Estimates of genetic and phenotypic correlations for ewe productivity traits of three breeds under 8-month breeding cycles and artificial rearing of lambs

Abstract

Paternal half-sib estimates for genetic and phenotypic correlations for ewe productivity of three synthetic sheep breeds (Canadian, Outaouais and Rideau Arcotts) were based on 11 106 ewe records from 3528 ewes and their 814 sires collected over 8 years (1983–1990) and 24 lambings in 8-month breeding cycles. Two separate flocks of ewes, housed indoors year-round in a controlled environment, were mated to lamb alternately at 4-month intervals, at the end of a period of short days, in February, June and October. Artificial lighting regimens and exogenous hormones were used to synchronize estrus. All lambs were raised on milk replacer diets from birth to weaning at 21 days of age. Genetic and phenotypic correlations based on mixed model methodology were estimated by breed and involved ewe weight at breeding, reproduction (prolificacy, multiple birth and fecundity) and total lamb weights (at birth, 21 and 91 days of age). Ewe weight at breeding was found to have neither genetic nor phenotypic relationships with the reproductive traits. Prolificacy was associated, both genetically (0.50–0.80) and phenotypically (0.77–0.79), with total lamb weight at birth for the three breeds. However, genetic correlations between prolificacy and total lamb weights at 21 days (0.85) and 91 days (0.94), important ( P<0.05) in the Outaouais breed, failed to demonstrate significance in the Canadian and Rideau breeds. Estimates of genetic correlations between fecundity and total lamb weights (0.88–0.96 at birth, 0.67–1.06 at 21 days, and 0.72–1.06 at 91 days) were always significant for the three breeds, suggesting that selection for fecundity would result in correlated genetic improvement of ewe productivity. Within total lamb weight, the genetic and phenotypic relationships between weights at birth and either 21 days or 91 days were similar. Corresponding estimates between 21 and 91 days of age tended to be larger. The magnitude of the phenotypic and environmental correlations were alike. In general, genetic and phenotypic correlations based on least squares analysis, maximum likelihood and restricted maximum likelihood procedures were similar, however, there may be some cause for concern in the absence of associated standard errors. The results provide estimates of genetic and phenotypic correlations for ewe productivity traits in the three Arcott sheep breeds developed in Canada under conditions of an 8-month breeding cycle when lambs were fed with a milk replacer diet and raised artificially in a controlled environment.