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Effects of Diet Forage-to-Concentrate Ratio and Intake on Energy Metabolism in Growing Beef Heifers: Net Nutrient Metabolism by Visceral Tissues^1,2,3,

U.S. Department of Agriculture, Agricultural Research Service, Livestock and Poultry Sciences Institute, Ruminant Nutrition Laboratory, Beltsville, MD 20705

Effects of diet forage-to-concentrate ratio and intake on metabolism of nutrients by portal-drained viscera (PDV), liver and kidneys were measured in seven growing beef heifers. Isonitrogenous pelleted diets containing approximately 75% alfalfa or 75% concentrate were fed every 2 h at two metabolizable energy intakes. Greater intake increased net PDV removal of glucose and urea nitrogen and net PDV release of NH₃N, -amino nitrogen (AAN), alanine and lactate. Lower net PDV release of NH₃N, liver removal of NH₃N and AAN and liver release of urea nitrogen accompanied lower N intake and digestion in heifers fed the 75% concentrate vs. the 75% alfalfa diet. Lower net PDV glucose removal resulted in greater total splanchnic glucose release when heifers were fed the 75% concentrate vs. the 75% alfalfa diet. In addition, net PDV, liver and total splanchnic release of ß-hydroxybutyrate (BOHB) were lower when heifers were fed the 75% concentrate vs. the 75% alfalfa diet. Greater tissue energy retention in heifers fed the 75% concentrate vs. the 75% alfalfa diet at equal metabolizable energy intake accompanied differences in net PDV metabolism of glucose, NH₃N and BOHB and liver metabolism of nitrogenous compounds, BOHB and lactate.

KEY WORDS: • cattle • intake • viscera • nutrient metabolism

¹ Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable.

² Presented in part at the 79th Annual Meeting of the American Society of Animal Science at Logan, Utah, August 1987 [Reynolds, C. K. & Tyrrell, H. F. (1987) Effect of diet intake level on net visceral tissue metabolism in growing beef heifers. J. Anim. Sci. 65 (Suppl. 1): 476] and the 11th European Association of Animal Production Symposium on Energy Metabolism in Farm Animals at Lunteren, Netherlands, September 1988 [Reynolds, C. K. & Tyrrell, H. F. (1989) Effects of forage to concentrate ratio and intake on whole body energy metabolism of growing beef heifers, EAAP Publication No. 43 (van der Honing, Y. & Close, W. H., eds.), pp. 151–154, Centre for Agricultural Publishing and Documentation, Pudoc Wageningen, The Netherlands].

³ Conducted with the approval of the Beltsville Agricultural Research Center Animal Care Committee.

⁴ To whom correspondence should be addressed: Building 162, BARC-East, Beltsville, MD 20705.

⁵ Deceased.

Manuscript received 13 June 1990. Revision accepted 13 December 1990.

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