Tips for herbicide weed control when it’s cool and wet

When weeds are growing slowly they absorb less herbicide, while stressed crops are more susceptible to injury

weed in a field

Cool, wet weather, especially in southwestern Manitoba, has delayed seeding and now it’s complicating weed control.

Muddy soils have delayed, or prevented farmers from doing a pre-plant weed burn-off allowing weeds to get bigger than the optimum stage for herbicide control.

Lionel Kaskiw, a farm production adviser with Manitoba Agriculture, Food and Rural Development (MAFRD) in Souris has seen some foxtail barley in wheat that’s close to a foot high.

“There really isn’t any product that’s going to control it (in crop),” he said June 11 during his weekly Croptalk webinar. “The best-case scenario will be getting something to suppress it… it’s going to be there and for the rest of the year.”

Where the crop isn’t up yet Kaskiw recommends hitting advanced foxtail barley with a higher rate of glyphosate. A half-litre per acre won’t control it, he said.

Less common weeds — usually moisture-loving ones — are also showing up in southwest fields, including purslane speedwell and cinquefoil, Kaskiw said.

Cool, wet weather reduces herbicide efficacy, including glyphosate, said MAFRD weed specialist Nasir Shaikh.

“The plants (weeds) start to shut down once they hit 5 C,” he said. “It’s advisable not to spray any herbicide once hitting that 5 C. The optimum temperature is usually the 15 to 25 C range when the weeds are actively growing and the herbicides can work very effectively. But since we are going through this cool spring I would say we can have a range from 8 to 10 C in the night before we stop spraying.”

Cool temperatures slow weed metabolism. Weeds then take in less herbicide and take longer to die. In some cases higher rates of herbicide are required to kill the weeds.

At the same time cool, wet conditions put crops under more stress and are more susceptible to herbicide injury, Shaikh said. Herbicide-damaged crops usually recover with little impact on yield.

Cloudy days can also make herbicides less effective, he said.

Here are some of Shaikh’s cool-weather herbicide spraying tips:

  • Try to spray during the middle of the day when temperatures are warmest.
  • Avoid spraying in early morning or late in the evening when temperatures are cooler and there can be dew on the weeds. Dew can dilute herbicides or wash them off weeds.
  • Don’t spray weeds if you expect frost and delay spraying for two or three days following a frost.
  • Check herbicide rainfast intervals and time applications accordingly.
  • Don’t spray during temperature inversions when herbicides can be moved off target.
  • Don’t spray if wind speeds exceed 15 kilometres an hour.
  • Avoid spraying when the relative humidity is below 40 per cent.
  • Use label-recommended wetting agents.
  • When possible apply a tank mix including herbicides from more than one group to slow the development of herbicide-tolerant weeds.
  • Try to apply herbicides when the crop and target weeds are at the optimum stage.

This isn’t always possible, nor is applying herbicides at the optimum temperature, Shaikh acknowledged. While it’s preferable to wait for warmer weather farmers should try to spray weeds before they get too big, he said.

Although most herbicides work better when temperatures are moderate some do better under cooler conditions, while others work better when it’s warmer.

Shaikh said the following herbicides work well under cool conditions:

  • Most Group 4 products;
  • Most residual herbicides of any group;
  • “Fop” Group 1 herbicides such as Horizon;
  • Aim/carfentrazone.

The following need warmer temperatures and also don’t work as well under cool and cloudy conditions:

  • Glyphosate;
  • Liberty;
  • “Dim” Group 1 herbicides such as Select, Poast;
  • “ALS” Group 2 herbicides;
  • Buctril, Sencor (Metribuzin), Fenoxaprop (Puma), Basagran, Flexstar, Reflex.

About the author


Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.



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