SASKATOON – Tom Wolf does not recommend spraying at night.
The Agriculture Canada research scientist told the recent Saskatchewan Soil Conservation Association conference in Saskatoon that the problem is temperature inversions that tend to occur at night.
“On a typical summer night, when the sun goes down, the air becomes stable. That means the spray drift you produce doesn’t disperse very well,” he said.
“You always produce spray drift. You can’t eliminate it. It hangs behind the sprayer. On a sunny day, that spray disperses rapidly because we have thermal turbulence. A lot of air is moving up and down all the time and that spray cloud dissipates quickly. At night unfortunately, it doesn’t. It stays concentrated, and because it’s calm it probably will hang behind your sprayer for quite a while.
“So when the winds do come up, you don’t know which direction they’re going to come up in or how long they’re going to be in that direction. You can’t effectively protect any area around your sprayed field from the impact of the drift. That’s why we recommend against night spraying.”
Wolf said he knows some farmers like the practice. When he was growing up he used to run a guideline down fields with a dirt bike every 18 metres so his father could spray at night.
“The advantage of spraying at night is that the winds are calm. The humidity is a little higher and temperatures are less extreme, so the droplet lives a little longer and has more opportunity for uptake,” he said.
“Forget about stomata being open longer. Herbicide isn’t taken up through stomata, so it’s irrelevant what they do.”