Effects of diet forage-to-concentrate ratio and intake on energy metabolism in growing beef heifers: whole body energy and nitrogen balance and visceral heat production.

Effects of diet forage-to-concentrate ratio and intake on balances of energy and nitrogen and portal-drained viscera (PDV), liver and kidney blood flow and O2 consumption were measured in seven growing beef heifers. Isonitrogenous pelleted diets containing approximately 75% alfalfa or 75% concentrate were fed daily as 12 equal meals every 2 h at two isoenergetic metabolizable energy intakes. A split-plot design was used, with 4 wk for adaptation to diet followed by 3-wk intake periods within 6-wk diet periods. Heifers consumed and digested less dry matter, energy and nitrogen when fed the 75% concentrate vs. 75% alfalfa diet at equal metabolizable energy. Heifers fed the 75% concentrate diet produced less heat energy and retained more tissue energy than when fed the 75% alfalfa diet. Blood flow for PDV, liver and kidneys increased with intake and was greater when heifers were fed the 75% alfalfa vs. 75% concentrate diet. Increased PDV and liver O2 uptake accounted for 44 and 72% of heat increment for the 75% concentrate and 75% alfalfa diets, respectively. Greater PDV uptake of O2 accounted for 72% of the decrease in tissue energy of heifers fed the 75% alfalfa vs. 75% concentrate diet at equal metabolizable energy.