Genetic parameters of Italian brown Swiss for levels of herd yield.

Genetic parameters were estimated for yields and percentages of milk, fat, and protein for registered Italian Brown Swiss cows. Data were 72,690 mature equivalent 305-d first lactation yields split by herd average milk into four files. An expectation-maximization REML algorithm was used on a multiple-trait model with equal design matrices for fixed and random effects. Data were preadjusted for geographical area, age-month of calving, and days open. The model included herd-year and sire genetic group as fixed effects and sires as random effects. Estimates of heritability were .28, .30, .26, .42, and .34 for milk, fat, and protein yields, and fat and protein percentages, respectively. Genetic correlations of milk yield and milk component percentages were negative, as expected. Correlation of protein yield and protein percentage was null. Results show an increasing tend of variance components from low to high herd yields, indicating that dispersion of yield about the mean increases as average yield increases. Relative differences among environmental components were larger than those among genetic components, yielding larger estimates of heritability in herds with lower yield. Estimates of heritability for milk from low to high herd yields were .42, .38, .35, and .33.