Spotlight on: Weed Control

Do pre- or post-harvest strategies work best?

By Colleen Halpenny

Growers of all crop types are constantly looking for the best ways to manage weeds in their fields. With emergences of more herbicide resistance and new weed species, producers are adjusting their crop systems to set up the next season for success.

“Both pre- and post-harvest weed controls have significant benefits, from a number of different directions. We’re looking to manage challenges, and provide flexibility for weed control options,” says Tammy Jones, technical sales agronomist with Corteva Agriscience.

Jones explains that Western Canada is unique in terms of its high variety of crops and weed types. So weed control strategies need to look at plans that include crop competition, and what application timing will be most beneficial to your end goals.

Starting off on the right foot

Regardless of the type of weed control used, Jones says that proper seedbed preparation will determine the efficacy of your control type.

weeds growing in field
    Tammy Jones photo

“Starting with a clean field and establishing our crop with those ideal conditions and minimal weed competition, the crops have significant advantages. Crop prices have been good producers want to take advantage of that.”

Producers are encouraged to spend time in each field, multiple times throughout the growing season, to make themselves familiar with insects, disease, water retention, and weed presence – fine-tuning selection of products based on what each field is presenting with, and how to cover it.

“Blanket coverages are a thing of the past; targeted applications are the most effective for costs.

“Producers recognize that no two fields are the same, so sitting down to figure out where to spend money to increase yields, and where you can limit costs, is a worthy use of time,” says Jones.

Pre-harvest control

A great result of pre-harvest weed control strategies is the reduction of production from current weeds, limiting seed banks for future occurrence, and the increase of harvest efficiency and quality of yields.

Jones explains, “a perennial weed that gets dried down prior to harvest makes combining easier, limits green staining on the edible beans, and producers don’t need to worry about drying down as much because they’ve already reduced the number of green matter and moisture associated with it.”

The Canola Council of Canada (CCC) outlines that moderate to heavy infestations of annual or grassy weeds should be sprayed out pre-harvest. Good control can be provided pre-harvest for perennial weeds as well. If coverage or weed staging are concerns with a pre-harvest application, note that weeds need up to four to six weeks of regrowth after harvest and may need three times the active ingredient for the same control.

For this mode of application to be effective, Jones says that “we need to get down into the crop canopy. We need to ensure that we have adequate coverage and contact with the weed surface since the crop is essentially in the way.

“This timing can work exceptionally well on those challenging weeds who then pull the herbicide down into the roots and remove future seed stores.

“Using glyphosate and adding more than one mode of activity to the tank will help fields limit herbicide resistance.”

The Manitoba Crop Alliance fact sheet for pre-harvest herbicides shares that glyphosate is a popular pre-harvest choice for many growers since, as the only systemic herbicide, it provides control and suppression of weeds, including winter annuals and perennials. Desiccants will also dry down green weeds, but only provide top growth control.

Jones explains that “we had a lot of questions this spring regarding foxtail barley. When it emerges in the spring, it’s tiny and hard to hit using a herbicide. This is where, when you compare the timings, if applied in the fall when more leaf material and coverage is available, it gives you good ability to clean it out and remove those root reserves.”

Post-harvest control

With more flexibility in chemical types available and timing window for post-harvest control, Jones agrees this is the “opportune time to make a positive start for next year.”

A post-harvest weed control strategy targets both perennial and winter annual weeds to prevent them from stealing early-season moisture the following spring. With the crop removed, and trouble spots noted from the harvester, a target plan can be achieved.

Jones says that “winter annuals like stinkweed, shepherd’s purse, and downy brome, which is increasing in presence, germinate in the fall and grow in the spring.

“If you wait to use a pre-seed herbicide application, it can be behind when they’re already grown and flowered. You’ll struggle to get control of them.

“Instead, making contact with them in the fall ensures they don’t remove from the soil what your new crop needs in the spring.

“Application during this stage can give coverage into the spring to limit their emergence and give producers more time to get to the fields.”

For effective results to be obtained, producers need to allow time for adequate regrowth for the weed to readily take up the applications, sufficient water volume for coverage, timed for the proper stage of growth of the weed, and what you’re looking for in terms of extended control of flushes in the spring.

Milk Thistle Weed
    Perennial weeds such as Canada thistle are good candidates for pre- and post-harvest weed control. - Tammy Jones photo

Jones explains: “Extended control for soil residual herbicides, you need to work with a later application, so lower soil temperatures. This will give you higher spring efficacy as it has stayed stored in the soil. If you need to hit a perennial weed like Canada thistle, you want to go earlier during more active regrowth, so you capture a higher uptake. Product selection is key to a positive return on your investment.

“It’s important to monitor for those winter annuals and try to control them from the onset. They will take advantage of all the moisture and nutrients prior to next spring’s crop, so fall removal of these weeds is seen much more frequently in those lush conditions.

“The most limiting factor we’ve found is how harvest has gone for producers. If it was a long season, we know producers are burned out. If an early season frost or snowfall hits, we’re limited in what can be done. And if they are handling livestock as well, making that extra trip out in the fall can be a hard sell.”

Weather challenges

Jones comments that, for spring applications, “if we had a start to the season as we did this year where it was over wet or too dry, neither weather occurrence would allow for effective control. So, weed competition is very high this year, and we can expect some impacts on harvest potentials.

“In comparison, we are seeing some concerns in Alberta and southern Saskatchewan this year with limited regrowth. So, a fall application in those drier areas most likely isn’t going to be worth the time and cost.”

Cover crops have also been on the rise, to not only combat weeds, but help preserve soil quality and hold nutrients – an approach that producers can use instead of a herbicide with the benefit of bringing nutrients up from lower in the soil profile. But as with establishing any crop, “weather and growing conditions to establish them, make them competitive with the weeds, and support them through the winter can be a tricky balance,” says Jones.

CCC notes that warm, sunny days when weeds are activity growing are generally the best for herbicide activity, but hot weather reduces efficacy, especially if conditions are also dry.

Regardless of which strategy you choose for your farm, Jones is confident that Prairie producers “will be able to incorporate different modes of action, interact with weeds at different stages of growth, and increase their diversity portfolio.” BF

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