Best Post-Harvest Weed Control Strategies for Canola

By Treena Hein

 Everyone wants to relax a little — and celebrate — after the canola is off the field, but it’s worth the time and effort to take this opportunity to do a thorough field analysis to nail down the best weed control approach to protect next year’s crop.

“If we deal really effectively with what’s left after harvest, that will set us up well for 2025,” says Ian Epp, an agronomy specialist with Canola Council of Canada based in Saskatoon. After harvest, your canola fields may still have weeds growing along with perennials like thistles and dandelions and winter annuals. “You want to manage these well so that next year when they are bigger, they don’t reduce your soil moisture.”

Applying tank mix partners with your herbicide or a soil-applied product is a very good strategy to control both perennials and winter annuals, says Epp. “Herbicide layering can set you up well,” he says. “Of course, you also have to look at what’s in your rotation next year to make sure you’re not applying something this fall that will prevent you from planting canola, or whatever else you plan to plant.”

Epp expects that, as in the past, the volume of herbicide-resistant weeds will increase next year, especially populations of kochia that are resistant to glyphosate and other modes of action.

“We have tools left but there are fewer of them,” he notes. “In the brown soil zone, kochia is the biggest concern, and growers in the black soil zone are also staying on top of it.”

Epp points out that if you see lines where some kochia plants died and some didn’t, resistance is very likely but take the time to rule out other things, such as an application rate issue or poor herbicide coverage due to large weed biomass. “You can send the (weed) seed away to check for type of resistance, but if you have a fair amount of kochia on the farm, you can assume it’s glyphosate resistant,” he says.

“If you have wild oats, you can also send the seeds away to see if you have Group 1 or Group 2 resistance,” says Epp and adds that knowing whether you have one or both groups will give you better clarity on how to control it next year.

Managing canola volunteers

With the rain that has already fallen across the Prairies this fall, Epp predicts that a good portion of canola seed will have escaped the combine and will likely germinate post-harvest and prior to freeze up.

“A good frost may wipe it out and you can try a harrow pass to encourage growth so that a frost can deal with it,” he says. “But, if you’re getting tighter on moisture and they’re getting larger and using up too much fertility with no frost coming, you may want to treat them — some canola volunteers and annual weeds may get too big to be killed off by the time a frost arrives.”

Indeed, Epp notes that where you used to be able to count on a frost happening in the second or third week of September, that’s no longer the case. Frost may not arrive until mid-October, which presents a threat but also the opportunity to get that harrow operation in.

“Every farm is different, and you need to think carefully about whether you need to control the annuals and volunteer canola,” Epp explains. “Some of the stubble may catch snow and (weeds) might function as a bit of a cover crop. It depends on the year and on your situation in the field. The nutrient removal will happen, but these weeds will be incorporated next spring. If you have really high growth, then consider treating the field, but there’s a lot of grower discretion involved.”

Epp adds that if frost comes later, and you have some Canada thistle stubble, you want to leave it alone to regrow before spraying so that there’s enough leaf surface area for the herbicide application to be effective. “You want to give them a while before spraying glyphosate, but it’s tough to find that window between harvest and early frost,” he says. “So, a later frost is actually a good thing to make the (spray) window longer.”

Are precision sprayers worth the cost?

There is a lot of buzz these days around precision herbicide application systems (after market installation on booms) that auto-identify weeds for spot spraying. In an after-harvest context, “green-on-brown” image analysis involves system software identifying green weeds in a brown (harvested) field. But are these systems worth the cost? We reached out to The Nozzle Guy, Tom Wolf, co-founder of spray research and training firm Agrimetrix, for his thoughts.

Wolf first notes that, of course, the purchase of a precision sprayer is only financially justifiable with a larger farm size, but overall, he’s enthusiastic about the technology. “It offers solid savings, so you get your money back in pretty good time,” he says. “It’s going to cost $100,000 to $300,000, but there are some dealer incentives and some really good bargains. For example, there are some green-on-green systems that are only about $25,000 that also do green-on-brown.”

According to Wolf, using this technology generally drops the herbicide cost to $5 per acre from the $15 per acre required for a broadcast spray. In canola alone, 5,000 acres with one or two burn downs a year — that’s a big savings.

“And of course canola growers grow other crops as well,” he says. “The green-on-green function can also be used for fungicide applications. You may have eroded knolls or other poor crop areas, and with these sprayers you can set them for a threshold level of green for fungicide application. For pre-harvest weed removal, you can use it to spot desiccate only the greenest areas, the problem areas with kochia for example.”

Besides product savings, Wolf points to two other reasons that Canadian canola growers should look hard at spot spray tech. One is stewardship (an excellent story to share with members of the public, giving them confidence in feeling good about Canadian food) and the other is the increasing problem of herbicide resistance.

He stresses that spot spraying allows farmers to spray more-expensive products more efficiently and help delay the inevitable worsening of herbicide resistance. “Using specific products in spot sprays, using multiple-mode-of-action tank mixes, this buys us time to adjust to using all our tools to slow down resistance — crop rotation, intercropping, weed seed destroyers, competitive cultivars, higher seeding rate and so on,” he says. “All these tools are important in preventing weeds from getting a strong foothold.”