Author: Tom Wolf

Spray Water pH

The scuttlebut on coffee row is that acidifying a spray mixture improves its efficacy. There are also claims that pesticides break down in the sprayer tank if the pH is too high.

But it’s not that simple. Low pH has a strong impact on pesticide solubility, and that means mixing and cleanout are affected. Acidifying the mixture can have profound negative effects for many products.

It’s important to know what you’re doing.

What is pH?

pH is defined as the negative log of the molar concentration of hydrogen ions in a water-based solution. The more abundant the hydrogen, the lower the pH. It’s a log scale, so every unit of pH refers to a 10-fold change in the concentration of hydrogen ions.

Both very low (acidic) or very high (basic) pH can be caustic. But having a low or high pH doesn’t mean it will burn your skin or clothes right away, it might just be a bit unpleasant. But at the extreme ends, protection is needed.

Why is pH Important in Spray Mixtures?

In spraying, the main effect of pH is on the pesticide’s solubility. Solubility matters when mixing and becomes important during cleanout as well.

A minor effect on pH, at least for herbicides, is on chemical breakdown, usually through hydrolysis, when the pH is too high. The effect on breakdown is rarely meaningful during any given spray day, but may play a role if a spray mix is stored overnight or longer.

The Basics: Strong vs Weak Acids

Strong acids like hydrochloric acid (HCl) ionize completely in solution. When added to water, only H⁺ and Cl^– are present, there is no HCl. The water’s pH does not affect solubility of a strong acid.

But weak acids do not completely ionize. The water pH affects the degree of ionization and therefore solubility.

Most herbicides are weak acids. A weak acid is one that does not dissociate completely in solution. A typical example of a weak acid functional group is carboxylic acid (-COOH). In solution, compounds with a carboxylic moiety exist in an equilibrium, with some as -COOH (containing the hydrogen, also called “protonated”) and others as -COO^– and H⁺. In the dissociated form, the acid is more water soluble than in its protonated form due to the negative charge that makes it ionic.

Weak acids have a dissociation constant known as the pKa. When the solution is at the molecule’s pKa, the acid is 50% dissociated. When the solution has a lower pH than the pKa, there is less dissociation and the protonated forms of the molecule dominate. That has two important implications for herbicides.

the molecule becomes less water-soluble at lower pH
the molecule has fewer opportunities to interact with positively charged items

pH Dependent Solubility

Water-solubility is a two-edged sword. On the one hand, having a highly water soluble product makes it easier to dissolve in water. This pays dividends when mixing a batch or cleaning a sprayer because a product formulated as a solution will easily go into a true solution and will stay mixed. Examples are glyphosate, glufosinate, and salts of 2,4-D, MCPA, and dicamba.

On the other hand, most pesticides need to enter a plant to reach their site of action. And a plant cell, with its waxy cuticle and oily membranes, creates an effective barrier for water, and for water-loving molecules dissolved in it. As a result, a formulation that allows the water-soluble product to interact with an oily barrier is needed.

The products that can do this are surfactants. Acting like detergents, surfactants have regions in their structure that are oil-loving (lipophilic) and other regions that are water-loving (hydrophilic). Surfactants can therefore bind to both oil and water and provide a bridge for water-soluble products across oily barriers.

That’s also one of the reason that the most water-soluble products such as glyphosate and glufosinate contain a lot of surfactants in their formulation, reducing the concentration of active ingredient in the jug and possibly leading to foaming with agitation.

Pesticides have a wide range of solubilities, and for some, water pH will play an important role. Below is a table of some water solubilities of selected herbicides.

				Solubility (ppm)
Trade Name	Active Ingredient	Mode of Action Group	pH ~ 5	pH ~ 7	pH ~ 9
Select	clethodim	1	53	5,450	58,900
Ally 2	metsulfuron	2	550	2,800	313,000
Express	tribenuron	2	48	2,040	18,300
Pinnacle	thifensulfuron	2	223	2,240	8,830
Everest	flucarbazone	2	44,000	44,000	44,000
Simplicity	pyroxsulam	2	16	32,000	13,700
Frontline	florasulam	2	0.1	6	94
Varro	thiencarbazone	2	172	436	417
Raptor	imazamox	2	116,000	>626,000	>628,000
Pursuit	imazethapyr	2	2,570	12,870	7,500
2,4-D	2,4-D salt	4	29,934	44,558	43,134
dicamba	dicamba salt	4	>250,000	>250,000	>250,000
Roundup	glyphosate	9	>500,000	>500,000	>500,000
Liberty	glufosinate	10	>500,000	>500,000	>500,000
Heat	saflufenacil	14	30	2,100	>5000
Distinct	diflufenzopyr	19	63	5,900	10,550
Infinity	pyrasulfatole	27	4,200	69,100	49,000

Compare the solubility at pH 7 to that at pH 5. For most of these herbicides, water solubility is worse at lower pH. That is because they are more protonated and become more lipophilic.

I’ve placed a lot of Group 2 products in this table because those products are most often implicated in tank cleanout issues. All Group 2 products in this table, with the exception of Everest (flucarbazone-sodium) have lower solubility at pH 5 than they do at pH 7. For some, like pyroxsulam and floarsulam, it’s a big change. Those products, when acifified, are prime candidates for poor mixability and poor cleanout.

When it comes to dicamba, low pH has another side-effect. It makes the molecule more volatile, increasing danger to sensitive plants nearby. For that reason, acidification of dicamba in its Xtendimax and Engenia formulations is not permitted.

Note that the Group 4 examples, 2,4-D salt and dicamba salt, as well as glyphosate and glufosinate, are highly water-soluble and pH has very little effect on that.

Particularly for glyphosate, the claim that it becomes more oily at low pH and will therefore be taken up more easily, is not supported by these data. Considering that the most acidic pKa for glyphosate (it has four acidic groups) is 0.8, pH would need to be much lower for any noticeable impact on oilyness.

Tank Mixability

Given today’s environment of herbicide resistance, applications with multiple mode of action tank mixes are very common. Acidifying a spray mix to benefit one herbicide may create problems for its tank mix partners.

If there is a concern that spray water is too alkaline, it is recommended that the pH of the finished spray mix be measured. Since many herbicides are weak acids, they will lower the pH of the mixture by themselves. For example, the addition of glyphosate to water with pH 7.5 will drop the pH to about 5 or so, depending on the water’s buffering capacity.

As a result, glyphosate tank mix partners that are pH sensitive may suffer in the presence of glyphosate, and pH may actually need to be raised.

pH Dependent Half-life

Herbicides

There are a lot of claims that pesticides break down rapidly in alkaline spray water. And yet, in my career working primarily with herbicides, I do not recall this ever being a problem in practice.

Below is a table of herbicides for which I could find half-life information, with the help of this comprehensive list produced by Michigan State University.

Product	Active ingredient	Half Life
Atrazine	atrazine	More stable at high pH
Banvel	dicamba	Stable at pH 5 – 6
Bromoxynil	bromoxynil	pH 5 = 34 d; pH 9 = 1.7 d
Fusilade	fluazifop-p-butyl	pH 4.5 = 455 d; pH 9 = 17 d
Liberty	glufosinate-ammonium	Stable over wide range of pH
Gramoxone	paraquat	Not stable at pH above 7
Reglone	diquat	pH 5 = 178 d; pH 7 = 158 d; pH 9 = 34 d
MCPA	MCPA	pH 9 = < 5 days
Poast	sethoxydim	Stable at pH 4.0 to 10
Princep	simazine	pH 4.5 = 20 d; pH 5 = 96 d; pH 9 = 24 d
Prowl	pendimethalin	Stable over a wide range of pH values
Roundup	glyphosate	Stable over a wide range of pH values
Treflan	triflularin	Stable over a wide range of pH values
2,4-D	2,4-D	Stable at pH 4.5 to 7

Note that all of the herbicides are relatively stable. Some are a bit less stable at high pH, but none of the listed herbicides is in danger of breaking down on the day it is being applied. Only one is actually unstable at high pH – paraquat, a herbicide no longer registered in Canada and resticted in many other countries. Those with short half-lives experience them at quite high pH which are rarely seen in practice.

Insecticides

Insecticides are a different story. Several are very sensitive to pH. This table is again adapted from a comprehensive list published by Michigan State University, here.

Trade Name	Active Ingredient	Half-life
Admire	Imidacloprid	Greater than 31 days at pH 5 – 9
Agri-Mek	Avermectin	Stable at pH 5 – 9
Ambush	Permethrin	Stable at pH 6 – 8
Assail	acetamiprid	Unstable at pH below 4 and above 7
Avaunt	indoxacarb	Stable for 3 days at pH 5 – 10
Cygon/Lagon	dimethoate	pH 4 = 20 hrs; pH 6 = 12 hrs; pH 9 = 48 min
Cymbush	cypermethrin	pH 9 = 39 hours
Diazinon	phosphorothioate	pH 5 = 2 wks; pH 7 = 10 wks; pH 8 = 3 wks; pH 9 = 29 days
Dipel/Foray	b. thuringiensis	Unstable at pH above 8
Dylox	trichlorfon	pH 6 = 3.7 days; pH 7 = 6.5 hrs; pH 8 = 63 min
Endosulfan	endosulfan	70% loss after 7 days at pH 7.3 – 8
Furadan	carbofuran	pH 6 = 8 days; pH 9 = 78 hrs
Guthion	azinphos-methyl	pH 5 = 17 days; pH 7 = 10 days; pH 9 = 12 hrs
Kelthane	dicofol	pH 5 = 20 days; pH 7 = 5 days; pH 9 = 1hr
Lannate	methomyl	Stable at pH below 7
Lorsban	chlorpyrifos	pH 5 = 63 days; pH 7 = 35 days; pH 8 = 1.5 days
Malathion	dimethyl dithiophosphate	pH 6 = 8 days; pH 7 = 3 days; pH 8 = 19 hrs; pH 9 = 5 hrs
Matador	lambda-cyhalothrin	Stable at pH 5 – 9
Mavrik	tau-fluvalinate	pH 6 = 30 days; pH 9 = 1 – 2 days
Mitac	amitraz	pH 5 = 35 hrs; pH 7 = 15 hrs; pH 9 = 1.5 hrs
Omite	propargite	Effectiveness reduced at pH above 7
Orthene	acephate	pH 5 = 55 days; pH 7 = 17 days; pH 9 = 3 days
Pounce	permethrin	pH 5.7 to 7.7 is optimal
Pyramite	pyridaben	Stable at pH 4 – 9
Sevin XLR	carbaryl	pH 6 = 100 days; pH 7 = 24 days; pH 8 = 2.5 days; pH 9 = 1 day
SpinTor	spinosad	Stable at pH 5 – 7; pH 9 = 200 days
Thiodan	endosulfan	70% loss after 7 days at pH 7.3 to 8
Zolone	phosalone	Stable at pH 5 – 7; pH 9 = 9 days

Among insecticides, dimethoate, amitraz, and malathion stand out as breaking down rapidly in alkaline water. For these products in particular, it may be important to acifify the spray mix if there is any delay in spraying.

Recommendations

I’ve never been a fan of messing with solution pH unless recommended on the product label. Even when there is evidence that lower pH improves efficacy, consider the impact on tank mix partners.

We’ve seen improvements in solubility and tank cleranout of Group 2 products with raised pH, and ammonia is the most cost-effective way to achieve that. But again, following label recommendations is strongly recommended. The consequences of changes in pH, particularly acifification, can be very detrimental. To be safe, consider doing a jar test before committing to a whole tank to a pH adjustment.

April 2, 2026

How to Interpret a Water Quality Test Result
It’s common advice: Test your water before using it as a spray carrier. You dutifully sample the well or dugout and await lab results. And what comes back is a whole lot of numbers. How to make sense of it all?
Three examples of water test results conducted by labs in Canada
All three of these tests report a large number of properties and identify specific minerals and other solutes. Which ones are important in spraying? Here is the order in which I look at the numbers.
Conductivity: This property is usually expressed as micro Siemens per cm (µS/cm) and simply identifies how many ionic solutes are in a sample (watch for alternate units such as mS/cm and convert if necessary). It doesn’t differentiate between any minerals or other molecules, and therefore has limited information. But it does tell us if there is a large or small issue with water quality. If conductivity is below 500 µS/cm, the water is probably good for spraying. If the value is around 1000 to 2000, further investigation is necessary. Some water samples return conductivity of more than 10,000 µS/cm, and it’s important to identify which salts are causing that problem.
Note that Total Dissolved Solids (TDS) are often listed, and these are related to conductivity. A common way to get TDS is to multiply conductivity by 0.65. The conversion factor depends on which salts are dissolved but the bottom line is that TDS and conductivity are closely related.
Bicarbonate: Bicarbonates are HCO₃ and their concentration is measured in milligrams per Litre (mg/L), which is the same as parts per million (ppm). Bicarbonates can antagonize Group 1 modes of action and the common threshold is 500 ppm. Research at NDSU has shown that Urea -Ammonium-Nitrate (UAN or 28-0-0 liquid fertilizer) can reduce bicarbonate antagonism in some Group 1 herbicides.
Bicarbonates are negatively charged and are associated with a positive ion, often the hard water cations sodium (Na), calcium (Ca) or magnesium (Mg). As such, waters that are high in bicarbonates are often also hard.
Total Hardness (calculated): This is one of the important parameters. Hardness antagonizes most weak acid herbicides, most importantly glyphosate and g;ufosinate, and also ties up surfactants and emulsifiers which can result in problems with mixing and compatibility. Hardness is caused by metal cations, in order of strength these are iron (Fe⁺⁺), magnesium (Mg⁺⁺), calcium (Ca⁺⁺), sodium (Na⁺), and potassium (K⁺). Of these, Mg and Ca are typically most abundant, although some water is high in Na.
The Total Hardness (ppm) reported in water tests is done by taking the most common two cations, calcium and magnesium, and using this formula: 2.497*Ca + 4.118*Mg. Note that some tests report hardness in Grains per Gallon, in this case, multiply grains by 17.1 to get ppm.
While this calculation usually gives an accurate prediction of hardness, you may need to have a look at iron and sodium as well. Iron is less common, but some well waters are high in sodium or potassium. These minerals are not captured in the Total Hardness measurement. A water test low in Total Hardness may still be high in sodium, these are typically the samples with high conductivity.
The threshold for Total Hardness depends on the herbicide, its rate, and the water volume. The most common quoted values are 350 ppm for the lower rates of glyphosate (1/2 L/acre equivalent), and 700 ppm for the higher rates. Lower water volumes increase the concentration of herbicide, and reduce the impact of water hardness or bicabonates.
pH: This parameter is a bit over-rated because it is later affected by the herbicide and adjuvant dissolved in it. There is usually no concern with pH between 6 and 8, and water is rarely outside this range. It is best not to change the pH of water unless it is required on the label for mixing, because some products require low, and others require high pH for optimum solubility. Compatibility is an ever greater concern as our tank mix complexity increases.
Water Conditioners
The most common water conditioner is ammonium sulphate [AMS, (NH₄)₂ SO₄]. In its pure form (21-0-0-24), a concentration of 1% to 2% w/v (8 to 17 lbs AMS/100 US gallons of spray water) solves most hard water and bicarbonate issues. Be cautious of using too much AMS (>3%), when added at high concentrations to some herbicides it can burn crops.
Research has shown that AMS works in two ways: The sulphate ion binds with hard water cations, forming an insoluble precipitate that prevents the antagonistic cations from binding to, and inhibiting, the herbicide. The ammonium ion has been shown to improve cellular uptake by weak ion herbicides.
Some product labels call for UAN as an adjuvant. UAN contributes ammonium, but not sulphate ions. As a result, while it may improve herbicide performance, it does not remove antagonizing cations from the mixture.
Acids have been used to combat hard water. Most common herbicides are weak acids, and the acid constituent, usually a carboxilic acid, has a unique pKa. The pKa is the pH at which half the molecules are protonated (contain a hydrogen atom, resulting in an uncharged acid constituent) and the other half are not protonated (negatively charged). If the spray mixture has a pH below the pKa, the weak acid herbicides become protonated. This means the herbicide becomes less water-soluble, but also that it has less chance of interacting with a hard water cation. Acids that work in this way are less effective at ameliorating the effect of hard water than AMS.
A small group of acids that includes citric acid and sufphuric acid can sequester or bind with hard water cations. But they do not contribute the ammonium ion that assists in weak acid herbicide uptake.
If your water is questionable for spraying, there are four common choices:
- Select a different well or dugout
- If the problem is barbonates or hardness, treat water with a conditioner such as Ammonium Sulphate (AMS), available in pure form as 21-0-0-24. Some acids (citric, sulfuric) can form conjugate bases with hard water cations, removing them from solution. But the associated significant lowering of pH should be treated with an abundance of caution as it may affect solubility of some pesticides.
- Reduce water volumes or increase herbicide rates.
- Use a municipal treated water source or invest in a reverse-osmosis (RO) system. RO is neither cheap nor fast and requires additional investment in storage, and a way to deal with solute-enriched waste water. But it may be the best option for some.
An Ammonium Sulphate calculator, originally developed by Winfield United using data from NDSU, can be downloaded here:
AMS-Calculator-Sprayers101-1 Download
An excellent resource for adjuvant and water quality topics is this addendum in the North Dakota State University Guide to Weed Control.
Using good quality water lowers the likelihood of problems with mixing and overall performance and that pays significant dividends later.
April 1, 2026
Nozzle Selection for See & Spray Select Spot Sprayers
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.
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:
1. Cover a band that is close to the same width as the nozzle spacing when a single weed activates a single nozzle, and
2. 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. TeeJet has issued DriftGuard (DG) versions of 65 degree nozzles for the Australian spot spray market, with the DG65055 a special nozzle that conforms to the VC spray quality requirement needed for 2,4-D products.
John Deere has recently (Spring 2025) released an 80 degree spot spray tip called the TSL. It ships with the angled adaptor for faster spray speed. However, 80 degrees is still not narow enough to permit single nozzle activation without some significant rate compromises between single and overlapping mode.
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.
July 2, 2025
The Economics of Spot Sprays
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. In the summer of 2024, John Deere announced they would only be charging fees on acres that were not sprayed, i.e., areas in a field in which the weed sensing technology identified no reason to turn on a nozzle. The fee question is likely not quite settled yet.
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.
Figure 1: Spot spray costs as a function of herbicide prices, assuming a 20% spot spray use rate, with $4.00 algorithm fee or $5.00 miss cost added.
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.
September 11, 2023
Greeneye makes impressive debut
Green-on-green sprayer competes with Blue River and Bilberry
One distinguishing feature of the new agriculture is the rapid development of new technologies. Ideas move from concept to implementation at record pace, helped by an influx of talent and capital into this profitable sector.
Greeneye Technology is an example of this pace. Founded by entrepreneurs who met in the Israeli armed forces, they developed a software platform that identified crops, weeds, and other objects in agricultural fields from drone imagery. They recognized the opportunity to transition their software to a sprayer platform, and in 2017 decided to join the race, most notably competing with Blue River, Bilberry, Carbon Bee, and Xarvio, to create a green-on green spot sprayer.
Greeneye, in an amazing display of efficiency and speed, has been a commercial product for approximately one year in the US and has sold several units in Nebraska, Minnesota and Oklahoma, and next year will expand to North and South Dakota, Iowa, Illinois, Kansas and Texas. Having consulted for the company in its early years, I paid a visit to Peterson Farms Seed near Fargo, ND in early July 2023 to see the sprayer first hand at a field demo. By the way, kudos to PFS for bringing this technology to their customers to see. Have to love a business so committed to the cutting edge.
Figure 1: The Greeneye system was mounted on a Hagie STS 12 sprayer.
The Greeneye system was mounted on a Hagie STS 12 sprayer (1200 US gallon tank) with a custom 120’ boom manufactured for Greeneye by Millenium. Recognizing the agronomic need to broadcast pre-emergence herbicides along with a post-emergent spray, they company retained the existing plumbing system (tank, pump, wet boom) for this purpose. They added a smaller spot spray tank (240 gallons) with its own pump and wet boom for spot spraying.
Figure 2: A smaller spot spray tank was added to the Hagie. If necessary, spray mix can be pumped from the larger tank to this smaller tank.
This approach permits the flexibility of broadcasting a pre-emergent herbicide during burnoff alongside a post spot spray on emerged weeds. The agronomist in me likes this a lot. Broadcast pre-emergent herbicides are an important part of resistance management, particularly in the US.
Figure 3: The second (spot spray) boom is mounted behind the factory wet boom.
The new wet boom has a nozzle spacing of 10”, is fitted with three-nozzle-turret TeeJet bodies. The 10″ spacing allows for higher resolution of the spot spray, increasing potential savings compared to a 20″ spacing.
Figure 4: The spot spray nozzles are mounted at 10″ (25 cm) spacing.
The spray was metered through custom-made TeeJet DG4003 tips using Gevasol solenoids running at a speed-dependent frequency, maximally 100 Hz, with turn compensation.
Figure 5: Solenoids activate the spray when a weed is detected in that nozzle’s lane.
DG Nozzles use a pre-orifice to meter the flow at the rated amount, with an exit orifice slightly larger. This creates a pressure drop, resulting in a lower drift spray.
Figure 6: These Drift Guard nozzles are custom-made for Greeneye by TeeJet.
Figure 7: The blue DG pre-orifice meters the flow at 0.3 US gpm at 40 psi.
Looking at the spray quality and coverage on water-sensitive paper I thought the deposit looked just right. Spot sprays shouldn’t be too fine for risk of displacement from their intended band. We’re not seeing bundled nozzles with other spot spray systems, who leave nozzle selection to the operator. That can pose difficulties and possibly forfeit either weed control or savings.
Figure 8: The spray deposit shows adequate coverage and a good droplet size distribution for good placement accuracy.
Sectional control retains the plumbed resolution at this time, although nozzle-by nozzle resolution is in the pipeline. Cameras are mounted at 1.5 m intervals and can run up to 50 fps. Camera resolution is proprietary, but the company claims that weeds as small as ¼” diameter can be detected. In its current configuration, weed diameters of 1” are detected, leaving smaller weeds for the pre-emergent products. LED lights flash to illuminate the camera field of view, improving image consistency and permitting the system to run at night. The Greeneye system analyzes a captured image just once to make a spray decision, and does not use segmentation in its algorithm.
Figure 9: The camera and lights look ahead to provide the necessary time for the on-board computer to make the required calculations that determines the plant’s identity. Note the aspirated lens cleaner.
Like its competitors, the user can select from individual nozzle activation or, in “windy mode”, the addition of the adjacent two nozzles to create a three nozzle broadcast. The length of the band is automatically selected by the software, with no user input available. Sensitivity adjustments are currently by request to the factory, but will be available for operator control in 2024.
Greeneye provides its own cab monitor that works with the sprayer monitor on sectional control. The Greeneye monitor keeps track of the spray volume usage and provides an ongoing report to the operator.
The software is able to report back whether a detection was a grassy or broadleaf weed, a powerful piece of information for keeping track of weed patches and monitoring for emerging problems. Weed maps are already being produced as a proprietary tool, and will be generally available in 2024.
New Greeneye customers have their sprayer picked up at the yard and transported to Greeneye facility where the new boom, tank, and digital components are installed. The customer receives the converted sprayer, calibrated and ready to go.
In my judgement, the install was clean and tidy. Camera mounts are welded on, and an air jet can be used to keep the lenses dust free. Brackets for the GPU and other control boxes are unobtrusive, although the wiring does get a bit crowded in places. Everything is accessible.
Cost is $239,000 US at time of printing (July 2023). This gets the customer a Greeneye system for a 120 foot boom, a brand new aluminum boom, retrofit of the sprayer to dual tank, installation and warranty. Return on Investment (ROI) for a spot sprayer will depend on farm size and herbicide use. Based on observed savings to date, Greeneye estimates that for a farm larger than 3,000 acres the ROI would typically be less than 2 years.
Greeneye does not charge subscription fees for its algorithms. This last aspect is interesting as John Deere and Bilberry both charge for use of their algorithms on a per acre basis. John Deere, for example, charges $3/acre US for corn, and $4/acre for soybeans and cotton, each time you make a spot spray pass with See & Spray Ultimate.
Available Greeneye algorithms are for Green-on-Brown, and Green-on-Green in corn and soybeans as of July 2023. Cotton and milo will be available in 2024, and Greeneye is working on canola and wheat as well. Like Bilberry, they capture images from the cameras for use in algorithm learning, and accuracy and hit rate should be improving with time. Travel speeds of 15 mph are working well according to Greeneye.
As for performance, the proof will be in the pudding. The company in its wisdom did commission an independent evaluation at the University of Nebraska, Lincoln, and has made a summary of the university report available on its website. According to UNL, broadleaf weed control in corn with the spot sprayer was equivalent to the broadcast treatment, and grassy weed control was slightly less effective. UNL researchers noted herbicide crop injury (“Status”, dicamba + diflufenzopyr) was reduced with the spot spray. Of course, savings will be a function of weed density and the detection threshold chosen by the operator, but the addition of reduced crop injury resulting in greater yields could also be very valuable.
A recent investor and business partner is Farmers Business Network (FBN). FBN sees the opportunity for a spot sprayer to act as a scouting platform that helps evaluate the success of new pest management strategies.
Support on the ground is in the form of US staff with backgrounds in the spraying industry. Software development and digital troubleshooting remains in Israel.
Although I no longer have business links to Greeneye, I was happy to see this sprayer operating as well as it did. I remain convinced that spot sprays will be an essential part of our spraying future, for sustainability and resistance reasons. It is heartening to see these early successes and it will be interesting to see them continue to evolve.
July 28, 2023