Short communication: Insulin responsiveness is affected by the level of milk replacer offered to young calves

Eight male Holstein calves (40.6 ± 2.9 kg of BW and 7.8 ± 1.6 d of age) were individually housed and allocated to either a low milk replacer (MR) allowance of 2 daily doses of 2 L each (478.5 g/d of dry matter from MR), or to a high allowance of 2 daily allotments of 4 L (957.0 g/d of dry matter from MR). In addition all calves had ad libitum access to the same starter feed and water. At d 7, 30, and 60 of experiment all calves were submitted to a glucose tolerance test (GTT) that consisted on an i.v. infusion of 180 mg/kg of BW of glucose at 4 h after the morning MR feeding. Blood was harvested at −15, −5, 0, 4, 8, 12, 18, 25, 35, 45, and 60 min relative to glucose infusions. Blood samples collected at −15, −5, and 0 relative to glucose infusion were used as baseline concentrations of glucose and insulin. Then, the area under the curve (concentration/min) for glucose and insulin was calculated as the increase with respect to the baseline. Next, the clearance rates of insulin (%/min), glucose (%/min), as well as insulin sensitivity were computed. The increase in blood glucose following the GTT (assessed as area under the curve) was similar in both low and high calves, which indicates that all animals were able to control glycemia effectively. Similarly, clearance rates of glucose and insulin were not different between low and high calves. However, calves in the high group needed a substantially greater serum insulin concentration (98.7 ± 13.2 µU/mL) than low calves (41.5 ± 13.2 µU/mL) to control glycemia. Furthermore, as age increased, the rise in serum insulin elicited by the GTT continued to increase in high but not in low calves. Insulin to glucose ratio was greater in high (157.5 ± 7.8 µU/mg) than in low (46.7 ± 7.8 µU/mg) calves. This ratio increased with age and in a more pronounced fashion in high than in low calves and the same pattern was observed for insulin sensitivity. We concluded that offering 8 L of MR/d in 2 separate meals decreases insulin sensitivity of young calves. Further research is needed to assess whether the impaired glucose responsiveness of calves can be minimized by feeding milk more frequently.