Beef feedyards produce nitrous oxide (N₂O), a potent greenhouse gas. Limited research has evaluated the processes that produce feedyard N₂O, and how rainfall and temperature impact N₂O losses. Manure in feedyard pens develops into a complex ecosystem of microbes, extracellular enzymes, feces, and urine, with varying H₂O content. This study aimed to improve understanding of feedyard N cycling under differing environmental conditions by incubation of manure in simulated feedyard pens using large chambers under laboratory conditions. We hypothesized that nitrification was the primary source of feedyard N₂O, with interactions among temperature, H₂O content, and manure properties. Emissions of N₂O were monitored with a real–time N₂O analyzer. Manure samples were taken at intervals for analyses of physicochemical properties, denitrification enzyme activity (DEA), and nitrification activity (NA). Due to equipment limitations, there was only one chamber per temperature tested. Correlation was poor among N₂O emissions and rates of DEA and NA. However, significant relationships were found among key manure characteristics, such as ammonia/ammonium and nitrate/nitrite concentrations, manure dry matter, redox status, and temperature. These data suggest that most N₂O was derived from denitrification in the top 5 cm of the manure pack. Further study is warranted to identify the processes involved in flushes of N₂O emitted immediately after rainfall, possibly due to abiotic chemical reactions that release N₂O sequestered in manure pores.

HighlightsEnteric methane (CH₄) from beef cows on pasture was measured over three seasons using three methods.Methods yielded similar results during the summer grazing season but diverged in autumn and winter seasons.Emission averaged 0.34, 0.27, and 0.29 kg CH₄ cow^-1 during lactation, mid-gestation, and late gestation, respectively.Annualized enteric methane emission rate for a beef cow herd grazing tallgrass prairie was 0.32 kg d^-1 cow^-1.Abstract. Methane (CH₄) is an important greenhouse gas, and about 20% of the carbon dioxide equivalent (CO_2e) greenhouse gases emitted by U.S. agriculture are attributed to enteric CH₄ produced by grazing beef cattle. Grazing cattle are mobile point sources of methane and present challenges to quantifying the enteric methane emission rate (MER). In this study, we applied three methods to measure herd-scale and individual-animal MER for a herd of beef cows grazing a native tallgrass prairie: a point source method that used forward-mode dispersion analysis and open-path lasers and cow locations, an open chamber breath analysis system (GreenFeed), and an eddy covariance ratio method that used the ratio of CH₄ and CO₂ mass fluxes. Three campaigns were conducted during the early season (July), late season (October), and dormant season (February). The point source and GreenFeed methods yielded similar MER (±SD) values during the early season campaign: 0.38 ±0.04 and 0.34 ±0.05 kg d^-1 cow^-1, respectively. However, the MER values from the two methods diverged in subsequent seasons. The GreenFeed MER decreased through the late and dormant seasons to 0.23 ±0.03 and 0.19 ±0.03 kg d^-1 cow^-1, respectively. In contrast, the point source MER stayed the same during the late season and increased during the dormant season to 0.41 ±0.07 kg d^-1 cow^-1. The CH₄:CO₂ ratio method, which was used only during the dormant season, yielded a MER of 0.29 ±0.05 kg d^-1 cow^-1. The point source and GreenFeed methods measured different MER (integrated herd-scale versus a subset of individual animals) and likely sampled methane emissions at different times during the day. We conclude that the point source method tended to overestimate emissions, and the GreenFeed method tended to underestimate emissions. Enteric methane emissions from beef cows over the three grazing seasons averaged 0.39 and 0.25 kg d^-1 cow^-1 as measured by the point source and GreenFeed methods, respectively. An annualized enteric MER for a beef cow herd grazing tallgrass prairie was 0.32 kg d^-1 cow^-1. Quantifying enteric methane emissions from grazing beef cows remains a challenge because of the mobile, often dispersed behavior of grazing cattle and the dynamic interactions of forage quality, dry matter intake, and changing physiological state of cows during the year. Keywords: Beef cows, Enteric methane, Forage quality, Grazing, Tallgrass prairie.