Vern Hofman, Extension
Agricultural Engineer
NDSU Extension Service
Sprayer performance is difficult to measure due to various factors. Some include nozzle type, nozzle orientation, operating pressure, application rate, plant shape, leaf orientation, and climatic conditions. The recommended chemical rate is listed on the container, but the best method of application is more difficult to determine. Information on application is helpful to improve pesticide efficacy, which is especially important when good plant coverage is needed for best pest control. Trials performed the last three years attempted to measure the effectiveness of nozzle type, operating pressure, nozzle orientation, application rate and application method.
The WRK pattern test equipment was used to measure canopy penetration of spray all three years. This was done by mounting a white floss cotton string at various levels in a plant canopy. Three strings were used, placed at different levels, through a 50-foot section of row. This allowed string placement at the base of the plant as close as possible to the soil level; the second was placed approximately three to four inches above the lower string and the third was placed on top of the row of plants. This provided information to determine relative deposition of spray in a plant canopy as compared to the amount landing on top of the plant.
Up to five strings may be placed in a plant canopy and plotted on one graph. This was not done due to the relatively low profile of sugarbeet plants.
The plants containing the three strings were sprayed with WT Rhodamine fluorescent dye at a concentration of 135 parts per million (ppm) applied with 20 gallons of water per acre. The trial at 10 gallons per acre (gpa) were done with a dye concentration of 270 ppm. The aerial trials were done at 5 gpa with a dye concentration of 540 ppm. This was done to keep the dye concentration per acre equal.
The dye collected on the string was read with a Model 112 Sequoia-Turner fluorometer. It is designed to pull the 50 feet of string through the fluorometer at a predetermined speed and provide an averaged reading of the dye over every four inches of string. This provided 150 data points over a 50 foot distance. All trials were repeated three times and the averaged values are shown in Table 1.
In 1989, two different nozzle arrangements, applying 20 gpa at three different pressures, were tried. The two nozzle arrangements included a broadcast application, which used 110 degree flat fan nozzles, spaced 20 inches apart on the boom mounted 13 inches above the crop canopy. The second nozzle arrangement used three 80 degree flat fan nozzles mounted over the top and on both sides of the plant. The nozzles were placed approximately eight inches from the plant. Another broadcast trial was used which applied spray similar to the other but at 10 gpa.
In 1990, two additional nozzle types were tried applying 20 and 10 gpa at three different pressures of 40, 100 and 180 psi. The two nozzle arrangements included a broadcast application, using 800 twin orifice nozzles mounted over the top and on both sides of the plant. The nozzles were placed approximately eight inches from the plant. Another broadcast trial was used which applied spray similar to the other but at 10 gpa.
The aircraft was a Thrush Commander that used 24 Delavan LF15 flat fan nozzles and 22 Lurmark 20-F80 flat fan nozzles.The spray pressure was 42 psi, the application height six feet with a flying speed of 112 mph.
In 1991, flat fan and hollow cone nozzles were used. The flat fans were used for broadcast application and hollow cones were used for directed application. Both were applying 20 gpa at 40, 100 and 180 psi of pressure.
The aircraft trial was done with a Pawnee model D at 5 gpa. It was equipped with 42-D8/45 nozzles operating at 40 psi. The aircraft speed was 105 mph.
All trials were completed on sugarbeets planted in rows spaced 22 inches apart. The trials were completed during August at temperatures ranging from 60°F to 90°F and wind conditions ranging from approximately 3-12 mph.
Table 1 shows the relative deposition of spray into sugarbeets by ground sprayers and aircraft. A considerable amount of variation shows up on individual strings. This is due to the string placement in the crop and the random leaf arrangement which may hide the string or expose the string to spray in canopy voids.
A significant difference in relative spray deposition occurs between string levels. It is difficult for spray to penetrate deep into the plant canopy for pest control with any spray system.
Comparing the relative deposition of broadcast applications over the three years work, it is very easy to apply chemical to the top surface of the crop. To get chemical to penetrate to the base of the plant, excluding a few exceptions, broadcast applications should be done at the 100 psi pressure and 20 gpa. Higher concentrations of dye show up at this pressure as compared to 40 or 180 psi or when applied at 10 gpa. Higher application rates tend to give better plant coverage.
When three nozzles are used to direct spray at three different sides of the plant, penetration into the base of the plant is better in almost all trials with the three pressures as compared to broadcast application. Comparing deposition in the bottom of the canopy for three directed nozzles shows an increase with increasing pressure for 1989 and 1991. In 1990, a decrease in deposition shows up with an increase in pressure. This may be caused by the twin orifice nozzle which will produce a smaller droplet which is more difficult to control. When fine droplets are produced, they must hit their target quickly or they will lose their velocity and be lost. Nozzle type may have an affect on spray penetration, but it is difficult to make a comparison due to the varying crop conditions from one year to another.
The aircraft trial in 1991 was completed in the same field as the ground sprayer work. In 1990, the aircraft trial was done in another field and should not be directly compared. The penetration of spray by aircraft in 1991 compares closely to the broadcast ground sprayer. In 1990, the aircraft showed improvement in lower plant penetration than the broadcast ground sprayer. Most of the aircraft trials compared to the three directed nozzle trials show reduced chemical deposits in the lower part of the canopy. Air moving over aircraft wings helps break up large spray droplets and carry the spray into the plant canopy.
A basic understanding of droplet size affect on pesticides is important when selecting techniques for foliar application. The relationship between droplet size and coverage is complicated which sometimes results in misconceptions. It is generally believed that applying small droplets at high spray pressures will provide increased control with low volumes of spray solution. Research does not substantiate this theory. It is true that atomizing an amount of spray solution into smaller droplets will increase the possible coverage but consideration must be given to droplet evaporation, drift potential, canopy penetration and deposition characteristics.
A typical spray nozzle will produce a wide range of droplets from a few microns up to several hundred microns. (There are 25,400 microns in an inch.) For example, an 8001 flat fan nozzle operating at 40 psi will produce about 40 percent of the total number of droplets of 50 microns or less in size but will contain only about one percent of the total spray volume. Another 15 percent of the number of drops are in the 50 to 120 micron size and contain a little more than two percent of the volume.Another 40 percent of the total number of drops is in the size range of 120 to 250 microns and contain almost 70 percent of the volume. Fine drops contain only a small volume but when the fine drops deposit off target, major drift problems can occur. Increasing sprayer operating pressure will substantially increase the fine drops and reduce the number of large drops.
Producing fine drops may sound good in theory, but getting them to travel a distance and deposit on the target is difficult. Increasing spray pressure to "drive" droplets into plant canopies to increase coverage is not verified with current research. As drop size is reduced, the distance a drop will travel after it is emitted from a nozzle under pressure is very small. From Table 2, a 50 micron droplet will reach falling speed in three inches after it leaves the orifice. Then it must depend on gravity to carry it to the target. The third column in Table 2 lists the falling speed of various drop sizes. A 50 micron drop will fall.25 feet per second (three inches/second) so for example, if a nozzle is mounted 18 inches above a crop, a 50 micron drop will move three inches due to the emitting pressure and then must move the other 15 inches by gravity. This will take about five seconds. This assumes the top of the plant is the target. If spray drops must travel into the plant canopy, more time will be needed due to the greater distance. The time to evaporate droplets of various sizes is listed in column 4 of Table 2. A 50 micron drop will evaporate in 1.8 seconds to a 17 micron drop which means about all the water carrier will evaporate before it reaches the target. As a result, the drop gets smaller which compounds the problem of getting the drop deposited on the target. Producing even smaller drops increases the problem of depositing spray on the target.
As a droplet approaches the target, a fine droplet may be deflected from the plant surface by the air flowing along the surface. Depending on drop size, speed and the resistance (drag) of air on the drop, it may deposit on the target or be deflected away from the surface by the moving air across the plant surface.
For a typical application of pesticide with water carrier,drops smaller than about 75 to 100 microns will evaporate to a residual core of pesticide before reaching the target. Producing small drops looks attractive to cut carrier and possibly active chemical, but getting fine droplets on target is extremely difficult. Placing nozzles closer to the target such as using three nozzles in a directed application will help spray depositon the target better when using higher pressures.
1991 Sugarbeet Research and Extension Reports. Volume 22, pages 60-64.
