In recent years, glyphosate-resistant giant ragweed has become increasingly problematic in many soybean production fields in Missouri. As spring approaches, giant ragweed seedlings are usually the first summer annual broadleaf weed to emerge and these seedlings usually do so before the typical time of soybean planting (Figure 1). In some areas, giant ragweed seedlings have emerged already.
Control of glyphosate-resistant giant ragweed prior to soybean planting is critical to the successful management of this species. By eliminating the initial flush of glyphosate-resistant giant ragweed prior to soybean planting, crops gain a competitive advantage and management of in-crop escapes will be more effective. Our research indicates that successful management of glyphosate-resistant giant ragweed is dependent upon the use of an effective preplant burndown herbicide treatment prior to soybean planting. If glyphosate-resistant giant ragweed is not controlled well with an effective burndown herbicide treatment, our research shows that in-crop applications of glyphosate tank-mix partners will provide poor control of this species in Roundup Ready soybeans.
Figures 2 and 3 show results from some of the research we conducted on glyphosate-resistant giant ragweed in 2011. As you can see from these results, glyphosate-resistant giant ragweed control ranged from 28-51% when using standard rates of glyphosate alone, which shows that we cannot use glyphosate alone for the control of this species prior to planting-- especially in locations where glyphosate resistance is suspected. The addition of a growth regulator herbicide, such 2, 4-D or dicamba (Clarity), or the saflufenacil-containing products like Sharpen to the glyphosate burndown treatment provided greater than 84% control of glyphosate-resistant giant ragweed in both locations (Figures 2 and 3). Excellent glyphosate-resistant giant ragweed control was also achieved with preplant burndown combinations of Ignite or Gramoxone plus Clarity (Figure 3). Preplant burndown combinations of glyphosate plus 2, 4-D plus Authority First or Valor SX or Valor XLT provided 98% control of glyphosate-resistant giant ragweed (Figure 2). However, control of glyphosate-resistant giant ragweed was reduced when 2, 4-D Ester was not included with glyphosate and the Authority or Valor products (Figures 2 and 3). Reduced control of glyphosate resistant giant ragweed was also observed with combinations of glyphosate plus Firstrate or Valor XLT due to ALS- and glyphosate-resistance in the giant ragweed populations at these locations (Figure 3).
In Missouri, we have not seen giant ragweed populations with an extended germination pattern as some have reported in eastern portions of the Corn Belt. Ultimately, what we feel this means is that we must eliminate the population that is present prior to planting with multiple, effective herbicide modes of action. If we do so, we are not likely going to see this species as a problem for the remainder of the season.
REVISED: October 1, 2015