California Air Resources Board

Principal Investigator/Author: Ermias Kebreab

Contractor: University of California, Davis

Sub-contractor: DNDC Applications, Research and Training, LLC

Contract number: 21RD019

Project Status: Active

Relevant CARB programs: SB 1383, Dairy and Livestock GHG Emissions Working Group

Topic areas: Climate Change, GHG Emission Inventory Program, Short-Lived Climate Pollutant Inventory

Research Summary: 

In an effort to mitigate statewide methane (CH4) emissions, California's Legislature passed Senate Bill (SB) 1383 in 2016 that mandates reduction of statewide CH4 emissions from dairy and livestock sector to 40% below 2013 levels by 2030. These emissions are split approximately evenly between manure management activities and enteric fermentation. The state has supported multiple CH4 emissions reduction strategies for manure management, but has yet been able to achieve significant CH4 emission reductions from enteric fermentation. Recent research findings have shown that Animal Feed Additives (AFA) may reduce enteric CH4 emissions by as much as 50%. However, the AFA evaluation strategies are inconsistent across studies, making it difficult to understand the AFAs' full potential in reducing enteric CH4 emissions, and only a few studies take the necessary steps to evaluate the potential impact of AFA use on upstream and downstream emissions. To ensure comparability and comprehensiveness of AFA studies, developing and following a testing and reporting standard (Standard) to quantify CH4 emissions from enteric fermentation is critical. This Standard will serve as a replicable model for measurement studies that aim to evaluate the effectiveness of various AFAs on reducing CH4 produced in cattle ruminants.

As California's dairy operations continue to evolve in response to SB 1383 (e.g. AFA use), it becomes imperative that there is a need for a tool to adaptively estimate CH4 emissions from the dairy and livestock sector. The California Dairy Emission Model (CADEM, CARB Contract #19RD028) adopted the mechanistic approach to improving the representativeness of CH4 emission estimates for dairy manure management but continues to rely on empirical equations for the enteric fermentation. Although California's statewide CH4 emission from enteric fermentation is currently estimated using the state-averaged gross energy and methane conversion factor as recommended by the U.S. EPA (IPCC refined Tier 2), a more accurate and regionally representative estimate for enteric fermentation will require its own mechanistic model that incorporates parameters on animal diet and digestibility at the facility level. Such improvement will allow California to comply with IPCC refined Tier 3 for comprehensive dairy CH4 emission quantification strategy.

Keywords: greenhouse gas emissions, climate change, greenhouse gas reduction strategies, methane emissions, dairy emissions