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

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Abstract
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, α-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 β-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.