Beat Weeds to the Punch

How do you prevent herbicide-resistant weeds from cornering your field? One effective solution, besides keeping careful records of weed outbreaks, is to rotate the herbicides you use.

By: Darrell Smith, Farm Journal Conservation and Machinery Editor

As a corn producer, you follow a scouting plan, a hybrid selection plan, a harvest plan and a marketing plan. Now you need a herbicide plan.

Why? Resistant weeds have entered the ring. For many farmers, resistant weeds rank among the leading rivals to a yield hike. According to a survey by the Weed Science Society of America, weeds have evolved resistance to 21 of the 25 known herbicide sites of action and to 148 different herbicides.

Resistance Defense Plan

  1. Scout for weeds
  2. Keep records
  3. Rotate herbicides based on:
  • Active ingredients
  • Modes of action
  • Sites of action
  • Recropping intervals

"Herbicide resistance is random genetic mutation, which happens in the wild all of the time," says Isaac Ferrie with Crop-Tech Consulting. "When you spray the same herbicide over and over in the same field, you select for resistant plants. The resistant plants will be the only ones that survive. That’s like natural selection on steroids."

Almost every day of the growing season some farmer, somewhere, is discovering resistant weeds in his fields (or he would if he knew what to look for). "It’s not unusual for a farmer to find weeds that show no effects at all after three applications of glyphosate," Ferrie says. "That’s resistance resulting from too many applications of the same product."

In his home state of Illinois, Ferrie says 90% of soybean acreage and more than 70% of corn acreage is planted to herbicide-tolerant varieties. Because there are only a few herbicide-tolerant traits available, that suggests some of the same herbicides are being applied again and again—a recipe for resistance.

"Sound practices can’t keep resistance from occurring, but they can keep it from spreading," Ferrie says. "That’s why you need a herbicide plan—to tell you which herbicides you will apply on each field next season."

Sprayed weed photo for herbicide managment plan story

This common cocklebur shows whitening or bleaching of foliage, characteristic of Impact herbicide, a pigment inhibitor.

Get a plan in place. Your herbicide management plan (which you could call your resistance defense plan) involves scouting, record keeping and understanding herbicides’ active ingredients, modes of action, sites of action and re-cropping intervals. Planning must start the summer before because herbicide management will impact your hybrid and variety selections the following year.

Accurate weed scouting lays the foundation for herbicide management. "As you scout fields and harvest, map where patches of weeds are present and record what species they are," Ferrie says. "Stop the combine and take a close look. Carry a scouting manual.

"The patches of weeds you find might be herbicide-resistant species. As you plan for next season, review what herbicides were applied on that field and for how many years you applied the same one. Then rotate to a different product," he recommends.

Using one herbicide on all of your acres is no longer an option, Ferrie says. Your herbicide plan will let you select a herbicide to control the weeds in each field, while rotating products to prevent resistance from establishing.

Check the label. To effectively rotate herbicides, you need to understand their modes of action, sites of action and active ingredients. "Herbicides are grouped by mode of action based on how they affect the plant," Ferrie explains. "Examples include inhibiting amino acid synthesis or disrupting cell membranes.

"Within each mode of action, herbicides can be further divided into various sites of action, based on which pathway inside the plant each herbicide uses to inflict damage. Within each site of action are different chemical families, and within each family are different active ingredients." (Click here to see the table for examples.)

The active ingredient is the chemical that causes a fatal reaction in plants. Herbicides with different trade names might contain the same active ingredient. For example, Roundup, Touchdown and other products contain glyphosate.

"If you only look at the trade name and fail to consider the active ingredient, you could apply the same product without knowing it," Ferrie explains.

Change the target. But herbicide rotation involves more than choosing a different active ingredient. Herbicides actually become resistant to the site of action, not just the active ingredient.

"When a weed randomly genetically mutates, the site of action—the pathway the herbicide uses—can change," Ferrie says. "If the site of action changes, all of the herbicides that use that site of action will no longer be effective."

Entire chemical families of products use the same site of action. "To prevent resistance from developing, you must vary the site of action from year to year and when you make multiple applications in one season," he says.

The same strategy applies to tank-mixes. "When tank-mixing products, you’ll get more effective control if you use products with different sites of action," Ferrie notes.

"If you mix a fast-acting contact herbicide, such as paraquat, with a slower acting systemic herbicide such as glyphosate, the paraquat might kill the top growth before the glyphosate translocates from the leaves to the roots or growing point. If the paraquat doesn’t kill the weed, the glyphosate will add little to the mix," he explains. "The reason tank mixes of glyphosate and 2,4-D are so effective is that the two herbicides have different sites of action and similar reaction times, so they work together"

Read the Label

read label

A site of action group number is listed on most herbicide labels.

It’s on the label. Herbicide labels tell you what you need to know to effectively rotate products. Along with the active ingredient, most products now display the site of action group number, a numbering system developed by the Weed Science Society of America.

"The site of action group number is usually on the label inside a black box," Ferrie says. "Using products with different group numbers assures you they have different sites of action."

Because of late-season rescue treatments, fall applications to control winter annuals and the use of cover crops, your herbicide plan must consider re-cropping intervals. For annual ryegrass and radishes, some soybean herbicides have an 18-month re-crop interval.

A dry fall might lengthen the re-cropping interval because micro-organisms won’t be able to decompose herbicides in the soil. If you’re in doubt when planting nears, have a soil sample tested for herbicide residue or conduct a grow-out test by planting oats in a pan of soil.

Keeping resistant weeds under control or preventing them from developing requires significant changes. You might need to apply conventional herbicides. "To diversify our herbicide portfolio, we’ll have to use some chemistries contained in older herbicides that haven’t been used frequently for some time," Ferrie says.

Make sure you understand the product before you use it. "As farmers begin using older products with which they are no longer familiar, we could see a flurry of misapplication, overapplication and drift issues," Ferrie notes.