Weed resistance concerns grow

In January, Iowa State University Extension weed scientist Mike Owen was a keynote speaker at the first-ever Pan American Weed Conference. Experts from universities and industry gathered in Florida to share ideas and information about weeds developing resistance to herbicides. Some 200 people from North and South America attended, exchanging experiences and ideas on sustainable solutions.

What Owen learned reinforced what he’s been recommending all along: Continuous application of Roundup and other glyphosate herbicides must be avoided in order to avoid the development of widespread problems with glyphosate-resistant weeds.

Key Points

• Weed resistance to glyphosate herbicide is increasing in Iowa and elsewhere.

• Changes in weed management must be made now to avoid widespread problems.

• Proactive management will make growers more money, not cost them money.

“We are seeing an increase in resistant weeds across Iowa and the Midwest,” says Owen. “Last summer we ran tests with growers in Iowa, investigating concerns and complaints about poor control of common waterhemp, giant ragweed and others. We confirmed glyphosate resistance in Iowa in two of those species — common waterhemp and giant ragweed.”

The ISU study also documented weed resistance to PPO inhibitor herbicides, such as UltraBlazer, Cobra or Phoenix. “We found more weeds with PPO resistance than I thought we would,” says Owen. “But it makes sense given the herbicide use patterns we see.”

Rotate modes of action

What’s the solution? Use different herbicides? Cultivate to remove weeds from corn and soybeans? More crop rotation? “The way to manage weed resistance is diversification,” he answers. “Anything you can do other than apply the same herbicide year after year will help. If you continuously use herbicides with the same mode of action, it’s inevitable: Weeds will evolve resistance to the herbicide.”

In addition to applying post herbicides with different modes of action, applying an early preplant or preemergence herbicide also helps avoid weed resistance if the herbicide chosen has activity on the weed in question. Problem is growers think they have to spend extra money. “This is a real issue given the big reduction in glyphosate prices for 2010,” notes Owen. “From the perspective of convenience, simplicity and cost, glyphosate will likely remain the popular choice by growers.”

However, data shows if farmers use a residual herbicide applied early preplant or preemergence, they’ll actually make more money than that herbicide will cost. “You also need to make a very timely application of a postemergence herbicide. However, determining exactly when a ‘timely’ application should be made is impossible,” he adds.

For example, most growers think applying postemergence herbicides by the V3 growth stage of the soybean plant is timely. But the data suggest in an average field an application at V3 likely costs the grower approximately $23 in lost yield potential. If the grower waits four or five days after V3, say when beans reach V4 stage of development, they’ve lost about $45 an acre. You can kill the weeds then, but you’ll never get that lost yield potential back.

Weeds compete fiercely with young corn and soybeans for nutrients and water, and the genetic diversity of weeds is far greater than that of corn or beans. A weed’s job is to survive. Weeds are actually more competitive than the crops.

You need to be proactive

Since weeds compete for nutrients, light and moisture, the crop will always suffer if you don’t control weeds early. “Knowing when to remove weeds from a crop varies field to field and year to year, so we can’t predict that very accurately,” says Owen. “But when you go with a total post weed management program, regardless of what herbicide it is, you run quite a risk of losing some yield potential.”

The best way to solve the weed resistance problem is to never allow resistance to evolve. Consider the old adage: If it ain’t broke, don’t fix it. But with weeds, it’s the reverse: If it’s not broken, now is the time to fix it. Because by the time you notice you have resistance, it’ll stay there, thanks to prolific production of weed seed and the long life of weed seed in soil. “Once a weed evolves resistance in a field, you’ve got resistance there forever. That’s why farmers need to proactively manage to keep herbicide resistance from occurring,” says Owen.

This article published in the February, 2010 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.