When it comes to the range of weeds — resistant and otherwise — plaguing Nebraska crop fields, common ragweed may not be the highest on growers' radars. However, a recent study by the University of Nebraska-Lincoln shows common ragweed may pose a greater threat to soybean yields than previously thought.
Ethann Barnes, weed science graduate student at UNL who led the research, notes the study compared different densities of common ragweed planted into soybeans during the 2015 and 2016 growing seasons at the Eastern Nebraska Research and Extension Center (ENREC) near Mead.
LIMITED CONTROL: In a two-year study, Barnes compared control options for resistant common ragweed in Liberty Link soybean and gross profit margins for those control options. This photo, taken in a field near Adams in 2015, shows a resistant common ragweed population sprayed with just a post application of Liberty – with little to no weed control.
This includes the low end of the spectrum, with two common ragweed plants per meter on a 30-inch row. The densest stands, however, were 12 plants per meter. "When you look at a field with two plants per meter, that's still complete coverage of common ragweed. That's not a weed here or there," Barnes says. "With 12 plants per meter, you're looking at a weed every 3 to 4 inches apart."
Emergence makes a difference
In 2015, both densities took their toll on soybean yields. With two plants per meter, soybean yields suffered a 76% reduction. With 12 plants per meter, yields were reduced by at least 95%. However, the following year, the reductions weren't as severe. Two plants per meter reduced yields by 40%, compared to an 80% reduction with 12 plants per meter.
"We attributed that difference to the time of weed emergence. In 2015, the common ragweed we planted emerged a little earlier than the soybean did. It was about a week advantage over the soybean," Barnes explains. "In 2016, it emerged on the same day the soybean did."
Based on Barnes's previous research, 90% of common ragweed emergence usually occurs before the average soybean planting time — typically before the first or second week of May.
So, what can be done to control common ragweed — especially glyphosate-resistant common ragweed — when there are few options for postemergence weed control in soybean?
Options for pre-plant control
During a three-year study, Barnes also evaluated how tillage affects weed emergence. "Sometimes tillage can stimulate emergence of weeds like lambsquarters," he says. "Sometimes just tilling the soil brings seeds from the seed bank to the surface. If you till, it's going to kill the already emerged plants, but it could also create a new flush from the seed bank. We wanted to make sure it didn't do that with common ragweed. We didn't find any indication that tillage would affect the emergence pattern."
"You could have 90% emergence of common ragweed, and do some light preplant tillage to kill already emerged seedlings before planting," he adds. "It's not for everybody, but I think growers need to weigh the potential benefits and the costs of tillage."
In a separate two-year study, Barnes compared control options for resistant common ragweed in LibertyLink soybean and their gross profit margins, including a single preplant burndown application, a preplant followed by a preemergence application and postemergence application, a preemergence application followed by early and late-post applications, and a post-only application.
"The two-pass programs always made the most sense economically as long as you had enough to burndown, the common ragweed is 90% emerged, and you have some residual," he says. "The burndown application was the most important before planting with a little residual, followed by a post application of glufosinate with another mode of action."
While common ragweed can be found throughout the state, it's much more widespread in southeast Nebraska, Barnes notes. Previous studies determining the yield impact of common ragweed have mostly focused on the southeastern U.S. and Canada, where growing conditions and cultural practices vary greatly from those in Nebraska. Meanwhile, common ragweed is often overlooked, especially with other weeds like Palmer amaranth on the radar.
"Common ragweed is a big potential threat, but there are other weeds growers are more focused on," Barnes says. "However, we found out there is a lot more yield impact from common ragweed than we thought."