What to do with those yellow soybean fields?

Now is also the time to scout for Bertha armyworms and diamondback moth

That bright yellow volunteer canola in your soybeans might look worse than it really is — so before trying to control it, consider whether it makes economic sense.

That’s the advice Tammy Jones, Manitoba Agriculture and Resource Development’s (MARD) weed specialist gave in an interview following a Crop Talk webinar July 15. (Jones’ last day with MARD was July 17. She joined Corteva July 20.)

It takes at least three canola plants per square metre to make it worthwhile to apply a herbicide to control volunteer canola in soybeans, Jones said, referring to research conducted by University of Manitoba weed scientist Rob Gulden.

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Why it matters: Canola volunteers in soybeans are unsightly, but the cost of trying to control them at the flowering stage in some cases may end up being more about revenge than saving money.

Three canola plants per square metre isn’t a lot, but it doesn’t take many flowering canola plants to make a field look messy, Jones said.

There’s a lot to consider. How much will the volunteer canola affect soybean yield? How much will the herbicide and application cost? Are the canola and soybeans at the right stage for the herbicide being applied? How much will soybean yields be affected by driving through the crop?

If the canola is flowering and in high enough numbers across the field, much of the damage to soybean yields has probably already occurred, MARD said in its July 15 Manitoba Pest Update.

Usually the volunteer canola Manitoba farmers have is Roundup Ready so glyphosate isn’t a control option. But there are several other herbicides that can be used, but with caveats, Jones said.

Bentazon (the active ingredient in Basagran) is one, but it’s registered to control canola up the eight-leaf stage, “…so if it’s flowering you’re not going to necessarily kill it in certain growing conditions,” Jones said. “It’s still a good product and will do a job, but it may not be as perfect as you want it to be.”

There’s also a risk the sprayed canola will just be set back, complicating the soybean harvest, she added.

“It may be just worth it to bite the bullet and let that canola mature and be out of the way rather than delaying its maturity by spraying it with a sublethal application and having it still green when you’re trying to harvest,” Jones said.

Reflex and Flexstar also can control volunteer canola in soybeans but not at the flowering stage, or outside of the Red River Valley, the Manitoba Pest Update says.

Imazethapyr (Pursuit, Phantom, Guardsman Gladiator or MPower Kamikaze) can’t be used either because the pre-harvest interval for soybeans is 85 days.

“The pre-harvest interval is the thing that worries more than anything,” Jones said.

“It’s just not worth the risk as far as I am concerned.”

It’s best to control volunteer canola early, but Jones said farmers don’t have a time machine. The next best thing is taking action in the future, including reducing canola losses at harvest time and following canola with crops that allow for easier canola control either pre-plant or in-crop.

“Nobody wants a messy field,” Jones said. “It’s annoying to look and it has management considerations for future years (by adding more canola seed to the field’s weed bank). There’s lots to think about. It’s not an easy decision.”

Worms & moths

Now is a good time to scout canola for Bertha armyworms and diamondback moth, MARD entomologist John Gavloski told the Crop talk webinar July 15.

Traps are up to monitor adult Bertha armyworms, with the highest population so far in the southwest around Killarney, he said.

Gavloski recommends checking for the pest using a three-sided 50-by-50 cm square (the fourth side is left open so it can be slipped around the plants). That area works out to be a quarter square metre.

“Give the plants (within the square) a shake just in case there are any caterpillars on the plants,” he said. “They should all be on the ground. Then start searching on the ground in that quarter metre square. Most of the caterpillars, if they are there, will be under trash debris and cracks in the soil.”

Research has determined for every caterpillar per square metre, canola yield is cut by 0.058 bushels per acre, Gavloski said.

“So if you’re not afraid of mathematics you can do the math and figure out how many caterpillars would it take to equal a spray (in cost),” he said. “So if you had 20 caterpillars per metre squared, on average, you’d be losing about 1.16 bushels per acre. If that’s going to equal a spray application that’s going to be your threshold. If that’s not quite enough to equal a spray application you might want to go higher with your threshold.”

Find more information on Bertha armyworms and thresholds at the Manitoba Agriculture website.

To assess diamondback moths in canola Gavloski recommends using a three-sided one-foot square. Shake the plants over something that can catch the insects and count them.

Spraying is economic when there are 20 to 30 diamondback moth larvae per square foot, he said.

“Now don’t get too frightened if you start seeing borderline levels at flowering,” Gavloski said. “At the flowering stage the plants can compensate quite well from a little bit of feeding, especially if you have good soil moisture, but when you get into the podding stage just be careful if they start moving onto the pods and doing feeding. That’s what they (plants) can’t compensate for.”

Find out more on diamondback moth at the Manitoba Agriculture website.

About the author

Reporter

Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.

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