
When you fly your spray drone and plan on covering 20 feet, how much area are you covering? This was a question that Hunter Medenwald, a graduate research assistant at Purdue University, set out to answer through his research.
The answer is simple: It varies. However, one thing is clear. There are instances where the spray pattern collapses and that intended swath width is nowhere near being met. The edges of the swath in those collapsed patterns see significantly lower coverage than what is predicted.
“Spray coverage is only occurring really highly in the center,” Medenwald says. “That’s where our weed control data was — in the center of that swath. It’s those outer regions that are really concerning.”
What happened?
Medenwald conducted three research projects examining what could impact spray coverage, including swath width and overall weed control. Here were the factors he tweaked and the outcomes:
Carrier volume. Volumes of 1 gallon per acre (GPA), 2 GPA and 3 GPA were applied, as well as 15 GPA via hand boom, in one project comparing how carrier volume affects coverage for glyphosate and glufosinate. There was minimal coverage below the canopy, and weed control remained the same across the different volumes for glufosinate. Control also remained the same across the different volumes for glyphosate.
“Increased carrier volume on drone applications does not always translate to greater weed control,” Medenwald says. “Generally, we think of products with systemic activity performing better at low carrier volumes than those with contact activity, but we’ve been surprised at the level of efficacy we’ve observed so far with spray drones.”
Deposition aids. All deposition aids used in this study brought the effective swath width below 20 feet. Effective swath widths with the addition of a deposition aid fell between 12.5 and 14 feet. Beyond those widths, spray deposition fell below 2% coverage.
“All of our treatments never actually reached that 20-foot target,” Medenwald says. “In every study that we’ve done, we’ve never met that target swath. It’s always been lower than expected.”
However, weed control was higher with the addition of a deposition aid than with the herbicide alone. The weed control was simply concentrated in a tighter pattern than the target 20-foot swath width.
Drift reduction agents. The addition of a drift reduction agent (DRA) to mixtures with activator adjuvants created tight swath widths that all fell below 20 feet. Without the DRA, the effective width was right around 20 feet or just short of that target. However, the DRA brought that width down to as low as 11 feet.
“When we add that DRA, we drastically change that effective swath width, down to 11 feet — about 50% of that target swath width,” Medenwald adds. “It may be inhibiting the performance of that spray pattern and ultimately a uniform application. The addition of the DRA to these spray solutions is likely creating larger droplets, which we have seen results in narrower effective swath widths. However, different DRA types will behave differently, and recommendations with one drone model may not translate to another.”
Key takeaways
One factor that also may have impacted results was the drone used in these studies. Medenwald admits that the DJI Agras T30 used to carry out these experiments may be less effective at making accurate applications than some of the newer models.
He recommends sticking to a ground rig for broadcast herbicide applications due to the spray pattern variability observed with spray drones. But he says the drone would come in handy for spot spraying and for getting into small or hard-to-reach fields.
“It’s important to remember that this is a developing technology that continues to be improved,” Medenwald says. “We’ve really only seen it the last five to 10 years in the U.S., but it’s continually improving each year.”
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