Giant Ragweed Still Looms Large

By Alyssa Essman

Each fall just before harvest, the OSU weed science program conducts a statewide driving survey evaluating the frequency and distribution of problematic weed species in Ohio. Diagonal transects are driven through the top 45-50 soybean producing counties. Visual ratings are given for ten weed species in each soybean field encountered. The weeds evaluated during this survey were: marestail, giant ragweed, common ragweed, waterhemp, Palmer amaranth, redroot pigweed, volunteer corn, common lambsquarters, grasses/foxtail spp., and velvetleaf. In 2022 over 4200 fields were surveyed. Roughly 57% of fields were clean, or at least free of the ten weeds evaluated. The most common weed in 2022 was giant ragweed, present in 12% of fields when combined across rating levels. Waterhemp was the second most frequent weed, in 11% of fields, followed by marestail in 10% of fields. Grass/foxtail spp. were found in 9% of fields and volunteer corn in 8% of fields.

Giant ragweed continues to be one of the most common and troublesome weeds in Ohio. It has a fast growth rate and is an extremely competitive plant. One of the first weeds to emerge each spring, giant ragweed can germinate through early summer. Continuous no-till practices and comprehensive herbicide programs can reduce populations over time. Ohio giant ragweed populations have been identified with resistance to group 2 (ALS inhibitors) and group 9 (glyphosate) herbicides, and multiple resistance to both group 2 and 9 herbicides. Resistance to these herbicides decreases control options for giant ragweed, especially in non-GMO soybeans. Effective giant ragweed control programs include a combination of herbicide modes of action and both pre- and postemergence applications. Weed scientists from OSU, Purdue, and across the corn belt have some general recommendations for management of giant ragweed:

• Effective burndowns reduce giant ragweed pressure at the time of planting. Examples of effective burndowns include a group 4 (2,4-D or dicamba) herbicide plus either a group 9 (glyphosate) or 22 (paraquat) herbicide. Check labels for restrictions on plant-back intervals for 2,4-D and dicamba.
• An effective residual product with the burndown application or at plant can reduce population pressure through the time of the post application. Full rates of chlorimuron or cloransulam (group 2) containing products tend to be most effective. Where giant ragweed is resistant to group 2 herbicides, fomesafen (group 14) can be used, but can be more variable and will restrict fomesafen use postemergence.
• Giant ragweed will likely require multiple postemergence applications. Two pass programs should include an initial application based on weed size followed by a second application 3-4 weeks later.
• Soybeans tolerant to glufosinate, dicamba, or 2,4-D can receive applications of these herbicides postemergence. Glufosinate followed by glufosinate is an option in the LibertyLink system. In the Xtend or Enlist systems, the second application may need be a group 14 herbicide (or glufosinate for Enlist) based on label restrictions for application timings.
• In non-GMO soybean production, group 14 herbicides (fomesafen, lactofen) can be used postemergence. Control can be variable and overuse increases selection pressure for resistance. OSU research has shown that fomesafen followed by lactofen 3-4 weeks later is the most effective approach.

For more recommendations regarding the management of giant ragweed, visit the Management of Herbicide-Resistant Giant Ragweed fact sheet or the Giant ragweed section in the “Control of Problem Weeds” portion of the Weed Control Guide [ANR-789].

A huge thanks goes to Tony Dobbels, Anna Skubon, and Axle who spent a great deal of time this fall looking at soybean fields. The 2022 survey would not have happened without this crew!