Litter Size

Among reproductive traits, litter size is the most amenable to selection. Although its heritability is low, the trait is reasonably variable, allowing relatively high selection pressure.[5] Annual gains of 1 to 2% in litter size have been achieved.[6] With improved tools for genetic evaluation available, and by basing selection decision on repeated records of litter size of ewes, larger gains are possible.

There is considerable variation between breeds in mean litter size, ranging from above one lamb to nearly four. By strategically crossing breeds, litter size can be readily increased.1-7-1

Regardless of the strategy followed, an optimum rather than maximum number of lambs born is the goal. The target mean litter size depends on the husbandry and feed resources available in a flock, and its season of lambing. Where litter size is too high, reduced survival and growth rates in multiple births may reduce rather than increase lamb output.

Lamb Survival

Lamb mortality is often a serious problem. There are important maternal genetic factors within breeds in addition to variation between breeds that can be used to improve lamb survivability. However, optimizing mean litter size is the predominant genetic mechanism to influence lamb survival. Lambs born in larger litters have lower birth weight, which predisposes them to starvation and hypothermia during inclement weather. Losses can be reduced by supplementary feeding of litter-bearing ewes during late pregnancy and early lactation, by lambing ewes during warmer seasons in colder climates, and, where practicable, by housing ewes and applying good husbandry practices including fostering at lambing time.

Slaughter Weight

Selection to increase meat production has primarily focused on size or weight. Substantial genetic change in live weights has been achieved (Fig. 1). Increasing weight, however, does not necessarily improve efficiency of meat production. Increased weights at immature ages correspond with heavier weights at maturity and thus, increased feed costs of the breeding flock. Larger lambs also typically take longer and consume more food to achieve a target level of fatness for market. Selection for size may also adversely affect fitness and reproductive success. Although the evidence is equivocal, the genetic relation-

1990 1992 1994 1996 1998 2000 2002 Year

Fig. 1 Estimated genetic trends in live weight in Charollais, Suffolk, and Texel sheep in industry breeding schemes in Britain. (Data courtesy of Signet Farm Business Consultancy, Milton Keynes, UK.) (View this art in color at www.dekker.com.)

1990 1992 1994 1996 1998 2000 2002 Year

Fig. 1 Estimated genetic trends in live weight in Charollais, Suffolk, and Texel sheep in industry breeding schemes in Britain. (Data courtesy of Signet Farm Business Consultancy, Milton Keynes, UK.) (View this art in color at www.dekker.com.)

ship between mature weight and litter size tends to be positive, while that with fertility and lamb survival negative.[4,5] In dam lines, and where fodder availability is limited, restricting increases in live weight is often desirable. By careful choice of sire breed in crossbreeding systems, weight can still be tailored to market

requirements. ]

Carcass Composition and Conformation

Strategies to alter the composition of the carcass act through direct effects on composition and indirect effects on the size at maturity. If live weight at maturity is increased, at a given immature weight, the earlier maturing tissues such as lean define a greater proportion of the body. As a corollary, at a constant level of fatness, lambs sired by breeds of larger mature size produce heavier carcasses. Composition can be altered by affecting the rate of gain of lean tissue to an immature weight, or to alter the proportion of lean in the carcass at an immature weight.[8] Crossbreeding is one useful tool to achieve that aim.

Besides carcass leanness, conformation is considered an indicator of carcass quality. Shorter blocky carcasses are perceived to have higher lean-to-bone ratio and increased muscle thickness at the same carcass weight, although the association appears weak. In some cases, better conformation carcasses are simply fatter. In contrast to carcass composition, the importance of conformation on quality is unclear. The percentage of the carcass consisting of more desirable cuts, and eating quality, appears more important than conformation.

Carcass composition cannot be measured directly and thus live weight and real-time ultrasound measurements of fat and muscle depth are often used as indicators of composition. Heritabilities are moderate to high for weight and ultrasonically measured traits,[4] and when these criteria are amalgamated into selection decisions, substantial increases in carcass lean content can be achieved. More recently, X-ray computed tomography (CT) has been introduced to sheep breeding programs, allowing a more accurate, in vivo measure of carcass composition,[8] offering further scope to accelerate genetic progress in lean meat production. However, CT is only cost-effective when used in coordination with ultrasound.

SIRE REFERENCING

In many countries, the size of flocks is small with little scope for intensive, within-flock selection. Furthermore,

Fig. 2 Schematic diagram of sire referencing scheme in sheep where offspring of reference sires provide a benchmark for comparison across flocks. (Courtesy Scottish Agricultural College, Edinburgh, United Kingdom.)

accurate selection of animals from outside flocks is difficult due to differences in husbandry. This problem has been overcome through cooperative breeding schemes such as sire referencing schemes (SRSs). In SRSs genetic links are created among flocks by mutual use of some rams (Fig. 2). These connections allow equitable, across-flock genetic evaluations, offering a larger pool of candidates for selection for some collective breeding goal. Quicker genetic progress is thus possible.[9]

Over the last decade, SRSs have been formed in many sheep breeds, particularly in Britain. High rates of responses to selection for lean growth rate have been achieved (about 1.75% per annum in the specialized meat breeds).[10] These schemes also have the ideal structure to exploit new technologies cost-effectively, such as molecular tools and CT.

CONCLUSION

Reproductive rate is a central contributor to production efficiency in sheep enterprises. By delineating roles of breeds, reproductive efficiency and fitness can be emphasized in dam breeds, or by crossing breeds with complementary dam characteristics, while weight and composition can be improved in sire breeds. By the strategic crossing of dam and sire breeds, lambs with carcass attributes tailored to market demands can efficiently be produced.

Technologies are available to allow sheep breeders to accelerate genetic gain within their flocks. However, utilization of these tools remains limited. Cooperative breeding schemes provide an avenue to remedy that situation, which is vital to the future of sheep enterprises.

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