Spraying: is its future up in the air?

Aerial application is an essential part of agriculture.

Planes have high productivity, leave no tracks in valuable crops and are the only option during wet spells.

Modern aircraft have large hoppers, commonly 400 to 500 gallons and as much as 800, but they use less water and adjuvants (carrier) than their ground sprayer cousins.

Ground sprayer operators take notice of this.

“If aerial applicators spray two to four US gallons per acre, why do ground applicators have to use so much more water?” we’re frequently asked.

The implication is that there’s a double-standard: aerial applicators get away with low volumes while ground rigs suffer additional refills.

In fact, aerial applicators use concentrated spray mixes because of the many practical restrictions involved and to stay cost-competitive. So really, ground applicators should be asking: “What compromises are aerial applicators making to be able to spray at lower volumes?”

Before we answer, let’s talk about coverage.

Coverage is really two things: the percentage of the target that has spray on it, and the droplet density on that target.

Imagine dipping the tip of a leaf in dye so that 10 percent of the leaf is covered. Ten percent coverage sounds pretty good, but it’s all bunched up in one spot.

Now imagine spreading that 10 percent over the entire surface as a consistent pattern of droplets. The odds of spores or insects coming into contact are greatly improved because the droplet density, which is the number of drops per sq. centimeter, has gone up considerably. It takes percent coverage and droplet density together to equal coverage.

So, to answer the question, the low volumes used by aerial applicators can work, but they must be applied as finer droplets to meet the minimum droplet density threshold required by the pesticide.

Herbicides are relatively forgiving when it comes to coverage because they often rely on translocation to move systemically in the plant.

However, coverage is especially critical for fungicides, which give protection by drying on foliar surfaces.

Achieving threshold coverage is made even more challenging when the spray has to penetrate dense canopies. In this case, you need more droplets to cover all that surface area.

When volumes are limited, the only way to get more droplets is to make them finer. Every time you reduce the average droplet diameter by half, you get eight times as many droplets.

However, the potential for off-target drift goes up as droplets become finer.

It’s the drift that we need to pay attention to because it negatively affects non-target organisms.

The public, as well as government, are increasingly concerned about pesticide drift. As a result, we’re seeing more regulatory restrictions to minimize its potential. And that, simply put, is a threat to the future viability of aerial spraying.

Many of these restrictions already appear on the pesticide label. Most aerially applied pesticide labels specify:

  • A minimum spray quality, usually medium to coarse droplet sizes. Spraying finer than this is illegal.
  • A required buffer zone distance, such as a setback or unsprayed area from the upwind side of a sensitive environment such as water.
  • A maximum wind speed, often 16 km-h. It might also specify temperature and relative humidity boundaries, although this is less common.

Provinces can impose stricter restrictions than the pesticide label. Most set a minimum 30 metre buffer zone around water bodies.

Aerial applicators have a limited number of tools to help them minimize drift potential.

Pilots are diligent about monitoring wind speeds, temperature, relative humidity and the potential for inversions, and they make accommodations for those conditions. However, there is really only one tool — droplet size — when volumes are held constant.

Larger droplets drift less, but larger droplets means fewer droplets and fewer droplets could compromise coverage.

Some aerial applicators look to the ever-evolving world of atomizer technology.

Historically, fan-and-deflector nozzles have been the aerial standard. They produce a droplet size spectrum with a fairly wide span, meaning there are both fine and coarse droplets present in the spray produced.

Rotary atomizers have a narrower span, producing fewer fines and almost no coarse droplets. This makes them useful for maintaining that “sweet spot” required for decent coverage at low volumes.

The problem is that the sweet spot is still in a driftable size range, with diameters less than about 200 microns. This size droplet will move laterally in a crosswind and can harm non-target areas.

Unfortunately, claims that rotary atomizers eliminate fine droplets are not true. In fact, our field drift studies show that they drift about the same as a properly adjusted fan-and-deflector nozzle from the same aircraft.

Any droplet less than 200 microns is considered a driftable fine.

There may be promise in a new generation of spray adjuvants, including oil-based products such as LI700 from Loveland and a newer product called Interlock from Winfield Solutions. They can reduce the production of finer droplets without increasing the number of larger droplets.

However, we’re still stuck with the problem that the “sweet spot” is in a drift-prone range.

So what are aerial applicators to do?

We suggest making drift control a priority, no matter which atomizer or adjuvant is chosen. Increasing carrier volume will be an important part of that strategy because at present it’s the only way to increase the number and size of spray droplets.

Abiding by label statements is another part of the long-term strategy.

Many product labels already require coarser sprays for drift control and higher volumes for coverage. Labels are the law. They may be inconvenient, but they are scientifically derived and no matter whether we operate a ground rig or a plane, we must follow the rules.

Don’t let an increase to operating costs tempt you to break the law. The government’s only response would be to increase its compliance efforts, which means more inspections and ticketing.

It’s the same issue we had with ground sprayers in the 1980s, when low volumes and reduced rates were in vogue. To reduce loading time and increase speed, those sprayers produced finer sprays to maintain the coverage we received from larger volumes. On average, we’ve im-proved matters since then.

The aerial business should not be competing in a race to the bottom, cutting corners to gain market. We believe aerial application has true value, and the industry should move in unison to ensure everybody continues to benefit from the service.

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