Effect of Open Days in Holstein Dairy Cows on Genetic Ranking of the Bulls

Milk production and fertility are important traits for genetic evaluation of bulls in breeding programs. The goal of this research was to investigate the reproduction and fertility status in Iranian Holstein dairy cattle and effect of days open on the milk production and genetic ranking of bulls. A total number of 706,653 test day records of first parity of 78,517 Holstein cows in 448 herds during the years from 1991 to 2016 were used. The phenotypic and genetic relationship of the amount of milk production in 270 days and the number of open days and their genetic parameters were estimated through a two-trait model. Also data were analyzed using a random regression model and predicted the breeding values of bulls. The results showed that in the studied population, 7.7 and 60% of the cows in the herds became pregnant by 45 and 112 days after calving, respectively and 54.6% of successful pregnancies occur after 90 days in milk. The heritability of 270 days milk and open days and their genetic correlation were estimated of to be 0.257 (±0.016) and 0.0314 (±0.004) and 0.538 (±0.06) respectively. The range of estimated heritability of 270 days milk for first to ninth groups were 0.11-0.26. Days open was a source of variation of the parameter. The results showed that number of open days affected the predicted breeding value of bulls and their ranking. It is concluded that, in sire evaluation programs in order to increase the accuracy of breeding value prediction of the sires it is necessary to include the variable of number of open days in the statistical models.