“While resistance will not cover the landscape in the next four to five years, it will start showing up more frequently.” —Bob Hartzler ISU agronomy professor

Taking advantage of the elevated view from a combine cab can help a grower assess the effectiveness of his 2011 weed control program.

Agronomists say this is especially important as more weeds develop herbicide resistance.

“Cases of herbicide resistance can be found in fields across Iowa, and this poses a bigger threat than ever before,” said Bob Hartzler, a professor of agronomy at Iowa State University. “While resistance will not cover the landscape in the next four to five years, it will start showing up more frequently.”

Herbicide-resistant populations generally do not infest an entire field over the course of a single growing season.

Rather, these populations usually begin as a small number of plants that survive to produce seed, said Aaron Hager, a University of Illinois Extension weed specialist.

If a herbicide-resistant waterhemp population is present or suspected in a particular field, consider harvesting that field last to prevent introducing the seed from the resistant plants into the combine and their subsequent movement to other fields, said Hager, who noted that female waterhemp plants have the potential to produce in excess of one million seeds per plant.

Tracking the spread

In the past 14 years, glyphosate has been the herbicide of choice for many farmers, but this has turned into too much of a good thing. In the United States, there are 12 glyphosate-resistant weed species. A number of these can be found in Iowa, including marestail, waterhemp and giant ragweed.

While seed movement is one way that glyphosate resistant weeds can appear in fields where they currently are not a problem, it’s not the only way, Hartzler noted.

GR weeds can also appear from independent selection of a resistant biotype from the weed community already present in the field.

Herbicide-resistant biotypes are present at low frequencies within the weed community prior to the discovery and introduction of a herbicide, Hartzler said.

Repeated use of an herbicide results in resistant biotypes becoming more prevalent.

Eventually they may become the dominant biotype, meaning the weed is no longer effectively controlled by the herbicide.

Diverse control

Now that GR biotypes have evolved in isolated fields across the state, the risk of resistance appearing in “clean” fields is probably as great due to movement from fields with existing GR problems as it is from independent selection.

Does the threat of resistance movement via gene flow mean it is futile for farmers to prevent GR from appearing in their fields?

No, Hartzler said.

“In fact, it places greater emphasis on the value of diversified weed management systems.

“This greatly reduces the benefit of the GR trait to weeds, and reduces the likelihood of resistant biotypes becoming established in fields that are currently free of the resistant trait.”

The key to managing resistance is to rely on multiple mechanisms of action, he said.

Herbicide labels now include a standardized system to explain each product’s MOA, which eliminates the need for farmers, crop consultants and ag suppliers to learn the MOA of all the active ingredients used in agriculture today.

Pre-emerge herbicides should be viewed as foundation products in a weed control program for soybeans, because 80 to 90 percent of the time a grower will need to use a post-emerge product to achieve season-long weed control.

“Unfortunately, pre-emerge products for soybeans are not nearly as good as pre-emerge products for corn,” Hartzler said.

“When selecting any pre-emerge product, choose a product that’s active on the weeds in your fields. Also consider any existing resistance problems, crop safety, carryover risk and cost.

“Since this isn’t a one-pass weed control system, you may not need to use a ‘Cadillac’ pre-emerge product,” Hartzler said.

Generally, the greater number of MOAs used, the less selection pressure is placed on weeds.However, designing an integrated program is not as simple as randomly adding MOAs, said Hartzler. He recommends hitting weeds with a variety of MOAs over a three- to four-year period.

The different MOAs used in the program must have good activity on the important weeds in the field to successfully reduce selection pressure.

“You have to know what weeds individual herbicides control,” Hartzler said.

Escape artists

Earlier this year, there was a great deal of concern about waterhemp and marestail surviving glyphosate applications made in late June and early July.

ISU Extension specialists are convinced that a significant percentage of these failures are linked to glyphosate-resistant biotypes in the field.

There aren’t a lot of options when growers get into these situations, Hartzler said. If glyphosate failed earlier to control the weeds, it is unlikely that a repeat application will do any good in controlling the surviving weeds.

Although it’s not popular with the majority of growers, mechanical control is really the only available option to manage escaped and/or herbicide-resistant waterhemp and marestail at a certain point in the growing season.

That’s why it’s so important for farmers to manage herbicide resistance.

“Weeds will develop resistance to any herbicide we rely on too heavily,” Hartzler said. “We need to use herbicides more wisely than we have in the past to be sure we have valuable weed control tools available to us in the years to come.”

You can contact Darcy Dougherty Maulsby by e-mail at yettergirl@yahoo.com.