Estimates of genetic parameters and genetic change for reproduction, weight, and wool characteristics of Rambouillet sheep

Abstract

Records were for Rambouillet sheep from data collected from 1950 to 1998 at the U.S. Sheep Experiment Station, Dubois, Idaho, USA. Number of observations were 44,211 for litter size at birth and litter size at weaning, 35,604 for birth weight, 34,114 for weaning weight, 39,820 for fleece weight, 39,821 for fleece grade, and 3574 for staple length. Genetic parameters from both single- and two-trait analyses for prolificacy, weight, and wool traits were estimated using REML with animal models. Direct heritability estimates from single-trait analyses were 0.09 for litter size at birth, 0.06 for litter size at weaning, 0.27 for birth weight, 0.20 for weaning weight, 0.51 for fleece weight, 0.16 for fleece grade, and 0.58 for staple length. Estimates of direct genetic correlation between litter sizes at birth and weaning was 0.76 and between birth and weaning weights was 0.60. Estimates of genetic correlation between fleece weight and staple length was positive (0.45), but negative between fleece weight and fleece grade (−0.47) and between staple length and fleece grade (−0.52). Estimates of genetic correlations were near zero between birth weight and litter size at weaning, small and positive between birth weight and litter size at birth, and moderate and positive between weaning weight and litter size traits. Fleece weight, fleece grade, and staple length were slightly but negatively correlated with both litter size traits. Estimates of correlations between weight traits and fleece weight were positive and low to moderate. Estimates of correlations between weight traits and fleece grade were negative and small, while estimates between weight traits and staple length were positive and small. Breeding values from both single- and seven-trait analyses calculated using the parameters estimated from the single- and two-trait analyses were compared across years of birth with respect to genetic trends. Estimated breeding values averaged by year of birth from both the single- and seven-trait analyses for the prolificacy and weight traits increased over time, while those for fleece weight decreased and those for the other wool traits were unchanged. Estimated changes in breeding values over time did not differ substantially for the single- and seven-trait analyses, except for traits highly correlated with another trait that was responding to selection (i.e., litter size at birth, which was highly correlated to both litter size at weaning and weaning weight).