ONTARIO GRAIN FARMER TECHNOLOGY 27 their actual coverage is a given percentage less than peak coverage, calculate an overunder dose for the product being applied, and accept that there will still be a lot of variability in the application. “And we hope that you’re spraying in a bit of a side wind, to blend the variability a little bit. Conventional aerial applicators have known this forever. It’s the best we can do with this narrow swath,” Deveau says. This marks the sixth year Deveau and his colleagues have been gathering real-world data on sprayer drone performance, much of which involved experimentation with DJI’s 40-litre capacity T50. Experiments are being conducted with fungicides, with visual results being increasingly easy to notice thanks to the proliferation of tar spot through southern Ontario. Results include the need to tighten swaths to achieve consistent coverage (drones naturally have a much higher coefficient variation than other application equipment), that coverage increases with water volume, and that greater canopy depth is inversely related to coverage, even with significant downwash activity. “We were initially very hopeful we were going to blow that spray in there and do a great job. It does, but unfortunately the downwash is a consequence of flight. It’s not really a variable people are trying to control. In fact, they want to fly as fast as possible to get it done, which minimizes that downwash. But if you were interested in slowing down and drilling it in, you would push it into the canopy a bit further – but you wouldn’t have the patience to do it,” says Deveau. Although fungicide application by drone is not currently permitted in Canada, some research has reported satisfactory results. But in many such cases, effectiveness was impossible to verify due to a lack of check strips, harvest data, or even aerial photographs. “There are lots of unsubstantiated claims of efficacy. And I think we’re going to see more and more of that,” he says. “For many, flying the treatment area and coming back empty afterwards is evidence enough”. Swath width also widens and collapses midflight, based on topography, altitude, wind, and other factors, further confirming the need for tighter flight paths. Operating at higher speeds and at higher altitudes was found to widen swath width, but heighten drift risk. Flying into a head wind increases apparent wind speed, and swath width. With a tailwind, swath width collapses to a point. This means flying back and forth across a field, alternating between head and tailwinds, increases the risk of lengthy coverage gaps. “Some of this may sound intuitive to you, but you do have to do the work so you can point to it,” Deveau says. Unanticipated findings were also many. Drone-deployed droplets were found to vector down and backwards – a result of the drone canting forward while in motion – providing little coverage on the part of the plant the drone is flying towards. Coverage on the reverse side was, conversely, extensive. “Imagine dropping a rock into water and creating the ripples. Similarly, when sprayladen downwash hits the ground, the spray moves laterally,” Deveau says. “When we added the collectors with the greatest degree of coverage (facing up, facing the flight path and facing the drone’s retreat) we found the swaths were mathematically wider than if we had just sampled in two dimensions.” “And in a rare shock, we actually did get some under leaf coverage…right at the edge of the swath, where the air slows down, rubbing the air around it, where the spray hits the ground and that turbulence causes it to roll. For one shining moment, it carries the finest drops straight up. Perhaps not useful, but an interesting observation.” LOOKING AHEAD Crucially, all of the aforementioned findings were garnered from spraying fungicides. But in late 2025, Deveau and his colleagues found herbicide – and specifically, glyphosate – was a different beast. Using both a DJI T50 and the larger DJI T100, as well as all the same parameters trialling fungicide applications, researchers found glyphosate covered 32 per cent and 40 per continued on page 28 The problem is not necessarily the drones. The drones can do what the drones can do. The problem is often how people are planning to use them. - Jason Deveau “
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