A meta-analysis of the effects of nitrate supplementation on enteric methane emission, production performance, and blood methemoglobin in dairy cattle

This meta-analysis evaluated the effect of nitrate supplementation on enteric CH₄ emissions and performance in lactating dairy cows. A literature search identified 9 publications including 10 studies. Nitrate dose ranged from 5.3 to 21.1 g/kg DM and DMI was 19.7 ± 2.47 kg/d, milk yield (MY) was 28.1 ± 6.50 kg/d, and CH₄ production was 348 ± 50.5 g/d (mean ± SD). The mean difference between control and nitrate supplementation was analyzed using the metafor package in R, applying 3 random effects models. Model 1 estimated the overall mean difference across all studies regardless of dose (mean dose: 13.8 g of nitrate/kg of DM). Model 2 included factor effects of nitrate dose centered around the 2 most tested doses: mode dose of 10.2 ± 1.94 g of nitrate/kg of DM (n = 15) and high dose of 19.8 ± 2.51 g of nitrate/kg of DM (n = 10). Model 3 was a linear model evaluating the nitrate dose response. Model 1 showed that nitrate reduced CH₄ production by 20.3%, CH₄ yield (g/kg DMI) by 16.4%, and CH₄ intensity (g/kg ECM) by 20.1% compared with control. Model 2 showed a reduction of 21.5% and 18.7% in CH₄ production, 15.6% and 16.9% in CH₄ yield, and 20.2% and 18.9% in CH₄ intensity compared with control for the mode and high dose, respectively. No difference in CH₄ mitigation was observed between the mode and high nitrate dose (model 2), nor was a linear effect of nitrate dose detected (model 3). Nitrate supplementation reduced DMI by 0.825 kg/d in model 1, whereas model 2 showed that reduction of DMI only occurred at the mode dose (-1.04 kg/d). According to model 1, nitrate supplementation reduced milk protein yield and content (-0.033 kg/d and -0.087 percentage units, respectively) and tended to reduce ECM yield (-0.55 kg/d). According to model 2, nitrate supplementation reduced milk protein content at both mode and high dose (-0.059 and -0.133 percentage units, respectively), milk protein yield at the mode dose (-0.035 kg/d), and tended to decrease milk protein yield at the high dose (-0.028 kg/d). Furthermore, feed efficiency increased by 3.21% in model 1 and by 3.59% in model 2 for the mode dose only. Blood methemoglobin increased upon nitrate supplementation according to model 1 (+0.695 percentage units) and model 2 (high dose only; +1.32 percentage units) but did not reach levels that posed risks for animal health. In conclusion, the most commonly tested dose of 10 g of nitrate/kg of DM reduced CH₄ production by 21.5%, CH₄ yield by 15.6%, and CH₄ intensity by 20.2%. Dose response analysis indicated no additional reductions when increasing the nitrate dose to a level higher than 10 g/kg DM. Despite the decrease in milk protein content and yield, which was observed at the mode dose of 10 g/kg DM, supplementing nitrate is an effective strategy to reduce CH₄ emission.