What spray volume is necessary to provide good coverage?

What spray volume is necessary to provide good coverage?

The general belief among most agronomists, technical staff and university professors is the spray volume needs to be adjusted according to the area of the leaves i.e., the foliar index. Many even go so far as to calculate that while a young recently emerged crop could be sprayed with only one gal/acre, it is necessary

The general belief among most agronomists, technical staff and university professors is the spray volume needs to be adjusted according to the area of the leaves i.e., the foliar index. Many even go so far as to calculate that while a young recently emerged crop could be sprayed with only one gal/acre, it is necessary to increase the spray volume accordingly as the crop grows. A similar comment was made at the recent NAAA convention by a chemical company representative when I inquired what spray volume did the company recommend for a specific crop. The answer was 2-3 gals/acre, but emphasized that for better coverage they recommend increasing to 5 gals/acre. I have listened to numerous professors emphasizing that it is necessary to increase the spray volume to 10-15 gals/acre for a crop such as citrus and some vegetable crops in California to 30 gals/acre.

The reality is totally the opposite; for improved crop penetration and coverage the droplet size needs to be reduced, not increased, and most definitely not to increase the volume. When the volume is increased the chemical concentration is reduced and each drop is therefore less effective. When the spray volume is increased, the risk of evaporation is also increased due to the lower concentration and hence a higher risk of drift.

There are also mechanical complications that few agronomists are aware of, especially so with faster aircraft. A turbine aircraft may easily be spraying 18-20 acres a minute and if the agronomist specifies an application volume of 10 gals/acre this means a flow rate of approximately 200 gals/minute! To achieve this flow per minute, it is necessary to install very many large nozzles that produce a very irregular or broad droplet spectrum. Simply stated, large nozzles produce larger droplets and results in poor crop penetration and bad coverage. 

Productivity: One of the largest aerial applicators in Brazil, Globo Aviação based in Imperatriz in the northern state of Maranhão, operates seven turbine aircraft on very large farms. Last year, one aircraft sprayed 300,000 hectares (750,000 acres), all of it using volumes of 5-10 liters/hectares (1/2 to 1 gal/acre).

Product labels: Spray volumes on product labels vary greatly from one country to another, even for the same crop, which should raise the question, “Why”? The reality is that practically all labels are produced using the system “copy and paste” on the premise that if a specific text has been approved by various authorities, it will be approved yet again without having to conduct and present data to confirm the product performance. Very few companies test products using aerial application equipment because regulations only permit spraying a very small area of four acres [1.62 hectares]. This would be quite a challenge to spray such a small garden using a turbine aircraft!

What volume is really required? –  (See table for label rates/volumes that work.)

Mosquitoes, locusts, boll weevil and spruce budworm control: For these targets, the aircraft must fly high and for mosquitoes and locusts also operate under adverse conditions of higher temperatures and low humidity. The solution is painfully obvious, remove the water and apply pure chemical or product mixed with a non-volatile carrier such as soybean oil that will not evaporate during the delivery of the droplets.

The result is Ultra Low Volume (ULV) applications with drift spraying. Since the droplets do not evaporate, it is relatively easy to estimate the downwind drift, not so with water since the rate of evaporation changes by the minute due to any change in temperature and/or relative humidity.

Extreme conditions: An estimated 80-90% of all insecticides and fungicides in South America are applied at low volumes of 5-10 liters/hectare (0.5-1.0 gal/ac) with excellent results under conditions of high temperature and often low humidity. In practice, the rule is quite simple as the conditions become more adverse (increase in temperature or reduction in relative humidity), the operator must reduce the volume of water and spray volume to protect against evaporation loss.

Text from a product label for Bacillus Thuringiensis in the USA: “For aerial application use at least three gallons of water per acre; exception being arid areas, where 5 to 10 gallons are required.” This recommendation has been proven to be completely wrong and has resulted in numerous product failures and increased drift. For forestry applications, the same product is labelled for application at ULV rates.

When the droplet size is increased, this results in less crop penetration and a dramatic reduction in effectiveness of “contact acting” products such as miticides, many fungicides and, of course, the pyrethroid family of insecticides.

Positive effects: Lowering spray volumes can result in some very positive effects if conducted in a scientific manner with careful attention to mixtures and application techniques. Having worked worldwide with all types of products on just about every crop on this great planet, field experience has proven that when done correctly all products work better when applied with less water and attention is paid to uniform droplet size.

In Europe, spray volumes were on the increase for years reaching 40 gals/acre with ground machines. Now, the trend is to reduce volumes with many recommending eight gals/acre and some even five gals/acre.

Recommendation: Pilots and agronomists should share their experiences and put pressure on researchers and chemical companies to examine the benefits of low volume applications so U.S. agriculture can maintain a leading role instead of being penalized.

Refuelling on the airstrip from the Globo Aviação support truck.

At the end of every working day, the calibration of each aircraft’s Micronair AU5000 rotary atomizers is checked.

Globo Aviação treats mainly soybeans with their AT-802 and AU5000 Micronair rotary atomizers applying a very low rate of 5-8 liters per hectare, depending on the viscosity of the tank mix. 

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