The Energy Costs of Ingestion of Naturally Occurring Nontannin Plant Phenolics by Sheep

We examined the energetic consequences to a ruminant herbivore of ingestion and absorption of low-molecular-weight phenolics, which commonly occur in ericaceous plant species. The first experiment simulated the entrance of absorbable xenobiotics into the general circulation, as might occur if the detoxification capacity of the animal were exceeded. Orcinol was administered intravenously to six sheep for 10 d at a low rate equivalent to about 5% of the total nontannin phenolics likely to be ingested by free-ranging sheep. This led to a 5% increase in energy expenditure (P < 0.05), measured by indirect calorimetry. In a further experiment orcinol and quinol were continuously infused into the rumen of five ewes, at a rate equivalent to the likely dietary intake of nontannin phenolics. When administered via this normal oral route, within the normal physiological range, the phenolics had no detectable effect on energy expenditure but led to a reduction in the intake of digestible energy (P < 0.05) and an increase in urinary energy excretion (P < 0.01). These experiments suggest that the effectiveness of the detoxification system is crucial to permit use of toxin-containing plants by herbivores, and it should be strongly associated with their foraging strategies.