During experimental period 4, hydrogen production was measured and was found to be higher for the nitrate diet. Our hypothesis was that nitrate would act as a hydrogen sink and the reason for the elevated hydrogen emissions for the nitrate diet is unknown. Hydrogen is an energy-dense gas and its emission by the animal could offset the energy benefit gained by the decrease in methane production. In experimental period 4, hydrogen production was measured from 0630 to 1530 h. The average hydrogen production over this 9- h period was 1.9 L/cow for the urea diet and 5.8 L/ cow for the nitrate diet. If results from the 9-h period are extrapolated to a 24-h period, hydrogen production is estimated to amount to 5 L/cow per day for the urea diet and 15 L/cow per day for the nitrate diet. The additional 10 L/d of hydrogen produced for the nitrate diet translate into 0.45 mol of hydrogen/d (22.4 L/ mol) or 0.9 g of hydrogen/d. Hydrogen is energy dense (142 kJ/g of H2: Afeefy et al. (2011), but the additional energy lost in hydrogen production for the nitrate diet was calculated to be relatively minor (approximately 1.0 kJ/kg BW0.75 per day or 3.6% of the observed methane decrease).