Rumen microbial responses to supplemental nitrate. II. Potential interactions with live yeast culture on the...

Nitrates have been fed to ruminants, including dairy cows, as an electron sink to mitigate CH₄ emissions. In the NO₃^- reduction process, NO₂^- can accumulate, which could directly inhibit methanogens and possibly other microbes in the rumen. Saccharomyces cerevisiae yeast was hypothesized to decrease NO₂^- through direct reduction or indirectly by stimulating the bacterium Selenomonas ruminantium, which is among the ruminal bacteria most well characterized to reduce both NO₃^- and NO₂^-. Ruminal fluid was incubated in continuous cultures fed diets without or with NaNO₃ (1.5% of diet dry matter; i.e., 1.09% NO₃^-) and without or with live yeast culture (LYC) fed at a recommended 0.010 g/d (scaled from cattle to fermentor intakes) in a 2 × 2 factorial arrangement of treatments. Treatments with LYC had increased NDF digestibility and acetate:propionate by increasing acetate molar proportion but tended to decrease total VFA production. The main effect of NO₃^- increased acetate:propionate by increasing acetate molar proportion; NO₃^- also decreased molar proportions of isobutyrate and butyrate. Both NO₃^- and LYC shifted bacterial community composition (based on relative sequence abundance of 16S rRNA genes). An interaction occurred such that NO₃^- decreased valerate molar proportion only when no LYC was added. Nitrate decreased daily CH₄ emissions by 29%. However, treatment × time interactions were present for both CH₄ and H₂ emission from the headspace; CH₄ was decreased by the main effect of NO₃^- until 6 h postfeeding, but NO₃^- and LYC decreased H₂ emission up to 4 h postfeeding. As expected, NO₃^- decreased methane emissions in continuous cultures; however, contrary to expectations, LYC did not attenuate NO₂^- accumulation.