Research Program Area: Climate Change
Enteric fermentation is the single largest source of methane (CH4) emissions in California. It contributes about 30 percent of the total methane emissions in the state, most of which are from cattle. This estimate was, however, based on the U.S. Environmental Protection Agency (U.S. EPA)'s Cattle Enteric Fermentation Model (CEFM) using default national or regional assumptions and model parameter values regarding feed Gross Energy Intake (GEI) and methane conversion factors (Ym). Enteric methane emissions can vary not only with different types of animals, but also with feed intake and nutrient composition. Those variables can change over time and across regions depending on feed availability and local economics. This study updated the feed intake and diet composition for California cattle, analyzed recent California-relevant literature data between diet characteristics and enteric methane emissions, and developed a set of new California-specific empirical models that can provide more accurate enteric methane emissions estimates from California cattle. Through regression analyses, the study identified the following most important predictor variables for enteric methane emissions: dry matter intake (DMI), neutral detergent fiber (NDF) or digestible NDF (dNDF), ether extract (EE), and GEI, depending on the type of cattle considered. The study also updated Ym values from 0.048 to (0.055 or 0.069) for dairy cow groups. The new models estimated 10 percent higher CH4 emissions due to the higher Ym values compared to the U.S. EPA model, but an overall 5 percent lower CH4 emission estimate if both the updated Ym values and feed intake matrix were used. The new models have resulted in more accurate dietary changes in California cattle operations, improved methane emission estimates from California cattle, and can be used to inform the California's Short-Lived Climate Pollutant Reduction Strategies.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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