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Herbicides with residual effects boost weed control in IP and glyphosate-tolerant soybeans

Adding a pre-emergence herbicide application with residual control to a glyphosate-tolerant or IP soybean crop buys a grower some time and quite possibly boosts yield by keeping the crop clean early on.

March 16, 2010  By Top Crop Manager

Untreated plots (left) contained 451 tubers after overwintering, whereas the plots treated with Guardian (right) had only 35 tubers in the same volume of soil.

   Photo courtesy of Mike Cowbrough, OMAFRA.

Adding a pre-emergence herbicide application with residual control to a glyphosate-tolerant or IP soybean crop buys a grower some time and quite possibly boosts yield by keeping the crop clean early on.

Mike Cowbrough, weed specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), says that nailing the timing of a second, in-crop application of glyphosate in glyphosate-tolerant soybeans can be hard. It is like a grower putting all his eggs in one basket: miss the window for the second spray, and he will more than likely miss out on yield, too. “Maybe it’s windy for a week or there’s horrible weather and you get delayed by 10 days,” says Cowbrough. “In our trials in 2009, where we delayed the in-crop glyphosate application to the third or fourth trifoliate, which equated to 10 days, we were able to protect yield where we had a residual herbicide and lost yield where we didn’t have one. A custom applicator looks at that and thinks that for the price of DuPont Guardian as compared to a single application of Roundup, I’m buying a little insurance if I’m not going to get there on time.”


That may be the thought that is going through the minds of a lot of growers lately. Sales of residual herbicides for early-season weed control in soybeans grew substantially in 2009. The strategy worked well in 2009, when it was cool and wet across much of Ontario and Quebec. The crop was slow growing and slow to fill the rows, says Kerry Teskey, senior sales territory manager with DuPont. “We rely a lot on crop competition in soybeans, and if the sunlight reaches the ground, it puts pressure on nature to grow something on it. Soybeans that weren’t sprayed with a residual herbicide had multiple flushes of weeds.”

Teskey says many glyphosate-tolerant soybean fields had to have a second or even third application of glyphosate. “Growers who used Guardian as a burndown or used it in-crop would have eliminated another trip across the field and kept their soybeans cleaner,” he says. “For those who used Guardian as a first in-crop application in conventional till, it may have been all they needed.”

It is a similar story in identity pre-served (IP) soybeans. “In IP soybeans, there’s no question that the most effective way to get good weed
control is to get some sort of residual herbicide down pre-emerge,” says Cowbrough.

There are several programs that work well. Cowbrough explains that in some situations, a tank-mix of Guardian and Boundary is best or Guardian plus Valtera. Both can be one-pass treatments.

Different challenges amoung regions
In some specific situations, e.g., controlling dandelions in the spring, a residual herbicide can boost the effectiveness of glyphosate substantially. Cowbrough says that researchers applied soybean herbicides on a yellow nutsedge patch with no crop competition at the Elora Research Station. They harvested the nutsedge tubers, which are the only part of the plant that overwinters to produce new plants in the spring. “Where we applied Guardian, we had an over 95 percent reduction in nutsedge tubers,” he says. “We had more than 451 tubers in a softball-sized amount of soil. In the soil treated with Guardian, there were only 35 tubers.”

Despite delays that equated to 10 days, field trials conducted by OMAFRA’s Mike Cowbrough showed that yield could be protected
using a residual herbicide.

Photo by Ralph Pearce.

Last year was a big year for IP soybeans in Essex County in Ontario. One retail outlet that does custom application, Agris Co-operative at Stoney Point, used a herbicide with residual to keep ahead of spraying. “It’s a necessity for us to start out with clean fields in this area,” says Chuck Belanger, crop specialist. “In conventionally tilled soybeans, some farmers have had good results with one pass of Guardian without a second pass. The typical no-till grower will have to go back with a second application. But even then, they’ll get much better control of perennial weeds.”

Belanger says there is a real problem with wild carrot in Essex County. He says tank-mixing a herbicide with residual with glyphosate is key for good weed control. But it is also good practice for resistance management. “Everyone knows that using glyphosate by itself is not proper stewardship. We have to shift away from that and add some residual from another group of chemistries.”

Other areas of the province suffer from different weed pressures. Brian Hoven, who farms at Oil Springs, Ontario, finished spraying a field for dandelions in spring 2009. But he had a few passes worth of herbicide in his spray tank so he polished it off in a second field. “When I fold up the boom, my GPS marks the spot where I stop spraying,” says Hoven. “When I was combining in the fall, I could see the line where the Guardian ran out and I had finished the field with glyphosate. There was velvetleaf from that point on that had germinated after I sprayed. They always said Guardian would do that but until I saw it with my own eyes I didn’t believe it.”

There are some hot spots of wild carrot and nutsedge across the province. But Cowbrough and others focus on the timing benefits of a residual herbicide. “The main thing to keep in mind is that a residual herbicide can help you spread out the workload,” says Roger Bourassa, DuPont senior sales representative. “You can do your burndown and buy yourself some time to make other in-crop applications.”


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