Effect of forage cell wall phenolic acids and derivatives on rumen microflora

The trans isomers of methyl and t‐butyl esters of p‐coumaric acid and 4,4′‐ dihydroxy‐α‐truxillic acid were compared with trans‐p‐coumaric acid in relation to their toxicity against rumen microorganisms. Toxicity was evaluated by the effect on growth of pure cultures of Bacteroides ruminicola and Ruminococcus flavefaciens and by the effect on dry matter loss in in‐vitro digestion studies using rumen fluid. Growth of the pure cultures of rumen bacteria was reduced by the methyl and t‐butyl esters as much as or more than by p‐coumaric acid. In contrast, 4,4′‐dihydroxy‐α‐truxillic acid, a photodimer of p‐coumaric acid, was only marginally inhibitory to growth of these bacteria. At 5 mM concentration in rumen fluid, the esters inhibited dry matter digestion of bermudagrass by rumen bacteria and by fungi, but results for 4,4′‐dihydroxy‐α‐truxillic acid were similar to the control treatment in which no phenolic compound was added.