Everything you ever wanted to know about Spot Spraying: 12 questions and answers

What do we mean by spot spraying? How large is a spot? Can it be fitted onto any field sprayer? When is spot spraying financially worthwhile? Future Farming looked into it for you. Read the 12 questions about spot spraying and their answers. – Photo: Agrifac

Future Farming regularly focuses on spot spraying techniques. But what exactly is spot spraying? How large is such a spot? Can you use spot spraying in every crop and for every application? Is it worthwhile, or are there situations where full-field spraying is better? Future Farming investigated it for you and talked with experts from Agrifac, BBLeap, Rometron, Homburg Holland and ABdrone.

When thinking of a spot, you might also imagine a spotlight that illuminates a specific area on the crop or ground with a cone-shaped beam of light. A spot sprayer essentially does the same thing: a spray nozzle applies a treatment to a specific spot on a crop or on the ground. The size and shape of that spot depends on the spray nozzle’s spray angle, the nozzle spacing, and the distance to the crop or ground.

The field sprayer or specialized spot sprayer must know exactly where to open and close a spray nozzle. To locate targets for spot spraying – weeds, volunteer plants, and sometimes even insects or diseases – satellite images, drones, and crop sensors are used.

1. What do we mean by spot spraying?

Spot spraying is the site-specific application of a single dose of pesticide to a limited, small area using one or more spray nozzles. That area could be, for example, a single plant. Spot spraying is not the same as varying the dosage, often referred to as VRA (variable rate application). Spot spraying can be done with standard spray nozzles as well as with pulsed (PWM) nozzles.

2. How large is a spot?

The size of a spot depends on the technique used and the application. The nozzle spacing is particularly important. For the Ecorobotix ARA spot sprayer, it’s 4 cm, for the Rumex spot sprayer RXF 600 it’s 6.6 cm, for the Garford Robocrop Spot Sprayer it’s often 6 to 8 cm, and for the RumboJet 880 it’s 10 cm. For field sprayers, it’s usually 25 or 50 cm, but other nozzle spacings are often possible. The (required) height of the spray boom and the spray angle of the nozzles used determine the size of the spot on the crop or ground. This typically ranges from 10 cm for spot sprayers to 25, 50, and 100 cm for field sprayers. Experts expect this to be reduced to 1 to 4 cm for spot sprayers, but full coverage of weeds and volunteer plants is still necessary for effective spraying.

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Garford uses this type of nozzle with six holes on its spot sprayer. There is no spray angle or mist. The nozzles are mounted on a rail for variable nozzle spacing. - Photo: Van Assendelft Photografy — Garford uses this type of nozzle with six holes on its spot sprayer. There is no spray angle or mist. The nozzles are mounted on a rail for variable nozzle spacing. – Photo: Van Assendelft Photografy

3. Can it be built onto any field sprayer?

In short, yes. Agrifac prefers a nozzle spacing of 25 cm because it offers better efficiency, more savings on chemicals, and less crop damage than a 50 cm nozzle spacing. This is generally true: the smaller the nozzle spacing, the greater the benefits, and the quicker you can recoup your investment. Rometron also uses a 25 cm nozzle spacing. BBLeap prefers a ring line for consistent pressure and good cleaning.

4.What data do I need for spot spraying?

That depends on the application and timing. If you’re using a spot sprayer or field sprayer with cameras or sensors, you don’t necessarily need prior data or information. Sometimes, a field map with previous weed hotspots is available. If you’re working with a task map, you’ll need data on the exact location of weeds or diseases. Proper timing and planning are still crucial.

5. In what situations does spot spraying yield the most?

In grassland, spot spraying works best for local and persistent root weeds such as sorrels, thistles, and couch grass. In sugar beets and onions, spot spraying is extremely effective for controlling volunteer potato plants. Because you’re not stopping the crop, it causes less stress and results in better yields.

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Spot spraying works best in grassland for local and persistent root weeds like sorrels, thistles, and couch grass. These are expensive pesticides. Moreover, you save the herbs. - Photo: BBLeap — Spot spraying works best in grassland for local and persistent root weeds like sorrels, thistles, and couch grass. These are expensive pesticides. Moreover, you save the herbs. – Photo: BBLeap

6. And in which situations should you go for full-field spraying instead?

Spot spraying is not very useful in cases of (very) high weed pressure, when the cost of the pesticide is low, and when no crop growth retardation occurs due to spraying. Using it for fungal control, such as against Phytophthora, is not very effective and even risky. VRA may be an option in those cases.

7. How effective is spot spraying compared to full-field spraying?

Companies like Agrifac, BBLeap, and Rometron say “very effective.” Rens Albers of Agrifac: “The goal is for the end result of the spot spraying application to be at least 95% of what you’d achieve with full-field spraying.” Ron Smit of Holland Homburg estimates that the effectiveness depends on the brand and type of spot sprayer, ranging from 60 to 97%.

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In crops like chicory, sugar beets, and onions, spot spraying is extremely effective for controlling volunteer potato plants. Because you're not stopping the crop, it causes less stress and results in better yields. - Photo: Jan Willem Schouten — In crops like chicory, sugar beets, and onions, spot spraying is extremely effective for controlling volunteer potato plants. Because you're not stopping the crop, it causes less stress and results in better yields. – Photo: Jan Willem Schouten

8. How accurately can weeds be detected?

That primarily depends on the type of camera or sensor under a drone or on a spray boom. Driving speed, a stable boom height, and light/shadow also play a role. For some spot sprayers, the amount of artificial light is also a factor. BBLeap claims that it can detect 99% of weeds with cameras on the spray boom. Agrifac says it can detect weeds as small as 5 millimeters with spray boom cameras, and it can “find” weeds as small as 2 millimeters on drone images.

9. How do you determine the amount of tank mix for spot spraying in advance?

“When spot spraying with cameras on the spray boom, you don’t know in advance how much you’ll be spraying. An estimate is often made based on the weed coverage,” says Albers of Agrifac. Johan Kikstra of BBLeap adds, “A practical solution is to spot spray nine fields, for example, and then spray the next field (partially) full-field to empty the tank. Or you can collect the leftover liquid in an IBC.” Chemical injection (injecting chemicals from a small tank into a carrier liquid from the main tank) can solve this workaround. If you’re working with a task map, the software calculates the amount of tank mix.

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The accuracy of real-time systems primarily depends on the type of camera or sensor. Driving speed, a stable boom height, and light/shadow also play a role.

10. What else can I do with the data?

A spot sprayer or field sprayer typically generates a so-called “as-applied” map. This is a field map that provides insights into the position and dosage per nozzle and per spray moment, linked to the nozzle spacing. This data can potentially give you information on weed pressure, crop vigor/potential, crop stress, and, of course, the amount of chemicals applied.

11. Do I need to follow certain (additional) regulations?

There is no difference in laws and regulations for full-field spraying and spot spraying. There are also no differences regarding the required drift reduction classes. However, it is expected that spot spraying will become mandatory for certain applications in the future. This is already the case for dock in grassland in Bavaria and Switzerland. It’s also expected that there will be chemicals on the market that you are only allowed to apply via spot spraying.

12. When is spot spraying financially viable?

The savings and payback time depend on the circumstances (area, crop type, weed or disease pressure), type of pesticide, reduction in chemical use (i.e., effectiveness of the spot sprayer and weed/disease pressure), reduction in crop growth retardation, and, of course, the brand, type, and capacity of the spot sprayer or field sprayer. If you’re working with a task map, the savings on chemicals will generally cover the cost of the drone flight and the task map. Spot spraying in grassland with cameras on a field sprayer is still difficult to make profitable, while the payback time in other cases varies from half a year to several years.