In two consecutive lactations in 1985/86 and 1986/87, 60 cows were injected with 500 mg of Sometribove, methionyl bovine somatotropin (BST), or a placebo at day 60±3 post partum; injections were repeated fortnightly for the remainder of each lactation. All cows received the same complete diet (metabolizable energy (ME) 11·5 MJ/kg dry matter; crude protein 169 g/kg dry matter) ad libitum for the first 20 weeks of lactation, after which, energy concentration of the diet was reduced as yield declined.
The administration of BST significantly increased milk yield by 4·3 (P < 0·001) and 3·1 kg/day (P < 0·05) in the first and second lactations, respectively. The mean dry matter (DM) intake of 16·3 kg/day in lactation 1 for the treated group was significantly (P < 0·01) higher than the 14·7 kg/day recorded for the control group. The response in intake was not significant until 6 weeks after treatment started. However, in the pretreatment period of the subseque1t lactation, the DM intake of the cows in the treated group was already significantly (P < 0·01) higher (+ 1·2 kg/day) than that of the control group. Although the DM intake in the treatment period of lactation 2 was also higher for the treated group, the difference from the control group was not significant.
With the exception of a significant (P < 0·01) increase in milk protein (control 35·3 g/kg v. treated 36·3 g/kg) in lactation 1, overall milk composition was unaffected by BST treatment. The administration of BST increased apparent feed efficiency (kg 4% fat-corrected milk (FCM)/kg DM intake) by 6 and 10% and gross feed efficiency (kg FCM/MJ ME) by 8 and 6% in lactations 1 and 2, respectively.
Animals treated with BST had significantly (P < 0·05) lower scores for body condition during the treatment period in lactation 1. This was, however, recouped during the dry period so that both groups had similar body conditions before the start of the next treatment period.
BST administered every 14 days in a prolonged-release formulation increased the yields of milk and milk constituents and apparent and gross feed efficiency during two consecutive lactations of treatment.