Assessing frost damage and a pre-emergent herbicide application in cereals

(Kara Oosterhuis/RealAgriculture)

Much of the Prairies has experienced freezing temperatures over the last few days, with a risk of frost not yet out of the forecast. This has growers concerned about cereal tolerance to frost, as well as when and if a pre-emergent application can still happen.

Wheat and barley are typically more resilient to cold temperatures than canola would be, says Jeremy Boychyn, agronomy specialist with Alberta Wheat and Barley Commissions. At this stage in wheat and barley on the Prairies, the growing point, or most of it, is still below the soil level.

“We may see some leaf burn, but typically we’re not going to see whole plant death, and that’s because there’s the soil and some of that moisture in the soil kind of buffering that cold temperature,” says Boychyn.

Things to look out for include some leaf tips dying, but three to five days after the frost event, new growth may appear. For wheat, the -8 to -10 °C range is the point at which leaves burn. For barley, the range is between -4 to -6 °C. It doesn’t mean the plant is dead, says Boychyn, it just means some of the leaf has died off.

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Boychyn also says that with wheat or barley genetics, a bit of stress will up-regulate the gene stress hormones, which primes the plant for more stress later.

When you’re going out and assessing the crop, consider that there are so many variables that play into frost severity, like soil moisture, residue cover, how cold it got and for how long. Boychyn says to look for leaf-tip burn, and then if you see this symptom, dig up the plant and determine if the crown is mushy or water-soaked and has died off. Typically, a healthy plant will be white and clean looking in the area between the roots and the leaves.

Spraying decisions are also impacted by this cold snap, whether it’s pre-emergent applications or in-crop, depending on the stage of crop.

“Typically the answer is we want to be waiting until those plants have the ability to grow again,” says Boychyn. Metabolism needs to function, both in the weeds and the crop because the herbicide needs to be shuttled through a weed’s system, and the crop needs to be active in order to metabolize the herbicide.

Night-time temperatures need to be at least 5 °C and a warm sunny day is necessary to get metabolism going again. New leaf growth is also crucial for proper metabolism and effective herbicide applications.

Boychyn recommends having an agronomist come out and assess the situation before making a pre-emergent herbicide application. It can make for a tight window between the time that those optimal temperatures come back after a frost event, and when the crop comes up.

“We know that weeds that have emerged prior to the crop emerging have the most impact on yield,” says Boychyn. “So we know we want to control those as much as possible, given the opportunity, rather than wait until after.”