Animal Production in Australia Vol. 15 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