Iowa’s resistant weed problem: what Canadian farmers can learn

Stacked traits


SCHLESWIG, Iowa — Driving on Interstate 80, west of Des Moines, the land rolls more than many people might expect.

There are also tree bluffs, scenic river valleys and billboards promoting wineries … in Iowa.

Another unexpected sight was a highway sign pointing out the town of Winterset, which apparently is the birthplace of John Wayne.

But there was also something most people likely would expect to see: many, many fields of corn, soybeans and more corn.

It wasn’t possible to see them in detail at 120 km-h, but the dozens of soybean fields along the highway had something in common. In early June, almost all had weeds popping up between the rows. And those weeds were likely resistant to one or more herbicides, including glyphosate.

Some growers in Iowa are reluctant to admit they have a problem with herbicide-resistant weeds because it could affect their reputation or the value of their land.

Brian Sieren isn’t one of them.

Brian Sieren, who farms near Schleswig, Iowa, holds up a waterhemp weed from his soybean field in early June. Sieren uses diverse chemistries to manage weeds on his soybean crops, pre- and post-emergence, because almost all of the waterhemp is resistant to glyphosate.| Robert Arnason photo

Sieren, who farms in western Iowa near the town of Schleswig, laughed when asked about herbicide resistance on his farm.

Nearly every farmer in Iowa has glyphosate-resistant weeds so there’s no need to feel ashamed, he said.

Walking into a soybean field, just east of his house, it took Sieren about five seconds to find a weed with resistance to glyphosate.

The 10 centimetre tall weed was likely resistant because almost all waterhemp in the region is resistant to glyphosate, the active ingredient in Roundup.

Pointing at the weed, on a 30 C morning with a scorching wind from the southwest, Sieren ex-plained that early season control is critical.

“You can still kill waterhemp when it’s two to three inches (five to eight cm) tall with glyphosate. But the key is getting there when it’s that tall,” he said, as the sun blazed off his Cyclone Nation T-shirt, a reference to Iowa State University football.

If waterhemp reaches 20 to 25 cm (eight to 10 inches) glyphosate will “ding it” but won’t kill it, he said.

Sieren and thousands of other growers in the U.S. Midwest are coping with waterhemp and other weed species resistant to glypho-sate because they relied almost solely on the weed killer for more than a decade.

On its website, DuPont Pioneer calls the period from the late 1990s to the early 2010s the “glyphosate era” of weed control in United States.

“A 2003 survey of Indiana soybean growers found that glypho-sate was the only herbicide applied on 74 percent of (Roundup Ready) soybean acres,” DuPont Pioneer said.

The glyphosate era was a fantastic time for Midwest growers because they had cheap and highly effective weed control.

“We had 10 years where it worked really well,” said Clarke McGrath, research and extension co-ordinator with the Iowa Soybean Research Centre. “And then in a really short time frame (he snapped his fingers), we had resistant weeds.”

Sieren noticed five years ago, on his 550 acre farm, that glyphosate was becoming less effective. The herbicide would damage but not kill weeds.

In the last couple years, its efficacy has diminished and it’s now difficult to kill waterhemp, marestail (horseweed) and giant ragweed on his land with just Roundup.

The situation on Sieren’s farm is challenging but other growers are worse off.

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There are fields in Iowa where waterhemp is resistant to four types of weed killers, including glyphosate, ALS inhibitors, HPPD inhibitors and PPO inhibitors.

Fortunately for western Canadian growers, waterhemp is still a rarity on the Prairies. The first confirmed find of the pigweed species occurred last fall on a soybean field southeast of Winnipeg.

Jeanette Gaultier, a Manitoba Agriculture weed specialist, said there is no data on herbicide use in soybeans, but most Manitoba growers probably use “glyphosate only” for weed control.

Sieren said Canadian farmers can learn a lesson from growers in Iowa.

“Get educated and don’t be in denial,” he said. “Because you’re going to have to deal with it eventually.”

Sieren, who also works as a marketing consultant and adviser for other farmers, has adopted a more sophisticated weed control program. He applies a diverse array of herbicides, pre-emergence and post-emergence, on his fields. Those include 2,4-D and Cobra, a PPO inhibitor.

McGrath said such a program is more expensive but the cost is reasonable.

“Two trips of just glyphosate, if a guy can get away with that, we’re talking $10 to $12 (an acre),” he said. “To do what guys like Brian (Sieren) are doing… you are probably talking $25 to $30 (per acre).”

McGrath said many Iowa growers are finally accepting that they have a problem.

“A few years ago, you would talk about resistance … and grower interest was ehhhh,” he said, glazing over his eyes to simulate boredom.

“Now (they) are asking for training on how to handle resistance … and how to put together the right herbicide and insecticide and fungicide programs to hold off resistance.”

Around Schleswig, most farmers realize the glyphosate-only era is over, but a few haven’t gotten the message. Those few can cause problems beyond the borders of their own farms.

Sieren, for instance, never had a problem with glyphosate-resistant horseweed, but two years ago it suddenly appeared on his land.

“I figured out where it came from. A neighbour that wasn’t doing a good job (controlling the weed) and the wind blew the seeds over,” he said. “I could do everything … right, as far as chemicals and trying to control stuff, but if your neighbours aren’t part of the program … the seeds blow around so much.”

Nevertheless, most Iowa growers have added chemistries for pre-emergence, post-emergence and burn-down applications. Others are planting LibertyLink soybeans or beans that are tolerant of glufosinate.

Last year, the acres of LibertyLink beans likely doubled in the state, McGrath said.

Diversifying herbicides and rotating between Roundup and LibertyLink beans are positive developments, but weed scientists at Iowa State University remain worried.

“Diversification of weed management approaches beyond herbicides must be considered in order to support the tools currently available to farmers,” said Mike Owen and Bob Hartzler in their 2017 Herbicide Guide.

“Despite recent pronouncements by some in the industry, agriculture will not be able to resolve weed management issues by simply spraying herbicides.”

In 2012, a group of Agriculture Canada weed scientists, including Hugh Beckie and Neil Harker, issued a similar message.

In a Weed Science paper, they said all proposed solutions to herbicide resistant weeds seem to involve more application of chemicals.

Over the objections of some weed scientists, major crop science firms pushed ahead with stacked traits, or crops with tolerance for more than one weed killer.

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Monsanto now has soybeans that can be sprayed with glyphosate and dicamba. And Dow has Enlist soybeans, which are tolerant of glyphosate and a new formulation of 2,4-D. The firms plan to roll out similar products for corn.

Sieren didn’t seed dicamba soybeans this spring but he thinks the technology will be beneficial.

“I think it will (make a difference) for some time. Then they (the crop science companies) will come up with something else.”

McGrath said the stacked traits should help, but he wonders how long they will last.

“It probably won’t take very long for dicamba resistance to evolve in waterhemp, marestail, lambsquarters…. It will happen quickly if we’re not careful.”

Many Iowa growers are also worried about dicamba drifting onto neighbouring fields.

“If you have a lot of people spraying dicamba, how do you know where it came from? The challenges will escalate over the next couple of years,” McGrath said.

If there is one weed that could play the role of super-villain, it would be Palmer amaranth.

The pigweed can produce 500,000 to one million seeds per plant, it can grow five to 10 cm per day and it rapidly develops resistance to herbicides.

In May, American and Canadian weed scientists ranked it as the worst weed in North America.

Palmer is most common in the U.S. southeast but in the last few years it has migrated to the Midwest.

It’s now in half of all counties in Iowa and has been spotted only 10 kilometres from Sieren’s farm.

Despite the threat, Sieren doesn’t lie awake at night worrying about Palmer amaranth. If it does appear on his farm, he said he will chop it down, put it in a bag and burn it.

If it becomes established in one of his soybean fields, he will switch to corn.

“You can just plant continuous corn and get rid of it,” he said. “Just because (more) chemicals are available for the corn (such as atrazine).”

Sieren is confident that Iowa farmers will take the necessary steps to tackle Palmer amaranth.

“I don’t think it will move in and cover whole fields,” he said. “I don’t think it’s going to be as bad as the scare factor it was two years ago.”

For Sieren and most farmers in Iowa, herbicide-resistant weeds are part of the new reality of farming.

It’s an added complication, but not the end of the world.

Few weed scientists believe that herbicide resistance will cause an agricultural apocalypse, but things may get worse before they get better.

“There are no new herbicides in the developmental pipeline that will be commercialized within the next 10 years and possibly longer,” Hartzler and Owen wrote in their 2017 herbicide guide.

“Issues in weed management continue to be increasingly complex, and there are no simple and convenient answers.”

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