A Study on the Association of Days Open with the Genetic Ranking of Iranian Holstein Bulls for the Trait of Milk Yield

The genetic evaluation of dairy bulls is based on their daughters’ production, type traits, and fertility. It is unknown how the different number of days open of the daughters of a bull influences its Estimated Breeding Values (EBVs) and ranking in the population. The present study aimed to examine the effect of days open on milk production of Holstein dairy cows and the ranking of the bulls according to their predicted breeding values. A total number of 706,653 test day records of the first parity of 78,517 Iranian Holstein cows in 448 herds during 1991 to 2016 were used. The daughters of the same bulls were allocated into nine groups of days open, the differences of which were 21 days. Data were analyzed using a random regression model and predicted the breeding values of bulls. The effect of herd-year-season on milk yield were significant (P≤ 0.001). The heritability of 270 days milk for the first to ninth groups were estimated to be 0.24(±0.04), 0.26(±0.02), 0.23(±0.02), 0.21(±0.03), 0.18(±0.03), 0.19(±0.04), 0.16(±0.05), 0.17(±0.05) and 0.11(±0.04), respectively. The Spearman rank correlation coefficient of predicted breeding value of the same sires in different groups were 0.60-0.75 (P≤ 0.01). The results showed a negative relation between the number of days open and the predicted breeding value of bulls. It can be concluded that the number of days open affects the prediction of breeding value and ranking of the sires and it should be corrected for, while predicting the breeding value of sires.