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Effect of Bovine Somatotropin on Metabolism of Lactating Dairy Cows: Energy and Nitrogen Utilization as Determined by Respiration Calorimetry^1,2,3,

Ruminant Nutrition Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705 ^* Department of Animal Science, Cornell University, Ithaca, NY 14853

Effects of exogenous bovine somatotropin (bST) on energy and nitrogen utilization by high producing dairy cows were examined. Nine cows received bST (51.5 KI/d) or exipient (control) in a single reversal design involving 14-d treatment periods. Energy and nitrogen balances were measured in open-circuit respiration chambers. Yield of 4% solids-corrected milk was increased by 22% with bST treatment. Dry matter intake and the apparent digestibilities of dry matter, energy and nitrogen were not altered by treatment. The only significant component of nitrogen utilization that was altered by bST treatment was an increase in milk nitrogen secretion. Cows were in negative tissue nitrogen balance during the control period (-21 g/d) and tended to become more negative during the bST treatment period (-34 g/d). Heat energy loss and milk energy secretion were increased with bST treatment. Tissue energy balance was -1.1 Mcal/d during the control treatment period and the use of energy reserves with bST treatment decreased tissue energy balance to -9.8 Mcal/d. Changes in heat production with bST treatment were equal to those predicted from the changes in milk and body tissue. Overall, the results demonstrated that bST treatment increased yield of milk and milk components even when cows were in negative nitrogen and energy balance. Effects of bST were predominantly associated with nutrient partitioning, and observed heat loss (associated with maintenance and partial efficiencies of milk synthesis and tissue utilization) did not differ from predicted heat loss.

KEY WORDS: • somatotropin • lactation • energy • nitrogen • calorimetry • bioenergetics

¹ Supported in part by the National Science Foundation (PCM 8108130), Cornell University Agricultural Experiment Station, the Upjohn Company, American Cyanamid Company and Monsanto Company.

² Presented in part at the 77th Annual Meeting of the American Dairy Science Association, June 1982, at Pennsylvania State University [J. Dairy Sci. 65 (Suppl. 1): 120 (abs.), 1982] and at the Ninth Symposium on Energy Metabolism, September 1982, at Lillehammer, Norway [European Association for Animal Production Publ. No. 29, p. 46 (abs.)].

³ Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

⁴ Present address: 52 Derby Road, Draycott, Derbyshire, England DE7 3W1.

⁵ Deceased.

⁶ Present address: Monsanto Europe S. A., Avenue de Tervuren 270–272, 1150 Brussels, Belgium.

⁷ Present address: Department of Animal Science, University of Wyoming, Laramie, WY 82071.

Manuscript received 3 December 1987. Revision accepted 15 March 1988.

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