Ruminants foraging under free-ranging conditions include a diversity of plants in their diet. A wide range of plant secondary compounds are broken down under microbial action in the rumen thus protecting the host animal from their otherwise toxic effects. For example, oxalic acid (OA), found in many tropical grasses, is effectively degraded by Oxalobacter formigenes following a period of adaptation of the rumen micro-flora (Allison et al, 1985). Similarly, butenyl nitrile (BN), a metabolite of glucosinolates, found in brassica plants, has been shown to degrade under rumen microbial action (Duncan & Milne, 1992). The purpose of this experiment was to investigate whether adaptation to the plant secondary compounds found in one food type might influence the degradation of other, unrelated secondary compounds and vice versa.