Genetic components of litter size variability in sheep

Classical selection for increasing prolificacy in sheep leads to a concomit ant increase in its variability, even though the objective of the breeder i s to maximise the frequency of an intermediate litter size rather than the frequency of high litter sizes. For instance, in the Lacaune sheep breed ra ised in semi-intensive conditions, ewes lambing twins represent the economi c optimum. Data for this breed, obtained from the national recording scheme , were analysed. Variance components were estimated in an infinitesimal mod el involving genes controlling the mean level as well as its environmental variability. Large heritability was found for the mean prolificacy, but a h igh potential for increasing the percentage of twins at lambing while reduc ing the environmental variability of prolificacy is also suspected. Quanti cation of the response to such a canalising selection was achieved.