Update:
Common ragweed was counted and measured in the cover crop trial and residual trial on the two farms through July 2019. Soybean yield measurements were taken using a plot combine on 24 Oct 2019. Data from year one was analyzed and results shared at the 2020 Northeastern Plant, Pest, and Soils Conference, Philadelphia, PA, in January 2020. The project has been funded for the 2020 season. The cover crop trial and residual trial treatments will remain the same as in 2019. The 2020 trials are underway at two cooperating private farms. One farm is located in Crisfield (Somerset Co); the second farm is located in Snow Hill (Worcester Co). Both fields had a wheat cover crop and a history of herbicide-resistant common ragweed.

View uploaded report

Herbicide resistant common ragweed (Ambrosia artemisiifolia L.) continues to be a problematic weed on the Lower Eastern Shore of Maryland. Because 95% of common ragweed is expected to emerge by early May (Myers, Curran, VanGessel, et al., 2004), we hypothesized that delaying cover crop termination in the spring in order to increase cover crop biomass and competition with weeds would reduce ragweed emergence and growth (Objective 1). In addition, later emerging common ragweed escapes can be difficult to control post-soybean planting. We hypothesized that applying residual herbicides, pre-soybean planting can reduce ragweed prevalence during the soybean growing season (Objective 2).

Objectives and key findings:
Objective 1: Assess the effect of delayed cover crop burn-down and residual herbicide application timing on herbicide resistant common ragweed emergence and growth.
The cover crop biomass did increase with later cover crop planting dates; however, the overall biomass accumulation, even for the “planting green” treatment, was lower than ideal for providing competition with weeds. At site 1, cover crop biomass was on average 92, 201, and 592 kg ha-1 for 4 April, 1 May, and 22 May dates, respectively. At site 2, cover crop biomass was on average 988, 1137, and 3347 kg ha-1 for 4 April, 1 May, and 3 June dates, respectively. In the untreated plots (only burn-down 4 April), common ragweed populations were >20 ragweed plants per m2 and >30 cm tall by July (Figure 1). When herbicide was applied only once at cover crop burn-down time, there was higher common ragweed prevalence when cover crops were terminated 4 April than 29 April or soybean planting (Figure 2). When comparing ragweed prevalence pre-soybean planting, for 1) cover crops terminated April 4 vs 2) cover crop still present, at site 1 there was less ragweed where the cover crop was present, but at site 2 there was the same amount of ragweed regardless of cover crop or not. Therefore, the effect of cover crop termination date may be more a result of the herbicide timing rather than the cover crop competition. Applying herbicides twice, 4 April and at soybean planting, did not reduce common ragweed in soybean as compared to just applying herbicide at soybean planting. However, including a residual herbicide at soybean planting resulted in lower common ragweed prevalence in soybean (Figure 3). Additionally, there was lower common ragweed prevalence in soybean when residual herbicides were applied at soybean planting than at 4 April burn-down (Figure 4). Soybean yield did not differ among various cover crop burn-down and residual herbicide application timings, with the exception of the control treatment (only 4 April burn-down) having lower yield at site 1.

Objective 2: Assess the effectiveness of various residual herbicide products on herbicide resistant common ragweed emergence and growth.
At site 1, the residual herbicide products of Command, Linex, Dimetric, Command + Dimetric, and Linex + Dimetric, resulted in lower ragweed prevalence in soybean than the no residual herbicide control. Residual herbicides did not result in reduced common ragweed at site 2 (Figure 5). Soybean yield did not differ among the no residual and residual herbicide treatments. However, reducing later season common ragweed emergence with residual herbicides can be very important for minimizing seeds in the weed seed bank. Furthermore, the presence of weeds at soybean harvest (even if yields are the same) can lead to harvest difficulties and contaminated soybean.

Objective 3: Screen populations of common ragweed in the Lower Eastern Shore for resistance to the herbicide families of glyphosate, ALS-herbicides, and PPO-herbicides.
In 2019, common ragweed samples from Somerset, Worcester, and Wicomico counties, MD were tested and 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 (VanGessel et al, unpublished data).

Results are preliminary as they are from only one growing season (two site-years). We will repeat the cover crop timing and residual herbicide studies in the 2020 growing season at two additional sites. This will provide us with four site-years of data spread over two growing seasons.

Deliverables:
Presentations at grower meetings:
• Hirsh, S. Oral presentation. Identification and management of herbicide-resistant common ragweed. Herbicide-Resistant Weed Field-day. Farm. Salisbury, MD. 25 Oct 2019. 2 hrs. 29 attendees.
• Hirsh, S. Poster presentation. Managing herbicide resistant common ragweed emergence and growth in soybean. Maryland Commodity Classic. Centreville, MD. 25 Jul 2019.

Presentations at professional meetings:
• Hirsh, S. 2020. Oral presentation. Managing herbicide resistant common ragweed (Ambrosia artemisiifolia L.) emergence and growth in soybean. 2020 Northeastern Plant, Pest, and Soils Conference. Philadelphia, PA. 9 Jan 2020.
• Hirsh, S., M. VanGessel, B. Beale. Anticipated 2020. Accepted poster presentation. Managing herbicide resistant common ragweed emergence and growth in soybean. NACAA (National Association of County Agricultural Agents) Annual Meeting and Professional Improvement Conference. Virginia Beach, VA. Anticipated September 2020.

Publications:
• Hirsh, S. Proactive measures to combat herbicide-resistant common ragweed. Agronomy News. Volume 11, Issue 1. April 2020. https://extension.umd.edu/sites/extension.umd.edu/files/_docs/Agronomy%20News%20Vol%2011%20Issue%20%231%20April%202020.pdf