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Impact of dietary nitrate and sulfate on methane to carbon dioxide ratio and performance of finishing steers
Sixty crossbred steers (initial BW = 416 ± 36 kg) were used in a 2 × 2 factorial design experiment to evaluate the effect of dietary addition of nitrate and sulfate on performance and methane to carbon dioxide ratio (CH4:CO2) of finishing steers. Cattle were limit fed 5 d prior to trial initiation and weighed on three consecutive days, the average of which was used as initial BW. Steers were stratified by BW and assigned randomly to one of four treatments (15 replications/treatment). Treatments consisted of a corn-based diet with 0 or 2.0% nitrate (diet DM) and 0 or 0.77% calcium sulfate (designed to not exceed 0.4% dietary S). Steers were adapted to nitrate gradually over a 28 d step-up period. One case of nitrate toxicity was observed during the study. At time of feeding, exhaled breath samples were collected from each steer every two weeks throughout the study using a custom built gas collection system and were analyzed for CH4 and CO2, using gas chromatography. Carbon dioxide was used as an internal marker and CH4:CO2 was used to quantify the effects of diet on methane emission. No interactions were observed between nitrate and sulfate additions for DMI and ADG, but tended to interact for G:F (P = 0.07). Feeding 2.0% nitrate decreased final BW and HCW (P = 0.02), while sulfate had no effect (P = 0.16). Addition of nitrate and sulfate each decreased DMI (P < 0.01), the combination of which decreased DMI by 20% compared to cattle fed neither additive. Similarly, nitrate decreased (P < 0.01) and sulfate tended to decrease (P = 0.07) ADG; when combined, ADG was 16% lower than that of steers receiving neither additive. For the G:F interaction, steers fed sulfate alone had lower G:F whereas steers fed nitrate alone had similar G:F as no additives. When combined, G:F was actually numerically greatest. A nitrate × sulfate interaction was observed for methane emissions (P = 0.04), where cattle fed only sulfate had the greatest CH4:CO2, steers fed nitrate or neither additive were intermediate. Steers receiving a combination of nitrate and sulfate had the lowest CH4:CO2. Feeding a combination of nitrate and sulfate can decrease CH4:CO2, but decreases DMI and ADG.
Keywords: methane, nitrate, sulfate