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Effects of Recombinant DNA-Derived Somatotropin and Dietary Energy Intake on Development of Beef Heifers: II. Concentrations of Hormones and Metabolites in Blood Sera^1,2,

University of Kentucky⁴, Lexington 40546-0215

Abstract

The effects of somatotropin (STH) and energy intake on serum concentrations of glucose (GLU), insulin (INS), nonesterified fatty acids (NEFA), urea nitrogen (UN) and insulin-like growth factor-I (IGF-I) were determined in 40 Angus heifers. At 7 mo (208 ± 8 d) of age heifers were assigned to four treatment groups: 1) vehicle (V) + high energy (HE; 2.68 Mcal ME/kg DM), 2) recombinant DNA-derived STH (20.6 mg/d; s.c.) + HE, 3) V + low energy (LE; 2.22 Mcal ME/kg DM) or 4) STH + LE. Animals remained on treatments until an average of 15.5 mo of age. Blood samples were taken every 30 min for 4 h at 9, 11, 13 and 15 mo of age to determine circulating concentrations of metabolites and hormones. Serum IGF-I was increased (P < .01) by STH injections, but this effect appeared to diminish with age (STH x age; P < .01). Energy intake did not influence IGF-I levels. Somatotropin increased (P < .01) serum GLU in heifers fed the HE diet but only tended (P = .08) to increase GLU in those fed the LE diet (STH x energy; P = .05). Although STH increased (P < .01) serum INS in both energy groups, the response in heifers fed the HE diet was greater (P < .02) than that in heifers fed the LE diet (STH x energy; P < .05). Heifers fed LE had higher (P < .01) concentrations of NEFA than heifers fed HE. Serum UN was lower (P < .01) in STH-treated heifers than for heifers treated with V. Animals fed HE had lower (P < .01) serum UN than those fed LE. These data suggest that exogenously administered STH and dietary energy may interact to influence intermediary metabolism in growing beef heifers.

¹ This paper (No. 88-5-211) is published with the approval of the Director of the Kentucky Agric. Exp. Sta.

² We thank David Rock of American Cyanamid for providing recombinant-DNA derived bovine somatotropin, Dennis Campion of USDA-ARS for assaying IGF-I, and the National Hormone and Pituitary Agency, NIADDK for growth hormone antiserum and standards.

³ Reprint requests.

⁴ Dept. of Anim. Sci.