Changes in liver and gastrointestinal tract energy demands in response to physiological workload in ruminants.

Liver and gastrointestinal tract weights (ingesta- and adipose-free) appear to increase or decrease in direct proportion to dietary intake within and across physiological stages of maintenance, growth, fattening or lactation. Liver and gut mass increase approximately 15 and 30 g per unit of liveweight raised to the 0.75 power (Wt0.75) for each multiple of 500 kJ/Wt0.75 [approximately 1 x maintenance (M)] increase in metabolizable energy (ME) intake, with linearity indicated up to the highest recorded level (4.5 x M). Extrapolation from in vivo arteriovenous O2 measurements across splanchnic tissues and from the previously cited weight information indicates that liver and gut tissue oxidize approximately 3.5 and 1.0 kJ of ME/g of fresh tissue daily, in contrast to whole-animal rates of 0.1 kJ/g. Thus, energy use by the relatively small amount of liver and gut accounts for 45 to 50% of whole-animal heat energy. On a differential basis, increases in energy use by these tissues appear to account for up to 70% of the heat increment of ME use above maintenance.