Back To Basics On Resistance Management: 3 Key Practices, 5 Tough Weeds

Photo: ©Debra L. Ferguson

Photo: ©Debra L. Ferguson

To keep resistant weeds from making a mess of your fields, weed scientists encourage you to practice these 3 general resistance management practices:

Practice #1

Tank mix or use a premix containing two effective and different modes of action with each application. Admittedly, that may be difficult when dealing with weeds with multiple resistance.

Practice #2

Use residuals to thin the overall weed population and take the pressure off postemergence weed products. With resistant weeds that emerge throughout the season, use overlapping residuals.

Practice #3

Don’t use the same practices and products over and over to control a weed. When you feel comfortable with a strategy, change it. Rotate crops, rotate technologies, take a hard look at cover crops – and, if you have no choice, get personal with weeds. Chop and/or pull escapes.

From there, fine tune your practices for control of these 5 highly competitive resistant weeds – Palmer amaranth, waterhemp species, horseweed, giant ragweed and Italian ryegrass.


Waterhemp is the Energizer Bunny of resistant weeds. It has rapid early-season growth and keeps on emerging and emerging throughout the growing season. It produces up to a million seeds per plant. Its lone weakness is its seed which is relatively short-lived in the soil.

No matter what the trait technology, design preemergence programs that take pressure off postemergence products like glufosinate, dicamba and 2,4-D, says Iowa State University weed scientist Bob Hartzler. “The program can be based on flumioxazin (Valor) and one of the various Authority (sulfentrazone) or Group 15 products (s-metolachlor, acetochlor or pyroxasulfone).”

In Iowa, waterhemp resistance to ALS-inhibitors, glyphosate and atrazine is common, as is resistance to ALS, glyphosate and PPO-inhibitors. Some waterhemp populations in Iowa have four- or five-way resistance.

Hartzler adds that some Iowa growers using tillage have gone to dinitroanilines (Group 3), such as trifluralin, because of the chemistry’s low cost. Hartzler says Group 15 herbicides can be applied postemergence to extend control to later in the season.

Palmer amaranth

Palmer amaranth, the Godzilla of resistant weeds, grows up to 2.5 inches per day, with rapid growth in all stages. That makes control with postemergence herbicides exceedingly difficult. It is extremely competitive, with up to 1 million seeds per plant. Like waterhemp, its seed is short-lived in the soil.

Palmer amaranth has shown resistance to six herbicide classes, but most populations have two-way resistance to glyphosate and ALS herbicides. However, producers are dealing more and more with resistance to PPOs. In Roundup Ready soybeans, there are no postemergence options on Palmer amaranth that is resistant to glyphosate, ALS and PPO herbicides.

That’s one reason weed scientists are concerned about producers leaning too heavily on LibertyLink and Xtend technologies to control Palmer amaranth and waterhemp. In many cases, producers are not taking full advantage of residual herbicides.

“We can talk about how dicamba is doing such a great job of killing our resistant waterhemp or Palmer amaranth, but the fact is we’re going across a lot of acres relying on one mode of postemergence action,” says University of Missouri weed scientist Kevin Bradley. “We need to stop thinking that herbicides are always going to solve our problems. We could be moving toward (metabolic) resistance.”

If you’re unfamiliar with the term, metabolic resistance means that weeds gain an enhanced ability to break down herbicides, even new ones.

Weed scientists recommend managing pigweed species with residuals as the primary herbicides, then use postemergence herbicides to clean up what’s left. With three-way or four-way resistance, residual options are limited to Warrant, Zidua or Dual and PPOs.

Weed scientists also urge farmers to pull out all the stops to deplete the soil seed bank of Palmer amaranth and waterhemp seed. Get personal with weeds, pull them out, chop them out or use practices like narrow windrow burning to destroy seed at harvest.


Horseweed (marestail) emerges from late March through June and from late summer through late fall. It generates up to 200,000 seeds per weed, and windblown seeds disperse easily across great distances.

Horseweed does not tolerate soil disturbances, such as deep tillage or shading from the crop canopy. It has shown resistance to 5 herbicide classes, but most populations are resistant to ALS herbicides and glyphosate.

“For us in Missouri, if we kill horseweed and giant ragweed before we plant our crop, we don’t really have any problems,” Bradley said. This is usually accomplished with fall and spring burndown programs.

A cover crop can also be effective, according to University of Tennessee weed scientist Larry Steckel. Steckel suggests cereal rye with either vetch or crimson clover to increase biomass, or wheat. An effective cover crop will suppress almost 100% of horseweed for Tennessee producers. A cover may provide a benefit for Palmer amaranth as well, perhaps eliminating a late postemergence herbicide application.

Giant ragweed

Giant ragweed emerges from up to 5 inches deep in the soil and is very competitive with Midwest crops. Many populations are resistant to glyphosate and ALS-inhibitors.

Producers can delay planting until after the weed’s emergence, which may eliminate 90% of giant ragweed with seedbed preparation tillage or a burndown herbicide.

Producers who do plant prior to giant ragweed’s emergence might face another problem. When giant ragweed does emerge, there might be a tendency to hold off postemergence applications until other weeds appear. But during this time, giant ragweed can exceed size restrictions for a postemergence application.

Iowa State’s Hartzler recommends planting into a weed-free seedbed, using Valor or an Authority product preemergence for some short-term activity and more flexibility for the postemergence application.

Hartzler says timeliness is critical for controlling giant ragweed postemergence, since there are few options preemergence. He says Enlist soybeans, when approved, or Xtend soybeans are excellent options because dicamba and 2,4-D are highly effective on giant ragweed.

Italian ryegrass

Italian ryegrass can move via contaminated wheat and cover crop seed. It has shown resistance to ALS inhibitors, EPSP synthase inhibitors.

Corn and soybean producers should take out resistant Italian ryegrass with fall residual and spring burndown programs, according to Mississippi State University Extension weed scientist Jason Bond. “The biggest issue with the weed is in corn, where Italian ryegrass can reach 12-24 inches tall at planting,” Bond said.

The fall residual is usually S-metolachlor, a pre-mix of S-metolachlor and metribuzin or S-metolachlor and flumioxazin for broader control.

The development of Italian ryegrass resistance to Select (clethodim) in Mississippi has changed the typical spring burndown program for the weed, according to Bond. He suggests adding metribuzin or atrazine to paraquat to control Italian ryegrass and other spring weeds in corn. In soybeans, add metribuzin or a premix that contains metribuzin to paraquat.