Heritability and genetic correlations coefficients in different levels of milk production and fat were evaluated in this study. A data set of 8,887 milk and fat records from 5,875 Holstein cows, sired by 1,125 bulls from 1984 to 2001 were used. Data were stratified in three levels according to the herd milk production, low (<6000 kg), medium (>6000 a <7500 kg) and high (>7500 kg). The (co)variance components were estimated by Restricted Maximum Likelihood Method including in an animal model as random, the genetic direct and the permanent environmental effects and as fixed, the effects of herd: milking control nucleus, year season, lactation order, and the age of the cow at parturition and the period of lactation as co variables. The observed means and standard deviations for milk (PL) and fat (PG) yields ranged from 4,922±743 kg to 8,760±126 kg and from 163±30 kg to 298±53 kg, respectively. The estimated heritability coefficients ranged from .13 to .19 and from .16 to .21, respectively for PL and PG. The highest values for the heritability coefficients, for PL and PG were observed at the low and medium levels, suggesting that at these levels the improvement expected by selection could not be reached. The genetic correlation coefficients ranged from .91 to 1.00, suggesting that the selection for one of the characteristics will produce a similar response in the other.
genetic parameters; fat production; milk production; variances heterogeneity
