Enteric nitrous oxide emissions from beef cattle1

Nitrous oxide (N₂O) is a greenhouse gas with a higher global warming potential than carbon dioxide (CO₂) or methane (CH₄). The objectives of this research were to quantify enteric N₂O emissions from beef cattle and determine effects of dietary nitrate (NO₃) concentrations. Experiments consisted of one in vitro incubation trial and 2 live animal (LA) trials. During the in vitro trial, gas was collected from 4 forage-based and 5 corn-based diets. During the LA trials, emissions were monitored from steers in respiration chambers. In LA trial 1, 5 measurements of 256 to 720 min were conducted on a single steer within a 48-h period. In LA trial 2, measurements were conducted on 4 steers in the absence of manure. Highest in vitro N₂O production was from diets containing added NO₃ or alfalfa. In vitro N₂O increased with dietary NO₃ concentrations (r² = 0.99), with little correlation to dietary CP (r² = 0.17). Added NO₃ decreased CH₄ emissions. Mean N₂O emission rates (±SD) from the LA trials were 6.93 ± 2.99 mg of N₂O∙kg⁻¹ of DMI in trial 1 and 2.20 ± 0.10 mg of N₂O∙kg⁻¹ of DMI in trial 2. Mean enteric N₂O emissions accounted for 0.35% (LA trial 1) and 0.12% (LA trial 2) of CO₂ equivalents. Enteric N₂O emission rates were 6 to 40 times lower than values cited in earlier publications. Enteric N₂O emission rates were also 58 to 108 times lower than manure emissions. Therefore, efforts to reduce greenhouse gas emissions from beef cattle should focus on enteric CH₄ and manure N₂O as opposed to enteric N₂O.