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Assessing droplet size and coverage |
| Written by Bruce Barker | |||||||||||||||||
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Making the nozzle choice.
In the drive to reduce spray drift, herbicide applicators have moved to coarse
sprays. While that may cut down on drift, the result is fewer droplets reaching
the leaf surface. At some point, weed control will diminish. "The movement to coarser spray has been led by high clearance sprayers which travel faster, have higher booms, and need to cover more acres per day. As a result, they need to control drift. In addition, lower water volumes are popular because they increase sprayer capacity. The question is, where do we draw the line to reduce drift and reduce the amount of water?" asks research scientist, Tom Wolf with Agriculture and Agri-Food Canada at the Saskatoon Research Centre. Based on field and laboratory research, Wolf says there are several considerations in the balancing act between drift control, increased coverage and canopy penetration. Target type and mode of action Grassy weeds and some broadleaf weeds (lamb's quarters, kochia, cleavers) can be difficult-to-wet, meaning that larger droplets tend not to stick to their surfaces. Therefore, these require somewhat finer sprays or higher carrier volumes. Contact modes of action require higher droplet densities than systemic products. Higher droplet densities are most easily achieved with higher water volumes or finer sprays. Carrier volumes At eight to 10 gallons per acre, any nozzle can be used successfully provided the correct pressure is chosen. Air-induced nozzles require higher pressures than conventional nozzles, but the ideal pressure depends on the specific nozzle. At four gallons per acre and lower, nozzle choice will be limited to ASAE 'Coarse' sprays. Nozzle manufacturers now provide charts that tell the user what spray quality is achieved with each nozzle and pressure. Conventional flat fan, pre-orifice and low-pressure air induced nozzles would work with the selection of the correct nozzle size. Pressure and boom height must be sufficient to generate good overlapping nozzle patterns. Nozzle orientation At slow travel speeds, orient nozzles forward. At fast travel speeds (15 to 20mph), Wolf does not have enough information to make a firm recommendation, but says that coarse sprays oriented forward had the best deposition on vertical targets in his research. Coarse sprays oriented backwards may improve spray patterns at fast speeds. Double nozzles (one pointed forward and the other backwards, such as the Lurmark TwinCap) are a good idea to improve coverage on vertical targets such as wheat heads (for fusarium head blight control) or grassy weeds. In laboratory tests, using coarser sprays with these double nozzles significantly improved coverage, but in field tests, this did not result in a yield advantage with fungicides or better weed control with herbicides. Travel speeds Fast travel speeds have both advantages and disadvantages, but the most important advantage is greater work rates. Disadvantages are greater dust generation, less uniform deposition (especially behind the tractor unit) and less canopy penetration. Boom heights Lower boom heights are almost always preferable over higher heights to reduce drift. For conventional nozzles with 80 degree fan angles, heights can be as low as 18 inches above target; 110 degree fan angles as low as 14 inches. Lower heights reduce drift and improve overall targetting. Low-drift nozzles require higher heights (an additional six inches) to obtain good uniformity. High booms increase nozzle overlap, which can be useful for low-drift nozzles and when pressures are low and patterns begin to collapse. But high booms can increase drift potential significantly. Use the lowest boom height that still offers sufficient overlap given the boom movement. Automatic boom levellers are available and have been useful for suspended booms on uneven terrain. Canopy penetration Droplet size does not have as much impact on canopy penetration as carrier volume. Penetration improves with slower travel speeds and higher carrier volumes for any nozzle. Air assist is one of the best ways to improve penetration and is more effective than the other methods. Determine your spray coverage Wolf has plotted out the effect of water volume and droplet size on spray coverage. He analyzed the spray coverage on water-sensitive paper and found that the number of droplets per square centimetre decreased when water volumes were decreased or droplet size was increased. Combining low volumes with coarse sprays resulted in the largest droplet density reduction.
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