Genetic aspects of cold resistance and related characters in newborn Merino lambs

Abstract

Resistance to body cooling and rate of recovery from induced hypothermia were measured in 287 single, newborn Merino lambs from 24 different sire families, using a water bath test in which partly immersed lambs were progressively cooled. Birth weight, birthcoat type (fine-hairy) and skin thickness were recorded at the time of test. There was an unexpected occurrence of congenital goitre, the incidence and severity of which was estimated by manual palpation of the thyroid gland. Heritability (� s.e.) of cold resistance (CR), estimated by paternal half-sib analysis, was 0.70 � 0.25. Sex of lamb, type of weather, time of test, Fecundin treatment and age of ewe were fitted in the model as fixed effects but none were significant. Other heritable traits (h2 � s.e.) included birthweight (0.50 � 0.22), birthcoat grade (0.61 � 0.24), coat depth (0.62 � 0.24), skin thickness (0.35 � 0.19) and the severity of goitre (0.21 � 0.16). Significant genetic correlations (r �s.e.) between cold resistance and other traits were: birthweight, +0.76 � 0.18; birthcoat grade, +0.56 � 0.24; birthcoat depth, +0.56 � 0.24; skin thickness, +0.51 � 0.27; goitre, -0.58 � 0.40. Most of the corresponding phenotypic correlations were small. Goitre did not affect CR significantly, despite the genetic correlation between them. Heritability of CR, further adjusted for the effects of birthweight, birthcoat grade and depth, and skin thickness as covariates, was 0.55 � 0.23. About 40% of the variation in CR was accounted for by fitting fixed effects and covariates, but significant sire effects remained. Rate of recovery from hypothermia was not heritable and it was unrelated to any other variable except goitre, which tended to be associated with slower recovery (rp = 0.18). It was concluded that genetic selection for increased CR would succeed but would promote birthcoat hairiness unless a corrective selection index was used. The relationship between birthcoat type and CR was considered to be mediated by genes affecting both coat type and CR, not primarily by a direct effect of coat insulation.