Selection response for milk production of United States Holstein bulls in the provinces of Bío-Bío and Malleco, Chile

Abstract

Correlated selection response of United States Holstein bulls was estimated in the provinces of Bío-Bío and Malleco (Chile). The aim of this research was to obtain indicators that could estimate the profitability obtained by chilean farmers due to the investment in imported semen. Information for milk and fat production of 7.523 first lactation cows, adjusted to 2X,305d-ME, freshening between 1992 and 2001, were considered. Four levels of herd production and variability were defined. Each herd was assigned to one production and variability level using the herd-year mean and herd-year standard deviation of milk yield, respectively. Herd-year means and standard deviations were determined using all first lactations in a herd-year, including those with unknown sires. The correlated response was estimated using the regression coefficient of daughters yield, deviated from the contemporary group, on the predicted transmitting ability of the sire as reported by the United States Department of Agriculture in November, 2002. The estimated selection response was 0.40 ± 0.06 for milk yield and 0.30 ± 0.05 for fat yield. These correlated selection responses were lower than the responses reported in United States herds, representing 34% and 26% of the selection response observed for milk and fat yield in that country, respectively. On the other hand, daughter selection response increased almost linearly with increasing variability but it was not so clear with increasing herd year mean. Classifying herds by herd-year standard deviation appears to be the most effective way to identify heterogeneous responses to sire selection.