Genetic control of lean tissue growth rate in sheep : genetic parameters and responses to selection

Abstract

Performance data sets were available from lean tissue growth rate (LTGR) selection experiments for Border Leicester (n=1070), Coopworth (n=3720) and Dorset Down (n=1439) flocks as well as a Corriedale progeny test flock (n=1495). LTGR flocks had been selected for an aggregate breeding value (ABV) incorporating lean and fat weights with positive and negative economic weightings, respectively. The economic selection indices used comprised liveweight (LW), fat depth (FD) and muscle depth (MD) were: (a) 0.44LW - 0.58FD 1986 - 1988 (b) 0.25LW - 0.58FD + 0.48MD 1989 – 1992.

Parameter and response estimates for index component traits were derived from multivariate AIREML analyses, fitting an individual animal model. Responses in index component traits and greasy fleece weight were obtained by regression of BLUP breeding values on time. Responses of ABV component traits and a correlated response in bone were measured as differences between LTGR and control lines of the Dorset Down flock in 1995 only.

Performance in index component traits was significantly affected by birth rank, age of dam, year and age at measurement. The magnitude of these fixed effects varied between breed, sex and season, indicating that standardised corrections are not valid.

Index component traits and greasy fleece weight had medium (0.10-0.30) to high (0.31-0.70) heritability estimates but low (0.20-40) to moderate (0.40-0.60) genetic and phenotypic correlations. Differences observed in genetic parameters between breed, sex and season could not be attributed solely to season or genotype, due to confounding with management and time of the year.

LW increased in response to selection for all data sets (+0.095 to +0.489kg/year). Generally MD increased (+0.04-to +0.09 mm/year) although in one instance it decreased (-0.05 mm/year). Generally fat depth decreased (-0.001 to -010mm/year) although it appeared to increase slightly in one data set (+0.01mm/year). Greasy fleece weight increased (+7.58 to +17.01 g/year). Relative responses were greatest in FD (-1.23±1.26% per year) and least in MD (+0.20±0.19% per year) with LW being intermediate (+0.54±0.22% per year). Correlated responses in greasy fleece weight were similar to those of LW in magnitude (0.46±0.14% per year). Response rates varied between traits, seasons, sexes and breeds, although most of the variation was small and insignificant. Generally, differences between data sets for responses in index component traits reflected differences in genetic parameter estimates.

Desirable responses were observed in ABV component traits (+72 and -40g/year for lean and fat weight, respectively) measured in vivo by X-ray computer assisted tomography in one year. There was a correlated response in bone weight (+30g/year). Responses exhibited by the sexes, differed significantly (P<0.05), rams having a greater response in lean weight (+136 vs +9 g/year) and a lesser response in fat weight (-3 vs -78 g/year) than ewes. A correlated response in bone weight was significantly (p<0.05) greater in rams than in ewes.