No chemical solution for resistance

There is no chemical solution to the problem of herbicide resistance, says University of Arkansas weed scientist Jason Norsworthy.

Instead, industry should focus on integrating weed management techniques to combat resistance.

“Otherwise, we’re going to go through herbicide after herbicide, and we are quickly approaching the day where we will have no herbicide solutions,” Norsworthy told a recent herbicide resistance conference in Saskatoon.

Norsworthy, who studies herbicide resistance in the United States, said new herbicides are needed to combat ever evolving weeds, but herbicides alone will not stop the development of resistant weeds that result in massive costs to growers.

“Thinking that a herbicide alone will solve our problem is not the way forward,” Norsworthy said.

The cost of chemical inputs skyrocket once herbicide resistance evolves to the point where producers are forced into a resistance management strategy.

American producers are spending US$40 to $150 per acre because of glyphosate resistance.

“If we take a look at the cost over the past 12 to 15 years, we’ve seen a 3 1/2 fold increase in seed prices,” Norsworthy said.

“We’ve also seen an increase of about three to 3 1/2 fold in the price of herbicides associated with managing this resistance.”

Norsworthy said a herbicide resistance treadmill has undermined the effectiveness of herbicides.

“Roundup Ready corn followed by Roundup Ready soybean followed by Roundup Ready cotton is not a rotation.”

ALS inhibitors were widely used in the U.S., but weed seed counts in the soil soared once this chemistry stopped working. As a result, glyphosate was brought in to control the weeds, and there was a tremendous risk for resistance evolving.

“Glyphosate controlled a lot of these weeds, but we had a few escapes, and those few escapes were ultimately resistant, and in hindsight it did not take us long before we lost that Roundup Ready technology.”

PPO inhibitors were deployed to manage resistant weeds once glyphosate began to lose its effectiveness, but again the fields had high weed seed counts in the soil.

“We brought in the PPO chemistry at that point to solve our resistance issues, and what did we have this past year, we had PPO resistance,” Norsworthy said.

“I don’t know what the next herbicide is going to be, but when you have high soil seed banks and you lose a herbicide, you’re setting yourself up for failure.”

Producers can reduce the weed seed count in the soils by diversifying their crops, including planting spring and fall seeded crops, increasing their seeding rate, decreasing row spacing, bringing some tillage back into their cropping system and trying to avoid spreading weed seeds with their combine with the use of technologies such as the Harrington Seed Destructor.

“It doesn’t make any sense to me to see weeds come into a combine and actually be dispersed back over the field. Why do we not try to capture those weeds and destroy those weeds?” Norsworthy said.

Using multiple modes of action when spraying is another aspect of an integrated weed management system. However, Norsworthy said growers need to be careful when using some products that advertise multiple modes of action.

“Products can have a broad leaf material and it can have a grass material that’s mixed, and at the end of the day that’s not two effective modes of action on a particular weed; that’s only one effective mode of action,” Norsworthy said.

As well, industry tends to reduce the rate of both products so that it can keep the cost comparable to other products in the market rather than doubling it.

“The problem with that is if you have resistance to one of those herbicides, the other product is now being applied at half rate,” Norsworthy said.

“It’s not going to be effective, and actually what it’s going to do is it will select for what we call low dose resistance over a very short period of time.”

It’s commonly believed that the American experience in dealing with herbicide resistance is far worse than the Canadian experience. However, Norsworthy said that on a per acre basis, herbicide resistance pressure is comparable on both sides of the border.

robin.booker@producer.com

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