Perspective: Impacts of dairy forage management on soil carbon change and net zero accounting

The US dairy industry has pledged to achieve net zero GHG emissions 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 substantial source of carbon (C) emissions to the atmosphere, with soil C losses ranging from 3.7 to 7.0 Mg C ha^-1 yr^-1 (13.5 to 25.6 Mg CO₂ ha^-1 yr^-1) 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.