Prairie farmers have about a six-week window between early September and mid-October to address two important weed control opportunities, says Neil Harker, an Agriculture and Agri-Food Canada weed scientist based at Lacombe, Alta.
With more producers growing Roundup Ready crop varieties, there hasn’t been as much interest in recent years in pre-harvest weed control treatments, and a low-cost application of 2,4-D to control winter annuals often gets overlooked as well, says Harker.
“I don’t think so much about specific weeds as groups of weeds that should be controlled this time of year,” he says. “With more in-crop use of glyphosate with Roundup Ready crops, there hasn’t been as much emphasis on pre-harvest treatments. For farmers thinking about growing pulses next year to help reduce the cost of fertilizer, a pre-harvest glyphosate application this fall is a good time to control difficult weeds they wonÕt be able to control next year, in crop.”
Also producers need to be thinking about post-harvest weed control measures right after harvest in September, says Harker. The window for a 2,4-D application will depend on weather, but could run into mid-October. Two things need to happen: Some rain is needed after harvest to help weeds germinate, and the product needs to be applied before seasonal temperatures get too cold.
Three of the most common winter annuals you can effectively control in fall are shepherd’s purse, stinkweed and narrow-leaved hawk’s-beard.
Ken Sapsford, weed specialist with the University of Saskatchewan’s department of plant science, says dandelion and narrow-leaved hawk’s-beard are two weeds that top his list both in terms of stubbornness early in the season, and their susceptibility to fall weed control measures.
Both dandelion and narrowleaved hawk’s beard prospered quite well in Saskatchewan this year, due to the cooler, drier spring, says Sapsford. Pre-seeding and in-crop weed control products can be effective on the weeds early in the year to a point. But once these weeds get established and start to flower they are much more difficult to control.
“September is the best time to control dandelion,” says Sapsford. Fall applications often seem to be more effective than the spring. With a fall rain to stimulate weed germination, a full one-litre per acre rate of the original formulation of glyphosate should be effective for dandelion control. (A lower rate of the newer formulations of glyphosate is also effective.)
Other treatments that can be effective on dandelions in the fall include products such as PrePass, Express, Express Pack and Spectrum. If you can’t get around to a fall application, an early spring treatment is the next best option. Effective spring treatments include PrePass or Express mixed with a half litre of glyphosate.
Narrow-leaved hawk’s-beard is another winter annual more susceptible to fall rather than spring weed control efforts, says Sapsford.
With a new flush of weeds after harvest, the treatment window can run from late September to early and even mid-October depending on the weather. The weed is susceptible to glyphosate. PrePass is another option in the fall, also providing a bit of residual weed control. Sapsford notes even 2,4-D at a rate of six ounces of active ingredient per acre can be quite effective at controlling narrow-leaved hawk’s-beard. Express Pack is also effective.
“There are several herbicides that are registered for control of narrowleaved hawk’s-beard,” says Sapsford. “And while it may be your primary target, the treatment is also going to be effective on other weeks such as stinkweed, flixweed and shepherd’s purse.”
New Manitoba weed issues
Manitoba weed specialist Bruce Murray says if the opportunity arises, pre-harvest is an excellent time to use glyphosate to control thistles and quackgrass. From a biological standpoint it is also a good time to control dandelion, but the crop canopy will prevent the herbicide from reaching the weeds.
A post-harvest herbicide application is also very effective, especially if there is rain to stimulate a flush of new weed growth.
“A post-harvest application of the original formulation of glyphosate at the 1.5-litre per acre rate prior to a heavy frost provides excellent control,” says Murray. “If you can whack a flush of seedlings as well as some of the perennials you can really bring the numbers down with that fall treatment.”
Weeds that are in several regions across Western Canada but may present a new twist for Manitoba producers are cleavers, nigh-flowering catchfly and chickweed. “What’s new here is that these are annual weeds that are taking on winter annual characteristics,” says Murray. “Just as we now see winter wheat grown in more areas of Western Canada, these weeds are adapting themselves to germinate in the fall and overwinter.”
All can be effectively controlled with glyphosate treatment post-harvest, but the point is farmers need to be aware of the weeds.
“Guys need to be out scouting their fields after harvest and if they see any sign of these weeds they should be spraying,” says Murray. “These weeds can survive over winter and then they can be more difficult to control in the spring.”
Night-flowering catchfly, which may be relatively new to some producers, looks very similar to white cockle and is of the same weed family.
Another emerging set of weeds Manitoba farmers need to be looking for are the bromes: downy brome and Japanese brome.
“We have seen these weeds in southern Alberta and southern Saskatchewan,” says Murray. “But they haven’t been prevalent north of Calgary or in the Saskatoon area or in Manitoba. But now we are starting to see them in Manitoba and generally they are becoming more common throughout the winter wheat growing area of North America.”
The bromes are difficult to control in-crop in cereals so the best opportunity is post-harvest. “Especially if we get some moisture in September we can really see these weeds flush in fall,” he says. “Once the winter wheat is off or once farmers are done with the rest of the harvest, they need to find out what these bromes look like and be checking their fields for them.”
The bromes, as with many other winter annuals, are easy to control with glyphosate and even tillage works well, too.