Focus shifts to weed control as Ontario planting wraps up

OMAFRA weed specialist Mike Cowbrough

As planting of corn and soybeans nears the finish line across Ontario, farmers and agronomists are turning their attention to weed control.

A stretch of dry weather that fuelled a two-week sprint of planting, with little time for spraying, means many growers are now checking the status of pre-plant applications and trying to determine next steps for weed control.

During a May 24 virtual meeting of eastern Ontario agronomists, extension specialists, and crop retail representatives, Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) weed specialist Mike Cowbrough was peppered with questions on herbicide performance and control strategies for specific weeds.

Several agronomists asked Cowbrough what type of performance growers could expect from soil-applied herbicides where the ground did not receive rainfall following application. He noted that if growers receive less than a half inch of rain within seven to 10 days after application, weed control is reduced by about 15 to 20 per cent. “Maybe you’re getting 95 per cent efficacy if you get a good close-to-an-inch or over half an inch of rain, and then it drops off to maybe 70 per cent control. So you’re still getting value, it’s just diminished.”

Cowbrough noted that while dry weather impacts herbicide efficacy, it also impacts weeds, reducing the number of annual weeds that emerge and grow. For corn growers, the good news is that there are plenty of options to deal with weed escapes.

Cowbrough also weighed in on the topic of poor dandelion control across the province, a big topic of discussion this spring. He noted that fluctuating spring weather, including significant swings from cool to warm conditions, can affect herbicide uptake and it likely impacted in-crop wheat herbicide applications and pre-seed burndowns.

Also see: Controlling those dastardly dandelions

Cowbrough cites a study conducted 10 years ago that looked at herbicide effectiveness based on dandelion staging. “Dandelion is most tolerant, or in other words you get the poorest control, when it’s in full bloom,” he noted. “So anything done prior to bloom, anything done after bloom, your results are better.  But in full bloom, that’s where things drop off significantly. So I think that’s probably what’s going on, especially in wheat, but also with some of these pre-plant burnouts.”

Another weed that appears to be making a comeback in eastern Ontario is horsetail. In this case, Cowbrough says growers have some good control options. While totally eradicating the weed is a tough chore, growers can get effective top growth control. Last year, he conducted trials in Enlist E3 soybeans and noted two standout programs.

“One was the Enlist Duo combination, or in other words, glyphosate plus in Enlist 1, so the 2,4-D choline product. And then the other effective treatment was Liberty plus Enlist 1,” says Cowbrough who notes that the combo is pretty important. “It gives the best level of top growth control. Each individual product — whether it’s Enlist or whether it’s glyphosate or whether it’s glufosinate — they’re all kind of either poor to mediocre.  I think it’s a little glimmer of hope for people that struggle with that weed in soybeans.”

During the meeting, Cowbrough also indicated he will be doing more research work this year on wild parsnip control. Last year he conducted trials with Barricade M and Group 2 and phenoxy herbicides like Classic and dicamba. He noted that the combo of a Group 2 and phenoxy herbicides has delivered the most consistent control.

When it comes to controlling wild parsnip, Cowbrough noted that once the plant grows beyond a height of 20 cm — typically late May or early June — it becomes highly tolerant to herbicide control. That creates challenges for spring spraying because most wild parsnip herbicide application happens after the plant has passed that growth point.

Cowbrough notes that some products will deliver better control with fall application and there is also less risk of off-target drift.

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