Spot sprays are becoming mainstream. As of 2024, John Deere’s See & Spray Select, their Green-on-Brown technology, is selling well in western Canada but it’s creating some confusion about how to outfit and run the system.
Quick Overview:
See & Spray Select is available on 120’ booms with either 15” or 20” spacing. It can be operated at up to 12 mph with conventional vertically oriented nozzles, or up to 16 mph with backwards oriented nozzles using a 40º adaptor available from John Deere. Optimal boom height is between 26” and 47”
Operating speed for See & Spray Select is measured at the boom. That means if an operator drives at the 12 mph limit and the boom yaws forward under normal driving or in a turn, the boom speed will exceed 12 mph and it will enter “fallback” mode. Fallback mode is intended to provide weed control when camera vision is compromised due to dust, height, or speed, and typically it means that all the nozzles in the affected boom region are turned on. To avoid unnecessary waste, an operator will want to minimize fallback mode and therefore will want to drive slower than the maximum allowed boom speed.
An operator has a choice of selecting a single-nozzle or overlapping-nozzle activation. In single nozzle mode, only the nozzle in the weed’s lane is turned on. In overlapping mode, one adjacent nozzle on each side is also turned on, for security. Overlapping mode is available on most spot spray systems to compensate for spray displacement in a side-wind, for example.
Research at the University of Wisconsin has shown that the overlapping mode resulted in more consistent weed control in a side-wind.
Nozzle Selection
Overlapping mode makes nozzle selection easier because the fan angle is not as critical. Nozzles are allowed to overlap as they’re supposed to on a broadcast boom, and the spray dosage is a function of nozzle size, spacing, and travel speed. It’s also easier because boom height movement doesn’t affect the dose, so long as the required overlap remains. But nozzle fan angles should still not be too wide. Low boom sway can result in under-dosing if there is too much overlap.
Single nozzle activation can save more product. But in this mode, nozzle fan angle is critical because it determines the band width. Unfortunately, current nozzle selection is poor – most manufacturers aren’t offering any narrow-enough fan angle nozzles yet. For this reason, John Deere’s nozzle recommendations are intended primarily for overlapping mode.
With single nozzle activation, the nozzle pattern (band) width needs to be fairly close to the nozzle spacing, but still have some overlap when adjacent nozzles are activated in a weed patch. The more the pattern width exceeds the nozzle spacing, the greater the underdosing in single nozzle activation compared to overlapping sprays. This conundrum is unavoidable. The closer these two values (pattern width and nozzle spacing) are to each other the better. But for this to work, boom height has to be consistent. Too low a boom creates gaps between adjacent narrow patterns. Too high and the pattern width widens, reducing the single nozzle dose. There is simply not much room for error.
Broadcasting Background Dose
With See & Spray Select, the A solenoid (front nozzle in ExactApply) can be used to apply a PWM broadcast spray simultaneous to the spot spray. This feature is useful with early season application because of just-emerged weeds that may be missed by the sensor. We might choose about 1/3 of the full rate applied this way, a dose which is sufficient to control these small weeds. With a tank mix for 10 gpa, one would spray 3 gpa with the front boom and 7 gpa with the B solenoid, the spot spray. This way the entire field receives the 3 gpa dose, while larger weeds that trigger the spot spray receive the 10 gpa dose.
The problem is again with nozzle availability. For example, 3 gpa with 15” spacing at 11 mph with PWM (broadcast mode) requires a small nozzle such as an 01 (orange) or 015 (green). These are hard to find in a low-drift version. Increasing the broadcast water volume to 5 gpa would allow an 02 (yellow) nozzle to be used. A 20” spacing would allow even larger nozzles to be used, for 3 gpa an 025 (lilac) is a possibility and this greatly improves the available choice. At 5 gpa, an 03 size is suitable, and now the John Deere LDM nozzle is an option (it is not manufactured in sizes smaller than 03).
Let’s assume a user selects 5 gpa for the broadcast based on nozzle availability. The next decision is whether to adjust the total applied volume upwards. If sticking with a 10 gpa tank mix, the spot spray would also be 5 gpa, making the broadcast 50% of the dose.
Alternatively, one could increase the spot spray volume to 10 gpa, mixing the tank for 15 gpa. This returns one to 1/3 of the total dose as broadcast, and 2/3 as a spot spray. A reason for doing this is to make nozzle size selection easier and also improving the product savings of the system.
The spot spray from the B solenoid is not PWM, which allows for a more straightforward nozzle sizing, as well as the use of air-induced tips which are available in a large number of sizes.
A summary of some possible nozzle combinations for two nozzle spacings and travel speeds is listed in Table 1.
Table 1: Possible nozzle sizes for overlapping mode in John Deere See & Spray Select Note that the travel speed is lower than the maximum allowed, to accommodate boom yaw.
If the operator chooses single nozzle activation, the fan angle of the nozzle becomes important. To recap, one would want to have a nozzle that can do two things:
- Cover a band that is close to the same width as the nozzle spacing when a single weed activates a single nozzle, and
- Provide sufficient overlap when multiple adjacent nozzles are activated in a larger weed patch.
It’s not possible to have a band width as narrow as the nozzle spacing and still get an overlapping pattern when it’s needed. This means the dose for a single nozzle pattern will unavoidably be spread out wider, resulting in a lower dose for any weed it encounters compared to the overlapping activation. But the wider the fan angle, the wider the band and the lower the dose, resulting in possibly reduced control for single nozzle activations.
On the other hand, a narrower band limits the boom height at which an acceptable overlap can be achieved. Let’s say an overlapping nozzle needs to have 30% overlap to get an acceptable spray distribution. At a 20” spacing, the band would need to be 26” wide (a 24% under-dose on a single nozzle compared to an overlapping section). Band width will change with boom height, but it depends on the fan angle. For a 60 degree fan angle, the band changes by about one inch for every inch of boom height. That means even with a modest 10” vertical movement of the boom, the dosage might change by 30%, a fair amount.
Actual changes depend on the nozzle spacing and the fan angle, but the point remains that this is a significant dosage change that could affect weed control. And this change in dose is because of boom sway.
Recommendations
What should a spot spray user do? One thing is clear, compromises will be necessary.
The most consistent application will be achieved with overlapping mode, but at the cost of forfeited savings. These lost savings may be recovered due to fewer weed control failures, or less need to re-spray.
On the other hand, the greatest savings will be achieved with single nozzle activation. But fan angle will need to be carefully selected and boom height consistency will be critical.
Availability of narrow fan angles is limited. Only Wilger (20, 40, and 60 degree DX), Greenleaf (40 degree Spot Fan), Arag (CFLD-CX 40 degree) and Magnojet (30 and 60 degree) offer spot spray-specific low-drift nozzles off the shelf. The availability will need to increase, not only in terms of fan angles, but also in flow rates and spray qualities. With spot sprays remaining a relatively small market this will take time. But the success of spot sprays also depends on it.
One question that only experience will answer is the relative frequency of single vs multiple nozzle activation for any given farm. If the majority of the activations are multiple nozzles, then setting up the nozzles for that situation (i.e., opting for wider fan angles that create more overlap) makes most sense.
But regardless of the choice made by the user, the need for stable booms remains paramount. This feature will be the basis on which any progress in spot spray adoption will be built. Call your dealer. Tell them how important boom stability is.