Effect of simple phenolic compounds of heather (Calluna vulgaris) on rumen microbial activity in vitro

Heather (Calluna vulgaris), a common dietary component of many mammalian herbivores, contains a broad range of tannin phenolics, flavonoids, and simple phenolic compounds. These may lead to a number of negative effects on herbivores as a result of toxicity or digestive inhibition, We assessed the effect of some simple phenolic compounds from heather on rumen microbial activity using the in vitro consecutive batch culture technique. The compounds tested were orcinol, quinol, and arbutin, a glucoside of quinol. At a concentration of 10 mM, both orcinol and quinol depressed production of gas and volatile fatty acids (VFAs). Orcinol and, to a lesser extent, quinol also reduced digestion of the substrate. In contrast to these negative effects on rumen microbial fermentation, arbutin stimulated both gas and VFA production, probably due to the presence of the fermentable sugar moiety. The presence of arbutin also resulted in different concentrations and proportions of VFAs compared to the presence of orcinol and quinol. Since many simple phenolic compounds are found in plants in the form of glycosides, the negative digestive effects of the phenolic moieties may be accompanied by the positive effect of the sugar moiety, which is liberated in the gastrointestinal tract on hydrolysis by glycosidase enzymes. This study suggests that when phenolics are present as glycosides, they do not necessarily adversely affect digestion by herbivores.