مقایسه ارزیابی گاوهای هلشتاین برای تولید شیر با استفاده از مدل‌های روز آزمون و 305 روز

A total of 179,460 monthly test-day milk records (thrice a day milking) obtained from 17,946 Iranian Holstein heifers distributed in 287 herds and calved from 1986 to 2001 were used to predict breeding value of animals. Monthly test-day milk production was analysed by applying a covariance function in which the effects of herd-year-season of production (HYSOP), age at test day (covariate), additive genetic and permanent environmental effects were included. Orthogonal legendre polynomials up to order 5 (quartic) were also implemented in the covariance model to take account of the genetic and environmental aspects of milk production variation over the course of the lactation. In the present research, breeding values of individual animals based upon 305-day and monthly test-day milk yields were compared. The results indicated that ranking of ten top sires, dams as well as progenies changed as the genetic evaluation of animals switched from the traditional (based on 305-day milk yield) to the new method that was based on the covariance function. Regression analysis of average breeding value of progenies on the first year of their calving showed that the amount of genetic trend obtained from the covariance function for 305-day milk yield was greater than that of the 305-day model (11.607 vs. 3.860 Kg/year) over the period of 16 years of calving. The phenotypic trend for lactation milk yield was 92.97 Kg per year which was statistically significant. This finding indicates that a large proportion of annual increase of milk production of Holstein heifers in Iran has been due to environmental rather than genetic improvement.