Tips for spraying in the wind

June 15, 2016 - Issue 13

How do you spray weeds in a timely fashion when every day seems too windy? Too windy, according to the Guide to Crop Protection, is wind above 15 km/h. But we know that some operators will go with winds of 20 km/h and maybe a bit above.

Herbicide drift injury on canola. Drift can be costly on your own farm and especially awkward and costly if it carries to a neighbouring farm.

Herbicide drift can be costly on your own farm and especially awkward and costly if it carries to a neighbouring farm.

Here are tips to improve performance and limit the drift risk in wind:

Use low-drift nozzles. Using a low drift nozzle so you can spray weeds early is better than waiting longer for a relatively calm day suitable for a finer spray.

Find a nozzle that can achieve a coarse spray at a broad range of pressures. But too coarse and you may not be getting adequate coverage, especially at low water volumes. Click here for a full article with images, including water-sensitive paper that demonstrates coverage based on droplet size and water volume.

Aim for 100% overlap nozzle to nozzle. This provides equal coverage across the whole boom width — not just in spray volume, but also in the number of droplets.

Aim for 100% overlap nozzle to nozzle. This provides equal coverage across the whole boom width.

100% overlap provides equal coverage across the whole boom width.

Keep your boom low to reduce drift. But use a nozzle with a wide fan angle to maintain 100% overlap at low heights. See the image above. Spray from one nozzle should reach to the middle of the spray pattern of the adjacent nozzle.

Maintain reasonable travel speeds. Slower speeds reduce the amount of fine droplets that hang behind the spray boom, and they also make low booms more practical.

Avoid early morning and evening spraying where possible. When spraying early mornings or evenings, the odds of spraying in a thermal inversion* are greatly increased. Spray can hang in the air, making it impossible to predict when and where it will settle. A dense cloud of spray hanging in the air can do a lot of damage if carried to a neighbour’s crop or yardsite as it moves downhill or with light winds. Bright sunny days with some breeze are ideal times to spray to minimize drift damage. If those days are hard to come by, click here for tips for spraying in wet conditions.

Let the weather help you. Don’t spray in periods of dead calm. Light winds disperse spray and help it to ground. Take the wind from the side if you can because travelling straight into the wind creates a lot of extra drift.

Talk to people who might be affected. If you have to spray on days that are not ideal, talk to neighbours who might be affected by spray drift. Damaging a sensitive crop or a relationship is probably not worth the risk.

*More on inversions. The following factors often signify that an air temperature inversion is in play:
—Large temperature swings between daytime and nighttime conditions
—Very calm (<3 km/h wind) and clear conditions —Intense high pressure and very low humidity —Mist, fog, dew and frost —Smoke or dust hangs in the air and/or moves laterally in a sheet —Odours seem more intense —Daytime cumulus clouds tend to collapse toward evening, —Overnight cloud cover is 25% or less, —Soil conditions that make inversions worse include low soil moisture, freshly tilled, coarse soils, heavy residue, closed crop canopy. Listen to a Canola Watch podcast on inversions.

Tom Wolf adds that air temperature inversions don’t happen during the day, with very rare exceptions related to cold air moving in from a body of water, or a strongly transpiring crop canopy. However, you can assume the atmosphere starts to become inverted as the sun sets, that the inversion will build in strength overnight, and that it reaches its peak height and severity at sunrise.

Therefore, if it’s late evening, night or early morning, assume you have a temperature inversion unless (1) winds are consistent and (2) cloud cover is pretty solid.

NDSU Extension Service has more in this factsheet: Air Temperature Inversions

Canola herbicides and droplet size.

—Glyphosate is suited to low drift (coarse droplet) sprays, but remember that at the low water volumes that favour glyphosate, coarse sprays may not provide enough droplets per square inch. A combination of coarse spray and at least 5 gal./ac. water volume is best to make sure you get droplets on even the smallest weeds. (NOTE: With pre-seed burnoff, glyphosate is often tank mixed with contact herbicides. In that case, higher water volumes and smaller droplet size will improve efficacy of the tank mix.)
—Liberty can work with a coarse spray nozzle but it needs at least 10 gallons per acre to maintain efficacy. Instead of increasing pressure to increase volume output consider using a larger nozzle. The Liberty label does not recommend using air induction nozzles.

—Group 2 products for the Clearfield system in general perform well with coarse sprays.

Further reading:

Sprayer strategies to improve weed control in canola
Sprayers 101 version of these tips
The risk with rain delays and full tanks

—Thanks to Jason Deveau with OMAFRA, Tom Wolf from AgriMetrix and Brian Hall Ag Business & Crop Inc. for their assistance with this article.

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