Farm Progress

How to slow down weed resistance? Here’s a look at why farmers need to practice zero tolerance with weed seed, plus why one typical herbicide program won’t work.

Jill Loehr, Associate Editor, Prairie Farmer

April 28, 2017

3 Min Read
NEMESIS CONTROL: Farmers who use 2.5 sites of action on average in their herbicide programs are 83 times less likely to develop herbicide resistance compared to 1.5 sites of action on average, says Dane Bowers, technical product lead at Syngenta.

What now with weed resistance?

“We’re not going to be able to spray our way out of this problem,” says Bob Hartzler, an Iowa State University weed scientist. “As long as we rely so heavily on herbicides, weeds are going to adapt. We need to find other ways to help manage weeds.”

But can we stop weed resistance?

“I think we can help slow down the way weeds adapt to herbicides,” Hartzler says. He offers three short-term and three three long-term strategies to fight herbicide resistance.

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NEW TOOLS? “Never doubting technology is a good philosophy,” says Bob Hartzler, referring to new technology like seed disruptors and weed seed cleaners. “Never doubt the pest, either.”

Short-term strategies:

1. Take a refresher course on weed biology. Weed management programs should factor in emergence patterns, Hartzler says. “Waterhemp has a very prolonged emergence pattern,” he explains. “We need to find a way to extend control much later in the season.”

Dane Bowers, technical product lead at Syngenta, recommends a program with overlapping residual herbicides: a preemergence application plus an early postemergence application when weeds are small, or even before they emerge. “That provides enough residual to carry to canopy,” he says.

2. Adopt a zero-tolerance policy. Manage the weed seed bank, Hartzler advises, as he remembers a time when weeds were never left behind in fields. “We knew weeds left uncontrolled were going to cause major headaches down the road,” he notes. “Somewhere along the way, we’ve lost that.”

Late-emerging weeds that don’t affect yields are often left behind, Hartzler explains. “It’s one of the reasons we’re losing the battle.”

One uncontrolled female waterhemp plant can produce 1 million seeds that last for five years.

Bowers says farmers can’t afford to leave any weeds behind. “That’s the concept behind zero tolerance: Don’t let weeds go to seed,” he notes.

3. Take a critical look at herbicide programs. Hartzler recommends multiple modes of action in every pass, but farmers should evaluate each herbicide in the tankmix individually. If a herbicide group isn’t effective on the target weed, or if rates are too low to control the target weed, it’s not an effective herbicide. (See the chart at the end of the story.)

Long-term strategies:

1. Consider cover crops. Cover crops with the right amount of biomass can boost soil quality, help manage nutrients and provide effective weed control, Hartzler notes.

2. Contemplate crop rotation. “I don’t see how we can maintain the current system we have,” Hartzler says. “Down the road, pests will adapt to continuous production of corn and soybeans.” Rotating to crops like clover or alfalfa can reduce herbicide use while managing weeds, but he understands it’s not economically feasible for most farmers at this time. The key is finding a profitable diverse rotation.

3. Go to narrow rows. Late-emerging waterhemp plants have a difficult time surviving in 15-inch rows, Hartzler explains. Adjusting to narrow rows may be the only way to achieve full-season control.

“Weeds are going to adapt to any management practice we use. It doesn’t matter if it’s herbicides, tillage or hand-weeding — weeds will adapt,” Hartzler says. “The less diverse the system, the more rapidly they adapt.”

Cross-check your herbicide program
A combination of seven different herbicide groups has to be a solid program against waterhemp, right?

The herbicide plan below is a strong program that will provide consistent weed control, but Hartzler says it won’t help manage herbicide resistance. Waterhemp is resistant to several of the herbicide groups, plus application rates are too low or timing is inappropriate to be effective against the weed.

“Roundup is carrying 95% of the load,” Hartzler explains. “The program is about as effective as two applications of Roundup with nothing else. It fails greatly in what we need to do down the road.”

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About the Author(s)

Jill Loehr

Associate Editor, Prairie Farmer, Loehr

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