Genetic parameters of body weight and fat-tail measurements in lambs

The data set used in this study was body weight of 5418 lambs at weaning, 4234 lambs at 6 months of age collected during 1989-2006, fat-tail measurements of 1205 lambs at 3 months of age and 988 lambs at 6 months of age recorded over a 4-year period (2003-2006) from a Lori-Bakhtiari research flock at the Shooli sheep breeding station in Shahrekord. Fat-tail measurements were upper fat-tail width (UFTW), lower fat-tail width (LFTW), fat-tail length (FTL), fat-tail gap length (FTGL), fat-tail depth (FTD) and upper fat-tail circumference (UFTC). (Co)variance components and genetic parameters were estimated using restricted maximum likelihood procedure and multi-trait animal model. The model included the fixed effects of year of birth, age of dam, sex of lamb, type of birth, body condition score (only for fat-tail measurements) and age of lamb (days) as covariate, and random effects direct additive genetic, maternal additive genetic and residual. The estimates of direct heritability of body weight and fat-tail measurements at 3 months of age varied from 0.11 +/- 0.02 for body weight to 0.25 +/- 0.06 for FTL, while the direct heritability of considered traits at 6 months of age were higher and ranged from 0.19 +/- 0.03 for body weight to 0.36 +/- 0.06 for FTL. The estimates of maternal heritability were lower than direct heritability except for body weight at 3 months of age, and decreased with age of lambs. Genetic correlations among body weight and various fat-tail measurements were positive and varied from low to very high. The genetic parameter estimates obtained for body weight and fat-tail measurements indicated that genetic improvement through selection could be possible for reducing fat-tail size, but single-trait selection for measurements of fat-tail causes some decrease in body weight due to correlated response to selection. Thus, because genetic correlations were not perfect, even with positive genetic correlations between body weight and fat-tail measurements, it is possible to increase body weight and reduce fat-tail size by multi-trait selection. (C) 2007 Elsevier B.V. All rights reserved.