Effects of monensin and amicloral on rumen fermentation.

Fermentative adjustments produced by monensin and amicloral and by combinations of monensin plus amicloral were investigated in vitro. Monensin increased (P < .05) production of propionate, usually decreased production of acetate and butyrate and partially reduced methanogenesis, but no accumulation of gaseous hydrogen was observed. Amicloral not only increased production of propionate (P < .05), but also usually increased butyrate production, decreased (P < .05) acetate production and almost completely inhibited (P < .05) methanogenesis, and substantial quantities (P < .05) of hydrogen accumulated in the gas phase. Both chemical agents were effective (P < .05) in decreasing utilization of an exogenous amino acid load. Monensin plus amicloral produced fermentative adjustments comparable to those predicted from cultures medicated with monensin or amicloral alone; thus, effects mediated by the combined agents were additive and no consistent positive or negative synergestic activities were noted. Characteristic adjustments in rumen fermentation caused by monensin, by amicloral and by additivity of these adjustments are reflections of the two chemical agents manipulating ruminal reactions by different mechanisms.