Diet selection by a ruminant generalist browser in relation to plant chemistry

Captive mule deer (Odocoileus hemionus hemionus) and black-tailed deer (O. h. sitkensis) were used in cafeteria-type, two choice feeding trials to test the hypothesis that digestible dry matter (energy) and nontannin phenolics of tree, shrub, and forb leaves are major determinants of diet preference. Deer selected plants in relation to a trade-off between the benefit derived from digestible dry matter and the cost of nontannin phenolics presumably associated with toxicity when absorbed. When one of the forages contained both the highest digestible dry matter and lowest nontannin phenolics, the deer always preferred that plant. When one forage had the highest digestible dry matter but the other plant had the lowest nontannin phenolics, the deer selected the high-energy plant when the difference in nontannin phenolics was relatively small, but they preferred the low-energy plant when the other forage had much higher levels of nontannin phenolics. Tannins influenced diet choice only as one of the factors reducing digestible dry matter in these forages. Apparently total dry-matter intake was constrained by the nontannin phenolic fraction but not by tannins. Tannins and nontannin phenolics both contribute to defending plants against browsers.