A review of feeding supplementary nitrate to ruminant animals: nitrate toxicity, methane emissions, and production performance

The purpose of this review is to discuss the risks and benefits of using supplementary nitrate to reduce enteric methane emissions in ruminants based on the results of a meta-analysis. The meta-analysis confirmed possible nitrate poisoning triggered by higher blood methemoglobin levels with increasing nitrate consumption of ruminants: methemoglobin (%) = 41.3 x nitrate [g kg(-1) body weight (BW) d(-1)] + 1.2; R-2 = 0.76, P<0.001. However, acclimatizing animals to nitrate reduced the toxicity of nitrate: methemoglobin (%) = 4.2 x nitrate (g kg(-1) BW d(-1)) + 0.4, R-2 = 0.76, P = 0.002. Animals fed nitrate reduced enteric methane emissions in a dose-response manner: methane [g kg(-1) dry matter intake (DMI)] = -8.3 x nitrate (g kg(-1) BWd(-1)) + 15.2, R-2 = 0.80, P<0.001. The reduction of enteric methane emissions due to supplementary nitrate was effective and consistent in both in vitro and in vivo studies and also persistent in several long-term studies. Dry matter intake and live weight gain (LWG) of cattle were not affected by nitrate: DMI change, R-2 = 0.007, P = 0.65; LWG change, R-2 = 0.03, P = 0.31. It is anticipated that supplementary nitrate as a substitute for urea may change urinary nitrogen composition in a manner that increases ammonia and nitrous oxide emissions from manure. Furthermore, supplementary nitrate may have various physiological roles in nitric oxide metabolism in ruminants. In conclusion, supplementary nitrate is a viable means of mitigating enteric methane emissions due to its consistent and persistent efficacy. Risk of toxicity can be lowered by gradual acclimation of animals to nitrate. However, lowered methane production may not re-direct additional metabolizable energy towards animal production.