Effect of Monensin on Breakdown of Protein by Ruminant Microorganisms in Vitro2

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Abstract
The effects of monensin on N metabolism by ruminal microorganisms in a semicontinuous culture system were determined. Rumen fluid inoculum was obtained from steers fed a hay-concentrate diet (60:40 ratio on a dry matter basis) containing 33 ppm monensin. Treatments were 0, 1 and 4 mg monensin/kg of incubation mixture, with starch, glucose, cellulose and casein used as the energy and protein sources. Casein degradation decreased linearly (P<.01) with increasing levels of monensin, as did production of ammonia-N (P<.05) and microbial N (P<.01). Increases were observed in nonammonia, nonmicrobial N (P<.01), alpha-amino N (P<.10) and total peptides (P<.001). The culture supernatant was fractionated on a Sephadex G-10 column to separate peptides. With the 4 mg/kg treatment, the percentage of the total ninhydrin positive material eluted at an elution volume:Void volume ratio of 1.5 or less was greater than the percentage eluted with the 0 or 1 mg/kg treatments (P<.025). Acetate production and molar proportion decreased (P<.001), while propionate production was unchanged. Methane production decreased slightly (P<.10). Cellulose degradation was markedly inhibited (P<.001) by monensin treatment.