Optimizing Cover Crop and Herbicide Strategies to Diversify Herbicide-Resistant Weed Management in Annual Grain Crops.

As the rise in herbicide resistance increasingly challenges weed management, adopting an integrated weed management (IWM) approach becomes ever more imperative. IWM utilizes knowledge of weed biology to design multi-tactic weed management strategies that effectively control pests and remove selection pressure away from herbicides. In the eastern United States, conservation tillage and cover cropping are growing practices on annual croplands, the latter of which may be utilized as an IWM tool to potentially target all phases of the annual weed life cycle. Thus, optimizing cover cropping and herbicide inputs is an IWM approach of interest in continuous no-till systems. A multi-state project was developed with the objective of integrating cover crop and herbicide strategies to diversify weed management. We hypothesized that: (1) fall-planted cover crops would decrease weed density and size at key times of herbicide application, and (2) integrating cover crops with targeted herbicides would increase overall weed control efficacy. To test these hypotheses, two field experiments were conducted in 2015-2016 and repeated in 2016-2017 near State College, PA, and Georgetown, DE. For the first experiment, cover crops were drill-seeded following small grain harvest and terminated with a burndown mixture prior to planting glufosinate-resistant corn. The cover crop treatments were drill-seeded in the second experiment following corn silage harvest and were terminated prior to planting glufosinate-resistant soybean. Notable first-year results (p ≤ 0.05) showed that at POST-emergence application, including a cover crop treatment resulted in 61% fewer horseweed [Conyza canadensis (L.) Cronq.] and 88% fewer horseweed when a PRE-residual was also applied. Additionally, cover crop treatments decreased horseweed population height by 59% at POST-emergence application. The results suggest that cover crops have the potential to lower weed population density and size, but the extent to which this suppression occurs is dependent upon the species of cover crops being integrated.