Long-term effects of sometribove in lactating cows during a fourth consecutive lactation of treatment: insulin and somatotropin responses to glucose infusion.

The effect of sometribove (USAN, methionyl bST) on the endocrine pancreas and blood bST concentrations was investigated in 6 control and 6 treated Friesian cows, averaging 111 and 118 d postpartum in their fourth lactation of treatment. Each lactation the treated cows received sometribove injections (500 mg) every 2 wk (injection cycle) starting 60 +/- 3 d postpartum, increasing milk yield 3.3, 5.9, 1.9, and 4.2 kg/d in lactations 1, 2, 3, and 4, respectively. On d 8 of a fourth lactation injection cycle, blood was sampled for 390 min, starting 30 min before an intravenous glucose infusion (100 mg/kg) over a 20-min period. Preinfusion concentrations of glucose, insulin, and bST were elevated in sometribove-treated cows versus controls: 82.1 versus 74.4 mg/dl, 28.1 versus 19.7 microIU/ml, and 4.6 versus .9 ng/ml, respectively. Glucose infusion resulted in a rapid increase in blood glucose and insulin concentrations, followed by a sharp decline to preinfusion values across both treatments, resulting in similar net area under the curves for glucose and insulin. Blood bST concentrations remained unchanged. This study supports the concept that sometribove increases milk yield in dairy cows by chronically influencing homeorhetic mechanisms.