URBANA, Ill. (U.S.A.) — To meet ambitious U.S. Department of Energy targets for sustainable aviation fuel (SAF), production of purpose-grown energy crops must ramp up significantly. Although researchers have made substantial progress in understanding the management and conversion of these crops, key knowledge gaps hold the industry back. Now, two new studies from the University of Illinois Urbana-Champaign help fill in the blanks for Miscanthus and switchgrass management.

“We have come a long way in our understanding of purpose-grown energy crops for SAF, but we still need to optimize agronomic management practices, like harvesting and nutrient management, to reduce production costs and incentivize growers,” said D.K. Lee, senior author of both studies and professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at U. of I.

Previous studies have been limited in spatial scale or focused on the first years after establishment, early in the perennial life cycle. But Lee’s group is working to provide more realistic, long-term solutions to maximize biomass productivity and grow a more sustainable fuel industry.

In the first of two recent studies, Lee’s team conducted an economic and environmental analysis of two harvest methods for switchgrass.

“Harvesting operations account for 60-80% of the total production costs for switchgrass,” said Muhammad Umer Arshad, postdoctoral researcher in Lee’s group and first author on the Bioresource Technology paper. “We wanted to understand why the harvesting cost is so high and how each operation contributes to cost, energy use, and greenhouse gas emissions, as well as identify where reductions are possible.”

Arshad explains that switchgrass harvesting can happen via the stepwise method, in which tasks like mowing, raking, baling, and roadsiding are separated into individual operations; or the integrated method, which uses different equipment to consolidate mowing and raking into one pass. Hypothetically, an integrated approach could reduce effort, energy consumption, and costs. But, after analyzing data from 125 Virginia commercial-scale sites varying in field size and biomass yield, Arshad found a more nuanced answer.

“We found that the integrated method makes more sense for smaller fields (less than 3 hectares, or 10 acres) and low-yield (less than 3.2 tons per acre) conditions, reducing GHG emissions by 9% and energy use by 5%,” Arshad said. “The stepwise method was better for large fields with high biomass yield, reducing harvesting costs to $37.70 per ton and achieving the lowest GHG emissions.”

The costs were estimated assuming the farmers are using their own machinery, tractors, and harvesting equipment.

Lee adds that the results reveal the importance of tailoring harvesting strategies to site-specific conditions and provide the first evidence-based guidance that harvest methods can improve both economic and environmental outcomes.

In a separate study published in Biomass & Bioenergy, the team tackled age-related declines in Miscanthus biomass yield, a function of tiller (stem) mass and density. These long-lived perennial grasses follow a predictable growth trajectory, including a juvenile stage that builds over several years to reach peak biomass yield, followed by a slow decline after about 10 years. Until now, it wasn’t clear which components of yield change over time and how nutrient management might help.

Postdoctoral fellow Nictor Namoi analyzed data from a long-term Miscanthus trial with nitrogen fertilization treatments that varied in amount and timing. First, he looked at how tiller mass and density changed over time and with various nitrogen treatments. Then, he asked what other soil fertility factors may influence the decline in biomass yield in older stands.

“We found that both tiller mass and density increase from the first year of establishment to the fourth year, and if you apply nitrogen, you get an increase in both factors,” Namoi said. “But over time, as you withdraw nitrogen by harvesting biomass, the first component to be impacted is tiller mass. So tiller mass is very sensitive to nitrogen management.”

Namoi adds that tiller density continues to increase until individual stands become saturated, with no more space for further expansion. After that point, biomass yield is determined by tiller mass.

He notes that the decline in biomass yield over time may not be entirely tied to nitrogen. With every harvest, soil nutrients tied up in plant biomass are removed from the system, depleting elements that play a role in photosynthetic efficiency. When Namoi analyzed soil phosphorus and potassium in mature Miscanthus stands, he found significant deficits in both essential nutrients.

“Our findings identify tiller mass as a key determinant of biomass yield in aging Miscanthus and highlight the need for nitrogen, phosphorus, and potassium management for long-term productivity,” he said.

Both studies provide practical guidance to increase profitability, a key factor for any producer looking to explore biomass crops.

The first study, “Optimizing bioenergy biofuel harvest: a comparative analysis of stepwise and integrated methods for economic and environmental sustainability,” is published in Bioresource Technology [DOI: 10.1016/j.biortech.2025.133288].

The second study, “Soil fertility management for sustainable Miscanthus × giganteus production: Increased tiller weight from nitrogen management explains yield gains in aged miscanthus,” is published in Biomass & Bioenergy [DOI: 10.1016/j.biombioe.2025.108394].

Research in the College of ACES is made possible in part by Hatch funding from USDA’s National Institute of Food and Agriculture. This work was also funded by the U.S. Department of Energy, Bioenergy Technologies Office (DOE-BETO) under Award Number (DE-EE0008521) and the DOE Center for Advanced Bioenergy and Bioproducts Innovation (U.S. Department of Energy, Office of Science, Biological and Environmental Research Program under Award Number DE-SC0018420).