Direct and correlated responses to selection for total weight of lamb weaned in Merino sheep

This study was conducted to evaluate direct and correlated selection responses to selection for total weight of lamb weaned per ewe joined (TWW), number of lambs born (NLB), number of lambs weaned (NLW) and weaning weight (WW). Data from the Grootfontein Merino stud and the Carnarvon Merino flock were used. Estimated heritabilities for TWW, NLB, NLW and WW were 0.19, 0.23, 0.17 and 0.21 for the Grootfontein Merino stud and 0.21, 0.19, 0.16 and 0.30 for the Carnarvon Merino flock. High genetic correlations (0.89-0.98) were obtained between reproduction traits in both flocks, while the genetic correlations between WW and TWW, NLB and NLW were moderate to high (0.32-0.78). Phenotypic correlations between WW and reproduction traits were low (0.04-0.19) and phenotypic correlations between reproduction traits were high (0.71-0.96). In the Grootfontein Merino stud, approximately the same selection response (gain per generation) for TWW could be expected from direct selection (9.03 kg) as from indirect selection for NLB (9.20 kg). In the Carnarvon Merino flock, the highest selection response for TWW (6.37 kg) would be achieved by direct selection for TWW. The estimated selection responses indicate that direct selection for TWW would be the most suitable selection criterion for improving reproductive performance in flocks with a high reproduction rate where an increase in the number of lambs would be undesirable. (South African Journal of Animal Science, 2001, 31(2): 115-122