What Is The Situation With Herbicide Resistant Weeds in Iowa?

What Is The Situation With Herbicide Resistant Weeds in Iowa?

Iowa farmers have dealt with weeds resisting certain herbicides before; now add glyphosate-resistant species to the list.

Farmers in Iowa have dealt with herbicide resistance in weeds for years, for example, atrazine and the ALS inhibitor products. But Iowa farmers have been spared the woes of glyphosate resistant species -- until now. Mike Owen, Iowa State University Extension weed scientist, gave an update on the situation at the annual Integrated Crop Management Conference at ISU in early December.

Q: What is the situation for herbicide resistant weeds in Iowa?

A: This is not something new. We've had herbicide resistant weeds for probably 30 years in Iowa, with some of the older herbicides such as the triazines and ALS inhibitor herbicides.

Recently, we've begun to see populations of giant ragweed, common waterhemp and very likely marestail that involve resistance to glyphosate. This isn't surprising. These resistant biotypes exist in states surrounding Iowa and we are now finally documenting the occurrence of these glyphosate resistant weeds here.

Q: How widespread is the problem of glyphosate resistant weeds in Iowa?

A: These are not widely spread. But looking to the future, they can be and may likely be unless growers begin to change how they manage weeds in glyphosate resistant crops.

The concept of the convenience and simplicity of using glyphosate weed control systems is admirable but from an evolutionary perspective, convenience and simplicity likely enhance the speed at which these weeds will change from being primarily sensitive to glyphosate, to being primarily resistant to glyphosate.

Q: So what do you recommend corn and soybean growers do?

A: Growers need to think about using alternative herbicides, particularly those herbicides that have activity on these weeds that are showing the resistance to glyphosate. And using the herbicides that have residual activity on these weeds, such as a soil-applied, preplant or preemergence herbicide.

Farmers need to look at their different application timings. We at ISU make a broad, across-the-board recommendation for corn and soybeans that a residual herbicide be applied early preplant. This should be used irrespective of what the crop may be--whether it's a genetically-modified crop or a conventional crop. We think using a preplant or preemergence treatment and then coming back with glyphosate postemergence later on makes good sense from a yield protection perspective. It certainly makes weed management that much more effective.

Q: What do you tell farmers who don't want to apply a preplant or preemergence treatment? They believe it is cheaper to skip the soil applied treatment and simply spray glyphosate over the top of the crop later on.

A: As it turns out, using a soil-applied early preplant herbicide really improves the profitability for growers- despite their concerns that they don't want to pay for that extra herbicide treatment. The fact is, they get better weed control, and earlier weed control, which enhances their odds of getting a better yield.

The attitude of skipping the preemergence or preplant treatment is a concern that weed specialists have because of the announced reduction in the cost of glyphosate. Some companies have significantly reduced the price of their glyphosate products. We fear growers are going to go back to thinking "glyphosate is cheap, that's all I'm going to use." If that's the choice growers make, if they opt for convenience and simplicity, it is inevitable that they will have problems with glyphosate resistant weeds and it is inevitable that they will be likely to lose yield potential.

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