Understand inversion to avoid drift

The issue is becoming more serious on the Prairies with the spread of dicamba-tolerant soybean varieties

ST JEAN BAPTISTE, Man. — The only way to avoid crop-deadly dangers with dicamba drift is to never spray near inversions.

That means farmers need to ensure not only that they’re checking current weather conditions but never spraying near sunset or in the evening, says the Nozzle Guy.

“It kind of lingers.… That vapour will lift and just move laterally very slowly towards your neighbour’s non-treated soybeans,” Tom Wolf of AgriMetrix says about nighttime spray drift.

“You don’t have to have a high dose, but you have a high duration.”

Wolf was talking to Red River Valley farmers at St Jean Farm Days about the dangers of dicamba drift, which has been a big problem in the United States. With dicamba-tolerant soybean varieties becoming more popular inside and out of the valley, it’s an issue that more prairie farmers will need to understand.

An inversion is when the temperature of the air near the ground is colder than higher above the ground. That has the effect of trapping the colder air — and any spray cloud inside it — close to the ground until it becomes warmer than the higher air.

Inversions happen just before sunset and can last for the entire night, becoming stronger near dawn. They end when the sun rises and begins heating the soil. Then the rising heat will dissipate the spray cloud, as generally happens during the day.

Wolf said about 80 percent of summer evenings in the Red River Valley are inversions, so the risk of drift is high.

The problem with dicamba drift is especially great because non-tolerant soybeans are extremely susceptible to dicamba. Even a tiny amount of the chemical will hammer an unprotected crop.

“The single most sensitive response to a herbicide at very low doses in agriculture is dicamba on soybeans,” said Wolf.

“If you think a Group 2 on canola hurts canola, multiply that by a factor of 10 for dicamba on beans.”

Some farmers think that spraying coarser droplets will alleviate the problem, but Wolf said that misunderstands what is drifting during an inversion. It’s not small droplets but a mist of “fines” that aren’t affected almost at all by gravity.

“Those (coarser droplet nozzles) are useful for droplet drift. They are not useful for inversion drift,” he said.

Farmers also often think that the tiny amount of chemical in an almost invisible mist won’t be a danger to a crop, but that doesn’t recognize the effect that the cloud can have over and on a crop all night.

“It’s not so much a question of how much is there, but rather how long does it sit over the crop,” said Wolf.

“That’s also a form of dosage,” he said.

“In the case of inversions, it can sit over the crop for hours. It can sit over the crop all night and cause extensive damage.”

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