Defining appropriate model for genetic analysis of milk yield in different lactations of Iranian Holstein cows

In this study, appropriate genetic co (variance) structure across ages of milk production in the Iranian Holstein cows was modeled using 35167 records originated from 1098 sires and 27236 dams. These data were collected from 110 herds during 2011 to 2015 by the Iranian Animal Breeding Center. For all lactations, birth year, birth month, calving year, calving season, calving month, age and herd were considered as the fixed effects. The effect of the animal age was fitted as a covariate. In this research, maternal genetic and non-genetic effects, the effect of herd-year-season as well as the animal genetic effects were fitted as the random effects. For each lactation, to investigate the importance of fixed and random effects, univariate animal model was used. In order to fit the best model for genetic analysis of the five lactations pre-structured and unstructured multivariate models and repeatability model were used.  The results show in comparison with pre-structure models, unstructured and repeatability models are less appropriate for genetic analysis of milk yield. The results derived from the best model indicate that milk yield is genetically changed up to 3rd lactation but no changes were found for the later lactations therefore, selection accuracy of Iranian Holstein cows could be increased when the additional milk records measured at the second and third lactations are used as well as the first lactation milk records. This issue should be studied economically as measuring the additional records takes more cost. In addition, it is suggested using appropriate models to increase the accuracy of estimated genetic parameters.