Top Crop Manager

Optimizing spray application to get the best results

An optimal spray application eliminates targeted weeds, disease or insects, or desiccates the crop so maximum crop yield can be achieved. Conversely, non-optimal spray application may not produce the intended results and can produce unintended or undesirable results:

  • If the application is ineffective, weeds, insects or disease are not eliminated and crop yield is reduced.
  • If the wrong chemical or incorrect application rate is used, the crop can be stressed or damaged, reducing the yield.
  • Spray drift on to adjacent fields can result in unintended chemical application and damage.
  • Re-spraying increases input costs.
  • There are a number of well-known factors that must be considered to ensure optimal spray application. Chemical choice, application rate and timing, as well as nozzle selection are all key factors, information for which is available from manufacturers, chemical suppliers, crop guides, agriculture extension divisions and agricultural consultants

  • Droplet size
    During the past several years, applicators have become more aware of the importance of spray droplet size in achieving optimal spray application. While smaller droplets give better coverage in some situations, they are also more susceptible to drift. Larger droplets provide better drift control, but may not provide effective coverage. So choosing the best droplet size for your spray application is often a trade-off between coverage and drift control.

    Individual droplets are measured in microns (a human hair is approximately 100 microns in diameter). The spray from a nozzle is made up of a wide range of droplet sizes and is measured as VMD (volume median diameter).

    The VMD means that half of the total spray volume consists of droplets larger than the VMD and the other half of the total spray volume consists of droplets smaller than the VMD. A large percentage of the droplets are near the VMD.

    A related factor to keep in mind is that, in a given volume of water, the smaller the droplet, the more droplets are available to deposit on the plant. The total volume of spray shown on each leaf in the illustration is the same. Only the droplet size is different. In fact, in an actual spray application there would be a range of droplet sizes produced by the nozzle.

    Some chemicals are more effective when applied with a certain droplet size, while others are effective when applied with larger droplets. Within chemical groups, contact products (mode of action Groups 6, 10 and 22) usually benefit more from greater volume than from smaller droplets. Systemic products (Groups 2, 4 and 9) are more flexible. Group 1 is systemic, but not as much as others, and also targets grassy weeds, so finer sprays are better. Mode of Action tables are available from crop guides, agriculture extension divisions and agricultural consultants. The recommended VMD / droplet size information may be shown on the product label or is available from the chemical manufacturer.

    If you are tank-mixing to spray both grassy and broadleaf weeds, this creates a challenge with single spray nozzle application systems because you can only spray one VMD (droplet size). While all spray tips produce a range of droplet sizes, the largest percentage of the droplets produced will be in a certain droplet size range and will only be most effective on one category of weed. Spraying with a dual nozzle system, such as Wilger’s Combo-Rate nozzle bodies, provides a solution to this problem by allowing you to spray with more than one spray tip at the same time. This dual-nozzle system would provide a wider range of droplet sizes, allowing for example, the tank-mix spray solution to be applied with a coarse and medium droplet size to help improve efficacy.

    Drift control
    With the need to spray more acres more frequently, drift control has become increasingly important. While wind speed is an important drift factor, droplet size also plays a significant role, with small droplets able to drift substantially farther than large drops.

    All spray nozzles produce a range of droplet sizes at a given pressure. The drift control nozzle models, such as Wilger’s SR, MR and DR models, are specifically designed to produce larger droplets at a given pressure. Other nozzle brands, such as the Air Bubble Jet, and Greenleaf’s TurboDrop or AirMix, are also available to control drift. Variable rate nozzles have also entered the market in the last few years. While drift control nozzles are an important step towards reducing spray drift, there are several other factors to consider:
  • A nozzle with a larger orifice produces larger droplets than one with a smaller orifice (at a given pressure).
  • Most nozzles produce smaller droplets at higher pressures.
  • A 110-degree nozzle has a smaller VMD than an 80-degree nozzle (for the same size of nozzle and pressure).
  • The closer the nozzle is to the ground, the less susceptible the spray is to drift.
  • On auto-rate controller-equipped sprayers, the VMD decreases at faster speeds and increases when slowing down, due to the flow rate/pressure change required to compensate for the change in speed.
  • Some considerations for reducing spray drift are:
  • Spray at the highest recommended application rate (water volume).
  • Use the largest size nozzle that gives the required flow.
  • Spray at the lowest recommended nozzle pressure.
  • Use a drift-reduction nozzle model.
  • Spray at the lowest height recommended for the nozzles being used.
  • When near adjacent fields, slow down and reduce the pressure (an auto-rate control automatically reduces the pressure as the speed reduces).

Spray nozzle calibration and maintenance
Even with auto-rate controller-equipped sprayers, it is important to verify nozzle output. Auto-rate controllers work by monitoring flow to the boom sections that are turned on. While this ensures that the total correct amount of liquid is delivered to the booms, the application rate from the individual nozzles can be incorrect.  

Nozzle output can be checked by using a jug and stopwatch to collect a spray sample or a flow type tester and then calculating the application rate. Another option for checking nozzles is the “Quick Calibrator,” which collects the spray sample for approximately 30 seconds and automatically displays the flow and application rate. The Nozzle Comparator feature on the Quick Calibrator allows you to store the nozzle measurements and compare them against the average nozzle output, which makes it easy to identify worn nozzles.

More information on optimizing spray application can be found in the “Tipnology” section of

*Ken Jackson is the marketing manager with Wilger, based in Saskatoon, Saskatchewan.

November 30, 1999  By Ken Jackson


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