estimation of genetic parameters for milk yield using random regression test day model in iranian holstein

in this study a total of 171,360 monthly test day milk records obtained from first lactation of iranian holstein cattle (three times a day milking) distributed in 96 herds and calved from 1999 to 2008 were used to estimate genetic parameters and to predict breeding values of the animals. the data was analyzed using random regression test day model. the results showed that average of heritability for the second half of lactation period was higher than that of the first half. the heritability value for the first month was lowest and for the eighth and ninth months of the lactation was highest compared to the other months of lactation. the genetic correlations between monthly test day records decreased by increasing the distances between months of lactation.