Response of dairy ewes in late lactation to recombinant bovine somatotropin

This study was conducted to evaluate the effect of recombinant bovine somatotropin (rbST) administration to dairy ewes in late lactation on milk yield, composition, clotting parameters, and plasmin/plasminogen system. Eighty multiparous lactating Comisana ewes (1864±18 days in lactation) were randomly assigned to one of two treatments: control and rbST. Slow-release rbST (120 mg/ewe) was administered for two injection cycles of 21 days each. Recombinant bovine somatotropin treatment increased milk (589.7 vs. 483.7 g/d; p < 0.01), protein (35.5 vs. 31.4 g/d; p < 0.05) and lactose (27.3 vs. 23.1 g/d; p < 0.05) yields. The percentage of milk fat (8.2% vs. 8.9%; p < 0.01) and protein (6.0% vs. 6.2%; p < 0.05) decreased, while somatic cells count and pH were unaffected by rbST treatment. Administration of rbST decreased milk coagulation time (r) (12 vs. 13.1 min; p < 0.05), while firming rate (k20) and curd firmness (a30) were unaffected. Plasminogen activity of milk was lower in rbST-treated ewes (18.11 vs. 21.89 U/ml; p < 0.01).