Effect of Direct-Fed Microbials and Monensin on In vitro Fermentation of a High-Forage Diet

An in vitro experiment was conducted to determine effects of directfed microbials (DFM) on rumen fermentation of a forage-based diet in the presence and absence of monensin (MON). Treatments included DFM (0 and 50,000 cfu; primarily Lactobacillus acidophilus and Enterococcus faecium) and monensin (0 and 5 ppm) and were applied directly to the appropriate flask. Substrates (80:20 forage: concentrate) were incubated for 30 h. Measurements included serial gas pressure, and terminal CH4 production, pH, and volatile fatty acid (VFA) and NH3 concentrations. Interactions between DFM and MON were absent (P>0.10) for gas production and fermentation end products, except for a tendency for DFM to increase total VFA concentration without, but not with MON (interaction between DFM and MON; P=0.07). Addition of DFM did not affect (P>0.10) gas production or fermentative end products except for a tendency (P=0.08) for a slight increase in proportion of isovalerate. >Contrariwise, MON decreased (P<0.001) total gas and CH4 production. Similarly, MON decreased (P<0.001) total VFA and NH3 concentrations, and molar proportions of acetate and butyrate and increased (P<0.001) proportions of propionate, valerate, isobutyrate, and isovalerate. Independently, DFM and MON increased (P<0.001) end point pH, resulting in an additive effect when the two treatments were combined. Addition of MON altered fermentation in a predictable fashion and was not influenced by the addition of DFM. In contrast, fermentation characteristics were largely unaffected by DFM with two exceptions: a slight increase in total VFA in the absence of MON and a small increase in pH that appeared to occur through a different mechanism than that for MON.