Adaptation of large herbivores to secondary plant compounds found in their food plants may influence their food choice. Plants, which are potentially toxic, may initially be avoided by herbivores but, following sampling and adaptation of detoxification pathways, herbivores may be able to exploit certain toxic plants without detrimental effects. Thus, adaptation of the rumen microbial population can protect the host animal from the toxic effects of plant secondary compounds. Oxalic acid is a plant component found at high concentrations in a range of food plants, such as Beta vulgaris L, and Rumex spp., commonly consumed by ruminants. It is readily degraded in the rumen by Oxalobacter formigenes following a period of adaptation (Allison and Reddy, 1984).