Effect of direct-fed microbials and monensin on in vitro fermentation of a high-froage substrate

Abstract Text:  An in vitro experiment was conducted to determine effects of direct-fed microbials (DFM) on rumen fermentation of a forage-based diet in the presence and absence of monensin (MON). In vitro treatments were arranged in a 2 x 2 factorial and included 2 levels of DFM (0 and 50,000 cfu; consisting of primarily Lactobacillus acidophilus and Enterococcus faecium) and 2 levels of monensin (0 and 5 ppm). In vitro runs were conducted on 3 separate days, with each run consisting of 4 vessels/treatment (n = 12/treatment). Inoculum for in vitro experiments consisted of buffer plus a composite of strained rumen fluid obtained from 4 steers fed an 80:20 forage:concentrate (alfalfa/corn/SBM/urea) diet. Fermentation vessels were supplied with common substrate (400 mg of donor steer diet) and incubated in a water bath (39C) for 30 h. Gas pressure was measured at 5-min intervals and samples were collected at termination for determination of methane production, pH, and concentrations of VFA and ammonia.  Data were analyzed as a randomized complete block design with run as the blocking factor. An exponential model without lag was determined to be the best fit model for gas production data and was used to calculate rate and total production of gas. Interactions between DFM and MON were absent (P>0.10) for gas production and fermentative end products, with the exception of a DFM x MON tendency (P=0.07) for total VFA concentration.  In the case of the latter, DFM increased total VFA concentration in the absence but not in the presence of MON. 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. In contrast, MON decreased (P<0.001) CH₄ production and rate and production of total gases.  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, and 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 and was not influenced by 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.

Keywords: direct-fed microbials, monensin, fermentation