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Resist herbicide resistance

Farmers don’t create herbicide resistance, but management practices can encourage it

Herbicide resistance is a global problem that requires a local solution, says
Kate Sanford Mitchell. Farmers, researchers and industry from North America 
to Australia are trying to find effective ways to control resistant weeds.

While many farmers think about herbicide resistance in Australia, the U.K. and the U.S., the reality is that it’s a Canadian problem, too, says Kate Sanford Mitchell. We need to listen to extension specialists who are sounding the alarm about herbicide resistance, she adds.

“Weed resistance is a global problem that requires a local solution,” says Sanford Mitchell, who manages Bayer CropScience’s herbicide and insecticide portfolios for oilseeds.

Weed populations can develop resistance to everything from herbicides to hand-weeding, says Sanford Mitchell.

“For the most part, resistance is a matter of probability. Growers don’t create resistant weeds but what they do is they select for them,” says Sanford Mitchell.

But herbicide resistance is the most common type of resistance in Canada, she says. Farmers spraying a large area are going to select for a few naturally resistant plants within the weed population. Using the same herbicide groups repeatedly will build that resistant population, Sanford Mitchell explains.

How many individual weeds start out naturally resistant in a population is anyone’s guess. Typically farmers don’t notice the problem until they’re in the sprayer or the combine, and they see patches of weeds. Even then, their first thought isn’t herbicide resistance.

“Their first thought is the chemical didn’t work. Or ‘I must have missed that spot.’ And so it goes unreported,” says Sanford Mitchell.

Finding resistant weeds

So how can farmers spot resistant weeds?

“Visually it’s really tough to say that’s a resistant weed prior to spraying,” says Sanford Mitchell.

After spraying, farmers should look for weed patches. A regular-shaped patch is likely a miss, Sanford Mitchells says. But if patches look irregular, and are kind of close together, “there’s likely something else going on.”

Farmers hoping for a new chemical cure are in for a disappointment.

“There’s no new chemistry coming in the pipeline. I hate to say it, but what we’ve been hearing is true,” says Sanford Mitchell.

Manufacturers are mixing existing herbicides into new combinations, Sanford Mitchell says. Those new products are useful, but farmers need to remember that weed populations have seen those chemicals.

“And so we need to find something that we can do that will help us control the herbicide resistant weeds in our fields or at least decrease the probability, if you don’t already have resistance, of it further developing and spreading across your field.”

Some herbicide groups were overused and saw more herbicide resistant weeds than others. “That low level of resistance still is within the population because those naturally-resistant mutants are likely still there.”

Although herbicide resistance is a growing problem in Canada, farmers still have plenty of weed control options. Now is the time for farmers to start managing herbicide resistant weeds. And there’s no need to be overwhelmed by the plethora of weed management techniques out there.

“Pick one management technique that you can do on at least one field. Try it out. See if it works on your farm.”

Looking beyond herbicides

Sanford Mitchell suggests looking at a range of techniques to control weeds. The goal is to make “mother nature second-guess itself and not be able to get ahead of us as quickly,” she says.

1. Rotate crops. A more diverse crop rotation will make it harder for weeds to take over.

2. Consider strategic tillage. Sanford Mitchell doesn’t suggest abandoning minimum tillage. But by tilling weed patches, you can bury the seeds, suppressing the weeds.

Remember you don’t “get a mulligan” with tilling, says Sanford Mitchell. If you till that same patch again, you risk bringing those seeds back to the surface, giving them a chance to grow again.

3. Seed competitive varieties. That doesn’t mean switching crop plans, but instead looking for a variety that’s more competitive, says Sanford Mitchell.

4. Seed differently. Try seeding cereals east-west instead of north-south. Australian research showed it increased light interception, Sanford Mitchell says. This allowed Australian wheat and barley to out-compete weeds and increase yield. Whether or not Canadian farmers would see the same response is an unknown, but she says it’s worth a try.

5. Look at the ditches. If you think you have resistant weeds, don’t neglect ditches bordering your field. Sanford Mitchell points out you don’t want those weeds spreading into your field.

6. Increase rates. Bump seeding rates high enough for crops to compete with weeds. Research consistently shows that higher seeding rates are an effective way to control weeds.

7. Seed later. A five year study out of the U.K. showed the best way to prevent herbicide resistance was to delay seeding, says Sanford Mitchell. She suggests seeding your worst field last, and controlling those weeds. Seed a different field last the next year.

Farmers in other countries are spending more to control herbicide-resistant weeds. In the U.S., some farmers in the south have resorted to hiring roguing crews to hand-weed fields stricken by weeds. That can cost as much as $150 per acre, Sanford Mitchell says.

“The best thing we can do is figure out what technique can help us prevent these extra costs from coming onto Canadian farms because we know it’s only going to eat at the bottom line.”

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