Effects of long-term administration of recombinant bovine somatotropin on the plasminogen-plasmin system and milk composition of crossbred Holstein cattle

Experiments were performed to examine the effects of long-term treatment with recombinant bovine somatotropin (rbST) on milk plasmin-plasminogen and milk composition by one injection of 500 mg of rbST in every 14 days throughout lactation in crossbred Holstein cattle. The animals receiving rbST gave a greater milk yield and rate of blood flow to the udder during early lactation than the control group. The milk lactose concentration remained constant while the milk protein concentration increased as lactation advanced in both groups. The level of milk fat in rbST-treated animals was significantly greater than in controls in the early lactation period (P < 0.05). The milk sodium and chloride concentrations of the rbST-treated animals significantly decreased in early lactation as compared with the control animals. The sodium : potassium ratio of the rbST-treated animals was significantly lower than those of control animals in the early lactation (P < 0.05) and it markedly increased in late lactation. As lactation advanced, the concentration of plasmin in the milk gradually increased, while the milk plasminogen concentration significantly increased in both groups. The plasminogen : plasmin ratio decreased in the control animals while it increased in the rbST-treated animals as lactation advanced. These findings demonstrate that rbST is involved the activity of the plasmin-plasminogen system but is not involved in maintaining tissue integrity in the mammary gland during late lactation in crossbred dairy cattle.