The repeatability of rearing performance of Merino and Dorset Horn ewes and its relationship with mature pelvic size

Abstract

Data from one Dorset Horn and three Merino flocks were used to examine factors influencing the ability of ewes lambing four times to successfully rear their lambs. Repeatability of rearing performance, defined as the ratio of lambs weaned:lambs born, was estimated in three flocks and the intra-class correlation estimates ranged from 0.05 to 0.17, with a mean of 0.10. Ewes rearing at least one lamb at their first lambing reared an average of 7.6% more lambs at each of the subsequent three lambings, compared with those that failed to rear a lamb at their first lambing. Mean pelvic dimensions, notably the conjugate diameter and the area of the pelvic inlet, of mature ewes, which reared lambs on three or four occasions were significantly larger in two flocks than in ewes which had poorer rearing performances (P<0.05), whereas in a third flock they were not significantly different. In the two flocks in which the significant differences were observed, mature pelvic dimensions were correlated with rearing performance at individual lambings. Rearing performance in these two flocks increased, respectively, by 16.3 and 6.3% per cm increase in conjugate diameter, and by 1.4 and 0.5% per cm2 increase in pelvic area (P<0.05). The slopes of these linear regressions were significantly greater for single than for twin-born lambs (P<0.05). The relationships between rearing performance and pelvic dimensions were also significantly curvilinear, the slopes of the regressions being significantly steeper below the mean pelvic dimensions than above them (P<0.05). Removing differences between ewes in pelvic dimensions reduced the repeatability of rearing performance. Allowing for differences in conjugate diameter reduced the repeatability in two flocks from 0.15 to 0.05, and from 0.09 to 0.08, respectively. In both flocks, allowing for differences in pelvic dimensions reduced the repeatability of rearing single lambs more than that for twin lambs.