Empirical model of annual nitrous oxide emissions from open-lot beef cattle feedyard pens (version 2)

Nitrous oxide (N2O) is a greenhouse gas with a higher global warming potential than carbon dioxide. About 12 million beef cattle are raised annually in the semiarid Southern High Plains on open-lot, earthen-surfaced pens. Feces and urine are deposited directly on the pen surface and contribute to N2O emissions. The objectives of this research were to: 1) develop a model for estimating annual N2O emissions from open-lot beef cattle feedyard pens in the Southern High Plains, and 2) assess various manure removal strategies for reducing N2O emissions. Empirical regression equations derived from laboratory and field studies were used to predict N2O emissions based on daily precipitation and temperature. The model accounted for water added to the pen surface from urine and feces, and allowed for different times and frequencies for pen cleaning. Feedyard runoff and infiltration were estimated using daily weather data for the 23-year period from 1996 to 2018. Average annual precipitation was 367 mm yr^-1, with range of 136 to 658 mm yr^-1 and daily maximum of 102 mm d^-1. Emissions of N2O were highest in the few days following precipitation events in the warm summer months. Timing of pen cleaning was important for one cleaning per year, where annual N2O emissions ranged from 0.196 kg N animal^-1 yr^-1 for cleaning on 1 May (late spring) to 0.533 kg N animal^-1 yr^-1 for cleaning on 1 Oct (fall). The optimum timing for two cleanings per year was 1 Jun and 1 Dec (0.118 kg N animal^-1 yr^-1). Timing had little effect on annual emissions for three or more cleanings. Cleaning pens 3, 4, 6, or 12 times per year resulted in annual N2O emissions of 0.086, 0.073, 0.063, or 0.057 kg N animal^-1 yr^-1, respectively. Emissions from urine and feces deposition accounted for 29.0% and 7.1% of overall emissions, respectively. As compared to one pen cleaning per year on 1 May, cleaning pens 2, 3, 4, 6, or 12 times per year reduced annual N2O emissions by 39.8, 56.1, 62.8, 67.9, or 70.9%, respectively. Thus, more frequent pen scraping than a single cleaning per year is recommended as a best management practice to reduce feedyard N2O losses. [Note: All emissions in this paper are presented per capita feedyard capacity. To convert to emissions per animal marketed, divide by 2.2.].