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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^1,2,

^* Department of Animal Science, Cornell University, Ithaca, NY 14853 and Animal Sciences Division, Monsanto Company, St. Louis, MO 63198

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.

KEY WORDS: • somatotropin • insulin-like growth factor • energy • protein • cattle

¹ Presented in part at the 82nd Annual Meeting of the American Society of Animal Science, August 1990, at Iowa State University, Ames, IA [McGuire, M. A., Bauman, D. E., Miller, M. A. & Hartnell, G. F. (1990) Effect of energy and protein nutrition on response of the insulin-like growth factors (IGF) to recombinant n-methionyl bovine somatotropin (bST, sometribove) in lactating cows. J. Anim. Sci. 68(Suppl. 1): 436(abs.)].

² Supported in part by Cornell University Agricultural Experiment Station.

³ To whom correspondence should be addressed.

⁴ Present address: Food and Drug Administration, Center for Veterinary Medicine, Rockville, MD 20857. This article was written by M. A. Miller in her private capacity. No official support or endorsement by the agency is or should be inferred.

Manuscript received 19 April 1991. Revision accepted 2 July 1991.

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