Strategies for mitigating greenhouse gas emissions from US dairy farms toward a net zero goal.

The US dairy industry has set the goal of attaining GHG neutrality by mid-century. Reaching this goal requires the implementation of multiple strategies for mitigating and offsetting GHG production. A large range of animal feeding, manure handling, cropping and energy saving strategies were evaluated alone and in combination to quantify their potential for reducing the farmgate life cycle carbon (C) footprint of milk production. Representative dairy farms in major dairy regions of the United States were simulated without and with these mitigation strategies using the Integrated Farm System Model. The benefit was quantified as the change in milk C footprint with a given strategy or combination of strategies compared with that of current practices in each region. Without consideration of economic constraints, strategies were modeled for plausible and maximum possible emission reductions. For plausible scenarios, C footprint reductions achieved with individual strategies ranged from 0.3% to 18%. Strategies with the most benefit included anaerobic digestion of manure, a covered manure storage using a flare for methane destruction, and the use of a feed additive for mitigating enteric methane. A combination of strategies provided an average reduction in the C footprint over all farms of 44%. Offsets that might be achieved on a few farms through soil carbon sequestration or use of food waste in anaerobic digestion provided further potential reductions of 7% and 4.9%, respectively. With the use of maximum strategies representing an upper limit in potential mitigation, GHG reductions for individual strategies ranged from 0.3% to 32%. A combination of maximum strategies gave a reduction averaging 70% over all farms, falling short of GHG neutrality. Considering alternative metrics for quantifying the global warming effects of methane as a short-lived gas in the atmosphere provided a possible short-term solution for demonstrating climate neutrality of US dairy farms. If implementation of mitigation strategies over the next several years reduced total methane from all farms at a rate greater than 1% per year, alternative metrics such as global warming potential star would predict no effect on global temperature rise. For the long-term, mitigation strategies have the potential for providing large reductions in GHG emissions, but achieving GHG neutrality over all US dairy farms appears improbable.