Use of Recombinant Bovine Somatotropin for up to Two Consecutive Lactations on Dairy Production Traits

Fifteen (control) cows were injected with saline; 28 cows were treated with rbST (15 with 10.3 and 13 with 20.6 mg/d). Fourteen of the 28 treated cows had between injected with rbST in the previous lactation. A total of 29 cows in the present trial had not received rbST in previous lactation. Treatment injections were started between wk 4 and 5 of lactation and continued for 266 consecutive days. Milk production, fee intake and efficiency, and BW data were recorded for 3 wk before and 3 wk after a 38-wk rbST treatment period. Administration of rbST increased average FCM yields in the current lactation by 19%, increased feed energy intake by 7%, and decreased feed conversion (energy intake per unit FCM output) by 17%. Previous treatment with rbST did not significantly affect current milk production but caused a nonsignificant increase in feed intake and a significant, unfavorable, increase in feed conversion of 12%. The results suggest that the advantages of rbST in terms of increased milk yield are maintained for a second consecutive lactation of rbST administration. However, the advantages of rbST in terms of improved feed conversion may be substantially reduced in a second consecutive lactation of administration.