Applying pesticides requires a high level of skill and knowledge. Increases in the size and complexity of sprayers over the years require even more attention to efficiency, efficacy, and safety. Although each crop requires a slightly different approach to the application of pesticides, some general principles apply to almost all spraying situations. Following these principles will help achieve better control of the problem.
These major principles include:
When applying pesticides, certain tasks are required for maximum biological efficacy. These include:
Only the most critical issues related to application of pesticides are discussed in this publication. For details of these and other topics, follow the web links to additional educational resources throughout this publication.
Although each component of the sprayer plays a role in achieving success in pesticide application, nozzles play the most significant role. Nozzles come in a wide variety of types and sizes. Each type is designed for a specific target and application. Most manufacturers’ catalogs and websites have charts showing which nozzle type is best for a specific job. Any of the factors below may be the deciding one when selecting the most appropriate nozzle for the job.
Once you determine the best nozzle that will be best for a specific spraying situation, you need to determine the appropriate size of that nozzle size that provides the application rates (gal / acre) prescribed by product labels under various operating conditions (spray pressures and travel speeds).
More information on selecting nozzle type and size are outlined in OSU Extension publication FABE-528, “Selecting the Best Nozzle for the Job.” (ohioline.osu.edu/factsheet/fabe-528)
Although complete elimination of spray drift is impossible, problems can be significantly reduced by awareness of the major factors that cause drift, while taking precautions to minimize their influence on off-target movement of droplets.
Follow these tips to minimize spray drift:
Extensive information related to factors influencing spray drift, is in OSU Extension publication FABE-525. “Effect of Major Variables on Drift Distances of Spray Droplets.” (ohioline.osu.edu/factsheet/fabe-525).
To achieve effective pest control, choose the nozzle and set up the application equipment based on what is being controlled and the part of the plant canopy that is being targeted. For example, when applying a fungicide to manage Fusarium head blight or “head scab,” on small grains, the target is the head, not the leaves. When a fungicide is applied using nozzles that direct the spray downward, most of the product is deposited on the leaves or the ground and not on the head. However, when trying to control diseases such as soybean rust, the target should be the leaves, especially ones in the lower part of the canopy. When spraying for soybean white mold, the most critical area that needs to be treated with fungicides is where flowering takes place. Nozzle selection has a significant influence on whether or not the droplets reach the specific target location in the canopy.
The following trends have emerged from two multi-year Ohio State studies on target deposition for diseases on soybeans and wheat:
A sprayer can only be effective, efficient, and safe if properly checked and calibrated well before the sprayer is taken to the field, and periodically during the spraying season. Some may argue that most sprayers are now equipped with sophisticated rate controllers and ground speed sensors, and calibration is not necessary. Unfortunately, not all electronic controllers can detect flow rate changes on each nozzle on the boom, and none can detect changes in spray pattern. If the ground speed sensor works based on revolutions of the tractor wheels, the ground speed determined may not be accurate, because of the slippage that may occur under some ground conditions. Manual calibration is always good to ensure the electronic controllers and sensors are working properly.
The primary goal with calibration is to determine the actual rate of application in gallons per acre, and then make adjustments if the difference between the actual rate and the intended rate is greater or less than 5% of the intended rate. Although rate controllers can regulate the flow rate of nozzles to keep the application rate constant, a manual calibration at least once a year is needed to ensure the rate controller is functioning properly.
Before starting calibration, make sure the sprayer has a good set of nozzles. Nozzles wear through extended use, causing over-application and/or non-uniform application. Some nozzles or screens may become clogged causing under-application. Clean all clogged nozzles and screens. Check the output of all the nozzles for a given length of time at a given spray pressure. Compare the output from each nozzle with the expected output shown in the manufacturers’ catalog for the selected nozzle at the same operating pressure. Replace any nozzles showing an output error of more than 10% of that recommended for a new nozzle by the manufacturer.
There are several ways to calibrate a sprayer. One easy method, the 1/128th method, is explained in the OSU Extension publication FABE-520, “Calibrating Boom Sprayers.” (ohioline.osu.edu/factsheet/fabe-520).
Although your sprayer may be in good condition and calibrated frequently, if the correct amount of chemical is not put into the tank, it can still result in unsatisfactory pest control. Labels list two recommended application rates: volume of spray mixture (pesticide and water) applied per unit area (gallons per acre, ounces per 1,000 square feet, etc.), and the amount of actual chemical applied per unit area (ounces, pints, or quarts per acre or 1,000 square feet). The first recommendation (volume of spray per unit area) is attained through proper calibration and operation of the sprayer. The second label recommendation requires not only proper calibration and operation, but also the right concentration of the actual product applied.
The amount of chemical needed per tankful depends on the recommended rate and the size of area that can be treated per tankful of spray. Calculations and concepts are the same whether using a manual backpack sprayer with a five-gallon tank, a lawn mower/ATV sprayer with a 15-gallon tank, a pull-type sprayer with a 500-gallon tank, or a 1,500-gallon self-propelled sprayer. The only difference is in units. For small sprayers, the rate may be expressed in ounces, quarts, or gallons per 1,000 square feet. For boom sprayers, the application rate is usually given in gallons per acre (gpa). Detailed information on how to calculate the proper amount of chemical to add to the spray tank is provided in the OSU Extension publication FABE-530. “How Much Chemical Product Do I Need to Add to my Sprayer Tank.” (ohioline.osu.edu/factsheet/fabe-530)
How the chemical is deposited is as important as the amount applied. Maintain uniform deposition of spray material on across the entire width of the target area. Non-uniform coverage results from using misaligned or clogged nozzles, using nozzles with different fan angles, or from uneven nozzle height across the boom. These common problems result in streaks, untreated areas, or over-application of chemicals. Nozzles which produce uniform or “even” distribution of spray across the spray pattern (no tapering of spray closer to the edges of spray pattern) should be used when spraying products directly on targets, such as young vegetable seedlings in a narrow band for insecticide application, or the area between rows of vegetables for weed control. With these types of nozzles, no overlapping of spray patterns are required since the product is evenly distributed across the spray pattern. However, when making broadcast applications covering the entire area under the boom, the regular flat-fan nozzles should be used. These flat-fan broadcast nozzles produce spray patterns with heavy spray volume discharged from the center of the spray, and the volume tapers off towards both end of the triangular-shaped spray pattern. When using such flat-fan broadcast nozzles, spray patterns from adjacent nozzles must overlap to obtain uniform coverage across the spray swath, as shown in the figure above. A low boom or a boom set too high creates a poor pattern and misapplication. Check the nozzle catalog to determine the proper boom height recommended for different nozzle types and spacings.
Make sure the nozzles are not fully or partially clogged. Clogging not only changes the flow rate, but also the spray pattern. Do not use a pin, knife or other metal object to unclog nozzles. This damages the nozzle orifice, causing changes in the flow rate and distortions such as streaks in the spray patterns. Use a soft brush, or pressured air to unclog nozzles. In addition to clogging, mismatched nozzle tips on the boom and uneven boom height are the most common causes of non-uniform spray patterns. They can all cause streaks or untreated areas that result in insufficient pest control and economic loss.
Most product labels outline some vague and general statements when referring to the application of products, such as “use nozzles that provide thorough coverage of the canopy.” There is no explanation about what “thorough coverage” represents or how to achieve it. It is your job to select a nozzle and operate it under certain pressure conditions to achieve “thorough coverage.” Some labels give specific recommendations on nozzles such as: “use nozzles that provide medium spray quality” or “do not use nozzles that produce droplets in coarse or larger spray qualities.” When there is this kind of specific information on the label, in addition to satisfying the gallon per acre requirement, also satisfy the droplet size requirement. Under these conditions, the operator’s job is to choose a nozzle type and size that satisfies the required droplet size while meeting other requirements such as gallons per acre application rate. For example, the nozzles shown in the figure below all produce the same flow rate (0.2 gallons per minute) at the same or slightly different pressures, but each provides a different spray quality (droplet size). Check the information given in nozzle manufacturers’ catalogs to make sure the nozzle provides the required spray volume at a given travel speed and spray pressure, as well as the spray quality (droplet size) under these same operating conditions.
Typically, systemic products do not require thorough coverage of the target, so coverage is not a significant issue when spraying them. However, contact-type products work best when applied evenly on the surface and coverage is maximized.
There are two options to improve coverage: 1) Increase the pressure. Higher pressures lead to creation of smaller droplets which provide improved coverage. 2) Increase spray application rate (gal/acre). When satisfying label requirements or recommendations related to coverage, in addition to the type and size of nozzle used, the rates of application can also help achieve higher levels of coverage. As shown in the following figure, regardless of the spray quality (droplet size) class, increasing the spray application rate increases product coverage.
Keep in mind that an increase in spray pressure always results in an increase in the spray volume contained in smaller droplets. This most likely results in higher levels of spray drift, especially when using conventional flat-fan or cone nozzles with smaller orifice sizes. Relying on increased pressure to improve spray coverage should be the last option, practiced only when the weather conditions are not conducive to increasing spray drift. The first option to improve spray coverage should be to select the right type and size of nozzles, followed by the next option, increasing the spray application rate.
Application Rate Affects Spray Coverage
Droplet Size Application Rate: 12 gpa Application Rate: 8 gpa Application Rate: 4 gpa Fine Medium Coarse Very Coarse Extra CoarseThe labels of 2,4-D or Dicamba herbicides include specific requirements for nozzles and operating pressure ranges. These strict requirements are put on the labels to eliminate off-target movement or drift of spray droplets. If you use any other type and size of nozzle and operate them outside the pressure range requirements given by the pesticide manufacturers, you are violating the pesticide label, and therefore the law. Remember, the label is the law!
It is your responsibility to comply with the requirements on pesticide labels. A list of currently approved nozzles and the operating pressure ranges on labels of several commonly used 2,4-D and Dicamba products can be found at: pested.osu.edu/sites/pested/files/imce/ApprovedNozzles.pdf
The table at this site is provided for information purposes and may not be up-to-date. Check the manufacturers’ websites and read the product labels for the most current information. Do not assume you do not have to worry about checking the label, because the same product was applied in a previous year. A nozzle required for the same product last year may not be on the label this year, or the operating pressures might have changed.
A rate controller on a sprayer was one of the most significant developments more than four decades ago. These gadgets enabled sprayer operators to keep the application rate constant, regardless of changes in ground speed. They are now a standard component of every new sprayer sold. No other significant developments occurred in spray technology for a couple of decades until auto steering/ guidance of tractors or self-propelled sprayers, and Global Positioning System (GPS) technology entered in how we operate farm equipment.
Until the arrival of the auto guidance technology, sprayer operators relied on using either foam markers or other mechanical means to mark the edge of each spray pass and avoid excessive overlaps. Neither approach was reliable, especially when using sprayers with wide booms and driving long distances in the field before turning back for the next spray swath. Under these conditions, the markings could be undetectable, especially with foam markings and when operator fatigue becomes an issue. When making multiple back-and-forth passes in the field, using auto guidance technology enables precise positioning of the sprayer, resulting in minimum overlap between each pass.
An extension of this technological development is the automatic on-off of nozzles, as a group on various sections of the sprayer boom, depending on the pre-programmed spray map using GPS data for the field sprayed. For example, when approaching grass waterways, a section of the boom intersecting the grass waterway can be turned off automatically until that section is clear off the waterway. This technology evolved into independent control of each nozzle on the sprayer boom regardless of where it is located.
Independent control of nozzles may not be important if the spray pass is always on a straight path, clear of obstacles. However, every pass includes a turn at the end and the sprayer may be following contours during spraying. Under these spraying conditions, the travel speed of nozzles located towards the ends of the boom are much faster than the nozzles close to the tractor and center of the boom. With conventional control systems, this leads to under application of chemicals on the ground under the nozzles towards the outer edges of the boom, while the ground near the center of the boom receives a higher dose of chemicals. This inaccuracy can be avoided using the individual nozzle flow control technology, usually referred to as the pulse width modulation (PWM). It is available on new sprayers manufactured by several companies and can be retrofitted on other sprayers lacking this technology. In addition to providing individual flow rate control at each nozzle, the PWM technology also allows the sprayer operator to keep the droplet size constant, regardless of changes in spray pressure. This is a major drawback of the conventional rate controllers without PWM technology. With conventional rate controllers, when the ground speed of the sprayer goes up, the system pressure goes up to maintain the constant gal/acre application rate . When pressure increases, size of droplets discharged from the nozzle decreases. Detailed information on how this technology works is provided in a Kansas State University publication available online (bookstore.ksre.ksu.edu/pubs/MF3314.pdf). Additional resources can be found on the web by searching for “pulse width modulation spraying.”
One other significant technological development available for integration into sprayers is sonar sensors on the boom that keep the boom sections retain the desired boom height. As cited earlier, spray coverage on the target is not uniform if the boom height deviates from the optimum recommended height. It is highly recommended that sprayers with large booms are equipped with sonar sensors to keep all sections of the sprayer boom at the same height during spraying.
The information presented in this fact sheet will help achieve better performances from applied pesticides. However, only the most significant issues are highlighted in this publication. For detailed coverage of these and other topics, visit the web sites of the resources listed throughout the publication. There are equally important topics not covered in this publication, including: general inspection of the sprayer, importance of proper product agitation in the sprayer tank, adequate size hoses and fittings, determining sprayer setup for acceptable application rate, selecting proper boom height based on nozzle angle and spray overlap, compatibility of products mixed, cleanliness and pH of water used to mix the products in the tank, proper cleaning of the sprayer tank, spray additives that can enhance product performance, and handling pesticide waste and empty containers. Detailed information on these and other topics not included in this fact sheet can be found in OSU Extension publication FABE-527, “Best Management Practices for Boom Spraying.” (ohioline.osu.edu/factsheet/fabe-527). Another excellent source of information on a wide range of topics related to pesticide application technology is sprayers101.com.
The author thanks Dr. John Fulton and Dr. Ajay Shah, Professors at Department of Food, Agricultural and Biological Engineering, The Ohio State University; and Mary Griffith, Agriculture and Natural Resources Extension Educator, Ohio State University Extension, for reviewing this publication and for their editorial contributions.
Ozkan, E. Selecting the Best Nozzle for the Job. Ohio State University Extension publication FABE-528. ohioline.osu.edu/factsheet/fabe-528
Ozkan, E. and R.C. Derksen. Effectiveness of Turbodrop® and Turbo TeeJet® nozzles in drift reduction. Ohio State University Extension publication FABE-523. ohioline.osu.edu/factsheet/fabe-523
Ozkan, E. and H. Zhu. Effect of Major Variables on Drift Distances of Spray Droplets. Ohio State University Extension publication FABE-525. ohioline.osu.edu/factsheet/fabe-525
Ozkan, E. Calibrating Boom Sprayers. Ohio State University Extension publication FABE-520. ohioline.osu.edu/factsheet/fabe-520
Ozkan, E. How Much Chemical Do I Need to Add to my Sprayer Tank? Ohio State University Extension Publication FABE-530. ohioline.osu.edu/factsheet/fabe-530
Ozkan, E. and A. Womac. Best Management Practices for Boom Spraying. Ohio State University Extension publication FABE-527. ohioline.osu.edu/factsheet/fabe-527
Sharda, A., T. Griffin, L. Haag, D. Mangus, J. Fulton and J. Slocombe. Pulse Width Modulated (PWM) Technology for Liquid Application. Kansas State University Extension publication bookstore. ksre.ksu.edu/pubs/