Tradeoffs in US dairy manure greenhouse gas emissions, productivity, climate, and manure management strategies

The United States is the largest supplier of dairy products globally, making it an important focus for environmental, economic, and societal outcomes. Increasingly greenhouse gases (GHGs) have become an area of focus for the industry, as industry groups have set their own goals to improve environmental impacts. A significant portion of dairy GHG emissions come from manure management, which can vary considerably by farm and region. Here we explore how the adoption and use of six common manure management strategies (MMS) have changed over a recent 12-year period, and how this relates to milk production, climate, and manure GHGs. Using data from the United States Department of Agriculture, the Environmental Protection Agency, and the National Oceanic and Atmospheric Administration across all fifty states, we find that overall US dairy manure management GHG emission intensity (CO(2)e per kg of milk produced) has increased 18% between 2003 and 2014, which is associated with an increase in adoption of liquid and anaerobic MMS. However, we also find that these systems are positively associated with higher productivity: nationally, total milk production grew by 21.0%, while the cow herd inventory grew by just 1.9%, an increase of 18.7% in per-cow milk production over the study period. We find clear regional adoption of certain kinds of MMS, which relate in many cases to temperature and rainfall. We discuss why these shifts may have occurred as a result of economic and policy drivers, including the shift towards these MMS for compliance with new water quality policies in the past decade, highlighting the tradeoffs that exist in on-farm decision-making. We provide some potential strategies to reduce GHG emissions in these systems while simultaneously considering water quality and other potential tradeoffs. We suggest that transitioning to some of these strategies requires additional research to better understand farmer decision-making as it relates to MMS, a currently understudied topic.