Cumulative hydrogen production was higher for the nitrate diet during the 9-h measurement period in experimental period 4 (1.9 L/cow per 9 h for urea vs. 5.8 L/cow per 9 h for nitrate; P = 0.003). Hydrogen production was higher up to 7 h after feeding for the nitrate diet (Figure 1). The variability in hydrogen production was much larger for pairs of animals on the nitrate treatment than for those on the urea treatment. Methane production was suppressed for a period of 5 h after feeding with nitrate, and this depression in methane production largely coincided with the increase in hydrogen production for this treatment