By Tom Wolf
Dusty conditions are common in spraying, all it takes is a layer of dry soil and some wheels. But in dry springs, dust is often associated with a further challenge: drought-stressed plants. Combine these two and weed control may be reduced. Following are a few guidelines to help.
- Dusty conditionsMost products are not strongly affected by the presence of dust on plant foliage. But two important products—glyphosate and Reglone—are very dust-sensitive. The active ingredients in both products are very “charged,” therefore they bind readily and strongly to soil particles. Soil is present in two areas of concern: dust on plant surfaces, as well as suspended soil in spray water that gives the “turbid” appearance.
- 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 overpowering 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.
- 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.
- 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. PWM-equipped sprayers may not need to change nozzles behind the wheels; they can select specific nozzles right on their monitor and boost their duty cycle to apply more. Of course, the nozzles need to be properly sized in the first place to allow some room for the extra-duty cycle.
- Even when field 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.
- 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.”
- 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.
- 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.
- Getting back to turbid water for a minute, ponds can be treated with a flocculant such as aluminum sulphate to remove the suspensions. Treatment must occur at least 24 hours in advance. The spray tank is not the place for this treatment as it results in a sediment.
One of the better references on dust and wheel tracks was produced by the GRDC in Australia, and can be found at https://grdc.com.au/__data/assets/pdf_file/0022/134266/grdc_fs_spray-wheel-tracks_low-res-pdf.pdf.pdf.
Any time plants are drought-stressed, herbicidal weed control will be more challenging. Following are a few tips on how to make sure herbicide effectiveness is maximized.
- Herbicide rate is the most powerful factor for effectiveness. Use the full label rate, or the highest rate in the range, when plants are stressed.
- Weed size is critical to good control. Smaller weeds are easier to control – they have had less time to adapt to the dry environment around them and may therefore have less cuticular wax, for example. Early removal has crop-yield advantages, as well.
- Ensure water supply is clean and does not contain antagonizing minerals. A water conditioner such as ammonium sulphate may be necessary to remove the antagonizing Mg, Ca, Na and Fe cations that are most common in well water. A water test is a good idea. Take action when “total hardness” (CaCO3 equivalent) is 350 ppm or greater and you’re applying glyphosate.
- Herbicides are best absorbed by plants when they are in the liquid form. Larger droplets evaporate much slower than small droplets. As a result, a coarser spray applied in a higher water volume will extend the droplets’ lifetime on dry days.
- Pay attention to the formulation you’re applying. Once the carrier is gone, oily formulations may still have good uptake because the oily active ingredient stays dissolved in oily solvents, and these evaporate much slower than water. Solutions, on the other hand, are more likely to leave their actives stranded on leaves in crystals once the water is gone.
- Time of day is critical. Although caution is required when spraying in the evening, overnight or in the early morning because of temperature inversions, those are also the times when evaporative losses are lowest.
- The “Delta T,” also known as “wet bulb depression,” is a better measure of water-evaporation potential than relative humidity. Avoid spraying if Delta T is greater than eight, as droplets will lose water too quickly for efficient absorption, and will shrink and drift more.
- Certain adjuvants may act as humectants or anti-evaporants. These markets are relatively well developed in the U.S., but not in Canada, and would require independent test data or past experience to make a recommendation.
A dry year is challenging on many fronts. Not only can it lower herbicide efficacy per se, it also affects crop growth. Crop competition is a crucial component in a good weed-management plan. Small adjustments in agronomy, such as earlier weed removal and improvements to the spray operation, can help prevent disappointing weed control.