genetic parameters estimation of age at first calving and its effect on productive traits of holstein dairy cows

the aim of this study was to estimate genetic parameters of age at first calving and its relationship with productive traits of holstein dairy cows. a total of 24491 first lactation records (2x 305d) which were collected from 1995 to 2009 were used. the general linear model (glm) procedure was applied to determine effective factors on studied traits. herd-year-season (hys) and age at first calving (afc) were fitted as fixed and covariate factors in the model respectively. variance components and corresponding genetic parameters were estimated based on bivariate animal models by restricted maximum likelihood (reml) method. hys and afc affected significantly studied traits including milk production, fat production, protein production, fat percentage and protein percentage. mean and heritability of age at first calving were obtained 25.08±1.8 and 0.093±0.01 respectively. twenty nine percent of heifers calved at 24.5 (24-25) months. the genetic and phenotypic trends of afc were -0.015 and -0.09 respectively (p<0.001). phenotypic and genetic correlations of afc with milk yield, fat yield and protein yield were negative. conversely, positive genetic and phenotypic correlation observed between afc and fat-protein percentage. the results showed that reducing age at first calving can increase milk-fat-protein production and the optimum afc for milk-fat-protein production was 22.5 (22-23) month in this study. also, reducing age at first calving to 22 month has a negative effect on milk, fat and protein production.