Butyrate proportion had adiet × monensin interaction (P < 0.01) as butyrate proportion was increased with monensinfor the alfalfa but decreased for the concentrate substrate. Ammonia concentration wasreduced (P < 0.01) by monensin on all substrates and was slightly higher (P < 0.01) for thealfalfa substrate compared with control and concentrate. Lauric acid distillate or PO additiondid not influence fermentation metabolites in the present study.