Concern over the carbon footprint of the dairy industry has led to various dietary approaches to mitigate enteric CH 4 production. One approach is feeding the electron acceptor NO 3 − yield. Based on the fact that few interactions were detected, LYC had a minimal role in attenuating negative cow responses to NO 3 − supplementation.

Nitrates have been fed to ruminants, including dairy cows, as an electron sink to mitigate CH 4 emissions. In the NO 3 − reduction process, NO 2 − can accumulate, which could directly inhibit methanogens and possibly other microbes in the rumen. Saccharomyces cerevisiae yeast was hypothesized to decrease NO 2 − through direct reduction or indirectly by stimulating the bacterium Selenomonas ruminantium, which is among the ruminal bacteria most well characterized to reduce both NO 3 − and NO 2 −. Ruminal fluid was incubated in continuous cultures fed diets without or with NaNO 3 (1.5% of diet dry matter; i.e., 1.09% NO 3 −) 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 NO 3 − increased acetate: propionate by increasing acetate molar proportion; NO 3 − also decreased molar proportions of isobutyrate and butyrate. Both NO 3 − and LYC shifted bacterial community composition (based on relative sequence abundance of 16S rRNA genes). An interaction occurred such that NO 3 − decreased valerate molar proportion only when no LYC was added. Nitrate decreased daily CH 4 emissions by 29%. However, treatment × time interactions were present for both CH 4 and H 2 emission from the headspace; CH 4 was decreased by the main effect of NO 3 − until 6 h postfeeding, but NO 3 − and LYC decreased H 2 emission up to 4 h postfeeding. As expected, NO 3 − decreased methane emissions in continuous cultures; however, contrary to expectations, LYC did not attenuate NO 2 − accumulation.

Nitrates have been fed to ruminants, including dairy cows, as an electron sink to mitigate CH 4 emissions. In the NO 3 − reduction process, NO 2 − can accumulate, which could directly inhibit methanogens and some bacteria. However, little information is available on eukaryotic microbes in the rumen. Protozoa were hypothesized to enhance nitrate reductase but also have more circling swimming behavior, and the yeast Saccharomyces cerevisiae was hypothesized to lessen NO 2 − accumulation. In the first experiment, a culture of S. cerevisiae strain 1026 was evaluated under 3 growth phases: aerobic, anoxic, or transition to anoxic culture. Each phase was evaluated with a control or 1 of 3 isonitrogenous doses, including NO 3 − , NO 2 − , or NH 4 + replacing peptone in the medium. Gas head phase, NO 3 − , or NH 4 + did not influence culture growth, but increasing NO 2 − concentration increasingly inhibited yeast growth. In experiment 2, rumen fluid was harvested and incubated for 3 h in 2 concentrations of NO 3 − , NO 2 − , or sodium nitroprusside before assessing chemotaxis of protozoa toward glucose or peptides. Increasing NO 2 − concentration decreased chemotaxis by isotrichids toward glucose or peptides and decreased chemotaxis by entodiniomorphids but only toward peptides. Live yeast culture was inhibited dose-responsively by NO 2 − and does not seem to be a viable mechanism to prevent NO 2 − accumulation in the rumen, whereas a role for protozoal nitrate reductase and NO 2 − influencing signal transduction requires further research.

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