Empirical Model of Annual Nitrous Oxide Emissions from Open-Lot Beef Cattle Feedyard Pens in the Southern High PlainsConference Paper
2018 American Society of Agricultural and Biological Engineers. All rights reserved. Nitrous oxide (N2O) is a greenhouse gas with global warming potential about 300 times that of carbon dioxide. More than seven million beef cattle are finished in feedyards in the semiarid Southern High Plains. Precipitation, feces, and urine deposited directly and continuously on the open-lot pen surfaces contribute to N2O emissions. The objective of this research was to estimate daily and annual N2O emissions from open-lot feedyard pens in the Southern High Plains through the use of an empirical model. Regression equations derived from lab experiments were used to estimate N2O emissions in an Excel spreadsheet based on daily precipitation, manure temperature, and urine and feces deposition over the 22-yr period from 1996 to 2017. Mean annual precipitation ranged from 136 to 658 mm yr-1(mean = 367 mm yr-1). Mean daily manure temperature ranged from -4.9 to 32.8C (mean = 15.2C). Urine deposition was estimated at 1.4 mm d-1(511 mm yr-1) at a cattle density of 15 m2animal-1. Model-simulated daily N2O emissions were best approximated by a lognormal distribution, with range 0.3 to 1,351 mg N2O-N m-2d-1and mean, geometric mean, and median of 71.1, 34.3, and 38.4 mg N2O-N m-2d-1 respectively. The model-simulated mean (SD) per capita feedyard capacity annual N2O emission rate for the 22-yr period was 0.39 (0.07) kg N2O-N animal-1yr-1 with range 0.23 to 0.51 kg N2O-N animal-1yr-1. Emissions due to precipitation, urine deposition, and feces deposition on the pen surface accounted for 61.8, 29.7, and 8.5% of overall N2O emissions, respectively. Emissions from June, July, and August accounted for 51.7% of annual emissions. The mean model-simulated emission rate for the 22-yr period compared favorably to three years of measured N2O emission data from two feedyards in Texas. Model-simulated daily emission rates were 1.3 to 13.1-fold higher than measured from two weeks of daily emission data from feedyards in Nebraska and Texas. The model will continue to be refined to better simulate field-measured N2O emission rates, and a sensitivity analysis will be conducted to assess how variability in regression parameters affects model simulations.