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Whack weeds when they are green and growing

You have two opportunities to apply a fall burndown herbicide: pre-harvest and post-harvest. A pre-harvest application is more often used as a harvest aid. A glyphosate treatment in early September can help dry down green annual weeds that can affect harvest operations, says Bill Hamman, a senior agri-coach with the Agri-Trend Agrology network, based in Lethbridge, Alta.

whack-photo1This Canada thistle is too mature for fall treatment to be effective.

“You’ve got these green weeds in the crop, it’s early September, the weather is cooler, the crop perhaps isn’t maturing as fast, so the question is, do you swath and take your chances with the weather, or do you apply a product that helps desiccate the crop so you can straight-combine?” says Hamman. You can use something like Reglone for a quick drydown or apply glyphosate for a slower drydown.

“You are going to get some benefit of weed control, but mainly this is a harvest aid that deals with those green weeds and lack of uniformity in crop maturity.” Glyphosate at the one-third- to one-half-litre per acre rate (of the high-strength formulation) should be effective.

whack-photo2New fresh growth of Canada thistle is very susceptible to a post-harvest glyphosate application.

Fall also provides an excellent opportunity to control perennial weeds such as Canada thistle, quackgrass and dandelion, says Hamman, but pay attention to the growth stage of the weeds. For effective control of perennials, a herbicide such as glyphosate needs to contact green, growing leaves of weeds.

“If you have bigger mature Canada thistle plants, for example, that are turning brown, they are close to maturity,” says Hamman. “So glyphosate won’t do much.”

Fall also provides an excellent opportunity to control perennial weeds such as Canada thistle, quackgrass and dandelion, says Hamman, but pay attention to the growth stage of the weeds. For effective control of perennials, a herbicide such as glyphosate needs to contact green, growing leaves of weeds.

Waiting for regrowth

Generally in the northern central half of the Prairies, where growing conditions are cooler, weeds may still be quite green in late August and early September making a pre-harvest treatment effective. But in drier years and in the southern Prairies it may be best to wait for regrowth of the weeds after harvest and a make a post-harvest treatment.

“With Canada thistle the optimum time to spray is when you have six to eight inches of new weed growth after harvest,” says Hamman. (Have a look at Photo 2 here.) “Even if there has been some light frost it should be okay to spray. Depending on the fall and weather we should be okay up until early October and I’ve even seen conditions favorable up until Thanksgiving.”

Hamman recommends a two-thirds-litre rate of the high-strength glyphosate formulation in the fall, followed by an in-crop treatment the following year, to deliver a one/two punch against the weed.

A pre-harvest treatment isn’t effective against dandelions, because the crop canopy limits the amount of herbicide to reach to leaf surface of the weeds.

Again, Hamman recommends you apply herbicide post harvest to new weed re-growth. With dandelions he recommends a higher 1.5-litre-per-acre rate in the fall, followed by a three-quarter-litre application in the spring. That one/two punch should greatly reduce weed numbers.

whack-photo3Bill Hamman looks at a dandelion with plenty of green leaf area that can be controlled with a post-harvest glyphosate treatment.

Quackgrass can be effectively controlled in-crop particularly with a glyphosate treatment in Roundup Ready crops. However, post-harvest treatments will be effective on quack grass as well.

A post-harvest herbicide treatment is very effective on winter annuals such as narrow-leaved hawk’s-beard, flixweed and tansy. “And something like a 2,4-D ester does a superb job in controlling these winter annuals,” says Hamman. “The big advantage of a post harvest treatment is that these weeds are very sensitive and susceptible at that time.”

Hamman also urges producers in southern Alberta and Saskatchewan, in particular, to be watchful for a relatively new weed, Japanese brome. “It is often confused with downy brome because they look much the same,” says Hamman. “Even I have difficulty telling the two apart until they head out. The head of downy broome will droop, while the Japanese brome doesn’t.”

Japanese brome is a spring annual with some winter annual characteristics. Products such as Puma and Simplicity “do an excellent job of controlling the weed in spring wheat and herbicide tolerant canola,” says Hamman.

Lee Hart is field editor of Grainews, based in Calgary. Contact him at 403-592-1964 or by e-mail.

About the author

Field Editor

Lee Hart

Lee Hart is editor of Cattleman’s Corner based in Calgary.

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