Integrating Continuous Cover, Manure Management, and Anaerobic Digestion Strategies on a Pennsylvania Dairy Farm: A Life Cycle Assessment

Continuous vegetative cover practices and manure management strategies have been proposed as best management practices in dairy production but can be difficult to support financially. Anaerobic digestion (AD) has the potential to offer a financial incentive for farmers by utilizing manure and grassy biomass for biogasa concept termed grass-to-gas (G2G). Life cycle assessment is applied to evaluate the environmental impacts (global warming potential (GWP), marine eutrophication potential (MEP), and acidification potential (AP)) of implementing these practices. Literature estimates informed changes in inputs and subsequent emissions from winter cropping, switchgrass cultivation, and manure and digestate management (i.e., subsurface injection and covered storage). The results show that G2G systems can reduce the GWP of milk by over 20% on average relative to the base case, with effects on the MEP and AP being mixed in part due to how continuous cover impacts feed production on the farm and imports from off-farm. Continuous vegetative cover reduced on-farm MEP with and without AD but led to increasing feed imports in most scenarios, decreasing the life cycle MEP benefits. Reducing herd size to avoid increasing feed imports led to milk losses lower than current dairy waste rates. Trade-offs were analyzed in the context of food–energy–waste relationships within integrated cropping, livestock, and AD systems.连续植被覆盖措施与粪便管理策略已被列为乳制品生产领域的推荐最佳管理实践，但在财务层面往往难以推行。厌氧消化（Anaerobic Digestion，AD）技术可通过利用畜禽粪便与草本生物质制取沼气，为养殖户提供财务激励，这一模式被称为草制气（Grass-to-Gas，G2G）。本研究采用生命周期评估（Life Cycle Assessment，LCA）方法，对实施上述措施后的环境影响进行评价，评价指标涵盖全球变暖潜势（Global Warming Potential，GWP）、海洋富营养化潜势（Marine Eutrophication Potential，MEP）与酸化潜势（Acidification Potential，AP）。研究基于已有文献的估算数据，确定了冬季作物种植、柳枝稷种植、畜禽粪便与消化液管理（即地下注入与封闭存储）各环节的投入变化及后续排放情况。结果表明，相较于基准情景，草制气系统可使牛乳生产的全球变暖潜势平均降低20%以上；其对海洋富营养化潜势与酸化潜势的影响存在分化，这在一定程度上源于连续植被覆盖对农场内部饲料生产及场外饲料进口的综合作用。无论是否配套厌氧消化系统，连续植被覆盖均可降低农场内部的海洋富营养化潜势，但在多数情景下会推高饲料进口量，从而削弱生命周期评估视角下的海洋富营养化潜势减排收益。通过缩减畜群规模以避免饲料进口量增加，可使牛乳产量损失低于当前乳制品废弃物的产生速率。本研究还在种植-畜牧-厌氧消化综合系统的食物-能源-废弃物关联框架下，对各项措施间的权衡关系进行了分析。