IMPACT OF BOVINE SOMATOTROPIN ON GENETIC EVALUATION OF DAIRY SIRES AND COWS

Data for milk yield and bovine somatotropin (bST) injections were obta ined for 92,463 lactations of 51,986 Holstein cows in 222 commercial d airy herds that had used bST during the previous 3 yr. The overall rat e of bST usage in these herds was 29%, and little difference occurred across lactations. A slightly higher percentage of cows with a low pre treatment phenotypic milk yield received bST, but there was no relatio nship 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 b ST administration was 147 d of lactation. Breeding values were estimat ed using three different animal models. In the first model, all data r egarding bST injection were ignored. The second model included a fixed effect of bST injection (0 or 1 for each lactation). In the third mod el, data for bST injection were used to define management groups such that cows within a particular herd, year, and season of calving that r eceived bST during the lactation were in a different management group from their contemporaries that did not receive bST. Correlations for E BV between sire and cow either ignoring bST or treating bST as a fixed effect were >0.999, and approximately 98% of selected individuals wer e chosen under both models. Use of bST information to define the manag ement 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 t he selected individuals chosen by the other two models. The genetic co rrelation between lactation milk yield with and without bST treatment, which was estimated using a multiple-trait model, was 0.98, which ind icated minimal interaction of genotype and bST. In general, bST treatm ent had little impact on the genetic selection decisions in this study .