Herbicide carryover still a possibility

A wet Prairie fall may help dilute herbicide in the ground, but there’s still a chance this year’s dry summer and current cool soil temperatures could result in herbicide carryover into next year.

A dry 2018 growing season and current cool soil conditions in Western Canada could lead to herbicide carryover to 2019.

“The precipitation will help, but it has come late in the season when the soil is cool,” says Jeff Schoenau, an agrologist and University of Saskatchewan soil fertility professor. “Herbicide decomposition/dissipation in the soil is reduced when the soil is dry and cold.”

Saskatchewan, Alberta and Manitoba experienced an unusually cool, wet 2018 harvest following a parched growing season.

Alberta Agriculture crop specialist Harry Brook adds warmth and moisture help herbicide chemicals break down faster, but single digit temperatures have stymied the process.

The recent precipitation will likely do more to further leech herbicides into the soil.

“Anything that causes water movement will help dilute the herbicide, but if the soils are cold or frozen, then degradation is pretty low, because the microbes aren’t active,” explains Robert Gulden, a professor at the University of Manitoba’s plant science department.


Carryover of a compound usually affects some crops more than others – or not at all – and farmers next year can select a different crop to grow that’s not susceptible to any potential compounds that may still be present, Schoenau says.

Planting a susceptible crop could result in plant injury, possibly causing a yield hit, says Gulden.

Farmers may also want to change what they spray next year to avoid stacking effects, Brooks says.

“One should be aware of potential additive effects from small amounts of different herbicides of the same group or mode of action that may still be present,” Schoenau points out.


Growers should pay particular attention to re-cropping guidelines and restrictions provided by the product manufacturer, Schoenau says.

Also useful is keeping good records of application rates and dates, and precipitation received on fields. As well, Schoenau recommends consulting provincial crop protection guides, which have sections with information on soil residual herbicides.

All three experts also support a bioassay prior to seeding.

“A bioassay where plants are grown on a test strip, or soil collected from representative treated and non-treated areas of a field, can be of assistance where no information exists,” Schoenau says. “When in doubt, consult the manufacturer.”

Bottom line

With higher risks of herbicide carryover this year, farmers are urged to closely follow product re-cropping guidelines and conduct soil bioassays.

Article by: Richard Kamchen