Hirsh, S. M.1; VanGessel, M. J.2; Beale, B. E.3
1Agent, University of Maryland, Princess Anne, MD, 21853
2Professor and Extension Specialist, University of Delaware, Gerogetown, DE, 19947
3Senior Agent, University of Maryland, Leonardtown, MD, 20650


Herbicide-resistant common ragweed (Ambrosia artemisiifolia L.) is prevalent on Maryland’s Lower Eastern Shore and Southern Maryland. In 2019, common ragweed populations were found to have two or three-way site-of-action resistance on the Eastern Shore. Early-season management of common ragweed is strongly dependent upon reducing weed emergence and controlling ragweed populations prior to soybean planting; therefore this study evaluated the combination of delaying cover crop termination in order to increase cover crop biomass and competition with weeds, and herbicide control. We performed two on-farm trials at two sites with a history of herbicide-resistant common ragweed. The first trial investigated ragweed emergence and growth following various cover crop termination timings (4 Apr, 29 Apr, or at soybean planting) and timings of residual herbicide application (at cover crop termination, at soybean planting, or not at all). The second trial evaluated various residual herbicide treatments applied at soybean planting (Command (clomazone), Linex 4L (linuron), Dimetric (metribuzin), Command + Linex, Command + Dimetric, or Linex + Dimetric). Experiments were in a randomized complete block design with four replications. When herbicide was applied only at cover crop termination, common ragweed was more prevalent in soybean when cover crops were terminated 4 April than 29 April or at soybean planting. Delaying cover crop burndown (“planting green”) and applying herbicide only at soybean planting resulted in lower common ragweed prevalence in soybean than applying herbicide twice—at 4 April cover crop burndown and at soybean planting—regardless of whether residual herbicide was included at planting or not. In addition, there was less common ragweed in soybean when residual herbicide was applied at planting (late-May to early-June) versus at cover crop burndown (early April). In the residual herbicide evaluation trial, at one location, none of the residual herbicides resulted in lower common ragweed prevalence than the control. However at the second location, treatments of Command, Linex, Dimetric, Command + Dimetric and Linex + Dimetric had less common ragweed than the control. Soybean yield was not significantly affected by delaying cover crop burndown or using residual herbicides.

All Accepted Posters