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Fall management key to weed resistance

It’s in the fall you see what worked and what you can change up for next year

When it comes to weed control, fall is often one of your best windows to find out how it’s going and what issues are on the horizon.

Tammy Jones, Manitoba Agriculture weed specialist, says the fall season offers a planning window for next year and lets farmers evaluate what went right or wrong this season.

Tammy Jones.
photo: Manitoba Agriculture

“The fall or early-winter season is the opportunity to evaluate success and make minor changes to crop and herbicide rotations to adjust to emerging issues before they become widespread problems,” said Jones.

“Things could be 10 times or a million times worse next year depending on the amount of seed produced. We need to be vigilant about weed control.”

Jones stresses the importance of scanning for weeds while on the combine and to avoid combining patches of weeds whenever possible. There are also combinations that can forestall resistance in the fall season.

“There are residual soil-applied herbicides that can be used to address weed issues, in combination with effective post-emergent herbicide applications next year,” said Jones. “This type of herbicide layering can be a great option to minimize weed competition, reduce the seedset of herbicide-resistant weeds and start to manage this serious issue.”

Soil temperature and moisture levels are key factors for some soil-residual herbicides.

“Soil-residual herbicides may be applied in the fall or the spring, although timing does need to factor in soil temperature – when it is warmer, there is more breakdown of the herbicide which shortens the amount of time the herbicide is active in the spring when most weeds are emerging,” said Jones. “But the advantage of a fall application of some soil-applied herbicides is that there is a greater chance of the herbicide being activated with moisture – not all soil-applied herbicides require moisture for activation, but there are some that do and application in the spring, followed by dry weather can reduce the efficacy of that herbicide.”

Jones advises that using these products, in addition to post-emergent herbicides, increases the number of modes of effective action for weed control. But the biggest key, she says, is wise planning.

“Growing a crop without a plan to address the weed issue is a recipe for disaster,” said Jones. “Crop rotations and herbicide rotations need to be diverse to avoid problems like herbicide resistance.”

Crop rotation plays an important role as different crops will promote the growth of different weeds.

Wide-row crops like corn and soybeans tend to favour the growth of C4 weeds like waterhemp and green and yellow foxtail while a wheat-canola rotation seems to favour C3 weeds such as wild oats, chickweed, and wild buckwheat. “Varying seeding dates, tillage, increasing seeding density, and fertilizing the crop while minimizing the amount of fertilizer for weeds, these are all ways to enhance crop competitiveness,” Jones said.

Jones also said some farmers are extremely reluctant to talk about weed resistance, fearing they’ll become the focus of attention and it may affect the marketability of their crops and even the value of their land.

Top tips for managing weed resistance in the fall

Harvest some mature seed from standing weed patches and send it away for resistance testing to help you plan a herbicide and crop rotation to address the issue. You may even learn there were environmental or timing issues rather than resistance that resulted in the patches of weeds.

Make sure that weed patches are managed separately. This becomes a consideration when applying fall fertilizer or any type of tillage.

Knowing the biology of the weed can help. Tumbleweeds, for instance, should not be left intact to tumble across your field and across the neighbouring fields.  A flail mower may be the way to minimize that issue.

Burning areas with patches of weeds may also destroy seeds – different weeds require different heat intensities and durations for destruction. In dry years this is not a viable option.

Plant a cover crop to suppress weed germination in the spring. This might be too late for this year as you do want to have biomass accumulation.

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