Effects of insulin, recombinant bovine somatotropin (rbST) and their interaction on DMI and milk fat production in dairy cows

This trial was designed to test the effects of insulin, rbST and their interaction on DMI and fat yield and composition in dairy cows. Eight Holstein cows (86 ± 10 DIM) were divided in two groups and used in two replicates of a Latin square design with four animals, four periods and the following four treatments: (1) intravenous infusion of saline, (2) infusion of saline and subcutaneous administration of 40 mg of rbST per day, (3) intravenous infusion of 12 mg of insulin per day coupled with glucose infusion, and (4) rbST administration combined with insulin and glucose infusion. The glucose infusion rate was adjusted to maintain euglycemia. Each experimental period lasted 14 days, i.e., treatments were administered during the first 6 days and no treatment was administered during the following 8-day resting phase. Data from the last 3 days of infusion were analysed using the SAS system for mixed models. For DMI, milk yield, fat percent, lactose percent and lactose yield, there was a significant interaction between insulin and rbST administration. However, fat yield was higher (1.63 vs. 1.39 kg/day), with a higher content of long chain fatty acids (242.2 vs. 176.4 g/milking), during rbST administration, regardless of insulin infusion. Insulin infusion decreased fat yield (1.39 vs. 1.63 kg/day for the control group), mainly through a decrease in long chain fatty acids (179.7 vs. 238.9 g/milking for the control group), regardless of rbST administration. Insulin decreased triglycerides in plasma (0.11 ± 0.01 vs. 0.15 ± 0.01 mmol/L). Plasma non-esterified fatty acids were increased by rbST but this increase was prevented by insulin. Overall, these results clearly indicated that insulin decreased milk fat yield but this decrease, unlike during classical milk fat depression (MFD), is associated only with a decrease in long chain fatty acids in milk.