Drone Pesticide Application: the second take | Weeders of the West

Contributed by Joel Felix, Oregon State University and Clarke Alder, Amalgamated Sugar

The era of using drones to apply agricultural pesticides is upon us. Recently, I was invited to speak at a pesticide license recertification course and among the attendees were individuals seeking drone spray license recertification credits! Drone spraying helps to infuse technology in agricultural operations that does not require someone to sit in a tractor for hours spraying large fields or flying in a small plane! Drone spraying is cool! Once the spray route is programed, the drone could be autonomously operated with the operator watching while seated outside the field, save for the periodic refilling of the spray tank. So, maybe, just maybe, this could help to steer the youngsters back to the farms to take over farming from older farmers who are contemplating retirement.

In the blog titled “Need Aerial Pesticide Spray? Call the Drone Guy” published on September 28, 2023, I concluded with the observation that “drone pesticide spray is convenient and probably suited for much more forgiving pesticides, for example, insecticides and fungicides, but risky for herbicides, particularly if there are sensitive crops nearby”. I also promised to write a follow up article, which is what you are reading now.

Drones used to spray agricultural pesticides are fitted with 5 to 10 gallon tanks, a boom, or nozzles (actually – atomized disc spray system) underneath the propellers as is the case with DJI AGRAS T40 (link opens in new tab). The spray volume is 2 to 3 gallon per acre, which calls for repeated refills that take a minute or so turn around! The proprietor I visited was operating the drone at 10 ft above the crop canopy while delivering a swath of about 30 ft wide! So, all things being equal (they teach that in economic 101) drones could be used to spray all types of agricultural pesticides. But as we know, things are seldomly equal, and mother nature throws curveballs during spray season. Our contention is that in some geographic areas drone spray is better suited to pesticides other than herbicides. For example, in areas where a single crop dominates the landscape, e.g. wheat in eastern Washington or eastern Oregon, one could venture into spraying herbicides using a drone knowing that stray product would land on a neighboring wheat field and most likely cause no harm. But in regions with multiple crops interspersed in adjacent fields, e.g. in the Treasure Valley of south eastern Oregon and southwestern Idaho, the risk of herbicide drift to non-target crops is much higher and is probably a risky operation!

Let us take a case of herbicide drift we happened on during spring/summer 2023. A drone was used to spray Affinity® herbicide to control weeds in a winter wheat field. All went well, except the operator did not realize at the time that fine herbicide droplets had strayed onto sugar beet plants growing in a field across the road separating the two fields (Figure 1).

Damaged sugar beets due to herbicide drift.

Figure 1. Herbicide drift into a sugar beet field (left of the road) in the Oregon Slope area near Ontario, Oregon. Note part of the target wheat field on the top right corner. Photograph by Joel Felix, Oregon State University, June 22, 2023.

Sugar beet plants were severely injured or completely killed (Figure 2). Injury severity at the entry point depended on plant size, with smaller plants killed while relatively older plants exhibited typical ALS herbicide injury (Figure 2, top right). Plants exhibited injury characterized by leaf deformities and chlorosis as far as 800 yds deep into the field.

Figure 2. Sugar beet plant injury from suspected Affinity® herbicide drift intended for the wheat field across the road in the Oregon Slope near Ontario, OR June 2023. Photographs by Joel Felix, Oregon State University, June 22, 2023.

Because of contractual agreements for sugar beet cooperative members to satisfy their quota, the grower continued tending to the field as if nothing had happened. Largely due to sugar beet crop’s ability to compensate for stand loss by growing bigger roots, a follow up photograph taken on August 11, 2023 suggested full canopy closure (Figure 3).

Figure 3. Suspected Affinity® herbicide drift on sugar beet in the Oregon Slope area near Ontario OR, June 22, 2023 (left photo) and the same field on August 11, 2023 (right). Photograph by Joel Felix, Oregon State University, 11-08-2023.

So, what about sugar beet root yield and quality? True, the beets in figure 3 certainly look well enough to produce a crop – and for the most part they did, producing 30.4 t/A and about 17.5% sugar content. Not what you’d call a “total crop loss” by any means, but about 10 t/A short of what the company average was for Crop Year 23. What the casual observer may not see, however, is what happened to the beets that did survive the application. Aside from the lost yield due to missing plants, the actual quality of the beets working to recover throughout the remainder of the season diminished greatly.

Figure 4 shows how the beets looked at harvest. The shriveled “driftwood” look of the beets is quite the contrast to the normal smooth outer layer of a “normal” sugar beet. When cut open, many beets still exhibit remnants of dark rings in the vascular tissue indicating chronic damage from the herbicide.

Sugar beet cut open showing remnants of dark rings in the vascular tissue indicating chronic damage from the herbicide.

Figure 4. “Petrified” look at harvest of beets suffering from group 2 herbicide damage. Dark rings in the vascular tissue still reside even after 5 months of healing and repair. Photos by Clarke Alder, Amalgamated Sugar, Taken 2 November, 2023

Though these beets may have made it through the season, some of the biggest challenges lie ahead for the cooperative. First, beets of this nature will not store long-term. At best, the cooperative has inside of 7 days to process these beets before they become unusable due to excess deterioration as the plants respire while trying to repair themselves from the damage of four months prior. Secondly, when these beets make it to the factory, they can be difficult to slice as many of them quickly become “rubbery” instead of staying crisp after harvest. Further into the process, they can cause issues in the filtration process as they deteriorate and perform much like rotten/discard beets producing compounds that commonly plug up filters in the factory. Small amounts of these beets must be blended with “good” beets in order to keep the factory slicing at capacity. To make a long story short, there’s more than meets the eye with a sugar beet crop when it comes to herbicide drift. Because the roots are what is used, the tops may not always indicate a usable quality crop.

To the original point about drones and pesticide safety. Well, there’s much research still to be done, but it stands to reason that instances like this would like to be avoided by all parties. That said, following the label helps to avoid many mistakes regardless of the application method. It may be that this could have just as easily happened with a ground rig with a breeze strong enough to move small droplets or while spraying during an inversion. So, in terms of the level of safety in applying herbicides with drones, we’ll let you decide. But as we stated at the beginning of this post, the era is upon us – And we hope we’re ready.

2 thoughts on "Drone Pesticide Application: the second take"

Tracy Eriksen says:

March 15, 2024 at 11:50 am

Thank you Drew/Felix for these articles. They are timely and revealing, and obviously for liability purposes alone, show the importance of being trained and properly licensed for drone operation. I first became aware, and excited, about ag drones in January 2023. I recognized that finally I have the potential answer to restrict wheel tracks in our fields. I’ve known for 10 years that tracks in the fields have been where the weeds grow, and you always have to be conscious of compaction. Nearly monthly I see a job for a drone. I was hoping we would have a T40 for the spring of 2024, but licensing has slowed us up. I have looked into it enough to see that accurate information, access to training, cost, and time commitment are problematic for an individual farm operation. Maybe we will get it in time for the T50’s introduction. I also have not seen evidence that the T30 or T40 have been pattern checked. I have been to a demonstration, and looked at videos. The drone pattern looks pretty good, but so did the plane in the early days, but they now have nozzle type, and spacing much different since they check their application pattern (which I think they do annually). I believe the drone technology is the future for us. Whether it will advance enough to remove the monster self propelled sprayers from the Palouse, in what’s left of my life, is questionable, –but it will happen.

1. Joel Felix says:
  
  March 22, 2024 at 8:49 am
  
  Hi Tracy,
  Many thanks for reading our blog post and more importantly for leaving us a comment. It is feedback like yours that keep us excited about what we do. Yes, we are all hopeful that the future of pesticide application will have drones in the mix. Drones offer convenience of spraying when conditions are not ideal to deploy a ground rig (e.g. wet conditions) and to avoid soil compaction as you pointed out. With drone swarming spraying of large fields will take not much time! Also, the turbulence created by propellers help to get best coverage for insecticides, fungicides, etc. So, yes, the future is bright for drone pesticide spraying. Finally, I wish you luck in your quest to procure a drone and I hope it will happen soon. Please keep us posted.
  
  Regards,
  
  Joel