Short communicationEffects of monensin on the chemical composition of the liquid associated microbial fraction in an in vitro rumen fermentation system

The hypothesis of this study was that monensin alters chemical composition of liquid associated rumen microbes. Furthermore, this study generates information on the chemical composition of rumen bacteria as affected by a ionophore by analyzing different parameters within the same experiment. Monensin at 5 and 8 μM was incubated with 380 mg of substrate (hay:concentrate 70:30 w/w) for 24 h in an in vitro gas production system. Monensin elicited no effects on nitrogen and true protein contents of the liquid associated microbial fraction, but sugar content and carbon did change on addition on monensin. Ratio of crude protein to purine bases decreased on adding monensin. True dry matter digestibility and short chain fatty acids production decreased on addition of monensin. Acetate, butyrate and valerate proportions decreased, whereas propionate and branched-chain fatty acid proportions increased. The change in acetate to propionate ratio concurs with the increase in the efficiency of microbial protein synthesis and the decrease in methane production. Results suggest that: monensin influenced the chemical composition of the liquid associated microbial fraction in vitro, and the changed ratio of crude protein to purine bases may lead to under- or over-estimations of microbial protein synthesis when purine derivatives are used as markers in the presence of monensin.