Effect of nitrate supplementation on diurnal emission of enteric methane and nitrous oxide

The objective of this study was to investigate the effect of nitrate supplementation on diurnal enteric methane (CH₄) and nitrous oxide (N₂O) emissions in dairy cows. Four Danish Holstein dairy cows fitted with ruminal cannulas were used in a 2 × 2 crossover design with 2 periods of 14 d duration. Cows were fed ad libitum with 2 experimental diets based on either urea or nitrate (8.6 g of ${NO}_3^{-}/kgofDM)$ supplementation. Samples of ruminal fluid, blood, and rumen headspace gas samples were collected. Gas exchange was measured in respiration chambers during a 96-h period. Emission of N₂O was calculated from the ratio between CH₄ and N₂O in the rumen headspace and the measured CH₄ emission. Nitrate supplementation resulted in a lower daily CH₄ production (g/d), CH₄ yield (g/kg of DMI), and CH₄ per kilogram of fat- and protein-corrected milk yield; a tendency of lower CH₄ intensity (g/kg ECM); and higher daily hydrogen (H₂) production, H₂ yield, and daily N₂O production compared with urea supplementation. The only difference in ruminal VFA composition was a higher valerate proportion in cows receiving nitrate compared with urea supplementation. In conclusion, nitrate compared with urea supplementation reduced CH₄ production, mainly just after feeding, but also increased N₂O production.