Methane production by mixed ruminal cultures incubated in dual-flow fermentors.

This study evaluated the effects of dilution rate and forage-to-concentrate ratio on gas production by rumen microbes. Continuous cultures were used to monitor methane production at three liquid dilution rates (3.2, 6.3, or 12.5%/h) and three forage-to-concentrate ratios (70:30, 50:50, or 30:70). Filtered ruminal contents were allowed 6 d of adaptation to diets followed by 7 d of data collection. Forage consisted of pelleted alfalfa and the concentrate mix included ground corn, soybean meal, and a mineral and vitamin premix. The experiment was replicated in a split-plot design. Total volatile fatty acid production averaged 58.0 mmol/d and was not affected by treatment. Molar proportion of acetate increased with increasing forage-to-concentrate ratio. Molar proportion of propionate tended to decrease at dilution rate of 12.5%/h and increased with the medium and low forage-to-concentrate ratio. Culture pH tended to be greater at a dilution rate of 12.5%/h. Methane production that was calculated from stoichiometric equations was not affected by treatments. However, methane production based on methane concentration in fermentor headspace resulted in an interaction effect of treatments. Stoichiometric equations underestimated methane output at higher dilution rates and with high forage diets. Total diet fermentability was lowest at dilution rate of 3.2%/h. Increasing dilution rates increased microbial yield; increasing the proportion of concentrate improved microbial efficiency. Dilution rate and forage-to-concentrate ratio altered the partition of substrate by microbes. Methane production based on actual concentrations differed from values estimated using stoichiometry of end-product appearance.