Category: General Operation

Articles that discuss general field sprayer operation and productivity factors

  • How Much Pesticide in the Tank?

    How Much Pesticide in the Tank?

    Sprayer math can be intimidating, but the effort gives solid value. When combined with a calibrated sprayer you reap the following benefits:

    • Determine how much spray mix is required to apply the intended rate.
    • Estimate how much crop protection product must be ordered for the season.
    • Populate spray records which allow you to review practices, respond to enquiries and satisfy traceability requirements.

    There are many ways to perform sprayer math, and you need only look to local pesticide safety courses, industrial catalogues, and extension resource centres for examples. If you’re already comfortable with your current method, don’t mix and match with others. Sprayer math is a series of related calculations that employ constants to keep the units straight. It’s all or none.

    Walkthrough

    Let’s start with the classic, US Imperial formula for calculating the sprayer output. We’ll weave in Metric, later. This base formula can be adjusted to allow you to solve for any factor, as long as you’re only missing one piece of information. Turns out your high school teacher was right – you DO need algebra.

    GPM = [GPA x MPH x W] ÷ 5,940

    In this case, you can determine an output rate (GPM – gallons per minute). You’ll need to know your target volume (GPA – gallons per acre), your average travel speed (MPH – miles per hour) and your nozzle spacing (W – which is width in inches). The number “5,940” is a constant that handles all the unit conversions. If you divide the GPM by the number of nozzles on your sprayer (assuming they are all the same rate), you can hone in on the ideal nozzle size.

    But, as we noted earlier, you can do a lot more with sprayer math than just pick the ideal nozzle size. The rest of this article includes examples of both Metric and US Imperial formulae, but watch out for unit conversions. If at any time you don’t see the units you’re looking for, you can consult our unit conversion tool.

    Grab your calculator – it’s math time!

    Don’t be intimidated. With a little practice, sprayer math gets easier and it’s always worthwhile. The real trick is navigating unit conversions.

    Step 1 – How large is the area you need to spray?

    Multiply the length of the area you plan to spray times the width. If you are using metres, then divide the product by 10,000, which is the number of m2 in a hectare (ha). For feet and acres, divide by 43,560 which is the number of ft2 in an acre (ac):

    Step 2 – How much product is needed to spray the area?

    Consult the rate(s) shown on the label. In Canada, rates are often based on planted area (E.g. hectares). In Australia and New Zealand, they may be based on row length (not covered in this article). If you measure your area in acres, you’ll have to convert the rate by multiplying by a constant: 0.4.

    Now multiply the area you want to spray (step 1) by the rate (step 2).

    Step 3 – How far can you go on a full tank?

    You know your sprayer output (determined through calibration) so you divide that into your tank size. Watch your units:

    Step 4 – How much pesticide per tank? 

    Multiply the area that can be sprayed per tank (Step 3) by the pesticide rate (Step 2). Again, watch your units:

    Step 5 – How much area is left to spray?

    Just subtract what you’ve already sprayed from the total area.

    Step 6 – How much pesticide in the last, partially-full tank?

    Multiply the area you have left to spray (Step 5) by the pesticide rate (Step 2). Yes, watch your units:

    Step 7 – How much spray mix will I need for the partial tank to finish spraying the total area?

    Multiply the area you have left to spray (Step 5) by the sprayer output (determined through calibration). Guess what? Watch your units:

    Sample problems

    Time to test your knowledge. Let’s suppose you want to apply a product rate of 3 L/ha to your blueberries. You calibrate your sprayer and determine your output to be 50 L/ha. Your tank holds 400 L of spray mix. Your planting is 500 m long and 200 m wide.

    Q1 – How large is the area you need to spray?

    Q2 – How much product is needed to spray the area?

    Q3- How much area can be sprayed on one tank?

    Q4 – How much product should be added to a full tank?

    Q5 – After the tank is empty, how much area is left to spray?

    Q6 – How much product to add to the last, partially full tank?

    Q7 – How much spray mix will be needed to finish spraying?

    Tank mix calculator

    You might feel we buried the lead by adding this calculator to the end of the article. It’s important for a sprayer operator to understand the math required to interpret labels and calculate tank mixes, so hopefully you read and understood the process before you got here. And now that you have, this is a very helpful tool. Try it online, or download a standalone version.

    Notable exceptions

    Certain situations aren’t covered in this article. If you are spraying a greenhouse, the math is different. If you are performing a banded application, the math is different. And, if you’re an airblast operator trying to reconcile why a pesticide label uses planted area rather than canopy volume for its rates, you’re in for some additional reading.

  • Sprayer Math for Banded Applications

    Sprayer Math for Banded Applications

    Where crops are planted in rows, growers can save on chemical costs and reduce potentially wasted spray by performing banded applications. A banded application is treating parallel bands (Figure one), unlike a broadcast application where the entire area is treated (Figure two). This means only a portion of the field or orchard/vineyard floor receives spray, so the total amount of product applied per hectare (or per acre) should be less for banded than for broadcast.

    Figure 1
    Figure 2

    Banded applications are used in many situations, including:

    • Applying herbicides right over a crop during planting, both for pre-emergent or post-emergent crops.
    • Applying insecticides/fungicides by “directed spraying” using drop hoses or row kits; the latter is pictured in Figure three.
    • Carefully spraying herbicide between the rows to control weeds in the alleys of an established crop (Figure one).
    • Applying herbicide under fruit trees or grape vines to control weeds (Figure four).
    Figure 3
    Figure 4

    It’s easy to make mistakes when calculating product rates for banded applications and these can be costly errors: too little means poor control and too much means wasted product and possible crop injury. This article describes how to calculate sprayer output and product rate for common banded applications.

    Step One: Determine broadcast volume

    Pesticide labels typically list broadcast product rates (e.g. amount of formulated product per hectare or acre). In this example, let’s say the label recommends a broadcast product rate of 500 ml of formulated herbicide applied using 100 litres of spray mix per hectare (i.e. added to 99.5 L water).

    Step Two: Establish sprayer settings

    Select a travel speed that is safe, gives decent efficiency and doesn’t compromise coverage. For this example, we’ll say the sprayer moving is at 8.0 km/h.

    Select a band width that completely covers the target row and some of the adjacent area where control is desired. Band width should be measured along the ground for soil-applied products or along the top of plants for post-emergence products. We’ll use Figure one for our settings: bands are 50 cm wide on 100 cm centres. We’ll say that a single nozzle swath can treat the band, and that we’re spraying 2 hectares of planted area.

    Step Three: Calculate the banded sprayer output

    We can calculate how much of the planted area actually receives spray using this formula:

    [band width (cm) ÷ row width (cm)] x total planted area (ha) = actual sprayed area (ha)
    [50 cm ÷ 100 cm] x 2 ha = 1 ha

    For completeness, here’s the US formula:
    [band width (in) ÷ row width (in)] x total planted area (ac) = actual sprayed area (ac)

    From this we now know that we should be able to go twice as far on a tank spraying a banded application as we would a broadcast, because we’re only spraying half the planted area.

    Step four: Calculate the nozzle output

    Use the following formula to convert the broadcast output into the banded output:

    [broadcast output (L/ha) x travel speed (km/h) x (swath width (cm) ÷ number of nozzles per swath)] ÷ 60,000 = nozzle output (L/min)
    [100 L/ha x 8 km/h x (50 ÷ 1)] ÷ 60,000 = 0.67 L/min

    For completeness, here’s the US formula:
    [broadcast output (gal/ac) x travel speed (mph) x (swath width (in) ÷ number of nozzles per swath)] ÷ 5,940 = nozzle output (gal/min)

    If multiple nozzles were contributing to the swath, such as in figure three or figure four, this formula will account for it. You still mix the labelled product rate at a ratio of 500 ml of herbicide to 99.5 L water, but as we determined in step three, we should be able to spray twice the planted area using a banded application as we would a broadcast application.

    Warning! Watch your units. You may be familiar with other formulae for calculating your output. Do not mix and match formulae or parts of formulae. For example, here is another Metric option for determining L/min. It employs different units so it requires a different constant:

    [broadcast output (L/ha) x travel speed (m/min) x (swath width (m)) ÷ number of nozzles per swath)] ÷ 10,000 (m2/ha) = nozzle output (L/min)

    Step Five: Use the nozzle catalogue to find the right nozzle

    Using a nozzle manufacturer’s catalogue, select a nozzle that gives the desired spray quality (usually coarser for herbicides) and will produce the 50 cm swath we’re looking for (which can be adjusted a little using boom height). Always choose to operate a nozzle in the middle of its pressure range.

    Step Six: Calibrate the sprayer (i.e. double-check)

    Follow your typical calibration process and make minor adjustments until the nozzle discharge per minute results in the desired banded output. A rate controller will handle this on larger sprayers, but if you don’t have one you can make small adjustments to speed and pressure until the desired output is achieved. Ideally, if your math was right, these changes won’t be needed.

    When performed correctly, banded applications are a great way to focus your efforts on the target, saving time and money.

    Other Resources

    Download this booklet on spraying potatoes using banded rows. Or try our calculator: