Getting Into the Weeds on Perennial Forages

Perennial forages like alfalfa, orchardgrass, tall fescue, birdsfoot trefoil, small burnet, and forage chicory are plants that provide ground cover and weed suppression while also serving as livestock feed (baled roughly four times a year). Among PA organic farmers, it's common knowledge that planting a field of perennial forage is a good way to reduce problem weeds but which perennial is best? Is there more weed suppression when planting a greater diversity of these forages? Which weed species are most affected? Is two years in perennial forages enough?  Questions like these could make a huge difference for farmers looking to turn a problem field around, and agronomy Master's student Allie Rice is in the middle of finding answers.  

Rice is working under Dr. Carolyn Lowry conducting research at a long-term agricultural research site that has been running organic cover crop cocktail studies for over 15 years. Her project examines a specific question: if a two-year perennial forage crop is incorporated into an annual corn-soybean rotation, will it reduce weed pressure and density over time? Since the research site is USDA organic certified, the team's focus is on ecological weed control rather than herbicide-based management, using plant biology and diverse crop mixtures to outcompete weeds naturally. 

In 2024, Rice collected baseline data to determine what the weed community looked like before any perennial forages were introduced. The various perennial species and mixes were planted in 2024 and remained in the field for two years. There's a control treatment that stayed in annual crops for the duration. This spring, the forages were tilled under and the fields were planted to corn, so researchers can directly compare weed populations against the original baseline. Researchers examined weed density and community in the subsequent corn to study the potential effects the forages had. Results are expected by early 2027.  

Rice is trying to see if incorporating 2 years of a perennial forage rotation decreases weed seedbank density and how the forages influence the weed community composition. To do this, the team has to compare the viable weed seeds in the soil before and after planting them. They do this by conducting "grow outs" of weed seeds in a greenhouse. They collect 10 random soil cores per plot, remove debris, and spread the soil onto trays in the greenhouse which are then watered to germinate any weed seeds present in the soil. This process takes about nine months, since the soil must be dried, broken up, and rewatered multiple times to fully exhaust the seed bank and ensure all viable seeds germinate. Seedlings are identified and counted at the cotyledon stage when only the first two leaves have emerged, allowing the team to work quickly and pull seedlings before they flower and set seed, which would otherwise compromise the experiment's accuracy. The team is comparing the weed seed bank composition and density from the "annual" and "perennial" sides of each plot, allowing the team to see how different legacies may influence the subsequent corn crop. 

The scale of this work is significant: Just to establish the baseline weed seedbank conditions at the start of the experiment, the team counted and identified 61,000 seedlings across 61 different weed species. The most common species observed were giant foxtail and fall panicum, along with chickweed and yellow woodsorrel. One particularly interesting finding is that the weed seed bank composition doesn't always match what's observed above ground in the field. This research will help to determine if seed bank data can help predict future field populations and inform proactive weed management strategies. 

The project is a collaborative effort: Ella Templeton, undergraduate student in Plant Science, has contributed substantially to the data collection. Rice’s work is one compenent in a greater organic cropping systems experiment led by one of the original creators of the Cover Crop Cocktails project, Dr. Jason Kaye (Distinguished Professor of Soil Biogeochemistry), with  Dr. Carolyn Lowry (Assistant Professor of Weed Ecology and Management), and Dr. Guojie Wang (Assistant Professor of Forage Crop Systems).