Spot resistance

Do you know how to identify glyphosate-resistant weeds in your fields this summer?

“It can be difficult,” says Jeff Stachler, North Dakota State University and University of Minnesota Extension weed specialist. “Glyphosate resistance looks different than previous types of herbicide resistance we’ve experienced.”

Key Points

• Identifying glyphosate-resistant weeds early is important.

• Suspect glyphosate-resistance if a single species survives.

• A mix of nearly healthy, dead and injured plants is a telltale sign.

Here are some tip-offs

Look for these signs of glyphosate resistence:

• Has only one species or just a few species, survived the glyphosate treatment? If several species, especially grasses, have survived, it’s probably not glyphosate resistance. Control failure might be due to poor spray coverage, low application rates, gaps between boom overlaps, environmental conditions, or many other possibilities. However, if only a few plants of a single weed species have survived, glyphosate resistance is likely.

• Are there dead plants next to live plants? Look closely at the ground level. Dead plants will be a brown, shriveled clump of leaves.

• Is there a range of injury to the surviving weeds? If there’s glyphosate resistance, there will be a continuum of responses to the herbicide. Some plants will be dead, others will appear normal, but will be stunted compared to untreated plants, and the remainder will show all types of responses in between.

• Are some of the plants branching excessively? That’s an indication that the herbicide killed the main meristem, but the plant survived and put out branches in an effort to produce seed. This is likely a lower level of resistance.

If you answer “yes” to all of the questions above, glyphosate resistance is extremely likely, Stachler says.

Lab and greenhouse tests are needed to absolutely confirm glyphosate resistance. But such tests are not needed to change your crop rotation or the herbicides you use to head off the spread of glyphosate-resistant weeds.

“The important thing is to be able to identify it early, when you only have a few — maybe as few as five or six — glyphosate-resistant plants in a whole field,” Stachler says.

“The earlier the problem is identified and the faster you respond to the emerging problem with diversified weed management, the smaller the economic losses. Waiting until large areas of the field have dense patches of resistant plants is too late. At that point, yield loss is likely occurring, and changing management strategies can be extremely costly compared to minor adjustments earlier.”

For a video of how to scout for glyphosate-resistant weeds, go to www.ag.ndsu.edu/weeds/herbicide-resistant-weeds.

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RESISTANCE SIGNS: Dead, stunted and healthy weeds of the same species growing side by side indicate the healthy ones may be resistant to the herbicide.

This article published in the May, 2010 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.