Genetic evaluation of Iranian Holsteins for milk protein yield trait using fixed and random regression test day models

In this research, fixed and random regression test day models were utilized for genetic evaluation of protein test day trait in Iranian Holsteins. The data set was consisted of 250,911 protein test day records belonging to 28,737 first lactation cows (three times a day milking) in 396 herds which calved between 1998 and 2008. The data were collected by Animal Breeding Centre of Iran. In fixed regression test day model, fixed effect of herd-recording year-production season-sperm type (HYSC), covariables of Holstein gene percentage, age at recording, days in milk (up to order 3), random effects of additive genetic and permanent environment were included. In the random regression test day model, HYSC, the covariables, and orthogonal Legendre polynomials (up to order 3) were included. Variance and covariance components were estimated using restricted maximum likelihood procedure. Additive genetic, permanent environment as well as phenotypic variances increased from the beginning towards the end of the lactation course. Heritability estimate obtained from fixed regression model was 0. 07 and the average heritability estimate obtained from random regression test day model was found to be 0. 104. Genetic trend for 305-d protein yield trait with random regression test day model 410. 64 g per year and with fixed test day model 124. 16 g per year was estimated. There was a significant difference between the models in terms of average predicted breeding value (P<0. 0001). For cows with records, Pearson and Spearman correlations between breeding values predicted by random regression model and fixed regression model were found to be 0. 926 and 0. 915, respectively.