‘Grass2Gas’ may be key to sustainable Pennsylvania dairy farms, study suggests

UNIVERSITY PARK, Pa. — Implementing novel management practices in dairy farming, one of the commonwealth’s major agricultural industries, could help alleviate a large source of both nutrient pollution and greenhouse gas emissions, according to a multidisciplinary team led by researchers at Penn State. Those practices include continuous cover — keeping fields covered with vegetation year-round — and anaerobic digestion — a microbial process that converts manure and plant organic matter, called biomass, into biogas — a combustible fuel consisting mostly of methane. To evaluate the effectiveness of these practices, which the team refers to as Grass2Gas when they are combined, the researchers conducted a study involving the simulation of a typical large Pennsylvania dairy farm. The team compared the environmental footprint of the farm employing different scenarios of Grass2Gas with that of a dairy farm under traditional management.

Using lifecycle assessment, which accounts for every practice and resource used by a farm from origination to disposal, the researchers found that using anaerobic digestion of manure with grassy biomass could reduce the carbon footprint of milk production by over 20% on average, compared with a typical dairy farm. But their modeling suggested that for the Grass2Gas approach to diminish overall water and air pollution, adjustments and tradeoffs may be necessary. They published their findings in Environmental Science and Technology. The work is the latest from a multi-institutional project, also titled Grass2Gas, that includes collaborators from Penn State, Iowa State University and Roeslein Alternative Energy.

“It has been suggested that promoting perennial plant species for nutrient management and converting manure and biomass into biogas with anaerobic digestion, which can be upgraded to renewable natural gas or directly burned in a generator to produce electricity and heat, can support sustainability on Pennsylvania dairy farms — and we wanted to see if that is true,” said study senior author Christine Costello, assistant professor of agricultural and biological engineering in the College of Agricultural Sciences at Penn State. “We found that it could be true, but our research highlights the complexities of integrating anaerobic digestion into farm systems, including the impact on soil biogeochemistry and nutrient balances.”

Surprisingly, the researchers said, the practices’ effect on water pollution in the simulation were mixed. Continuous cover reduces nutrient runoff on the farm, which should reduce eutrophication — excessive enrichment of water bodies with nitrogen and phosphorus that triggers algae growth, depletes oxygen and kills aquatic life. But growing more vegetation for anaerobic digestion also increased the need for off-farm imports of feed for the cows in most scenarios. More feed imports results in more environmental impact from producing that feed somewhere else, the researchers reported, offsetting many of the water quality benefits from a life cycle perspective.

In their modeling, the researchers found that reducing herd size to match available feed resulted in small milk losses, comparable to typical levels of milk wasted in the dairy supply chain.

“If the farm avoids feed imports by reducing herd size, the reduced milk production was less than current estimates of milk wasted in the dairy supply chain,” Costello said, explaining the team compared their modeled result to the reported food loss and waste rates calculated by the U.S. Department of Agriculture. “In other words, if waste could be avoided, reducing herd size wouldn’t significantly hurt overall milk availability.”

They estimated the potential for multiple impacts — global warming, marine eutrophication and acidification, which is linked to emissions that contribute to acid rain — across multiple scenarios. They used data from previous studies to estimate how winter cover crops, manure handling and management of the digestate — the nutrient-rich fertilizer left after the anaerobic digestion process — affect emissions and resource use.

“The digestion process changes the chemistry of manure, especially how nitrogen is stored and released,” she said. “That means when farmers spread digestate on fields, it behaves differently in soil and air than undigested manure. That influences emissions considerably, especially nitrogen, including ammonia, nitrous oxide and nitrate emissions, which impair climate, air quality and water quality.”

This study highlights the complex balance between milk production, energy generation from biogas, manure and digestate management, food waste and environmental impacts such as climate change and eutrophication, Costello said. Integrated crop-livestock-digester systems can reduce climate impacts but may unintentionally shift nutrient pollution elsewhere unless carefully managed.

“The main takeaway from the study is that agricultural and environmental scientists, engineers and policymakers should think about crop and livestock production and energy technology as one interconnected system,” Costello said. “When we think about adding an energy-production technology — in this case, anaerobic digestion — to a farm, we really need to think about how the residual materials — in this case, digestate — will be handled.”

Study first author Elmin Rahic, who was a postdoctoral scholar in Costello’s lab at Penn State, is now a research scientist in the Bioeconomy Institute at Iowa State University.

Contributing to the research at Penn State were Heather Karsten, associate professor of crop production and of ecology; DeWaunis Kelly, who earned a master’s degree in agricultural and biological engineering; and Lucas de Lima Casseres dos Santos, doctoral degree student in the Department of Agricultural and Biological Engineering.

The research is part of a U.S. Department of Agriculture (USDA)-funded project that seeks an answer to the question: Could Grass2Gas practices give farmers an economic incentive to improve the environmental footprint of their farms? This work was supported by the Agriculture and Food Research Initiative, Sustainable Agricultural Systems Program, in USDA’s National Institute of Food and Agriculture.

Outcomes of this project and the work of others in the region working on anaerobic digestion will be covered in the Anaerobic Digestion on the Farm Conference at Penn State June 9-11.