Response of somatomedins (IGF-I and IGF-II) in lactating cows to variations in dietary energy and protein and treatment with recombinant n-methionyl bovine somatotropin.

Mid-lactation Holstein cows (n = 4) were assigned to four dietary sequences in a 4 x 4 Latin square to determine energy and protein effects on somatomedins. Diets were designed so that intakes were either high or low for net energy (NE) or crude protein (CP) with the range being representative for a lactation cycle. Each dietary treatment lasted 16 d and consisted of an adjustment period (d 1 to 7), a basal period (d 8 to 12) and a period of bovine somatotropin (bST) (40 mg/d) administration (d 13 to 16). Blood was obtained via jugular catheters every 4 h on d 11 to 16. Basal milk yield was decreased by NE or CP restriction. Milk yield was increased for cows fed all diets with bST, but response was greatest for those fed the high NE/high CP diet (31%, 7.7 kg/d). Plasma insulin-like growth factor (IGF)-I and IGF-II concentrations were not affected by diet. For all diets, bST caused an increase in plasma IGF-I (125%) and IGF-II (21%), with the increase being substantially greater for cows fed the high NE/high CP diet. Basal insulin levels differed among diets and increased with exogenous bST in cows fed the high NE/high CP diet. Results are consistent with a role of somatomedins in the mechanism by which exogenous bST increases milk yield, and variations in somatomedin response due to nutritional status may explain part of the differences in milk yield response to exogenous bST.