At first glance, spot sprays are a no-brainer. Why spray a whole field when you can save product by spraying just the weeds?
But then the first commercial green-on-green systems introduced user fees, complicating the cost equation. Companies sell the hardware, and charge a fee for use of their detection algorithms.
Currently, costs range from $3 to $4 per acre, and this fee is either applied once per season (no matter how many times the algorithm is used on a specific field) or each time the system is deployed. As of September, 2023, Bilberry (via Agrifac as AICPlus, but also via Goldacres in Australia and Dammann in the EU) was using the former approach, and John Deere with See & Spray Ultimate in the US was using the latter. Greeneye is not charging fees. For Green-on-Brown systems, the likes of Rometron’s WeedIT and Trimble’s WeedSeeker, no fees are charged.
Fees essentially identify a pesticide price point below which spot sprays are not economical. Let’s take an example of a $4.00 per acre price of herbicide, broadcast (column 1 in Table 1). The “Gross Cost” of the broadcast treatment is simply the cost of the herbicide. For a spot spray, if a specific field requires just 25% of the herbicide (a 75% saving), the herbicide cost is $1/acre (column 2). Add a $4.00 per acre algorithm fee, the Gross Cost is $5.00 per acre. Broadcast spraying would cost $4.00 per acre, less than the spot spray cost. The cost for herbicide at which spot sprays become economically interesting is therefore above $5.00 per acre.
The gross cost of a spot spray pass can be calculated as follows:
Gross cost = (pesticide price * use rate) + use fee
Where “use rate” is the proportion of the field sprayed with the product.
Table 1: Spray cost scenarios for low value crops and herbicides
Now let’s assume a weedy field, one in which only a 50% saving is possible (column 3). Herbicide cost is now $2/acre, added to the $4.00 algorithm fee, for a total cost of $6.00 per acre (column 3). The weedier the field, the higher the herbicide price needs to be for a spot spray to be justifiable. Spot sprays without fees, on the other hand, allow the user to keep all the savings (columns 4 & 5), and will be the most economical option no matter the herbicide cost.
But that’s not the whole story. Spot sprays aren’t perfect. Companies are quoting a minimum weed size of about ¼” diameter (say, 6 mm), below which the plant can’t be detected. Some weeds are invisible due to shading by crop residue or other plants.
The technological answer to this problem is to implement a low-rate broadcast spray in the background. The lower rate is sufficient to kill the smallest weeds, but it reduces the overall savings. Current systems are capable of doing this due to their use of PWM valves that can deliver broadcast and spot sprays at the same time from the same nozzles.
Use of a background spray adds to the herbicide costs. If a 30% broadcast background spray were used in this example, it would add $4.00 * 0.3 = $1.20 to the cost (not shown in table). The assumption is that the lower rate broadcast would easily kill the smallest weeds that were undetected, without adding to the likelihood of resistance development from under-dosing. The assumption is also that large weeds weren’t undetected.
Users who opt for a spot spray with no background run the risk of having misses that would not be incurred with a broadcast spray. The cost of these misses depends on the situation. In some cases, it is inconsequential. A tiny weed may not cause much harm if the crop is larger and growing vigorously.
But what if the weed is competitive, and could ultimately cause yield loss? A re-spray may be required. What if this weed later causes harvesting difficulties that may necessitate a desiccation spray? What if it is resistant, and its seed production causes problems in the future? Those costs need to be considered.
In this case, we are assuming the cost of a miss at a conservative $5.00 per acre, which could be the cost of operating the sprayer for a re-spray. The cost would apply to all spot sprays equally, but not to the broadcast spray. Now the broadcast spray, still at $4.00 per acre, is the most economical.
There are also potential benefits to consider. One is the yield loss caused by the application of a herbicide with low crop safety. Think of Status (dicamba and diflufenzopyr) in corn, or metribuzin in lentils. Limiting the exposure of the crop to the herbicide reduces the potential yield loss. We rarely consider this effect because it is quite uncommon, but when it does occur it’s offset by the yield benefit of removing the weeds. Spot spraying can also open up new uses for herbicides with low crop safety.
Let’s assume the yield benefit of avoiding phytotoxicity to the crop is $10 per acre. We’ll apply this saving to the proportion of the field that is not sprayed. The spot sprays regain their advantage, but only in cases where the weed density was low or no fees were charged (“Crop Health Benefit”, Table 1). Where weeds were sprayed in larger proportions of the field area, yield benefits were reduced.
We repeat the whole exercise for a higher value crop, with more expensive treatments but also higher penalties for misses and greater crop phytotoxicity costs.
First considering only the “Gross Cost” scenario, the advantage of the spot sprays grows in this scenario (Table 2). But when the cost of a miss is added, it’s surprisingly close. As in the lower cost example in Table 1, the broadcast spray remains relatively competitive even with higher costs.
Table 2: Spray costs for higher value herbicides and crops
When we add a potential crop health benefit of $20/acre, the spot sprays regain their larger advantage.
Using the herbicide price as the variable and plotting the broadcast and spot spray costs, the place where these lines cross is the herbicide cost below which the broadcast application is most economical. In the example below, the use fee was $4.00, and the miss cost was $5.00. As expected, the “no fee” situation was always more economical than broadcast when no miss costs were added. As algorithm and miss costs were added, herbicide prices needed to be above $5.00 and $14.00, respectively for the spot sprays to be more economical than the broadcast application.
This exercise is not intended to declare winners and losers. Its purpose if simply to initiate a discussion about the overall cost of various approaches. What if owners of spot sprayers make, on average, more passes over the field? What would the value of a lighter, cheaper sprayer be on their bottom line? With less expensive sprayers, the fixed cost of a spray, or a re-spray would drop. Is there a benefit from reduced soil compaction? What if the use of more complex tank mixes, necessitated by resistance, jeopardizes crop safety? The benefit of spot sprays would increase.
Continued development of nozzles specifically for spot spraying, as well as better boom levelling, will improve spot spray economics because the smaller width and length of an applied band that stable booms allow will increase savings. As these take hold, they will tilt the calculations in favour of the spot sprays.
I’ve often repeated that the savings created by spot sprays ought to be re-invested in herbicide tank mixes, with a goal to prolong the utility of herbicides before resistance develops. This could ultimately create the biggest long-term return on investment because once herbicides are no longer effective, alternative strategies will be needed.
I’m as hopeful as anyone else that agriculture can retain the benefits of effective and safe herbicides for a long time to come. But it will only take one weed on a farm to become resistant to all available herbicides for major change to be necessary. The more time we have to develop these alternatives, the better. Spot sprays are definitely a part of that strategy.