Impact of bovine somatotropin on genetic evaluation of dairy sires and cows.

Data for milk yield and bovine somatotropin (bST) injections were obtained for 92,463 lactations of 51,986 Holstein cows in 222 commercial dairy herds that had used bST during the previous 3 yr. The overall rate of bST usage in these herds was 29%, and little difference occurred across lactations. A slightly higher percentage of cows with a low pretreatment phenotypic milk yield received bST, but there was no relationship between bST usage rate and the genetic potential of the cows, as measured by the PTA for milk of the sire. The mean starting date of bST administration was 147 d of lactation. Breeding values were estimated using three different animal models. In the first model, all data regarding bST injection were ignored. The second model included a fixed effect of bST injection (0 or 1 for each lactation). In the third model, data for bST injection were used to define management groups such that cows within a particular herd, year, and season of calving that received bST during the lactation were in a different management group from their contemporaries that did not receive bST. Correlations for EBV between sire and cow either ignoring bST or treating bST as a fixed effect were > 0.999, and approximately 98% of selected individuals were chosen under both models. Use of bST information to define the management groups resulted in correlations > 0.99; the EBV from models that either ignored bST or treated bST as a fixed effect had 92 to 95% of the selected individuals chosen by the other two models. The genetic correlation between lactation milk yield with and without bST treatment, which was estimated using a multiple-trait model, was 0.98, which indicated minimal interaction of genotype and bST. In general, bST treatment had little impact on the genetic selection decisions in this study.