By Blake Jackson
The U.S. dairy industry faces challenges in reducing agricultural greenhouse gas emissions, particularly methane released through manure storage and handling.
According to Jeff Mulhollem at Penn State College of Agricultural Sciences, anaerobic digestion offers a potential solution by converting cow manure into biogas for electricity generation or renewable natural gas production. However, high construction and operating costs have limited adoption on many farms.
Researchers at Penn State have found that combining manure with plant biomass could increase methane production and improve the financial viability of farm-scale anaerobic digestion systems. Their findings, published in Biomass Conversion and Biorefinery, identify winter rye as a particularly promising feedstock for producing renewable energy.
“Anaerobic digestion can help dairy farms manage manure while producing renewable energy, but many projects struggle economically,” said study senior author Juliana Vasco-Correa, an assistant professor of agricultural and biological engineering at Penn State.
“A key question is whether adding crop residues or dedicated biomass to manure can make these systems more financially viable. The results of our study demonstrate that co-digestion with plant material can enhance the economic performance of manure-based anaerobic digestion.”
The research team modeled a 1,000-cow dairy farm in the northeastern United States, comparing manure-only digestion with systems that combined manure and switchgrass, winter rye or corn stover.
Switchgrass was included for its ability to grow on marginal land and support soil health, while corn stover provided an agricultural residue option. Winter rye offered an additional advantage because it can grow between cash crops without requiring separate acreage.
Researchers evaluated two energy pathways: electricity generation and renewable natural gas production. Winter rye emerged as the most promising option because of its higher biogas yield.
The economic analysis found that combining manure with plant material could substantially reduce the financial support required for a digester project to break even.
The necessary capital assistance, typically provided through grants or loan guarantees, fell from approximately 47% for manure-only digestion to as little as 4% under the best-performing co-digestion scenario.
Study first author Camila Valderrama, who recently earned a master's degree in agricultural and biological engineering from Penn State, highlighted the importance of matching system size with the intended energy product.
“Larger systems improved the economics of renewable natural gas but could make electricity production less attractive, showing that scale and end use need to be considered together,” Valderrama said.
Vasco-Correa noted that the modeling framework can be adapted to different farm sizes, feedstocks, locations, and market conditions. The researchers also made their models, code, and datasets publicly available to support further analysis.
“The study shows that improving agricultural bioenergy deployment is not simply a matter of maximizing methane production,” Vasco-Correa said. “Technical performance, farm scale, biomass supply, energy markets and policy incentives all have to work together for a project to succeed.”
The findings suggest that selecting suitable biomass and evaluating energy markets together could help dairy farms reduce emissions while improving the economic prospects of renewable energy projects.
Photo Credit: penn-state
Categories: Pennsylvania, Livestock