Dry conditions and wind can cause dust build-up on weed leaves, which may limit herbicide uptake.
Tom Wolf with Agrimetrix says there is no magic cure for dust problems, but provides the following guidelines:
1. Most products are not strongly affected by dust, but two important products are very dust-sensitive: glyphosate and diquat (Reglone and others). The active ingredients in these 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. The “turbid” appearance is an indicator of suspended soil.
2. Dust on leaves 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 than the dust particles, essentially over-powering the antagonist. For glyphosate, two approaches are common: (a) reduce water volume; (b) increase herbicide rate within label guidelines. 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, either for leaf coverage (Group 6 and 14) or for adequate soil coverage for soil active herbicides (Group 15). For diquat, (Group 22) low water is a bad idea for the same reason as Groups 6 and 14.
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. Sprayer operators will often use slightly larger nozzles in wheel tracks.
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 (when water droplets had dried), 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.
Source: Canola Watch