Genetic factors and hormonal stimulation affecting weight and fertility of ewe lambs.

O NE hundred seventy-nine ewe lambs from a three-breed polyallel cross involving the Willamette, Suffolk and Hampshire breeds were used to study the genetic aspects of fertility. Ewe birth weight, ewe weight at mating and fertility were analyzed in relation to heterosis, general combining ability, maternal effect, reciprocal differences, and the effects of a hormone treatment. Treatment consisted of three injections of anterior pituitary extract 2 weeks apart beginning 7 weeks before the breeding period. One-half of the ewes from each breed group were treated, others were not. There was no significant difference in fertility of the treated and control ewes. In some cases, treatment adversely affected fertility, in particular in ewe lambs of the Suffolk breed. Willamette and Hampshire ewes were unaffected. For ewe birth weight, heterotic effects approached significance (P~0 .10) and accounted for 2.8% of the total variance in birth weight. Comparisons of reciprocal crossbreds with the mid-parent mean showed crossbreds 0.3 kg heavier at birth. Heterosis, purebred differences and reciprocal differences were highly significant for ewe weight at mating. Maternal effect and general combining ability were significant. Crossbred ewes were 2.5 kg heavier than the purebreds. The genetic factors measured for ewe weight at mating accounted for over 62% of the total variance. Ewe lambs treated with gonadotropins during the normal breeding season do not respond with the success that has been shown for mature ewes. Overall lambing percent was 71 based on the number of lambs born to 179