estimation of genetic parameters of holstein dairy cattle for production traits

the genetic parameters for milk production and milk fat traits for first to third parities were estimated using single, multi trait and repeatability models. the genetic parameters for test day milk records were also estimated using random regression model. the test day records and 305 day milk records of 17055 cows from 10 herds collected between years 1996-2007 in khorasan razavi province were used in the analyses. the highest heritability values for milk yield and milk fat were observed in the first parity. there were high genetic and phenotypic correlations between parities. the correlations were reduced by increasing the differences in parity numbers. single-trait random regression model was applied to estimate variance components and genetic parameters for test-day records of milk yields in the first lactation. phenotypic variance of milk yield trait was not constant during the lactation and it was higher at the beginning and at the end of lactation period. residual variance for milk yield was maximum in early lactation. minimum and maximum values for additive genetic variances for this trait were estimated at the beginning and 8th month of the lactation, respectively. estimates of heritabilities were found to be lowest during early lactation (0.07) and were increased up to mid lactation and were maximized nearly at the 8th month of lactation (0.28).