Genetic analysis of test day milk records in Holstein cows resulted from American sperms

In this research, to estimate genetic parameters for the trait of milk production in Holstein cows resulted from American sperms, a total of 76,504 monthly test day milk records (three times a day milking) in 189 herd and calving from 2001 to 2012 were used. Test day milk records were genetically analysed by a random regression model. In the model, fixed effect of herd-year-season of calving (contemporary group), and covariables of age at recording and Holstein gene percentage were included. Orthogonal Legendre polynomials with third order were implemented to take account of genetic and environmental aspects of milk production over the course of the lactation. The results revealed that the average heritability for the second half of lactation period was higher than that of the first half. The heritability value for the second month was lowest (0.047) and for the tenth month of the lactation was highest (0.150). Gradual increase of genetic variation over the lactation and high residual variance in early months of the lactation were main factors affecting the heritability. High values for the repeatability of test day milk indicate that high accuracy could be expected as a cow is selected for later lactation based on her earlier records.