Estimating Daughter Yield Deviation and Validation of Genetic Trend for Somatic Cell Score in Holstein Cattle Using Random Regression Test Day Model

The objective of this study was to estimate Daughter Yield Deviations (DYDs) of bulls and Yield Deviations (YDs) for cows using a random regression model and validation of genetic trend using estimated DYDs and Method II of Interbull for test-day records of Somatic Cell Score (SCS) in the first lactation of Iranian Holsteins. Data set included the 108995 test day records collected by the Animal Breeding Center of Iran from 2001 to 2010. Results of the present study indicated that variation in YDs of cows at different stages of lactation corresponds closely with their Estimated Breeding Values (EBVs). Because YDs and DYDs are considered as an additional measure of an animal's genetic merit, their correlation with EBVs is very important. The correlation between DYDs and EBVs of bulls for SCS was 0.88. High correlation estimates between DYDs and EBVs indicated that, in addition to EBV, the DYD can be an appropriate measure for dairy cattle breeding programs. The correlation increased with increase in the number of bull daughters and the average number of test-days of daughters. Estimated DYDs for each production year were used to validate the genetic trend obtained from the model which was used for genetic evaluation. Results indicated that genetic trend for SCS in the first lactation of Iranian Holsteins was slightly overestimated.