mature weight of 50 kg mated to a sire breed with a 30% greater mature size. Heterosis for weaning rate in the crossbred bred progeny exhibited 10% heterosis for growth, through an increase in appetite. Other parameters were the same as above.

Relative to the purebred enterprise, a first-cross system in which mature size of the sire and dam breeds diverged by 30% resulted in a 7% increase in biological efficiency (Figure 11.10). This is almost twice the increase in efficiency obtained from increasing mature size by a similar amount in a purebred flock. It resulted from a 15% increase in mature size for the crossbred progeny, which effectively increased lean meat output from the system for the same maternal cost. A second-cross system, which made use of both complementarity effects and an increase in fertility of the crossbred ewes, resulted in an increase in biological efficiency of almost 18% relative to a purebred enterprise. Again this increase was due to a greater output for the same maternal cost, although in this case it was due to both a 15% increase in mature size of the progeny relative to the dam and a 25% increase in weaning percentage.

Crossbreeding can also be used to manipulate carcase composition of the progeny, as the crossbred progeny's mature size is the average of the sire and dam breeds. There is little heterosis for carcase characters, after adjustment for stage of maturity. Therefore in comparisons of crossbred and purebred progeny at the same weight and age, the differences in carcase composition would simply be a function of the differences in stage of maturity. Therefore use of a large-mature-size sire breed increases the mature size of the crossbred progeny, which if slaughtered at the same weight will be at a lower stage of maturity and therefore leaner than purebred progeny. As the curve of biological efficiency is relatively flat in the region of 45 to 80% of maturity, a slight drop in stage of maturity for slaughter of the crossbred progeny will have little effect on biological efficiency. The economic benefits from manipulation of carcase composition will obviously depend upon the price premiums being paid for particular fatness/carcase weight combinations.

ReadingsThe following readings are provided on CD.

1. Butterfield, R.M., Zamora, J., James, A.M. and Thompson, J.M., 1983. 'Changes in body composition relative to weight and maturity in large and small strains of Australian Merino rams: 2 individual muscles and muscle groups.' Animal Production, vol. 36, pp. 165-174.

2. Perry, D., Thompson, J.M. and Butterfield, R.M., 1988. 'Food intake, growth and body composition in Australian Merino sheep selected for high and low weaning weight.' Animal Production, vol. 47, pp. 275-293.

3. Wolfe, B.T., Smith, C., King, W.B. and Nicholson, D., 1981. 'Genetic parameters of growth and carcase composition in crossbred lambs.' Animal Production, vol. 32, pp. 1-7.

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SummaryIncreases in the costs of production and competition from other meats, such as poultry and pork, mean that lamb producers are under increasing pressure to increase the efficiency of their production systems. To achieve this, they should have a clear understanding of the factors affecting both biological and economic efficiency of their enterprise.

In a self-replacing flock, an increase in mature size of the flock, either by substitution or by selection for growth rate within a breed, has little effect on the biological efficiency of the ewe/lamb unit. The decision to change mature size should be made on the basis of the fat/weight class combination that provides the greatest return per kg of carcase.

As sheep have a relatively low reproductive rate compared with other species such as pigs and poultry, a larger proportion of the total food required by the system goes towards maintaining the dam. Crossbreeding systems reduce the maternal food costs by complementation of sire and dam effects, or by an increase in fertility. The greater is the ratio of mature size of the sire breed relative to the dam breed in a cross-breeding system, the greater the increase in biological efficiency, provided no reproductive problems such as dystocia occur. In addition, the greater the divergence in mature size in a crossbreeding enterprise, the greater is the scope for decreasing carcase fatness of the crossbred progeny. Selection for growth rate within a breed provides an option to increase the mature size of our present terminal sire breeds.

ReferencesButterfield, R.M. 1988. 'New Concepts in Sheep Growth.' Griffin Press: Netley, S.A.Butterfield, R.M., Griffiths, D.A., Thompson, J.M., Zamora, J., and James, A.M., 1983. Changes

in body composition relative to weight and maturity in large and small strains of Australian Merino rams. 1. Muscle, bone and fat. Animal Production, vol 36 pp 29-37.

Hopkins, A.F., Congram, I.D., and Shorthose, W.R. 1985. Australian consumer requirements for lamb and beef. 2. Consumer preferences for three common cuts of lamb carcases of varying weights and fatness and subject to various chilling rates. Livestock and Meat Authority of Queensland Research Series Report No. 19.

Purchas, R., Butler-Hogg, B. and Davies, A., 1988. The production and processing of meat. New Zealand Society of Animal Production Special Publication No. 11.

Taylor, St C.S., 1987. An evaluation of genetic size-scaling in breed and sex comparison of growth, food efficiency and body composition. Proceedings Australian Association of Animal Breeding and Genetics, vol. 6, pp. 1-12.

Thompson, J.M., Parks, J.R., and Perry, D. 1985a. Food intake, growth and body composition in Australian Merino sheep selected for high and low weaning weight. Food intake, food efficiency and growth. Animal Production, vol. 40 pp. 55-63.

Thompson, J.M., Butterfield, R.M. and Perry, D. 1985b. Food intake, growth and body composition in Australian Merino sheep selected for high and low weaning weight. Chemical and dissectible body composition. Animal Production, vol. 40 pp. 64- 70.

Thompson, J.M., Butterfield, R.M. and Perry, D. 1985. Food intake, growth and body composition in Australian Merino sheep selected for high and low weaning weight. Partitioning of dissected and chemical fat in the body. Animal Production, vol. 40 pp. 395-401.

Thompson, J.M., and Parks, J.R. 1983. Food intake, growth and mature size in Australian Merino and Dorset Horn sheep. Animal Production, vol. 36 pp. 471-9.


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