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Effects of dietary nitrate supplementation on enteric methane and nitrous oxide emissions from beef cattle
Feeding nitrate has been proposed as a means to reduce enteric greenhouse gas emissions from ruminants. Nitrate can compete with methanogens for hydrogen in the rumen and therefore reduce methane from eructation. However, increasing the nitrate concentration in the rumen could induce enteric nitrous oxide emissions, potentially nullifying the greenhouse gas reduction achieved from lowering methane emissions. The present study investigated the effects 2% nitrate (on DM basis) versus an isonitrogenous concentration of urea supplemented to finishing steers on enteric methane and nitrous oxide emissions. Sixteen steers were allocated to nitrate and urea treatments in a randomized complete block design (n=8). Eructated emissions were collected using head chambers for 12 h following the morning feeding. Methane was measured using the TEI 55C direct methane analyzer and nitrous oxide using the 46i nitrous oxide analyzer (both were Thermo Environmental Instruments, Franklin, MA). All data was analyzed using the Proc Mixed Model in SAS. The nitrate versus urea treatment lowered methane production at measurement h 1 and 2 (P<0.01), but did not lower overall methane production during the 12 h measurement period. The nitrate versus urea treatment increased nitrous oxide production at h 1, 2, and 3 (P<0.05) of measurement and the overall 12 h measurement period (P<0.0001). Nitrous oxide was detected in both treatments at each time point, with a 6 fold increase in production in the nitrate (~600 mg/12 h) versus urea treatment (~100 mg/12 h). Overall, combined greenhouse gas production expressed as carbon dioxide equivalents was similar between treatments. This study indicates that nitrate supplementation in finishing beef cattle is effective at reducing eructated methane in the time immediately following feeding, and might need to be supplemented at a higher concentration and/or more frequently to achieve more optimal methane reduction. Furthermore, this study suggests that cattle could be a source of the potent greenhouse gas nitrous oxide, which is further stimulated by nitrate supplementation. Additional research is necessary to evaluate more effective means of reducing methane with nitrate in finishing beef cattle and the production of nitrous oxide with and without supplementation of nitrate.
Keywords: Greenhouse gas, Hydrogen Sink, Ruminant