More Ontario producers are choosing to control weeds in Roundup Ready crops by applying a pre-emergent residual herbicide and following up with glyphosate post emergence.
“I think there’s a huge trend, in Ontario, to two-pass weed control. What I mean by that is putting down a soil applied residual followed by glyphosate, post (emergence),” said Peter Sikkema, a University of Guelph weed scientist who spoke at the recent Crop Connect conference in Winnipeg.
“The adoption in soybeans is less (than corn), but I would have to think that 25 to 50 percent of Ontario soybean acreage is now treated with a soil applied residual herbicide…. Every single herbicide manufacturer in Ontario is recommending that Ontario farmers use a residual herbicide.”
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The arrival of glyphosate resistant weeds in Ontario over the last five years likely convinced a few growers to alter their herbicide practices.
However, many producers still have doubts about the merits of soil-applied herbicides.
“The question I get from farmers — is there an economic return to using a residual herbicide?” Sikkema said.
To answer that question, he and his colleagues evaluated four weed control scenarios at soybean plots in Ontario:
- They sprayed with glyphosate early post emergence.
- Sprayed glyphosate late post emergence.
- Sprayed two applications of glyphosate.
- Applied a pre-residual herbicide and cleaned up with glyphosate post emergence.
Sikkema concluded that options three and four provided nearly 100 percent weed control, but the soil-applied scenario did reduce the economic return per acre.
The difference was around $18 per acre, assuming farmers used the cheapest available residual herbicides.
“(But) that’s not statistically significant,” Sikkema said. “So you can make as much money by putting down a pre-residual and cleaning up with glyphosate than putting on two applications of glyphosate.”
It may be slightly more expensive, but soil applied herbicides do provide other benefits.
Sikkema said the timing of the first glyphosate application is critical for soybeans. Yield losses can add up quickly if adverse weather or lack of labour delays spraying.
“If you’re going to make a mistake in terms of Roundup application timing, always make the mistake of spraying too early,” he said.
“If you forget everything else from my presentation today, always re-member small weeds are easier to kill than large weeds. And … most of the time, late emerging weeds do not reduce crop yield.”
To illustrate the financial losses associated with early emerging weeds, Sikkema presented a delay scenario.
“(If) it rains on Sunday night and can’t get into spray until Thursday morning … you (could) lose $30,000,” Sikkema said.
Growers who use a soil applied herbicide don’t have to worry about rain and glyphosate timing because the pre-emergent product controls early season weeds, Sikkema said.
“You want to start clean and stay clean through the critical period of weed control.”
Dennis Lange, a Manitoba Agriculture crop production adviser in Altona and soybean expert, said early season weed control is critical in soybeans.
“The product (glyphosate) is registered for the first trifoliate … but it’s also important to scout your fields ahead of time. If your (weed) populations are there, you may need to go ahead of that first trifoliate,” he said.
Sikkema said many soybean growers wonder if it necessary to control later emerging weeds?
In nearly all cases, late season weeds do not cut into soybean yields.
“If you keep your soybeans weed free until the first trifoliate, any weeds that come in after that, our data would say you only lose one bu. per acre,” he said.
“It’s the same if you keep (it) weed free until the second trifoliate…. If weeds come in after that they have almost no impact on yield at all.”
Rob Gulden, a University of Manitoba weed scientist, said spraying for volunteer canola in soybeans isn’t worth the trouble.
Sikkema said the most significant argument for soil applied herbicides is preventing Roundup resistant weeds. If farmers kill 95 percent of the weed population with a non-glyphosate herbicide, it reduces the probability of Roundup resistant weeds on that field by 95 percent.
“If you rely exclusively on glyphos-ate, you’re increasing potential for selecting for the glyphosate resistant biotype,” he said.
“If you are driven totally by short-term economics, I think your most cost effective way (to control weeds) … is a glyphosate-only program…. But if you choose to do that, you are choosing to live dangerously.”
