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.
