Estimation of genetic parameters for lamb weight at various ages in Mehraban sheep

Abstract The aim of the present study was to estimate variance components and genetic parameters among growth traits of Iranian Mehraban sheep using single and two-trait animal models. Analyzed traits included, birth weight (BW), weaning weight (WW), weight at 6 months (W6), weight at 9 months (W9) and yearling weight (YW). (Co)variance components were estimated with REML for an animal model that included fixed effects of year, season, age of dam, sex and type of birth. Random effects included direct and maternal additive genetic effects, maternal permanent environmental effects with a direct-maternal genetic covariance and random residual effects. Direct heritability (h2) was increased from 0.19 at birth to 0.36 at yearling age. Inversely, maternal heritability (m2) estimates were decreased from 0.11 at birth to 0.01 at yearling age. Estimates of maternal permanent environmental variance as a proportion of phenotypic variance (c2) were 0.06 and 0.04 for BW and WW, respectively and they were very negligible for other traits. Direct-maternal genetic correlation (ram) was positive and it increased with age from 0.05 for BW to 0.73 for YW. For the two–trait analyses, the genetic correlations were larger among WW, W6, and W9 (0.59 to 0.82) and were small between BW, W9, and YW (<0.4). Positive genetic correlations between traits show that selection at any age is possible.