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 NO3−/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. N2O emission was calculated from the ratio between CH4 and N2O in the rumen head space and the measured CH4 emission. Nitrate supplementation resulted in a lower daily CH4 production (g/d), CH4 yield (g/kg of DMI), a lower CH4 per kg of FPCM, 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 volatile fatty acid 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.