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Herbicide carryover may not be in the past

Some soils are more susceptible to herbicide carryover than others, and 
herbicides from some groups are more prone to cause problems

Harry Brook hasn’t had a lot of calls on herbicide carryover recently, but he’s not sure it’s a problem of the past.

“Actually, when you really come right down to it, it could raise its ugly little head again because as we’re having problems with weed resistance, guys are moving back to some old chemistries to try and control problem weeds like wild oats,” says Brook, crop specialist with Alberta Agriculture and Rural Development.

Wet weather in the last three or four years helped break down a lot of products, Brook says. And most modern herbicides are less persistent than older chemistries.

Carryover issues

Kristin Phillips, Canola Council of Canada agronomist, says she sees carryover in canola crops more often than she’d like. “But guys that are keeping good records don’t have an issue with it.”

Phillips sees carryover issues with Group 2, 4, 5, and 14 herbicides. “And also, we’re starting to hear and see some Group 27 carryover,” says Phillips.

She adds that herbicide carryover shows up more often in soils with low organic matter. Soil pH also affects some herbicides. An April 2013 issue of the Canola Council of Canada’s Canola Watch notes that some Group 5 herbicides, along with some Group 2 sulfonylureas, break down more slowly in alkali soils. But Group 2 IMIs disintegrate slowly in acidic soils.

Signs of herbicide carryover depend on the chemical group, Brook says. “If it’s really bad, you could get nothing coming up. And then you have to start digging around to check out the plants. Did they germinate?”

Group 2 herbicides affect the growing point, Brook says. Often the growing point will be dead and white, but shoots will pop up at the bottom of the plant.

Anastasia Kubinec, oilseed specialist with Manitoba Agriculture, Food and Rural Initiatives, outlined herbicide carryover symptoms at CanolaLAB in Brandon, Manitoba. Group 2 carryover symptoms can also include yellowing, purpling, and cupping.

Group 4 carryover symptoms include twisting and stem swelling, while Group 9 carryover can cause wilting and yellowing. Bleaching is a sign of Group 27 carryover.

Other carryover symptoms can include leaf sclerosis, stunted plants and bare patches, says Kristin Phillips. Cotyledon thickening is also sometimes seen.

“Typically you’ll see it earlier than later, especially if it is herbicide carryover and not herbicide drift. You want to be looking for it at that cotyledon to three leaf stage,” she says.

Farmers and agronomists should also look for patterns, Brook says, such as problems in spots with low organic matter. “So if you notice at the hilltops, the crop’s doing very poorly and you’re noticing stunting, that could be due to a carryover issue.”

Manitoba farmers who think they may have an herbicide carryover issue can send samples to the crop diagnostic lab or bring samples to their local government (GO) offices. Farmers in other provinces can also send samples to labs for residual testing. The Canola Council lists labs that conduct herbicide carryover tests at

Labs can conduct soil bioassays in the fall or early spring to see if any products are carrying over, says Phillips. “But if you are seeing symptoms on the plant and you’re not sure, then you would send (the plant) to the lab for diagnostics.”

Phillips also suggests taking pictures of herbicide injury for records and to send to agronomists. Posting pictures to social media such as Twitter is becoming more common as well, but Phillips advises farmers to cross-reference information coming from Twitter users. “Sometimes people don’t necessarily give the right answer on Twitter. But at least it gets your brain thinking and you can Google it and go see and look at more symptoms.”

Farmers can also use the Canola Council’s Diagnostic Tool, at, to help diagnose crop issues.

Dealing with carryover

The actions farmers can take to deal with damaged crops depends on the situation.

“Sometimes the crop will grow through it if it’s just a small bit of residual that’s carrying over. You’ll see that uptake in the cotyledon and the plant will actually grow through it,” says Phillips.

If plants don’t grow through it and damage is extensive, reseeding may be an option. But the crop will likely need moisture to wash away the herbicide, she adds. And farmers may need to choose a different crop to reseed, she adds.

Farmers re-seeding after an early-season hail storm should also think about carryover potential and its effect on the re-seeded crop, says Brook. As well, farmers doing late fall herbicide applications to control late winter annual flushes should be aware of residual times, he adds.

Application rates and re-cropping information is displayed on product labels. Saskatchewan Agriculture’s Crop Protection Guide also includes a chart outlining re-cropping restrictions for residual herbicides.

Both Phillips and Brook say good record keeping is the key to preventing problems.

“Know what you’re using, when, and have a record of it so you can check back two or three years, or even just for next year,” says Brook.

Alberta farmers can reach Brook and other crop specialists toll-free at 310-FARM (3276). The Canola Council of Canada lists contact information for Phillips and other agronomists at, under “Contact Us.”

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.



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