Drones have increased in popularity with farmers over the last decade, but their return on investment is not yet proven, as researchers continue to test the technology in many different applications, such as cover crop seeding.

Unmanned aerial vehicles (UAVs) are known in popular culture as “drones.” They can cover hundreds of acres in an hour and provide a birds-eye view of fields at the canopy level.

Working with a team, Purdue University professor of weed science Bryan Young decided to test UAV use after hearing positive testimonials and advertisements about fertilizer drone applications. There were some promising results but also challenges along the way.

There are several operational differences for growers to consider when switching from a ground rig to a drone. UAVs could potentially reduce pesticide waste overall — depending on how well growers use their alternative application systems — by being more precise and intentional with their applications.

Battery-powered drones can spot spray sections of a harvest, allowing growers to note what is needed from an aerial point of view and adjust the nozzles to the droplet size needed in real time.

Selling Points

It may be obvious, but mobility is likely the biggest advantage and selling point for drones.

“Fields that you can’t get a fixed wing aircraft or a helicopter into, you can't get a ground rig in,” Young says. With a UAV, there is less water to haul. Spraying two to three gallons per acre instead of 15 to 20 means growers don't need to carry as much water with them, he says.

Burcucumber in corn is a perfect example of how the mobility of drones surpasses the usefulness of a ground rig. Burcucumber is naturally more challenging to spray due because of the need to untangle it from a ground rig upon during cleanup – potentially sacrificing some corn stalks that are crushed in the transportation process.

“There was a site up by campus where they had to shut off the center pivot irrigation because the corn was falling down with the travel of the wheels as it went through,” Young says. “They had to bring in a drone to spray a couple hundred acres in the bottom area.” 

UAVs are highly effective in scenarios like this where spot spraying of a strip that was missed or targeting specific weeds like burcucumber or Johnsongrass.

Growers could also use an ATV sprayer during wet spring conditions for tasks such as terminating spring cover crops, but Young believes such a system could eventually be replaced with a UAV.

Cost wise, Young suggests UAVs are more cost effective initially than buying a new ground rig — under $50,000 compared to $750,000 for a new rig. The replacement of parts and required maintenance for drones is limited compared to a ground rig.

Hiring out a ground application could be cheaper, from $8 to $10 per acre, whereas UAV application would potentially cost $16 to $20 per acre or $200 per hour. UAV spraying companies are hesitant to provide fixed pricing per acre due to terrain differences or soil conditions, Young notes.

Having the correct insurance on the UAV is imperative though, and it differs from ground application insurance. According to Young, only certain companies will insure aerial pesticide applications — the same companies that insure fixed wing aircrafts.

The biggest question for growers may be how quickly they must use their insurance, and whether it will cover user error.

“You can crash a UAV,” Young says. “A ground rig — you buy that, and you might be spraying with that for 20 years if you need to."

Pros & Cons of Different UAV Types

There are three different aerial application types: a fixed-wing plane, a single-rotor helicopter and the multi-rotor UAV.

The biggest advantages a multi-rotor UAV has over the fixed-wing and single-rotor are carrier volume and travel speed. It has lower carrier volumes and travel speeds, but the boom orientation can be larger. With a low volume limit, Young says, the controller cannot expect good coverage with a large droplet size. Battery-powered drones can remotely adjust the droplet size.

“You have to make sure that you’re calibrated for something that is within the capacity of that spinner atomizer,” Young says.

The rule of thumb is that aerial aircraft cannot have a boom that is more than 70 percent of the width of its wingspan: if the boom extends past that growers will see drone turbulence and application drift.

Fixed-wing and single-rotor applicators can have a greater ferry distance, but a multi-rotor UAV must be mixed and loaded at the field edge.

Young says that’s a challenge for commercial applicators with drones as long trailers with water, shuttle tanks and an observation deck may not fit into field or driveway.

UAV controllers need to check field boundaries prior to operating to stay about 10 to 15 feet away from any tree line and high-power transmission lines.

Maintaining a line of vision of the drone is what will prevent mishaps, Young says.

When the drone is finished spraying, it will automatically return to the starting position and, in many cases, fly up and over to the controller — which can be problematic. Losing radio connectivity to the drone creates a dangerous situation as well if the drone is carrying pesticide.

“You and the visual observer need to see where it is at all times,” Young says. “Unfortunately, you must be downwind for your best line of sight. Are you going to drift down yourself? 

“If that’s the case, you might want to shift around for some of those first spray applications next to where you’re going to be loading from, and then maybe move that spot so you don’t drift down yourself.”

For adjusting the spray swath, the manufacturer’s manual may say the swath is 30 feet wide, but another rule of thumb is to subtract four or five feet from whatever the manufacturer might say.

“So don’t take the manufacturer's recommendation for that drone, what their spray swath is. Assume it is less. It's just a matter of how much less and it might depend on what type of canopy you're trying to push it into,” says Young.

Young adds that some degree of overlap is recommended, “because a drone is like one big tapered flat fan nozzle. You must overlap the last path to make sure that application is uniform across the field.

“I would subtract a couple feet, at least one foot off each side to feel good about that uniformity.”

The swath width is difficult to estimate, not in the least due to wind. If the wind is behind the applicator, it tends to narrow the swath. If the applicator is flying into a headwind, it blows the swath out wider, and if it’s a crosswind the applicator must factor that into the pass.

Young suggests using a contact herbicide, such as Liberty in the spring on a bare field before planting season. “Spray it, then come back four days later and see what your spray swath was,” says Young.

Meeting Guidelines

Regulatory guidelines are unique. A drone application from another type of fixed-wing aircraft or single-rotor helicopter may be a supplemental label that is necessary and simple.

For a drone, there are some pesticide labels that do address drone applications recently.

“For Roundup PowerMax 3, the label says ‘Do not apply this product using aerial application equipment except under conditions specified on this label or on separately published supplemental labeling,’” says Young.

Young says drone labels currently need to be updated to acknowledge the way people are using them with various chemicals and ensure these practices are legal. Everything on the label for the aerial applications is there to manage drift, which is most concerning for the EPA in terms of regulating drones.

“If it isn't clearly labeled, who’s liable if the pesticide doesn’t perform? I think there are some pesticide manufacturers that are supporting this industry and others that are a little bit, let’s wait and see what happens,” says Young.