Enteric Nitrous Oxide Emissions from Beef Feedlot Cattle

Nitrous oxide (N₂O) is a greenhouse gas with higher global warming potential than carbon dioxide (CO₂) or methane (CH₄). Of the few respiration chamber studies focused on quantifying enteric (ruminal) N₂O emissions from beef cattle, almost all have been confounded with N₂O emissions from manure. The objective of this research was to quantify enteric N₂O emissions from beef cattle while minimizing or eliminating potential bias from manure emissions. Experiments consisted of one in vitro and two live animal trials (LAT). Four in vitro feedlot diets (10 g DM; CP=4.9 to 9.6%) were incubated in buffered ruminal fluid for 96 h. Gases were collected in sample bags and analyzed for N₂O and CH₄ with both gas chromatography and a high precision (sub-ppbv), real-time continuous laser instrument utilizing integrated-cavity output spectroscopy (ICOS). In the LAT, gases were monitored in single-animal respiration chambers every 1 sec using ICOS. In LAT 1, five measurements (256 to 720 min) were conducted on a single steer (BW 269 kg) over a 3-d period (CP=9.6%) while feces and urine accumulated but were collected separately within the chamber. In LAT 2, short-term measurements (9 to 28 min) were conducted on four steers (BW 278-339 kg), 1.5 to 3 h after feeding (CP=11.1%), in the absence of feces and urine deposition within the chamber. In vitro N₂O was very low, ranging from zero (CP=4.9 to 8.1%) to 0.011 μL g^-1 DM (CP=9.6%). In vitro CH₄ ranged from 21.8 to 24.4 mL g^-1 DM. Mean N₂O emission rates were 24.4 ± 10.5 mg animal^-1 d^-1 in LAT 1 and 13.2 ± 0.59 mg animal^-1 d^-1 in LAT 2. Mean CH₄ emission rates were 82.5 ± 14.7 g animal^-1 d^-1 in LAT 1 and 126.5 ± 24.2 g animal^-1 d^-1 in LAT 2. Enteric CH₄ emissions were typical of previous publications, but enteric N₂O emissions were more than an order of magnitude lower than previously reported for feedlot diets, and considerably lower than used in current emission models. Enteric N₂O emissions accounted for only 0.26% (LAT 1) and 0.092% (LAT 2) of the total CO₂ equivalents of N₂O and CH₄ combined. Mean enteric N₂O emissions were 1.43% (LAT 1) and 0.78% (LAT 2) of the 1.7 g N₂O animal^-1 d^‑1 we recently reported for manure emissions from commercial beef feedlot pen surfaces. Thus, enteric N₂O emissions from finishing beef cattle are a very small percentage of both enteric emissions and feedlot manure emissions.