Genetic parameters estimation and selection progress for milk yield in Manchega sheep

Abstract

In the study about the model of genetic evaluation the one including the contemporary group herd-period, the number of lambs at lambing, the combined effect parity-age at lambing and the number of days between lambing and first test-day as fixed effects (GLM6) showed the highest determination coefficient (0.43 for 120D6FCM (individual milk yield standardized to 120 days of lactation and 6% of fat content), 0.51 for 120M (averaged milk to 120 days), 0.54 for 120%F (averaged percentage of fat to 120 days) and 0.38 for 120%P (averaged percentage of protein to 120 days)). Contemporary group herd-period was the effect which explained the highest percentage of variability (44.7%) for all traits. A repeatability animal model including different numbers of fixed effects was tried to evaluate animals. The one considering fixed effects previously referred to (MOD6) had the highest determination coefficient and the lowest residual variance for all traits studied. Changes in ranking of animals were observed depending upon the model of evaluation used. Heritability estimates in our own population with MOD6 for 120D6FCM (0.29) and 120M (0.29) were higher than those cited in the literature (0.25 for both traits), however, for 120%F (0.28) and 120%P (0.35), our estimated values were lower than those of the literature (0.50 for both traits). Positive trends are shown for both traits, 120D6FCM and 120M, from 1990 to 1992, because in this period appear data of the daughters of the breeding males. The difference in variance components does not substantially affect genetic trend for 120D6FCM and 120M milk.