Milk and fat production of purebred and grade Holstein dairy cows in Minas Gerais State, Brazil were analyzed by, mixed models methodology and restricted maximum likelihood in an animal model. The fixed effects were herd-year-season, genetic composition (purebred and grade) and the linear and quadratic effects of age at calving and the random effect was the animal. A total of 49,666 lactations of 26,822 cows from 380 herds, being 9,468 of purebred and 17,354 of grade were used. Two seasons, each with three classes of two months, dry season (April-May, June-July, August-September) and rainy season (October-November, December-January, February-March) were used. In order to estimate genetic gains, 18,482 records of cows in the first calving, being 8,938 of purebred and 9,544 of grade cows were used. The data were grouped in five classes: ≤4,000kg, from 4,000 to 6,000kg, from 6,000 to 8,000kg, from 8,000 to 10,000 and ≥10,000kg of milk production in 305 days. The overall milk and fat average were 5,865.54 and 196.85kg, respectively. The milk and fat averages for the same age classe were consistently larger for the purebred cows. The estimated genetic trend for milk and fat in each class of milk production were: 10.52 and 0.33kg, 8.31 and 0.25kg, 8.90 and 0.29kg, 11.00 and 0.36kg, 9.51 and 0.36kg by year, respectively. The average milk and fat production for cows of first calving were: 6,084.6 and 205.1kg, 5,739.5 and 191.8kg for purebred and grade cows. The estimated genetics trends of 8.7 and 9.6kg, for purebred and graded cows, although small, reflect an increasing in milk production potential from 1986 to 1996.
Dairy cattle; Holstein; genetic trend
