Minimising the risks of nitrite toxicity when dietary nitrate is used to mitigate methane output in sheep and cattle

Summary

Inorganic nitrate ingested by ruminants is reduced to nitrite then ammonia by rumen microbes. This two-step process deprives methanogens of hydrogen thus reducing methanogenesis while providing a nitrogen source for microbial growth.Absorption of the intermediate nitrite into the blood can be associated with hypoxia as a result of formation of methaemoglobin (MetHb) and other metabolically active intermediates such as nitric oxide. The syndrome is often termed nitrite toxicity.This review assesses how the risk of MetHb formation may be reduced in nitrate-fed ruminants to allow confidence in nitrate feeding as a commercial management practice.Potential control points were identified in regard to (1) reducing nitrite accumulation in the rumen contents and its entry into the blood and (2) reducing MetHb accumulation in the red blood cell.This report providesDetailed appendices containing reviews of current knowledge and the underpinning literatureA summary of current knowledge and opportunities for manipulation about each control pointA list of critical gaps in understanding of nitrate and nitrite metabolism in ruminants and potential management interventions that require research to enable safer management of nitrate-fed ruminantsDescription of the research questions needing answers to permit practical and safe management strategies for use of nitrate in diets for ruminants