Genetic Effects on Carcass Composition Breed and Selection

There are many different breeds of sheep in the world and in individual countries, reflecting the different environments where sheep are found from high mountains to temperate lowland pastures, and from arid to extremely cold regions. Sheep are a natural grazing species, but in many dry countries, they are also reared on grain (concentrate) diets and in feed lots. These different environments have resulted in wide variation in body shape (conformation) and size.

In general, smaller, lighter breeds are fatter at a particular slaughter weight than the bigger, heavier breeds, reflecting differences in the stage of maturity (smaller breeds are closer to maturity). These differences are best expressed when all groups have been fed in a similar way. However, some breeds seem to depart from the general rule linking carcass composition with stage of maturity; e.g., the Soay and Texel are leaner than expected on this basis. In one study, Texel carcasses contained 60% muscle compared with 56% in the Oxford Down, whose mature weight was greater (100 kg in Oxford, 87 kg in

Because carcass composition traits such as the percentages of muscle and fat are moderately heritable (typically 0.3 0.5), it is possible to select leaner animals within breeds. This is likely to be most successful where large populations can be evaluated by linking flocks on different farms. One approach is the Sire Reference Scheme, in which the same sires are evaluated on several linked farms.[3] Progress on within-breed selection for leaner carcasses could be accelerated if accurate methods for evaluating carcass composition in the live animal were widely available, for example, computed tomography (CT).

The shape of the body or carcass, termed conformation, is affected by breed type. The ewe-type breeds, noted for maternal traits, have thinner muscles and more angular carcasses (poorer conformation) than the meat-type breeds, whose carcasses are blockier. These carcasses contain more muscle at the same fat cover, so a higher price is justified when carcasses are classified for fatness and conformation. However, premiums for conformation often penalize acceptably lean carcasses from ewe-type breeds.

Conformation differences between breeds are linked to differences in how the body fat is partitioned between fat depots. The ewe-type breeds have a higher proportion in the abdominal cavity and a lower proportion subcutane-ously on the carcass compared with the meat-type breeds.[4]

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