The genetic basis of growth, reproduction, and maternal environment in Romney ewes

Abstract

This paper is the first in a series of studies on the genetic variation and covariation in growth and reproductive characters of the New Zealand Romney sheep. The data used were obtained over a period of 11 years (1955-1965 inclusive) from a random-bred experimental flock maintained at Massey University, Palmerston North, New Zealand. Among the environmental factors studied, type of birth and rearing has a marked effect on liveweights measured at different times from weaning to the yearling stage. On average, the singles were about 9 lb heavier than the twins at weaning, or c. 17 % heavier in terms of the mean weaning weight. Presumably because of their pre-weaning growth handicap, the twins compared with the singles exhibited post-weaning compensatory growth. The age of the dam and of the individual moderately affected the weaning weight of the lamb. However, their effects on post-weaning liveweight characters over the hogget stage (from weaning to about 14 months) showed that the influence of age of dam remained undiminished whereas that of the individual tended to decrease with time from weaning. The number of times a ewe hogget comes into oestrus during the first autumn of her life (number of hogget oestruses) was unaffected by type of birth and rearing, moderately affected by age of the individual, but relatively strongly influenced by the age of dam effect. Some doubt existed as to whether the average difference in fertility (number of lambs born per ewe) between ewes themselves born as singles and twins (type of birth effect) could be regarded as environmental or as genetic in nature. The present results indicated that in the fertility data, type of birth effect contained a genetic component, and adjustment should not be made for difference in type of birth if the aim is to study genetic variation and covariation in fertility of the ewe. The estimates of heritability for the various characters were made. by using paternal half-sib correlation (method 1) and regression of daughter on dam (method 2). In general, good agreement existed between corresponding values of heritability obtained by different methods of analysis. The estimates of heritabilities by methods 1 and 2 were: weaning weight (December) 0.30 and 0.23; March liveweight, 0.45 and 0 35; June liveweight, 0.39 and 0.42; hogget liveweight (yearling in October), 0.51 and 0.46; number of hogget oestruses, 0.32 and 0.27; number of lambs born per ewe at the first lambing (2-year-old), 0.053 and 0.045; over the first two lambings, 0.121 and 0.111; over the first three lambings, 0.032 and 0.205. The results are discussed in relation to genetic improvement in the ewe's fertility by direct selection.