Estimation of heterosis in Merino × Corriedale crosses with sheep

Abstract

Reproduction rate data (lambs born, lamb survival rate, and lamb body weight) are presented for purebred Merino and Corriedale ewes and their reciprocal crosses. The four breeds were born in each of two years (1961 and 1962) and mated in each of two years (1963 and 1964, 1964 and 1966) to Dorset Horn rams. Heterosis was estimated as the superiority of the mean of the crossbreds over the mean of the purebreds, expressed as a percentage of the latter. The heterotic effect for lambs born per ewe joined was 1.9 % in 2-year-old ewes and 5.3 % in adults, the corresponding figures for lamb survival being 3.5 and 1.4 % for single-born, 17.2 and 12.0% for twin-born lambs. For mean age-corrected lamb body weight, 2-year-old ewes showed a heterotic effect of 3.7 % and adult ewes 2.0 %, while for pounds of lamb weaned per ewe joined the figures were 11.3 and 10.2%. Although some heterosis was exhibited, the mean of the crossbreds seldom exceeded that of the superior parent (the Corriedale), and there were sometimes differences between the reciprocal crosses, the Corriedale x Merino cross being superior in performance to the other. The potential for exploiting this degree of heterosis in reproduction rate cannot be assessed until the corresponding wool data have been analysed. In reproduction rate, the Corriedale was superior to the Merino, but again a final conclusion about their merits must await results on wool data.