Genetic analysis for the trait of first lactation milk production and evaluation of the impact of selection for it on the phenotypic value of age at first calving of Iranian Holsteins using quantile regression

The main aim of the present research was the genetic analysis of the first lactation milk yield trait and evaluation of the impact of selection for milk on the phenotype of age at first calving in Iranian dairy cows using quantile regression. Lactation milk records of 327190 first parity cows (progeny of 5260 sires and 235172 dams) in 637 herds (in 15 provinces) and calved between 2007 and 2015 were utilized. Genetic analysis was carried out with the use of an animal model. In the model, fixed effects of the province, herd, year and month of calving, covariates of lactation length, age at first calving, percentage of Holstein gene, as well as the random effect of the cow were included. The model was fitted on the data using the DMU package. Regression of age at first calving on predicted breeding value was fitted through a quantile regression model. The findings revealed that the heritability of lactation milk yield was equal to 0.276. Annual genetic gain in grade and Holstein purebred were found to be 17.352 (R2=0.913) and 23.398 (R2=0.962) Kg, respectively (P<0.0001). All estimated regression coefficients were negative in different percentiles of age at first calving and ranged between -0.0009 and -0.0344 (month per Kg) which indicates that the influence of genetic potential of cows for milk yield on different parts of calving age distribution is not the same.