Health and reproductive performance of dairy cows treated for up to two consecutive lactations with bovine somatotropin.

Forty-three purebred Holstein cows were put on an rbST milk production trial. There were 15 control cows (saline-injected), 15 cows injected with 10.3 mg rbST/d and 13 cows injected with 20.6 mg rbST/d. Injections were initiated between wk 4 and 5 of lactation and continued for 266 consecutive days. Fourteen of the rbST-treated cows were treated with one of three dosages of rbST in the previous lactation (10.3, 20.6, or 41.2 rbST/d). The objective of this study was to determine the health and reproductive efficiency of cows treated with rbST for up to two consecutive lactations relative to control cows. The incidence of 11 health-related variables before, during, and after the 38-wk injection period were not different among treatment groups. Previous treatment with rbST had no effect on reproductive variables; however, there was evidence that injections with the highest dosage of rbST in the current lactation resulted in lowered reproductive efficiency. It is possible that the effects of rbST on early lactation energy and nitrogen balance have confounding effects on the estrous cycle.