To be palatable, deer forage plants must also be digestible. Digestibility in turn has been found to depend to a large extent on the balance between nutrients which favor the growth of rumen microbes and any inhibitory substances which plants may contain that depress microbial growth. Deer select their forage primarily by olfaction and secondarily to taste and sight. The inhibitory substances which have been isolated thus far fall into two general groups of chemicals, essential oils and phenolic compounds. However, only a few compounds in these large groups have been found to be inhibitory. Soil fertility, particularly the level of nitrogen available to plants, seems to have a marked effect on palatability, however, this is thought to result from an improvement in the nutrient level accumulated in the plants. Deer can tolerate the inhibitors in unpalatable plants if they only eat a small amount at a time, and they appear to thrive best on a mixed diet. The development and accumulation of microbial inhibitors in plants is considered to be a form of physiological defense against animal use that has had a bearing on natural selection. Conversely, natural selection has operated to favor strains of deer that are able to make the best use of the available plants. An understanding of these relationships should be useful in planning management programs.
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