Genetic Analysis of Milk Yield in Iranian Holstein Cattle by the Test Day Model

Using monthly test day records the genetic parameters of Iranian Holstein cattle in first lactation were studied. Data of 277400 test-day milk records from 65320 cows and 2210 sires were analyzed by an animal random regression model using restricted maximum likelihood methodology. The model included herd-test-date, interaction between year-season of calving, days in milk (linear and quadratic) and dam age (linear and quadratic) as fixed effects and random regression coefficients for additive genetic and permanent environmental effects. The average of 305 days milk yield was 9760 (±1324) kilogram. Differences of milk yield among provinces were significant (P<0.05). The average of heritability estimates of milk was 0.50. The genetic correlations between adjacent test-day records were high and decreased with increase in interval between tests.