برآورد پارامترهاي ژنتيكي صفات توليد شير و چربي در گاوهاي هلشتاين با استفاده از ركوردهاي روزآزمون

Estimation of genetic parameters for milk and fat yield traits of Holstein dairy cattle using test day records

Test dat milk and tat yield records from first calving Holsteins were analyzed using single trait random regression test day model (RR/CF model) and restricted maximum likelihood method. Fixed random regressions (additive genetic and permanent environmental) were modeled with orthogonal legendre polynomials on days in milk. Residual variance components during lactation were estimated using 3`" order variance function. According to the results, for milk yield trait a random regression model with 3 and 4 orders of tit for additive genetic and permanent environmental covariance functions, respectively and for fat yiled trait 3 order of fit for both covariance functions are suitable. Phenotypic variance during lactation was not constant and was higher at the beginning and the end of lactation. Maximum residual variance for milk and fat yield traits was estimated in early lactation. Minimum and maximum additive genetoe variance for both traits was estimated at the hegimmimg and 8ʹh month of lactation, respectively. Estimates ofʹ heritability were tbund to he lowest during early lactation. This parameter increased to mid lactation and maximized nearly in the 8ʹ1ʹ month of lactation (0.28 for milk yield and 0.22 for It yield). Additive genetic and permanent environmental correlations between adjacent test days were more than between distant test days.