Impacts of dairy forage management on soil carbon change and net-zero accounting.

The US Dairy Industry has pledged to achieve net zero greenhouse gas emissions (GHG) by 2050, but reliance on corn (Zea mays L.) silage as a primary forage source undermines progress toward this goal. Soils managed for corn silage production are a significant source of carbon (C) emissions to the atmosphere, with the soil C losses ranging from 3.7 to 7.0 Mg C ha yr (13.5 to 25.6 Mg CO ha yr) reported in the literature. However, biogenic emissions from soil C loss are not typically represented within C-footprints or life cycle inventories. Using an example dairy farm, we demonstrate that including emissions associated with soil C losses under dairy forage production can increase the C-footprint of milk nearly 2-fold. We suggest that this approach represents a more accurate estimate of the emissions impact of milk production, and that gains in the GHG efficiency of milk have come, in part, at the expense of soil C where forage rotations are predominated by silage corn. The C balance of forage production systems can likely be improved with advanced manure management technologies and application strategies that return more manurial C to the soil while minimizing N and P loading. However, we argue that more extensive changes to forage cropping systems will also be required. Expanding the role of perennials and winter annual crops in forage rotations; breeding forages with greater yield, persistence, and deeper more extensive root systems; and additional creative solutions to retain more plant-derived C in soils are necessary to balance soil C budgets and achieve net-zero emissions targets.