"Enteric CH4 production from ruminant livestock is the largest source of anthropogenic CH4 emissions in the United States and in most countries with significant livestock production. Because animal products from ruminants are an important part of the diet and CH4 from ruminant animals contributes to greenhouse gas (GHG) emissions, finding ways to reduce CH4 from the livestock production are essential to long-term viability of ecosystems and consumer food choices. In addition, CH4 represents a significant dietary GE loss and reduction in methanogenesis can result in improved feed utilization. Supplementing the diets of ruminants with nitrate salts consistently decrease CH4 production without signs of microbial adaptation in the long-term. Nitrate reduction is a biochemical process that consumes H2, which, in the rumen, is the substrate used by methanogenic archaea for energy production. Monensin sodium has been extensively used in ruminant production because it increases feed conversion and the reduction in CH4 production from monensin feeding is mainly related with more efficient fermentation and reduced protozoa numbers. Therefore, nitrate and monensin affect methanogenesis through different mechanisms. The objective of this study was to evaluate if the combination of nitrate and monensin could additively reduce CH4 production without compromising animal health and performance. A series of experiments were conducted to test the effect of additives on enteric CH4 production and ruminal fermentation parameters. Independent of diet composition, nitrate and monensin additively reduced CH4 production with in vitro models, with maximum reduction of approximately 90% on batch culture trials and 45% on semi-continuous culture fermenters. In batch culture trials the combination of additives at higher doses (2.5% of nitrate in the diet and 6 mg of MON per L of culture media) reduced digestion of feeds. In the semi-continuous fermenters a similar effect on nutrient digestion was observed when additives were combined in higher levels tested (2.5% of nitrate in the diet and 4 mg of monensin per L of culture media), but no effect was observed on VFA production. An intermediary dose of nitrate (1.5% of DM) was tested in growing steers and a significant reduction of 10% for CH4 production was observed. Animals fed nitrate did not demonstrate any clinical sign of toxicity during the experiment with methemoglobin levels similar to control animals. Monensin reduced CH4 production by growing steers by 5% when compared to control. When combined, additives reduced CH4 production by approximately 16% without affecting ruminal fermentation, demonstrating a potential for utilization of additives in the diet of ruminants as a means of CH4 abatement. Long-term performance trials coupled with CH4 measurements are needed to confirm that this strategy does not impair animal performance and are able to sustain reduction on CH4 levels for longer periods of time."--Pages ii-iii.
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