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

Includes references

In this research, fixed and random regression test day models were utilized for genetic evaluation of protein test daytrait in Iranian Holsteins cows. The data set was consisted of 250,911 protein test day records belonging to 28,737first lactation cows (three times a day milking) in 396 herds which calved between 1998 and 2008. The data werecollected by Animal Breeding Centre of Iran. In fixed regression test day model, fixed effect of herd-recording yearproductionseason-sperm type (HYSC), covariables of Holstein gene percentage, age at recording, days in milk (upto order 3), random effects of additive genetic and permanent environment were included. In the random regressiontest 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. Additivegenetic, permanent environment as well as phenotypic variances increased from the beginning towards the end of thelactation course. Heritability estimate obtained from fixed regression model was 0. 07 and the average heritabilityestimate obtained from random regression test day model was found to be 0. 104. Genetic trend for 305-d proteinyield trait with random regression test day model 410. 64 g per year and with fixed test day model 124. 16 g per yearwas 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 byrandom regression model and fixed regression model were found to be 0. 926 and 0. 915, respectively.