Ruminal fluid from steers fed on pasture was incubated with artificial media at pH 5.5 and 7.0 in two experiments. In the first, the effect of monensin level on resistance of ruminal bacteria to potassium depletion was evaluated; in the second, effects of the ionophores monensin and lasalocid on ammonia and protein production were quantified. In experiment 1, culture media affected potassium level. The monensin concentration needed to cause half maximal potassium depletion was 2.77µM at pH 5.5 but was 0.056µM at pH 7.0, showing that bacteria incubated at pH 5.5 were more tolerant to monensin than those incubated at pH 7.0. Both ionophores as well as increased acidity caused decreased ammonia production. Both ionophores inhibited ammonia production by 56%, independently of pH. In cultures incubated at pH 5.5 compared to pH 7.0, ammonia production was decreased by 50.5%, independently of the ionophores. Therefore, effects of ionophores and acidity were additive, and the maximum inhibition occurred in the presence of an ionophore at low pH (75.2%). Microbial protein production was lowest when lasalocid was present in a low pH culture medium, causing inhibition of microbial growth.
bacteria; lasalocid; monensin; pH; rumen
