Genetic evaluation for persistency of lactation in Holstein cows using a random regression model

A model for analyzing test day records including both fixed and random coefficients was applied to the genetic evaluation of first lactation data for Holstein cows. Data comprising 87045 test-day milk yield records from calving between 1997 and 2001 from Holstein herds in 10 regions of the Brazilian state of Minas Gerais. Six persistency of lactation measures were evaluated using breeding values obtained by random regression analyses. The Wilmink function was used to model the additive genetic and permanent environmental effects. Residual variance was constant throughout lactation. Ranking for animals did not change among criteria for persistency measurements, but ranking changes were observed when the estimated breeding value (EBV) for persistency of lactation was contrasted with those estimated for 305-day milk yield (305MY). The rank correlation estimates for persistency of lactation and 305MY were practically the same for sire and cows, and ranged from -0.45 to 0.69. The EBVs for milk yield during lactation for sires producing daughters with superior 305MY indicate genetic differences between sires regarding their ability to transmit desirable persistency of lactation traits. This suggests that selection for total lactation milk yield does not identify sires or cows that are genetically superior in regard to persistency of lactation. Genetic evaluation for persistency of lactation is important for improving the efficiency of the milk production capacity of Holstein cows.

Estimation of genetic parameters for test-day milk yield in Holstein cows using a random regression model

Test-day milk yield records of 11,023 first-parity Holstein cows were used to estimate genetic parameters for milk yield during different lactation periods. (Co)variance components were estimated using two random regression models, RRM1 and RRM2, and the restricted maximum likelihood method, compared by the likelihood ratio test. Additive genetic variances determined by RRM1 and additive genetic and permanent environmental variances estimated by RRM2 were described, using the Wilmink function. Residual variance was constant throughout lactation for the two models. The heritability estimates obtained by RRM1 (0.34 to 0.56) were higher than those obtained by RRM2 (0.15 to 0.31). Due to the high heritability estimates for milk yield throughout lactation and the negative genetic correlation between test-day yields during different lactation periods, the RRM1 model did not fit the data. Overall, genetic correlations between individual test days tended to decrease at the extremes of the lactation trajectory, showing values close to unity for adjacent test days. The inclusion of random regression coefficients to describe permanent environmental effects led to a more precise estimation of genetic and non-genetic effects that influence milk yield.

An investigation into heterogeneity of variance for milk and fat yields of Holstein cows in brazilian herd environments

Registros de produçäo de leite e de gordura de primeira lactaçäo de vacas da raça Holandesa foram utilizados para estudar a heterogeneidade de variância entre rebanhos, os quais foram classificados em dois grupos (baixo e alto) com base no desvio padräo fenotípico da produçäo de leite por rebanho-ano (HYSD). Componentes de (co)variância dos efeitos de reprodutor e resíduo, heritabilidade e correlaçäo genética entre grupos de baixo e alto HYSD foram estimados através de análises bivariadas, usando-se um modelo reprodutor e o método da máxima verossimilhança restrita com um algoritmo livre de derivaçöes. Os componentes de variância de reprodutor e de resíduo para a produçäo de leite (produçäo de gordura) nos rebanhos de baixo HYSD foram respectivamente 79 por cento e 57 por cento (67 por cento e 60 por cento) dos valores estimados para os rebanhos de alto HYSD. As estimativas de heritabilidade para a produçäo de leite e de gordura foram maiores (0,30 e 0,22) nos rebanhos de baixo HYSD que nos rebanhos de alto HYSD (0,23 e 0,20). As correlaçöes genéticas para a produçäo de leite e de gordura entre os dois grupos de rebanhos foram respectivamente 0,997 e 0,985. A resposta correlacionada esperada nos rebanhos de baixo HYSD, usando-se informaçöes dos rebanhos de alto HYSD, foram 0,89 kg/kg para a produçäo de leite e 0,80 kg/kg para a produçäo de gordura. As avaliaçöes genéticas no Brasil devem considerar a heterogeneidade de variância para aumentar a precisäo das avaliaçöes e a eficiência da seleçäo para a produçäo de leite e de gordura na raça Holandesa. A resposta à seleçäo é menor nos rebanhos de baixo HYSD do que nos rebanhos de alto HYSD devido à reduçäo das diferenças na resposta da progênie e nos valores genéticos dos reprodutores nos rebanhos de baixo HYSD. Os investimentos na seleçäo de reprodutores para aumento da produçäo têm maior probabilidade de sucesso nos rebanhos de alto HYSD do que nos rebanhos de baixo HYSD.