Genetic analyses of milk and fat yields measured at different lactation of Iranian Holstein cows using appropriate models

In this study, to determine the appropriate genetic co (variance) structure across ages of milk production and fat milk in the Iranian Holstein cows during five lactation using 53138 records originated from 1510 sires and 4835 dams. This data were collected by Iranian Animal Breeding Center from 436 herds. This cattle were born in two years: 2005 and 2006 in country varies provinces. The milk production records in the first to fifth lactation respectively were 50578, 35970, 24337, 15445 and 8012. The fat milk records were 19463, 17935, 14451, 10729 and 6322 respectively. To determine the best model for genetic analysis, univariate, bivariate and multivariate models was used. The fixed effect including birth year, birth month, calving year, age, number of milking, state and herd-calving year-season and animal effects as the only random effect was considered. The result of multivariate models was examined by BIC method. A model that attributes the milk production and fat milk in to 3 different groups including 1sth lactation, 2nd lactation, 3rd, 4th and 5th lactation was the best model for the genetic analysis of both traits in Iranian Holstein cows. With this model, heritability for milk production in five lactation, respectively, 0.21, 0.15, 0.12, 0.11 and 0.11 and for fat milk respectively, 0.15, 0.12, 0.11, 0.09 and 0.09 were evaluated.