As glyphosate resistance stretches across the U.S., the need for alternative chemistries is at an all-time high. Cornfields are likely better off than your soybean fields because most troublesome resistant weeds are post-emergent broadleaves that can often be controlled by herbicides used in corn production systems.
In soybeans, you have only three options for broadleaf weeds: ALS (group 2), glyphosate (group 9), and PPO (group 14) in a Roundup Ready system and glufosinate in place of glyphosate along with ALS and PPO, if using LibertyLink technology.
All three herbicides that are commonly used in the Roundup Ready system have resistance issues. In the U.S., the first ALS-resistant weed was discovered in 1987 (kochia), followed by glyphosate in 1998 (rigid ryegrass) and PPO in 2001 (waterhemp).
Some researchers believe PPO resistance would have developed sooner had Roundup Ready crops not hit center stage in the late 1990s.
“I think, had Roundup Ready soybeans not come along, we were probably two or three years away from having a disaster in PPO resistance, but it got swept under the rug,” contends Patrick Tranel, professor of molecular weed science at the University of Illinois. “When we saw resistance to Roundup the short-term answer was add a PPO, and after switching back, what was swept under the rug came back quickly.”
A fast reaction. Instances of PPO resistance took off quickly across farmers’ fields when more selection pressure was applied. “Our biggest shock this year was how quickly PPO resistance spread,” notes Larry Steckel, University of Tennessee Extension weed scientist. “Next year, counties with resistance won’t get any traction [against weeds] with post-emergent PPO sprays.”
Now is the time to preserve the chemistries that do work. PPOs, even glyphosate, are still effective herbicides in some areas. Good stewardship is critical to ensure the herbicides continue to work.
“If you have resistance to PPOs and glyphosate, my recommendation is to use LibertyLink soybeans,” Tranel says. “If farmers don’t switch to LibertyLink, my advice is to load up on pre-emergent herbicides as much as you can. Even if you use LibertyLink be aggressive with pre-emergent herbicides.”
New technologies are in the pipeline and include Bayer’s Balance GT soybeans, Dow’s Enlist soybeans and Monsanto’s Roundup Ready 2 Xtend soybeans. Balance GT will enable the use of HPPD inhibitors post-emergent, Enlist allows the use of 2, 4-D post-emergent and Xtend permits use of dicamba post-emergent.
Mindful stewardship is key to extending the effectiveness of chemical weed control. Use overlapping effective residuals. Plus, use multiple effective modes of action to slow the spread of resistant weeds. In some cases, you may need to consider mechanical weed control such as tillage or hand-weeding.
“Try to plan as much as possible,” says Dawn Refsell, Valent manager of field development for the Midwest. “My favorite time to plan is right now—while driving the combine. Farmers can see what weeds are out there and plan accordingly.”