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Here is an article by Dr. Simerjeet Virk UGA Extension Precision Ag Specialist speaking about drone use in agriculture.

Until now, drones have been primarily used for crop monitoring – especially scouting – in agriculture. However, with recent advancements in aerial application technology and the weather challenges limiting timely pesticide applications with ground equipment, the use of drones for spraying pesticides is gaining significant interest in the United States. While spray drones offer several benefits and are gaining popularity among consultants and growers, it is important to address that they are not meant to compete with or replace ground application equipment. In fact, the technology pretty well complements ground applications in certain situations such as where the field is inaccessible to ground equipment due to crop damage, being too wet, or topography, or only small/specific areas of the field needs to be sprayed (spot-spraying) to treat weed escapes or other pests.

Anytime there is a new technology in agriculture, there are always questions regarding its suitability, reliability, durability, and of course return on investment. The case is not any different for spray drones either. To some the technology is even so exciting that there have been multiple queries about how to get into a spray drone business and offer custom spray applications/services to growers. While spray drones are not completely new to agriculture, the recent improvements in technology capabilities as well as the availability of different commercial platforms have increased interest in this technology for aerial pesticide applications.  

Whether you are a grower thinking about buying a drone sprayer for your own farm or considering to get into a drone business, it is important to understand the rules and regulations around the legal and safe operation of spray drones as well as the current technology capabilities and spray performance characteristics.

Rules and Regulations

According to the Federal Aviation Administration (FAA) – the entity that regulates air space in the United States, the use of spray drones for applying pesticides require special licenses and permissions that basically fall under the following two categories depending on the weight of the spray drone:

  • If the spray drone (including the weight of spray solution in the tank) weighs less than 55 pounds –
    • Part 107 remote pilot certification for operating drones weighing less than 55 pounds
    • Part 137 certification for dispensing chemicals and agricultural products with drones
    • Private or commercial pesticide applicator license
  • If the spray drone (including the weight of spray solution in the tank) weighs 55 pounds or more –
    • Requires special permission from the FAA including several exemptions for Part 61, 91 and 137 regulations.

There are other regulations as well that effects how spray drones can be currently operated including the need to maintain a visual line of sight at all times and flying at night. Also, drone operator cannot legally fly more than one spray drone simultaneously without first obtaining FAA exemptions. Most of the exemptions, especially if the weight of the drone is 55 pounds or over, are harder to obtain and requires working with an attorney or representative who clearly understands FAA rules and regulations.

Detailed information on the FAA regulations mentioned above or on exemptions required for operation of spray drones can be found at https://www.faa.gov/uas/advanced_operations/dispensing_chemicals.

Operational Considerations

Drone Capabilities

Several make and model of spray drones are currently available from major drone manufacturers including DJI, Hylio, XAG, and TTA. Most of them are available in different configurations in terms of the size of the tank, number of nozzles as well as the number of rotors (quad and hex copter are common configurations). An example of two different spray platforms is shown in Figure 1 below – a M4E quadcopter with 1.3-gallon (5 L) tank and 2-nozzle configuration on the left, and a DJI AGRAS T30 hexcopter with 8-gallon tank and 12-nozzle configuration on the right. In general, spray drones are available with anywhere from 1- to 8-gallon tanks with a spray swath ranging between 10 and 25 feet. Spray swath is also dependent on the height of the drone from the ground or crop surface and can range anywhere from 5 to 15 feet. Although spray drones can fly higher to attain wider swaths but that also results in poor spray pattern and makes it more susceptible to cross wind and spray drift.   

Figure 1. Two different type of spray drones – TTA M4E (left) and DJI AGRAS T30 (right).

Spray drones usually come equipped with a pump and nozzles (usually 01 or 015) that can deliver an application rate of 1 to 5 gallons per acre depending on the type of the drone and other spray settings. The number of acres covered – also referred to as drone capacity – can vary from one drone to another and is determined by many factors including the tank size, swath width, application rate, flight speed, and battery life. Most spray drones have a battery life in the range of 5 to 20 minutes (with spray solution in the tank) and are capable of covering anywhere from 10 up to 40 acres per hour. Generally, an application rate is selected based on the expected battery life and the size of the tank so that both the tank mix and batteries can be depleted and replaced at the same time. Short battery life and need for multiple refills also requires to have a system (usually located on a trailer) that allows quick charging on-site and mixing products during the loading period. This recharging and refilling trailer also needs to be moved around in a large field to reduce the non-spraying flight distance for spray drone to return to home position and resume operation after refilling/replacing battery. Most manufacturers provide specifications for their spray drone including tank size, application swath, battery life (empty and full), and spray volume (gallons per minute) but it is up to the drone operator to determine the application rate based on the type of application to be made and keeping other considerations in mind.

While there are multiple options available for spray drones to choose from, among all other things, the number of acres that drones will be used to spray, the type of pesticide applications, and the amount of upkeep and maintenance required for different platforms needs to be carefully considered before making such a decision to invest in a spray drone.

Spray Performance

Similar to the ground application equipment, the proper selection of application parameters such as spray volume, swath, speed and height is equally important for spray drones to maximize application performance and efficacy. While significant research exists on ground application equipment informing the best management practices for effective pesticide applications, limited or little information is available on parameter optimization for drone sprayers. Consequently, this is an area where lot of emphasis is currently being placed in order to thoroughly understand the spray performance and efficacy of pesticide applications with drones. Given the rules and regulations around operation of spray drones, the testing and research on spray drones is also not relatively as easy as in the case of ground equipment. 

The dynamics of spray particles from a drone sprayer are very different from than a boom sprayer as the nozzles are placed either directly under the rotors or on a boom under the drone sprayer where they are heavily influenced by the downward wash from the propellers. While this helps in increasing coverage and penetration into the crop canopy, it also increases the potential for spray drift. Spray coverage across the swath for a drone sprayer also differs from the ground equipment as the nozzles on a spray drone are placed in various orientations – facing down and/or outwards – around the sprayer. For a drone sprayer, selecting an optimal spray height and speed is important as both application rate and coverage uniformity across the spray swath are influenced by these factors. An example of this is illustrated below in Figure 2 where changes in coverage magnitude and uniformity can be observed for a spray drone operated at different spray heights and speeds. Before using a spray drone, it is also recommended to perform some test runs to determine effective swath and desired coverage for your application as it helps to determine the amount of overlap needed from adjacent passes.  

Figure 2. Comparative coverage of a spray drone (TTA M4E) at three different spray heights at a speed of 2.5 m/s (left), and at three different speeds at a height of 2.0 m (right). (Virk et. al. 2022, unpublished data).

Given the interest in spray drones and the focus on proper utilization of this technology, research is being conducted to better understand the influence of different application parameters, products, and crops on spray deposition, drift and efficacy. These efforts also include large-scale trials on grower farms to evaluate technology performance in real-world field/crop situations as well as to provide recommendations on how this technology might fit into a grower’s current crop protection practices. There is also considerable amount of work ongoing in the regulatory space to make technology more accessible and less restrictive for pesticide applications. 

While spray drones definitely have the potential to become a useful application technology, their use in agriculture will evolve as we learn more about their capabilities, limitations, and challenges through more research and testing. For drone sprayers to become one of the effective tools in a growers crop protection toolbox, it will require a collaborative effort among researchers, federal agencies, industry, and growers to find solutions for technology implementation in a way that maximizes its potential while ensuring safe operation and judicious use of pesticides.

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