Splanchnic tissue energy use in ruminants that consume forage-based diets ad libitum

The effects that characteristics of forage-based diets on gut and liver energy use in sheep with ad libitum consumption were discussed, and practical consequences of diet and animal effects on splanchnic energy use were projected. Explained variation in portal-drained viscera (PDV) and hepatic energy use by sheep in recent experiments increased when independent variables were included in regressions on DE intake (DEI) (PDV: R2=33 vs 22%; liver: R2=48 vs 25%). The regression equations used were the following: PDV energy use, Mcal/d=-.0194 + (.1207 x DEI, Mcal/d) + (1.1173 x fecal NDF excretion, kg/d) - (.2860 x DEI x fecal NDF excretion) and hepatic energy use, Mcal/d=.0014 + (.0266 x DEI, Mcal/d) + (.3325 x PDV energy use, Mcal/d) + (.0745 x urea N hepatic net flux, mol/d) + (.0016 x BW, kg). Thus, gut digesta, as implied by fecal NDF excretion, seemed to influence PDV energy use in addition to the effect of DEI, and energy consumption by the liver seemed affected by PDV energy use and hepatic ureagenesis. A regression equation (R2=49%) for splanchnic bed energy use, expressed as a percentage of DEI, was 43.61 - (9.531 x DEI, Mcal/d) + (15.914 x fecal NDF excretion, kg/d) + (6.057 x urea N hepatic net flux, mol/ d). If physiological control of feed intake with effect of efficiency of energy metabolism is assumed, and if whole-body efficiency of energy metabolism with ad libitum intake is constant, then efficiency of energy metabolism in extrasplanchnic tissues decreases as level of ad libitum intake and the ratio of splanchnic bed energy use to DEI decrease. Therefore, animal characteristics (e.g., maturity, body condition, and biological type) and grazing conditions (e.g., forage mass) may affect feed intake by influencing heat production by peripheral tissues. In conclusion, an increased understanding of dietary influences on efficiency of splanchnic tissue metabolism may lead to enhanced efficiency of forage utilization and ruminant production systems.