Predicting Greenhouse Gas Emissions from Beef Cattle Feedyard Manure.

Improved predictive models for nitrous oxide (N₂O) and methane (CH₄) are crucial for assessing the greenhouse gas (GHG) footprint of beef cattle production. Biochemical process-based models to predict GHG from manure rely on information derived from studies on soil and only limited study has been conducted on manure GHG. Little is known about specific factors that drive production and volatilization of N₂O and CH₄ from feedyard manure. We used GHG flux and weather data collected from non-flow-through non-steady-state chamber studies conducted in 2012 and 2013 on two beef cattle feedyards in the Texas Panhandle. Manure samples (unconsolidated surface manure and the underlying manure pack) were analyzed for basic physicochemical properties, soluble carbon (C) and nitrogen, and UV-visible spectral characteristics related to degree of decomposition and humification. Preliminary correlation analyses indicated that CH₄ production increased with increasing air temperature (P<0.01), manure pack water content, molecular weight of humic substances, and manure temperature (P<0.1). Current process-based models include dissolved organic C (DOC) content in equations to predict CH₄ production; however, there was no correlation between CH₄ and DOC or any other variables studied. Although there was high variability in N₂O emissions (2.6 + 13.4 mg m^-2 h^-1), no significant (P<0.1) relationship was identified between N₂O fluxes and any variable investigated. Other studies conducted on soil and soil/manure mixtures have found moisture to be the primary factor controlling N₂O fluxes, with maximal production at around 60% water (mass basis). The overall non-response to moisture in this study is likely due to the generally dry conditions of feedyard pens in the Panhandle (<48% water). These data will be used to improve parameterization of existing process-based models and develop new empirical models to predict feedyard GHG emissions. Further study is needed to improve understanding of N₂O production from feedyard manure.