Effect of monensin on breakdown of protein by ruminal microorganisms in vitro.

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 less than .01) with increasing levels of monensin, as did production of ammonia-N (P less than .05) and microbial N (P less than .01). Increases were observed in nonammonia, nonmicrobial N (P less than .01), alpha-amino N (P less than .10) an total peptides (P less than .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 less than .025). Acetate production and molar proportion decreased (P less than .001), while propionate production was unchanged. Methane production decreased slightly (P less than .10). Cellulose degradation was markedly inhibited (P less than .001) by monensin treatment.