Spraying in Dusty Conditions

Dusty conditions are common in spraying, and in dry springs they are often associated with a further challenge, drought-stressed plants. There is no magic cure for these problems, but here are a few guidelines:

1. Most products are not strongly affected by dust. But two important products are very dust-sensitive, glyphosate and Reglone. The active ingredients in both products are very “charged”, therefore they bind readily and strongly to soil particles, which includes not only dust on plant surfaces, but also suspended soil in spray water that gives the “turbid” appearance.

2. Dust can be viewed as similar to hard water cations, as a game of relative concentration. We try to get the herbicide concentration to be higher, essentially over-powering the antagonist. For glyphosate, two approaches are common: (a) reduce water volume; (b) increase herbicide rate. Reduced volume is tricky if the glyphosate spray contains a tank mix partner such as a Group 6, 14, or 15 to combat resistance. Those products require more water. For Reglone, low water is a bad idea for the same reason.

3. Some specialists recommend the use of higher water volumes to reduce the effects of dust. Although spray volumes are usually too low to actually wash dust off surfaces, the higher water volumes permit the use of larger droplets which may have better absorption characteristics in the presence of dust.

4. Another remedy is to increase the application rate in the spray swath where dust is most severe, usually behind the wheel tracks. Slightly larger nozzles in those regions are widely used by sprayer operators.

5. Even when dust is not a problem, roadside field edges may contain dust from traffic. Higher rates may be justified on the outside rounds for that reason.

6. A report in No-Till Farmer makes the following useful statements:  “Greenhouse research conducted by researchers at North Dakota State University in 2006 found that control of nightshade species with glyphosate was reduced when dust was deposited on the leaf surfaces before, or within 15 minutes after, glyphosate application. If the dust was deposited later than 15 minutes after application, phytotoxicity was not reduced.  Dust generated from silty clay soil tended to reduce glyphosate phytotoxicity more than dust generated from loamy sand soil.”

7. Several additional management opportunities exist for dusty conditions. Slowing down tends to reduce turbulence and dust generation. Although front-mounted booms apply the spray before the dust is generated, it will deposit before the spray is dry, limiting the benefit, as indicated by the NDSU study.

8. Don’t mistake aerodynamic turbulence for dust. Weed control may be lower behind the tractor unit or near the wheels because the spray is displaced by air currents. The use of water-sensitive paper can help identify if this is part of the problem.

One of the better references on dust and wheel tracks was produced by the GRDC in Australia, and can be found here.