<< View PDF >>
When the value of wool is low, as at present, there is a greater potential for increasing both biological and economic efficiency of sheep production through improvement in meat production. It has been suggested that biological and economic efficiency can be increased more through genetic selection for improved reproductive rate than in growth rate or body composition (Fogarty et al., 1982). Reproductive rate is the most important component of total litter weight, which is clearly the single most important economic trait in American commercial sheep production. Loss of the Wool Incentive Program and lower wool prices in recent years have increased the economic importance of the total litter weight weaned per ewe. Current farm prices for wool and lamb indicate gross income from lamb exceeds that from wool by up to sixteen fold for most commercial producers of western white faced sheep. Hence, genetically increasing marketable litter weight per ewe is one of the most important contributions genetics can make to the economy of the sheep industry.
Increases in litter weight weaned can be made quickly through crossbreeding especially with prolific breeds. However, introduction of new breeds, often exotic, can result in unadapted genotypes with or without other desirable characteristics. Also, after crossbreeding has been thoroughly exploited, the only recourse for continued genetic progress is via selection for genetically superior individuals within breeds or crosses. It is important, therefore, to determine the relative effectiveness of alternative selection procedures for improving litter weight weaned.
The trait, litter weight weaned, is a composite trait affected by the expression of several genetically influenced traits. Variation in these component traits contributes to the phenotypic variation in the composite trait. Litter weight weaned is a combination of several different aspects of ewe reproduction (fertility, and litter size), ewe viability and offspring growth rate (mothering ability, milking performance, lamb survival, lamb growth rate). Thus, it is a convenient biological and economic measure of ewe productivity (Martin and Smith, 1980; Ercanbrack and Knight, 1985).
Long term selection for a composite trait may (but not necessarily) improve each individual component trait. Component traits within a composite trait should not be expected to improve at the same rate because they may differ in the genetic parameters involved. However, selecting for a composite trait should result in a balance among the component traits that produces an adapted animal, while selection for an individual trait can result in a reduction in adaptability. For example, selection response for a non-composite trait such as ovulation rate in sheep may be positive but gains in ovulation rate can be offset by decreased embryo survival (Bradford, 1985). Similarly, selection for increased litter size at birth may not be accompanied by increased milking performance and lamb growth rate. There may be limiting factors associated with favorable major genes such as the Booroola (FecB) allele which increases ovine ovulation rate substantially. While the FecB allele will increase litter size, there are associated decreases in lamb survival and weaning weight (Willingham and Waldron, 2000).
Direct selection for the composite trait of litter weight weaned in mice was three times as effective as selection for litter size for increasing litter weight weaned (Luxford and Beilharz, 1990). Long term selection in Targhee sheep for individual lamb weaning weight, rather than total litter weight weaned, resulted in decreases in lamb survival to weaning and ewe fertility (Bradford et al. 1999). From this last study, it is obvious that single trait selection for growth rate to weaning can improve weaning weight but it does not necessarily increase total lamb production per ewe. Thus, litter weight weaned per ewe exposed is the most appropriate composite trait to be used in selection for increasing total lamb production. The objective of this review is to characterize the composite trait litter weight weaned and its component traits.