Losing the war on weeds

Tammy Jones has been doing the math on where herbicide resistance is taking Manitoba farmers and the numbers are grim

Provincial weed specialist Tammy Jones says leaving small patches of survivors can quickly result in a huge problem as new and highly competitive herbicide-resistant weeds have arrived.

Tammy Jones spent four hours last summer wrestling with about 300 waterhemp plants in a Manitoba field, yanking them out of the ground by the roots and carrying them to the field edge for destruction.

The provincial weed specialist was trying to demonstrate the value of controlling what might seem to be small patches of resistant weeds, using whatever means necessary — even hand roguing.

“I thought that was worth about $200, because I work for the government,” Jones said as chuckles rippled through the packed assembly of farmers.

However, her time expenditure was vastly overshadowed by the reality that the exercise took an estimated 90 million herbicide-resistant seeds from one of the most aggressive weeds on the landscape out of play.

That one exercise saved thousands of dollars in lost yield potential for the farmer if those seeds had been given a chance to grow. “Patch management is really important for a weed like this, especially if it’s resistant,” Jones said.

Jones urged farmers to act quickly if they see patches of weeds in their fields that don’t respond to conventional treatments. If they wait until their suspicions of herbicide resistance are proven in the lab, they’re giving those weeds a head start that could cost the race.

Why it matters: The amount of herbicide resistance in Manitoba farm fields is likely understated, which means the costs in terms of lost yields and alternative controls could soar in coming years.

Jones told her audience there are “at least” 34 kinds of herbicide-resistant weeds in Manitoba fields. But because proof requires collecting seeds of plants that don’t respond to treatment, growing them in a lab and testing them for resistance, she’s concerned farmers aren’t acting fast enough.

As a result, the list of herbicide-resistant species continues to grow and the production losses mount.

“The numbers keep increasing. We’re probably losing the battle on herbicide resistance.”

Resistant green foxtail first surfaced in Manitoba in 1988, followed closely by wild oats in the early 1990s. That wild oat resistance was found to be to both Group 1 and Group 2 products.

A rogue’s gallery of weeds. Wild oats (top left), green foxtail (bottom left), and waterhemp (right). photo: File/Manitoba Government/Iowa State University

But despite the long presence of both of these weeds in Manitoba fields, it’s largely been business as usual for crop producers because they produce relatively few seeds and don’t cut into production that much.

One wild oat plant produces a couple of hundred seeds per season. But those seeds can survive a long time in the soil and Jones is suggesting that trait is masking the development of herbicide-resistant biotypes.

Repeated use of Group 1 and Group 2 products kills the non-resistant plants, which provides an opportunity for the resistant ones to flourish.

“Without Group 1 and Group 2, what’s left?” she asked.

There are options like Group 3 and Group 8, but none are for in-crop control, they’re all pre-plant options. It means growers will, eventually, have a much more challenging time controlling wild oats in Manitoba.

Another threat that’s been overlooked is expansion in glyphosate-resistant kochia. A 2013 survey found about one per cent of tested populations had some level of this resistance present, but there was a clear and understandable pattern.

“It was mainly in corn and soybean fields,” Jones said. “That made sense.”

A 2018 survey, however, found that in 297 sample sites, 59 per cent of the tested kochia showed some kind of herbicide resistance. And glyphosate-resistant kochia was a serious and growing problem.

Alluding to her earlier statement about how testing takes time, Jones assured the assembled farmers that again the numbers were probably a conservative view of the scope of the issue.

“I guarantee you, I am a day late and a dollar short reporting,” Jones said. “I know there’s more out there.”

A similar survey in Alberta, where glyphosate-resistant kochia was first identified, shows their resistance level at about 50 per cent of samples, placing Manitoba firmly in the lead for the dubious distinction of having the highest levels of glyphosate-resistant kochia.

Despite the fact that it’s showing up in a high number of fields, Jones said those fields are so far showing only low levels of resistance, which gives farmers a false sense of security.

The majority of the surveyed fields are showing that less than 20 per cent of the tested population is resistant.

“In your fields, that means you probably won’t be noticing a resistance problem, if you have just one or two out of 10 plants surviving,” Jones said. “It’s hard to separate those from the misses.”

But eventually selection pressure will cause those populations to turn over and the fields will fall into the moderate- or high-resistance categories, which is when it becomes much easier to see.

“We’re going to have a resistant kochia problem,” Jones said. “It’s going to get worse, not get better. We need to have different strategies.”


She warned an influx of new and much more challenging types of resistant weeds are on the horizon.

The one that’s here already is waterhemp, a member of the amaranth (pigweed) family. It’s been marching northward toward Manitoba through the U.S.

In 2016, the first cases of the weed were found here. By 2019 the first case of glyphosate-resistant waterhemp was confirmed, Jones said.

It’s a big deal because it’s an extremely aggressive and competitive weed that can procreate and spread with startling speed. Because each plant contains both male and female seeds, it can rapidly outcross and rapidly evolve resistance. In some U.S. states, it is already resistant to nine separate chemical families.

One waterhemp plant can produce more than one million seeds from a single plant, and averages about 300,000 seeds per plant. The plants can set seed in as little as 14 days.

A single vial, roughly the size of a AA battery, can contain 40,000 seeds, Jones said. That means a bit of mud or dirt on boots, equipment on a quad, or a bit of chaff in the combine, can bring waterhemp into the fields in large numbers.

It can also be moved by water during flooding, as the name suggests. Even more troubling is its ability to survive a trip through a wild waterfowl’s digestive tract and emerge viable.

“If the water doesn’t get us, the wildlife will, or the equipment will,” Jones said.

Some U.S. research says yield losses can quickly hit 40 per cent in soybeans and Canadian studies from Ontario back that up.

Soybean crops with waterhemp infestations there saw an average yield loss of 43 per cent and a peak yield loss of 95 per cent. The taller and more competitive corn crops were better, but still saw substantial losses averaging 18 per cent and peaking at 40 per cent.

These figures, coupled with the fact that the seeds remain viable in the seed bank for about five years, make controlling patches in that crop imperative. That can mean tillage, mowing, spot spraying if there are chemical options that still work and even hand roguing.

Letting even a few resistant waterhemp escape will not only lead to production losses, but it could affect the marketability of the grain, lower land values and more complex management.

She suggested strategies that include increased seeding rates to encourage quick canopy closure, narrower row spacing, use of residual control products, inter-row cultivation, cover crop incorporation and machinery cleaning, to prevent its spread. As onerous as some of those changes might seem, doing nothing can carry even larger risks, including having to abandon annual crops.

“Can you afford to stop farming a field?” Jones asked. “What are you going to do? Get some sheep?”

A larger control project last season was a 35-acre patch in a commercial field that was heavily infested with waterhemp and had “no yield potential.” In that case the costs began with the close to $9,000 the grower had invested in planting the crop — and they increased from there.

The carrying costs of the land, and the variable costs of the control method, such as mowing or tillage, will vary depending on the operation. But in the end Jones estimates the total cost of this at around $15,000 in total. It might seem like a steep price tag, but Jones urged growers to consider the biology at work and what the costs for doing nothing would be.

She estimates that the weeds would have produced something in the order of eight million seeds an acre, or a total of 305 billion seeds.

Taking into account tillage, dormancy and predation, assume that 10 per cent of those seeds would become a plant the next season, or about 30 billion. Then assuming a control product might be 90 per cent effective, that would mean three billion plants still remained.

A post-emergent product, if it remains effective, might take out a further 90 per cent, leaving 300 million weeds remaining.

“Was it worthwhile to destroy 35 acres this year, rather than look at 300 million plants next year?” Jones asked. “There’s no way you can farm something with 300 million weeds on that area.”

About the author


Gord Gilmour

Gord Gilmour is Editor of the Manitoba Co-operator.



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