Open-lot beef cattle feedyards are a source of greenhouse gas emissions.
Pen surface methane emissions were quantified during the cool and warm seasons.
Methane was generated in the lower anaerobic layers of the manure pack.
Methane emissions decreased after rainfall due to methane oxidation or blocking of pore space.
Pen surface methane emissions accounted for >1% of overall feedyard GHG as CO2eq.
Abstract. Texas is one of the top beef-producing states, where annually more than five million beef cattle are finished in large feedyards on earthen-surfaced pens. Manure deposited on open-lot pen surfaces can contribute to greenhouse gas (GHG) emissions such as methane (CH4). Two week-long sampling campaigns were conducted in April (cool season) and August (warm season) to quantify CH4 emissions from the feedyard pen surface. Emissions were monitored before and after 12.7 mm simulated rainfall events. Temporal and spatial variabilities in emissions were quantified using automated recirculating flux chambers, a multiplexer system, and a real-time CH4 analyzer. During the cool season, mean CH4 flux was 1.09 (SD 2.39) and 0.12 (0.25) g animal-1 d-1 before and after rainfall, respectively. During the warm season, mean CH4 flux was 0.65 (1.01) and 0.26 (0.44) g animal-1 d-1 before and after rainfall, respectively. This suggested that CH4 was produced in the lower, anaerobic layer of the manure pack and CH4 emissions were inhibited following rainfall, most likely due to microbial oxidation of CH4 in the upper layers through methanotrophy or from slowing of diffusion by blocking the manure pore space. The overall mean pen surface CH4 flux was 0.53 g animal-1 d-1. This flux accounted for a small percentage (>1%) of the overall estimated feedyard GHG emissions expressed as CO2eq. Thus, efforts to mitigate GHG from open-lot beef cattle feedyards in the Texas Panhandle should focus on sources with higher percentages of overall GHG, such as enteric methane, pen surface nitrous oxide, and nitrous oxide from fertilized cropland. Keywords: Beef cattle, Flux chamber, Greenhouse gas, Manure, Methane, Rainfall.