برآورد روند ژنتيكي توليد شير در گاوهاي هلشتاين

An Estimate of Genetic Trend for Holsteinʹs Milk Yield

To estimate the genetic trend for lactation milk yield, a total of 38457 monthly test day records obtained from first calving of Holstein cows, recorded during 1995-2005 at Astan-e Quds-e Razavi Dairy Farm were used. A random regression test day animal model was applied to analyse the records. In the model, the environmental fixed effects of milking times, sperm origin and herd-year-month of recording were included. To take account of the shape of the lactation curve at genetic and permanent levels, orthogonal Legendre polynomials of order 4 were also fitted in the model. For progeny, genetic trend was estimated based upon regressing of mean predicted breeding value on year of zygote formation, birth and calving of animals. For sires and dams, the genetic trends were estimated based on regressing of mean predicted breeding value on the above corresponding years of their daughters. The genetic trend (based on the year of zygote formation) was 10.64 Kg per year (p<0.05) in the progeny. Non-significant genetic trends were observed for progeny based upon their year of birth and calving. For damsʹ population, significant genetic trends were found to be 8.77, 8.88 and 9.1 K.g per year based on year of zygote formation, birth and calving of their daughters respectively. The genetic trends in siresʹ population were all statistically non-significant (p>0.05).