In the Battle of Resistant Weeds, What's the Best Plan of Attack?

As glyphosate resistance stretches across the U.S. the need for alternative chemistries is at an all-time high. Your corn fields are likely better off than your soybeans 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 only have three options to control 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 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 come sooner had Roundup Ready crops not hit center stage in the late 1990s. Glyphosate eased selection pressure on PPO herbicides because fewer PPOs were used when glyphosate was so effective.

“I think, had Roundup Ready soybeans not come along, we were two or three years away from a disaster in PPO resistance, but it got swept under the rug,” says 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.”

Using PPO herbicides was an easy answer when glyphosate’s efficacy decreased. By that time, though, weeds had already started to build resistance from previous use. Instances of PPO resistance took off quickly when more selection pressure was applied. 

Waterhemp has the most widespread resistance, currently running rampant in six states: Arkansas, Illinois, Iowa, Kansas, Missouri and Tennessee. Resistant ragweed is found in Delaware, Ohio and North Carolina, and resistant palmer amaranth is found in Arkansas, Tennessee and, as of September 2016, Illinois.

“Our biggest shock this year was how quickly PPO resistance spread,” says 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.”

When dealing with waterhemp and palmer amaranth, it’s easy to tell if a PPO worked or if a weed could potentially be resistant when you’re scouting. If it’s susceptible the top leaves will burn and fall off—if resistant the plant might mottle at the top, but it will grow out of it, Steckel explains.

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. 

As PPO resistance spreads, be aware of what weeds are resistant in your state and surrounding states. Be vigilant while scouting to recognize if weeds in your fields might be displaying resistance.

However, areas with resistance to all three common soybean herbicides have limited options until new herbicide systems are developed and approved. Multiple modes of action paired with the remaining effective chemistries will help slow the resistance situation. You might also have to consider mechanical weed control such as tillage or hand-weeding.

“If you have resistance to PPOs and glyphosate, my recommendation is to use LibertyLink soybeans,” Tranel says. “If you 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.”

If you’ve seen resistance in your fields, you’ll likely have a large resistant weed seed bed as well, which will increase pressure on the LibertyLink system. Be mindful of good stewardship practices when using LibertyLink soybeans—it’s a technology that can quickly become ineffective if overused or used incorrectly.

This year, 40% of the soybeans planted in west Tennessee were LibertyLink, Steckel says. “We need another technology to mix in and preserve Liberty’s shelf life.”

New technologies awaiting final steps of approval 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. Remember the easiest stage to kill a weed is before it reaches 4" tall. Use overlapping, effective residual herbicides and don’t just kill a weed once, use multiple, effective modes of action to slow the spread of resistant weeds.

“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. You can see what weeds are out there and plan accordingly.” 

With herbicide resistance intensifying across the U.S., it’s important to understand what herbicides work in your area and how you might need to adjust in the future. Identify resistances in your fields and manage accordingly.

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