Milk Production, Fat Percentage, and Protein Percentage of Holstein Cows in Khorasan Razavi with Behavior of Additive Genetic and Permanent Environmental Variance over Time
In this study, the components of variance and heritability of milk production, fat percentage, and protein percentage of cows were estimated in Khorasan Razavi province at different stages of the first lactation period. Genetic correlations between different stages of production, were estimated using 56978 test-day records of milk production and fat and protein percentages of 8028 first calving Holstein cows calved between 2004 and 2006. Monthly test-day records of milk were genetically analyzed with an animal model using a uniform random regression model for all the three traits. The model was fitted with the restricted maximum likelihood method using WOMAT software. In the model used in this research, calving age and time of herd-year-month recording were considered as fixed effects. The random effects of additive and permanent environmental genetics of cows for milk production curve, fat percentage, and protein percentage were estimated during lactation by Spline and Legendre orthogonal polynomials, both fitted with a power of three (three coefficients). In this research, there were no differences between the results of spline functions and the orthogonal Legendre polynomials.
The results showed that minimum and maximum amounts of additive genetic variance (AGV) were in the two traits of milk production and protein percentage at the beginning and the end of lactation period, respectively. Minimum and maximum amounts of AGV in the fat percentage were recorded on day 206 and at the end of lactation, respectively.
In both traits, additive and permanent environmental genetic correlations decreased with increasing interval between stages of lactation.