Dose response of lactating dairy ewes during suckling and milking to bovine somatotropin

The objectives were to determine the effect of administering recombinant bovine ST (bST) every 14 d on milk yield and milk composition in dairy ewes and to assess the possible effects of treatment on milk yield loss that occurs at lamb weaning. Seventy-two lactating dairy ewes were separated into 4 groups at lambing, and each group received no bST (control) or 40, 80, or 120 mg of bST every 14 d until wk 20 of lactation. During the first 5 wk of lactation, ewes suckled their lambs during the night and were milked once daily (period 1). After weaning, from 6 to 22 wk of lactation, ewes were exclusively milked twice daily (period 2). Actual milk yield, potential milk yield, and milk component percentages were recorded weekly throughout lactation. In period 1, bST-treated groups increased yields of potential milk (P = 0.04) and the corresponding 6% fat-corrected milk (FCM; P = 0.04) but not actual milk yield (P = 0.42) compared with the untreated group. In period 2, treated groups increased potential (P < 0.01) and actual (P < 0.01) milk yields as well as their corresponding 6% FCM (potential, P < 0.01; actual, P < 0.01), in comparison with the untreated group. The group treated with 120 mg of bST every 14 d yielded 39% more actual milk and 44% more 6% FCM than the control group for period 2. At weaning, between the last week of period 1 and the first week of period 2, treated groups showed similar absolute (P = 0.15) and relative (P = 0.33) values for the potential milk losses compared with the control group. Treatments increased (P = 0.04) actual milk fat percentages during period 1, but did not affect the other milk components or somatic cell count. In summary, bST increased potential milk yield throughout lactation and actual milk yield only after weaning in dairy ewes. In dairy ewes, bST was not useful for reducing the milk yield loss that occurred at weaning.