Genetic and environmental variance components for milk yield across regions, time periods and herd levels for Holstein cattle in Mexico

Additive genetic, permanent environmental, and temporary environmental variance components were estimated for 86,812 milk yield records for different regions, time periods and herd-years according their standard deviation levels for Holstein cattle in Mexico. Variances were estimated using REML and an animal model. Additive genetic and residual variances were bigger in the Northern region and for the most recent period 1991-1997, both being associated with higher production levels. Heritability estimates for regions were from 0.27 to 0.31 +/- 0.02, and for time periods were from 0.20 to 0.27 +/- 0.03 using all lactations. Heritability values for region and time periods in first lactation cows were from 0.21 to 0.25 +/- 0.02 and 0.21 to 0.31 +/- 0.02, respectively. Heritability according to the standard deviation levels were from 0.21 to 0.24 +/- 0.01. Repeatability values ranged from 0.36 to 0.51. Differences (P<0.05) in the additive and environmental variances were found classifying herds-years for milk yield standard deviation level, through regions, periods of time and their combinations. Different degrees of heterogeneity of variances were found according to the classification criterion used. Results obtained in this study suggest the need to let genetic evaluation procedures account for these differences in variance according to region, in order to reduce bias and to increase the accuracy of predicted breeding values.