Heterotic Basis of Ovulatory and Reproductive Performance in Ewes of Three Breeds

Variation in ovulation rate of ewes was studied with particular reference to the heterosis effect in a 3-breed (Dorset Horn, South Australian Merino, Corriedale) experimental sheep population. For the population as a whole, heterosis effect on the young (1 1/2 year-old) ewes (+0.17 k 0.06** ova, or 14.5%) was similar to that on the mature (5 1/2 year-old) ewes (+0.19 f 0.06** ova, or 12.5%), indicating that age of the ewe is of minor or no importance in heterotic variation of ovulation rate. However, some variation was present in the breed-specific estimates, notably for the Dorset Horn/Merino combination due largely to the fact that the mean ovulation rate of the purebred Dorset Horn ewes did not increase with age, as did all other breed-genotypes. This sheep population also provided data for study of variation in number of lambs born per ewe joined, or reproductive rate, at the 2 year-old lambing. The results showed that on average the Fl crossbred ewes were higher in reproductive rate (+0.17 t 0.07* lambs, or 26.2%) than their purebred contemporaries but no definite pattern is discernable from a comparison of corresponding heterosis effects on reproductive and ovulation rates of the primiparous 2 year-old ewes. The positive relationship between means of these 2 traits for the 9 breed-genotypes was clearly established.