Air-assisted boom sprayers have been around since the 80s (likely longer). Common in Europe, they have demonstrated value in mitigating drift and improving canopy penetration. In North America, they are used by a few vegetable and berry growers, but are exceedingly rare among field croppers. There are a few possible reasons for this.
- Air-assist is not effective on bare ground, making the feature inappropriate for some herbicide applications.
- While there are exceptions, this feature is rarely available on self-propelled sprayers, which are preferred by North American operators.
- There are few, if any, after-market air-assist upgrade kits available.
- Many operators feel the advantages of air-assist don’t outweigh the price-tag.
Given my background with horticultural airblast sprayers, I’m a proponent of using air when spraying. It opens the crop canopy, exposes otherwise-hidden surfaces, entrains and carries drift-prone droplets to the target (reducing drift and improving coverage) and it extends the spray window by out-competing moderate winds. I have no proof, but I suspect it may also help alleviate the negative impact of sprayer tire turbulence on coverage uniformity under the boom. And, just to play Devil’s Advocate, I’m not a proponent of using air-assist as a means for reducing spray volumes.
We felt air-assist needed a champion to demonstrate the potential benefits. So, in 2015 we held a demonstration at Canada’s Outdoor Farm Show. We used water-sensitive paper to evaluate coverage in a soybean canopy (moderately dense, planted on seven inch centres) from a Hardi Commander with and without TWIN air-assistance.
The demo treatments
The sprayer was calibrated for 93.5 L/ha (10 gpa) at 2.75 bar (40 psi) at 9.7 km/h (6 mph). The boom was suspended 50 cm (20 inches) above the top of the canopy. On one side of the boom, we ran yellow mini drift nozzles (MD 11002’s) to create a Coarse spray quality, and on the other side we ran conventional yellow flat fans (F 11002’s) to produce a Fine spray quality.
Water-sensitive paper was attached to rods at three canopy depths: at the top, midway down and at the bottom of the canopy. Papers were oriented both face-up and face-down. Following each application, papers were collected for digital analysis using “DepositScan” which calculates the percent surface coverage and the deposit density. Both of these factors contribute to overall coverage.
We collected papers from three treatments:
- Fine spray quality, No air assist
- Coarse spray quality, No air assist
- Fine spray quality, Air assist
We held two demos per day at noon and 3:00 pm for three days, giving us six sets of papers to analyze for each treatment. The weather ranged from 25-29°C, 30-58% relative humidity and winds of variable direction from 3-11 km/h.
This was a simple randomized complete block design, but it was not a rigorous experiment. We simply took the opportunity to gather numbers from the demonstration. A more fulsome experiment would require many, many more passes under more stable conditions. For example, we set the angle of the air and nozzles to about 30° forward and the air speed at maximum, which wasn’t necessarily correct. Ideally, these settings should have been fine-tuned to match the forward speed of the sprayer, the density of the crop and the weather conditions. There was a lot of boom sway (watch the video). And, droplets under 50 microns do not register in the DepositScan software, even though they can still be discerned with the eye (if only just).
And so, caveats aside, the following graph illustrates the mean percent coverage and mean deposit density for papers in each treatment, for papers that were facing up (figure 3). Standard error of the mean is presented alongside the average (x% ± y).
Results
Treatment 1 (Fine, No Air) reflects a typical coverage pattern for a dense canopy. Coverage declines as a function of canopy depth because spray droplets are intercepted by plant material before they reach the ground. This is particularly evident with broadleaf canopies that create shading. The coverage data doesn’t show it, but there was an obvious (and unacceptable) plume of spray drift during these applications (see Figure 4).
Treatment 2 (Coarse, No Air) follows the same coverage trend as Treatment 1. This treatment represents much larger, and fewer, droplets than Treatment 1, and yet the only obvious difference is reduced coverage in the middle of the canopy. There was little or no plume of spray drift during these applications.
Treatment 3 (Fine, Air) also followed the trend of reduced coverage as a function of canopy depth. Mean coverage was higher at the top of the canopy compared to the other two treatments. In fact, according to an ANOVA, deposit density was significantly higher in this canopy position than the other treatments, with 95% confidence. While mean coverage in the middle of the canopy was more than 2x that of Treatment 2 (see Figure 5), it was not statistically significant. There was no apparent difference at the bottom of the canopy. It is important to note that unlike Treatment 1, there was little or no spray drift plume during these applications (see Figure 4).
DepositScan was unable to detect coverage on any of the downward-facing papers. However, close visual inspection did reveal differences. Unsurprisingly, Treatment 2 (Coarse, No air) did not produce any underside coverage; Large droplets do not change direction mid-flight unless acted upon by some other force. Droplets can bounce and shatter, but that did not occur here. The Medium-Fine droplets created in Treatment 1 (Fine, No Air) and Treatment 3 (Fine, Air) did leave trace coverage on the downward-facing surfaces. Generally no more than 10-30 deposits on the entire 1 x 3″ surface, representing less than 1% total surface coverage. It could not be determined if the air used in Treatment 3 improved underside coverage over that of Treatment 1.
So, does it work?
Many experiments in peer-reviewed journals say yes. A perfunctory literature review reveals improved coverage in the middle and lower portions of cotton, potato, soybean and wheat canopies. Some of these experiments were based on coverage using fluorescent dyes, and some with water-sensitive paper. Others were based on efficacy and report improved crop protection. The actual implementation was highly variable with some authors recommending angling the air and nozzles forward 20-25°. Others proposed 30° backwards. Most agreed that the air speed should be set relative to the canopy density where higher speeds improved coverage deeper in the canopy, but did so at the expense of coverage in the higher canopy. Picture a bell curve on it’s side where the Y axis is canopy depth and the X axis is coverage. More air shifts the curve down the Y axis.
As for our demonstration, some interpretation is required. If an operator is spraying a contact product with limited or no translocative properties, then coverage becomes especially important. In order to improve coverage, higher volumes and finer droplets combined with slower travel speeds are often advised. This may be impractical as most operators prefer to use less water and drive faster.
When we used Medium-Fine droplets with no air assist, coverage was good (Figure 3) and better than coverage obtained using Very Coarse droplets. However, spray drift was unacceptable (Figure 4). When air-assist was engaged, we reaped the coverage advantage of smaller droplets and drift reduction as good or better than what we saw with coarser droplets. Unexpectedly, we did not see an obvious improvement in coverage from the air assist. This begs the question “If the spray didn’t drift, where did it go?” We would expect more uniform coverage under the boom, and some improvement in canopy penetration, but it’s likely our ad hoc experiment wasn’t sophisticated enough to reveal it.
In the end, there are obvious advantages to the air-assist mechanism. The ability to employ a finer spray quality when required, while greatly reducing spray drift and combating inclement weather to extend the spray window are appealing features. Research has clearly demonstrated that deep-canopy spray coverage and overall efficacy are improved when this system is properly adjusted to match spray conditions. Given my limited experience with the system, I’m not comfortable with suggesting it warrants lower dosages because of the expertise required to adjust the system. However, experienced operators have accomplished it in Europe.
As more high-clearance, self-propelled sprayers (such as the Hardi Alpha) are introduced to North America, we hope we’ll be seeing more air-assist options on boom sprayers.
