No-till soybean producers are increasingly integrating fall-seeded cover crops into crop rotations as a method for improving soil health. Typically, these farmers are not focused on the weed suppression benefits of cover cropping, but some recent research being conducted here at Penn State is showing the benefit particularly for suppression of winter annual weeds. The Penn State project is focused on demonstrating the potential for cover crop strategies to improve management of herbicide resistant weeds, with particular emphasis on glyphosate-resistant horseweed (marestail). Our three-year project is currently supported by Pennsylvania Soybean Growers and a USDA-CPPM (IPM) grant.

Starting in 2014, field experiments evaluated weed-suppressive strategies using one- and two-species mixes of grass (rye or oats), legume (hairy vetch or crimson clover) and brassica (radish) cover crops. We evaluated the effect of various cover crop mixtures on suppression of horseweed/marestail at the time of a typical burndown herbicide application in the spring before planting soybeans. The study was replicated at research stations at Rock Springs (Penn State) and Georgetown (University of Delaware) in both the 2014-2015 and 2015-2016 growing seasons. In 2015-2016 and 2016-2017, we have extended this research to include additional field experiments at the Penn State and University of Delaware research stations that are evaluating different herbicide strategies in combination with cover crops.

Preliminary Results from our 2014-2015 field studies look promising regarding the potential for cover crops to improve management of horseweed in no-till production systems. Several cover crops and mixtures resulted in at least a 75% reduction in horseweed populations at the time of a spring burndown herbicide application. As other research has also shown, cereal rye alone or in mixture tended to stand out as the most weed suppressive species. Our research also showed that sufficient soil fertility and especially N to allow for vigorous cover crop establishment and growth can help cover crops suppress weeds. In addition to reducing density, cover crops also resulted in decreased horseweed size at the time of burndown applications, suggesting that spraying smaller horseweed plants should result in greater herbicide control and could likely reduce selection for resistant populations. These preliminary results suggest that cover cropping strategies that produce high levels of FALL ground cover, through the selection of the right cover crop species/mixture along with adequate soil fertility, are most effective for suppressing winter annual weeds like horseweed/marestail (Figure 1).



The effect of increasing fall cover crop ground cover (10 weeks after seeding) on horseweed suppression at spring burndown.

Our future work will continue to examine how we can improve cover crop-weed suppression performance by manipulating cover crop species, mixtures, planting date, termination date, and herbicide timing. This research will focus both on winter annual weeds including horseweed/marestail as well as some problematic summer annuals such as the pigweeds.