Coverage required: The completeness of coverage required depends primarily on how the chemical acts and characteristics of the pest being controlled. Some of the more common ways a pesticide might function are as a protectant, contact poison, systemic poison and translocated herbicide. Systemic insecticides and herbicides that are translocated within plants do not require complete coverage. Also, complete coverage is not required for insecticides that kill by ingestion. However, the coverage required is somewhat dependent on the mobility of the insect. Coverage required for contact insecticides is dependent on both insect size and mobility. Chemicals used for control of plant diseases usually require more thorough coverage to provide a protective barrier.
Coverage obtained: Coverage obtained is determined primarily by droplet size and the volume of material applied. As droplet size is decreased, better coverage is obtained with a given volume of material. If the diameter of spray droplets is reduced by one-half, the number of droplets produced with a given volume is increased by eight times. Conversely, doubling the diameter will reduce the number produced by eight times. Using small droplets to achieve good coverage can result in problems due to drift.
Drift: With some chemicals, drift of droplets outside the target area may cause damage. An applicator is financially liable for such damage. Always read the label on the chemical container to learn of possible drift hazards and to obtain application recommendations.
The amount of drift is determined by droplet size and wind velocity. The smaller the droplet and the higher the wind velocity, the farther a droplet will travel before being deposited. Little or no wind can be tolerated when using a chemical with a high drift hazard and with targets susceptible to dam-age in the vicinity.
Vertical air movement is also important when applying pesticides. Vertical air movement is affected by the change in air temperature as the elevation above ground level increases. If the temperature decreases with increasing distance above the ground, air near the ground rises, creating a ventilating condition (a lapse condition). If the air temperature increases with height (a temperature inversion), air near the ground will have very little or no vertical movement. This is the condition that exists when smoke is trapped near the ground. On a daily basis, ventilating (lapse) conditions normally exist from about midday to evening. Temperature inversions normally occur from late evening to morning.
A ventilating condition is desired when applying sprays with large droplets, which have only a small percent of small driftable droplets. The small droplets are dispersed over such a large area that the possibility of damage from drift is greatly reduced. A temperature inversion is desired for pesticides being applied as aerosols or dusts.
Volume applied: The quantity of material applied per unit of area by a sprayer depends on the shape and size of the hole (orifice) in the nozzle, pressure in pounds per square inch (PSI) at the nozzle, nozzle spacing on the boom, speed of travel and concentration of the A.I. in the chemical-water mixture.
A discussion of each of these items follows:
Nozzle size: Nozzles are selected that deliver the correct amount of material per unit of time. However, during use the size of the hole in the nozzle may change rather rapidly due to wear, plugging with dirt and damage caused by trying to clean a plugged nozzle with wire, etc. The rate of nozzle wear depends on the type of material being sprayed, the amount of dirt in the water and the material from which the nozzle is made. Wettable powders (WP or W) will cause rapid wear in aluminum, brass and plastic nozzles. Stainless steel nozzles and nozzles having tungsten carbide orifices are more resistant to wear.
The Prairie Agricultural Machinery Institute in Canada has found nozzle tip wear sufficient to cause a 10 percent increase in nozzle delivery after only 50 hours of use. This is the major reason for checking the flow rate of a nozzle frequently.
Pressure: As the pressure in pounds per square inch (PSI) at the nozzle increases, the flow rate through the nozzle increases. The flow rate is directly related to the square root of the pressure. Thus, doubling the pressure increases the nozzle flow rate by 1.4 times; tripling the pressure increases the flow rate by 1.73 times; and increasing the pressure by four times doubles the flow rate.
Nozzle manufacturers publish flow rate in gallons per minute (GPM) versus pressure for their nozzles. You may find that nozzles do not deliver the amount specified by the manufacturer. This might be caused by an inaccurate pressure gauge. It could also be caused by the fact that the actual pressure at the nozzle is less than that indicated by the pressure gauge. Such pressure losses can be caused by restrictions in the line, buckling of supply hoses or partially clogged nozzle strainers. Using nozzles larger than those for which the sprayer was designed may cause a reduction in pressure at the nozzle.
Nozzle spacing: If a given size nozzle is spaced farther apart on the boom, the application rate in gallons per acre (GPA) is reduced. For example, spacing nozzles (of the same size) 40 inches instead of 20 inches apart on the boom reduces the GPA by one-half.
Speed of travel: Application rate varies inversely with the speed of travel. That is, if the speed of travel in miles per hour (MPH) is doubled, the GPA is reduced by one-half. If the MPH is reduced to one-half, the GPA is doubled. Thus, speed of travel is very important in obtaining proper application rate. If the chosen MPH is not maintained, improper application rates result. To find the speed in MPH, divide 120 by the number of seconds required to travel 176 feet. For example, if 20 seconds are required, the speed in MPH is 120 divided by 20, or 6 MPH.
Concentration of the mixture: The spray applied is usually a mixture of the material containing the active ingredient (A.I.) and water. Because a fixed number of GPA of the mixture is applied, the amount of active ingredient applied per acre (A.I.A.) is determined by how much of the A.I. is mixed with each gallon of water.
Placing the proper quantity of A.I. in a spray tank of water does not assure proper amounts of A.I.A. Some spray materials tend to settle to the bottom of the spray tank. Thus unless adequate agitation is provided, the first material from the tank will have a higher concentration of A.I. Wettable powders (WP or W) have the greatest settling tendencies and require continuous agitation. Emulsifiable concentrates (EC or E) require little agitation. Soluble powders require little agitation after they are dissolved. Flowables (F or L) require moderate agitation.
Adequate agitation requires about three gallons per minute of flow through the agitation line per 100 gallons of tank capacity. Inadequate agitation can result if the agitator control valve is not adjusted properly or if the pump has inadequate capacity. The pump must have sufficient capacity to provide nozzle flow plus agitation flow plus a reserve capacity to allow for pump wear. Centrifugal pumps require only a small reserve capacity (10 to 15%) because of low wear rates. Roller pumps usually wear more rapidly and should have more reserve capacity (25 to 30%).
Over-agitation may result in foaming, which can be corrected by restricting agitation flow rate by adjusting the agitator control valve.