After a drought, lingering effects of herbicides can really pose a threat for the next cropping year.

In this Canola School episode, Breanne Tidemann, research scientist at Agriculture and Agri-Food Canada, Lacombe, Alta., explains why there’s a risk for herbicide carryover from residual products for the upcoming crop year.

Tidemann says that it’s important to remember that canola is susceptible to not just imazamox and imzaethypyr, but also to other active ingredients that have residual effects.

“Keep an eye on any herbicide active ingredients that have residual, particularly those that are known to control canola,” says Tidemann. Both Group 2 and Group 5 herbicides can persist in soil, although Group 5s have a much shorter half-life.

Herbicides are broken down in different ways, says Tidemann, and the sun and the heat over the summer was a good thing, but a lot of residual herbicides break down by soil microbes and they require water.

“The conditions that are good for microbe growth are usually those that are good for crop growth, so if you haven’t had good crop growth, you probably also haven’t had a lot of microbe [activity],” she says, adding that there has to be moisture present for herbicides to dissolve into a soil solution, for plant uptake. Otherwise, the herbicide will be “stuck” to soil clay or organic matter particles, which can act as a buffer or alternate deposit zone. (Story continues below video)

A grower advisory released by BASF has increased questions about how much precipitation in-season is enough to break down herbicides. The total amount necessary varies by product, and is listed on the product label.

For those farms with their own weather data, knowing exactly how much rain fell during the summer months is useful, but Tidemann cautions that taking the risk could mean losing a whole canola crop. That’s a risk anytime a residual product is used.

The variability of conditions within a field might also affect carryover, since soil is highly variable. Testing soil for carryover risk is an option, but as Tidemann explains, planting a test strip is usually recommended a year ahead of the planting plan, with planting into a soil sample as an option, but wouldn’t stake an entire crop on such a small test.

Furthermore, precipitation over winter and into spring isn’t bankable. Tidemann’s final words of advice are to take a close look at herbicide records for any residual product that was applied, look at the recropping restrictions for the following year, and even beyond — up to 22 months post-application — and make a plan. It may be easier to adjust cropping plans than take the risk.

(Editor’s note: this post has been edited for clarity)

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