Update:
At the Snow Hill site, common ragweed was counted and measured 10 times (4/14/20, 4/29/20, 5/7/20, 5/15/20, 5/25/20, 6/1/20, 6/15/20, 6/25/20, 7/3/20, 7/8/20). Ragweed was prevalent through the field. Soybean was planted 5/21/20. At the Crisfield site, common ragweed was counted and measured 7 times (4/14/20, 4/29/20, 5/8/20, 5/14/20, 5/29/20, 6/4/20, 6/15/20). Soybean was planted 5/16/20. By mid-June, it was evident that the field did not have a sufficient population of ragweed to merit further data collection and the site was dropped from the study. Wheat cover crop biomass was measured at both sites at the three termination dates—early April, early May, and at planting. Soybean yield was measured at the Snow Hill site using the University of Maryland plot combine on 12/10/20.
Ragweed prevalence data from Snow Hill has been analyzed using analysis of variance (ANOVA) via SAS statistical software. In addition, the aggregated data from the three site-years (Kingston 2019, Rehobeth 2019, and Snow Hill 2020) has been analyzed using ANOVA.

View uploaded report

The project goal was to assess management strategies for reducing herbicide resistance common ragweed (Ambrosia artemisiifolia L.) in soybean. This is very relevant to the Lower Eastern Shore of Maryland and has implications state-wide and region-wide. Common ragweed collected in 2019 on the Lower Eastern Shore of MD (Somerset, Worcester, and Wicomico counties) was found to be resistant to glyphosate (group 9) and cloransulam (Firstrate; group 2 “ALS Inhibitors”) herbicides. Furthermore, three-way resistance to glyphosate, Firstrate and fomesafen (Reflex, group 14 “PPO Inhibitors”) was found for two eastern shore samples from Dorchester Co, MD and Kent County, DE. The objectives of the 2020 project were to 1) Assess the effect of delayed cover crop burndown on herbicide resistant common ragweed emergence and growth, and 2) Assess the effectiveness of various residual herbicide products and timings on herbicide resistant common ragweed emergence and growth.

Objective 1. To test objective one, we assessed the effect of three different cover crop termination timings (early April, early May, or at soybean planting) on common ragweed population. Depending on the treatment, residual herbicide was or was not included along with the burndown herbicide. Results will be discussed for the three site-years in which the trial was repeated (Kingston 2019, Rehobeth 2019, and Snow Hill 2020). When terminated in early April, cover crop biomass was 92 kg/ha, 988 kg/ha, and 803 kg/ha for Kingston, Rehobeth, and Snow Hill, respectively. When terminated in early May, cover crop biomass was 202 kg/ha, 1137 kg/ha, and 1413 kg/ha for Kingston, Rehobeth, and Snow Hill, respectively. When terminated at soybean planting, cover crop biomass was 591 kg/ha, 3347 kg/ha, and 1875 kg/ha for Kingston, Rehobeth, and Snow Hill, respectively (Figure 1). Untreated ragweed populations were significant at all three sites, averaging between 20 and 30 ragweed plants per m2 by June (Figure 2).
Differences of ragweed prevalence among research treatments was assessed through analysis of variance (ANOVA) across the three site-years. The following results are from 30 days after planting. When residual herbicides were not used, herbicide application at planting resulted in lower ragweed prevalence compared to herbicide application in early April. Furthermore, herbicide application in early April plus at planting did not reduce ragweed as compared to a single herbicide application at planting. Cover crop burndown plus residual at planting resulted in lower ragweed than when an early April plus at planting herbicide was applied. This held true regardless of whether a residual herbicide was added at the early April or at the at planting application. Cover crop burndown plus residual at planting resulted in the same amount of ragweed as when an early May plus at planting herbicide was applied. This held true regardless of whether a residual herbicide was added at the early May or at the at planting application. When two herbicide applications were applied, the first early April and second at planting, the timing of the residual herbicide did not change ragweed prevalence (early April vs at planting). When two herbicide applications were applied, the first early May and second at planting, the timing of the residual herbicide did not change ragweed prevalence (early May vs at planting). Finally, two herbicide applications never provided better control than a single at planting herbicide application with or without residuals (Figure 3). Overall, a single herbicide application at planting that included a residual herbicide provided very good control of common ragweed at each of the three sites (Figures 4). At the Kingston and Rehobeth sites, soybean yield did not differ among treatments. At the Snow Hill site, soybean yield was lower for the treatment that had herbicide application early April + application with residual herbicide at planting than for the treatment that had herbicide application early May plus application with residual herbicide at planting or the treatment than had herbicide application at planting.

Objective 2. To test objective two, we assessed the effectiveness of 6 various residual herbicide products on common ragweed emergence and growth. Differences of ragweed prevalence among research treatments was assessed through analysis of variance (ANOVA) for each site-year and across the three site-years. At 30 days after planting, we found no difference between any of the residual herbicide treatments versus the no residual herbicide control. At Kingston and Rehobeth sites, the ragweed prevalence was low across all treatments, including the control, which may have masked significant differences. At the Snow Hill site, the ragweed prevalence was highly variable in the control, which may have masked significant differences. Soybean yield was not different among any treatments at Kingston, Rehobeth, or Snow Hill sites.

Overall, we found that common ragweed emerged in early-May. Delaying cover crop burndown and providing just a single herbicide application at planting that included residual provided very good control of ragweed, and resulted in lower ragweed prevalence than treatments with two herbicide applications that terminated cover crops earlier. We did not find differences among residual herbicides or between a herbicide application with vs without a residual herbicide in the current study.

Research findings were presented at the 2021 Northeastern Plant, Pest, and Soils Conference on 5 Jan 2021 (oral presentation), and via poster presentations at the National Association of County Agricultural Agents Annual meeting and professional improvement conference (29 Sep 2020) and the University of Maryland College of Agriculture and Natural Resources Cornerstone Event (9 Nov 2020). Two articles were published in University of Maryland Extension Agronomy News: Managing herbicide resistant common ragweed in soybean through cover cropping and application of preemergent herbicide (Volume 11, Issue 8) and Proactive measures to combat herbicide-resistant common ragweed (Volume 11, Issue 1).

The study findings are relevant to help Maryland soybean growers to manage herbicide-resistant common ragweed. We found that delaying cover crop termination and providing just one herbicide application at soybean planting time can improve herbicide-resistant common ragweed control. Using just one pre-emergent herbicide application would also result in few input costs.