Farm Progress

Cahoon used the odds of winning the lottery (1 in 292 million) to illustrate the initial frequency of a herbicide-resistant weed within a population.

Brad Robb, Staff Writer

July 31, 2018

6 Min Read
North Carolina State University’s Dr. Charlie Cahoon talked about the latest in resistant management at the 2018 Milan No-Till Field Day in Milan, Tenn.Brad Robb

The 30th annual Milan No-Till Field Day was the perfect place for Charlie Cahoon to talk about herbicide resistance — despite his being over 600 miles from home. Cahoon knows that widespread adoption of no-till was facilitated in large part by the introduction of herbicides coupled with herbicide-resistant traits like Roundup Ready.

They also proliferated the conservation of soil and water. “If we want to preserve no-till, we must do everything we can to get a handle on herbicide resistance,” says Cahoon, assistant professor and Extension weed specialist: corn and cotton, North Carolina State University. “If we can’t control weeds with herbicides or other tactics, we’ll have to revert back to tillage to control weeds and we will lose the benefits of no-till.”

Cahoon used the odds of winning the lottery (1 in 292 million) to illustrate the initial frequency of a herbicide-resistant weed within a population.

“You may think, like your odds of winning the lottery, your odds of having that herbicide-resistant weed are low; however, one surviving female pigweed per-acre producing 450,000 seeds per-plant with a 5 percent germination rate the following year across a 1,000-acre farm, can mean you would have to deal with 22.5 million plants,” says Cahoon. “My father-in-law says I’ll never win the lottery because I’m too cheap to buy a $2 lottery ticket, but if this scenario comes to fruition on your farm, you essentially just bought $22.5 million in lottery tickets. Your odds at hitting the herbicide-resistant lottery are better than you think.”

Development of Resistance

How can you tell resistance is developing? Cahoon says there are at least four ways:

  • When performance or control is poor on one species while another species is being controlled

  • When the product that once controlled the weed is no longer providing control

  • When, at least initially, poor control is confined to spots in the field

  • (This is the big one.) When you have weeds in the field in close proximity that were similar in size when they were sprayed, and a few of them were controlled and the others were not

In a research project under the direction of Dr. Jason Norsworthy at the University of Arkansas, scientists selected for resistance in a greenhouse by cutting dicamba rates on pigweed. “They wanted to see how fast resistance could occur,” says Cahoon.

“After using sub-lethal rates on two generations, the third generation was resistant to the full rate of dicamba,” says Cahoon. “That proves Mother Nature will win. Dicamba resistance can and will occur, especially if we abuse the technology.”

See also: No-till milestone celebrated at Milan Field Day

Farmers have no control over the genetic mutation that occurs naturally in weed species populations, but they do have control over the “selection” pressure, which is the application of a specific herbicide or herbicides with the same mode of action. For herbicide resistance to occur, a resistant plant must be present in the population, and selection pressure must be placed on that plant. “How long that will take to occur depends on several factors,” adds Cahoon.

How effective is the herbicide? If it is a residual herbicide, how long will it remain in the soil? “The frequency of resistance comes into play. It might not be 1 in 200 million. It might be 1 in a million, which increases the odds of resistance development,” says Cahoon. “Biological factors of the weed also come into play. Palmer amaranth has a lot of genetic diversity and is a prolific seed producer which increases the chances of resistance.”

Integrated Weed Management

The weed science discipline is moving toward a systems approach to weed control. From reducing the intensity of herbicide use (especially those with the same modes of action), to combining chemical control with cultural and mechanical tactics, these options will reduce the chances for resistance development.

“I encourage all growers to maximize crop competition by increasing seed density or changing row spacings to get that canopy closed quickly,” explains Cahoon.

Rotating crops will allow the use of other herbicide portfolios and allow producers to rotate modes of action. Cahoon knows a healthier crop is more able to compete against weeds. Place fertilizers where the crop can benefit from it and not the weeds.

“I know we’re at a no-till field day, but mechanical control could play a vital role if needed,” adds Cahoon.

Many times, weed seeds mature when the crop matures and during harvest they are slung out the back of a combine. Cahoon talked about a relatively new machine, The Integrated Harrington Seed Destructor (iHSD), ( designed by an Australian wheat farmer. It provides harvest-time weed control by grinding weed seeds into a fine powder, preventing germination the next season.

The iHSD is attached to the back of a combine and takes in the chaff stream. “It’s actually a cage mill that crushes weed seeds on-the-go while harvesting,” says Cahoon. “Eventually, these machines will be offered as an integrated unit within the combine.”

Diversity, Reducing and Mixing

“Rotating modes of action and reducing the overall dependence on herbicides is where we can make up the most ground when battling resistance. Always consider diversity in your mode of action choices.

“Rotating crops is great, but you’re not accomplishing anything if you use the same herbicides on corn or beans that you’re using on cotton,” emphasizes Cahoon. “If you rotate to corn from cotton where you used Liberty, use atrazine or HPP inhibitors.”

It is also important to use tank mix partners with overlapping spectrums of control. “If you have Roundup-resistant pigweed in your soybeans and you’re spraying Roundup and dicamba, then you’re relying solely on dicamba to control those resistant weeds,” says Cahoon. “However, if we put out dicamba plus a PPO herbicide (and if PPOs still work on your pigweed), now you’ll have two products in the tank that effectively control Palmer pigweed. That drives our chances of resistance-development way down.”

Cahoon reminds producers to use full rates when spraying small weeds. Using a full-rate on a big weed is the same thing as using a cut-rate on a small weed. “Remember, that’s how they selected for dicamba resistance in that University of Arkansas study,” says Cahoon. “If we abuse it, we will lose it!”

Start clean, stay clean was the take home message Cahoon stressed. Start with a preplant burndown, a post-emergence, plus a residual. A post-emergence at or around planting with a residual again is advised.

“I always get asked, ‘Why do I need a residual at burndown and at planting?’ If it turns off dry after planting and my pre-emergence herbicide applied behind the planter isn’t activated, I’ve got that residual I applied at burndown to carry me a few weeks into the growing season,” says Cahoon. “It’s like hedging when you market your crop.”

Always be willing to pull up a pigweed no matter how far it is out in the field. “If it’s late in the day, you’re tired and you just want to go home, just remember, that pigweed could be the lottery winner,” concludes Cahoon.

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