The Case for Aerial Application
Over Chemigation
By Scott Bretthauer,
Director of Education & Safety NAAA
Agricultural Aviation, Fall 2017
Chemigation is often used by growers in place of aerial application because they perceive that the cost is lower. Growers figure since they already have the irrigation equipment, and they will already be making an application to their crop, it makes the most economical sense to apply crop protection products via chemigation rather than hire an aircraft to make the application. A close examination, however, of the two application methods reveals they are vastly different in how they apply crop protection products. To help make the comparison, an example of a fungicide applied to field corn will be used to illustrate the differences.
To begin, let’s look at the intended target of both application methods. The purpose of irrigation is to provide water for the crop, in this case corn. Corn, like other crops, uptakes water from the soil via its root system. This means the target of irrigation is the soil. Compare that to an aerial application. Since the aerial application is focused solely on the crop protection product, the target is the leaf, not the soil. An examination of the labels for fungicides applied to corn indicates these are products designed to be foliar applied, not soil applied. Chemigation, therefore, does not direct the fungicide to its intended target.
"The Target Is The Leaf, Not the Soil."
Another thing to consider when comparing the efficacy of aerial application versus chemigation is the droplet size formed by the two different application systems. Droplet size is important for efficacy because it influences coverage and deposition. Deposition is especially critical for foliar applications because the droplet needs to deposit and be retained on the leaf once it reaches it, and not bounce or roll off the leaf. Research has indicated the ideal droplet size for making fungicide applications is a medium droplet spectrum, or around 300 microns in diameter. Agricultural aircraft can be easily set up to provide this droplet size by selecting an appropriate nozzle type, orifice size, operating pressure and nozzle deflection angle.
Gavin Morse Flying one of Gem Air Inc's AT-602 early morning over peas. 2017
An irrigation system, by comparison, makes an average droplet size around 2 mm, or 2,000 microns, almost seven times the recommended size for a fungicide application. Droplets can get as large as 5,000 microns at the outer edges of the spray pattern, 16 times the recommended size. Droplets of this size do not deposit on the target foliage very efficiently. They have a strong tendency to roll off the leaf, which of course is fine for the intended purpose of an irrigation system—to put water into the ground for the roots to absorb. For foliar applications, though, this results in much of the fungicide missing the intended target. These fungicides cannot be absorbed from the soil into the plant’s root system. Once they hit the soil, they are lost to the plant and provide no effectiveness against diseases.
Over-dilution of the fungicide in the water carrier is another problem chemigation faces that is easily overcome by aerial application. Several of the fungicide labels recommend using the lowest amount of water possible when using chemigation, and several go so far as to set a maximum amount of water that can be applied when using chemigation. One of the fungicide labels sets this maximum at 0.25 inches of water. This is equal to 6,789 gallons per acre (GPA). For an aerial application, a commonly used GPA is 2 GPA. The maximum amount of the fungicide that can be applied per acre is 14 fluid ounces per acre, regardless of the application method. For the aerial application at 2 GPA, this means the spray is 5.5 percent fungicide.
"The chemigation solution is only about 0.002 percent fungicide; 2,750 times weaker than the spray solution applied by the agricultural aircraft." -Dr. Bretthauer
One study found there was a 14-bushel-per-acre yield advantage for aerial application compared to chemigation.
A research study conducted by a fungicide manufacturer revealed how much better aerial application is than chemigation. There was a 14-bushel-per-acre yield advantage for aerial application compared to chemigation. When you consider the differences in application target, droplet size and spray concentration between aerial application and chemigation, it is not hard to understand why there was such a difference in yield.
