A meta-analysis of effects of feeding nitrate on toxicity, production, and enteric methane emissions in ruminants

Abstract Text: Nitrate is usually considered an undesirable compound in ruminant feeds due to its potential toxicity (methemoglobinaemia). Despite this concern, nitrate has recently received attention as an electron acceptor in the rumen that could possibly reduce enteric methane emissions. It was also proposed that nitrate might be a useful non-protein nitrogen source as a substitute for urea. Therefore, a meta-analysis was conducted to investigate the effects of feeding nitrate on its toxicity and effects on animal production and enteric methane emissions. Data were collected from published literature (PubMed, ScienceDirect and CAB direct) and analyzed using the MIXED procedure of SAS with study as a random effect. When nitrate was fed without a gradual acclimation (stepwise increases in nitrate in diets) or infused into the rumen, blood methemoglobin levels (MetHg, % of total hemoglobin) linearly responded to nitrate levels [9 studies with 25 treatments; MetHg (%) = 41.3 × nitrate (g/kg BW/d) + 1.2; R² = 0.76, P < 0.001]. However, when nitrate was fed using a gradual acclimation, the potential toxicity of nitrate was considerably lowered [3 studies with 11 treatments; MetHg (%) = 4.2 × nitrate (g/kg BW/d) + 0.4, R² = 0.76, P = 0.002]. Animals fed nitrate reduced enteric methane emissions in a dose-response manner [8 studies and 25 treatments; CH₄ (g/kg DMI) = -8.3 × nitrate (g/kg BW/d) + 15.2, R² = 0.80, P < 0.001]. Three studies examined the persistent efficacy of nitrate on reducing enteric methane emissions (54, 90, and 92 d, respectively). Animals fed nitrate (2.1 to 2.7% of dietary DM) reduced methane emissions persistently by 16.5 to 35.4% (treatments × time, P > 0.31). However, dry matter intake (DMI) and live weight gain (LWG) of cattle were not affected by nitrate (DMI, 20 studies and 46 treatments; R² = 0.007, P = 0.65; LWG, 12 studies and 35 treatments; R² = 0.03, P = 0.31). In conclusion, the potential toxicity of nitrate can be reduced by acclimatizing the animals to nitrate gradually. Nitrate is a potential feed additive to reduce enteric methane emissions and its efficacy is persistent. However, lowering methane production using nitrate may not re-direct additional metabolizable energy towards animal production. This meta-analysis, however, needs to be interpreted with caution because of the small number of studies used.

Keywords: meta-analysis, nitrate, ruminants