Weed Control
Can You Mix Herbicides in One Tank? The Order Matters
NDSU: most broadleaf herbicides tank mixed with grass herbicides reduce grass control. The fix is sequence — grass herbicide one day before, or seven days after.
In this guide6 sections
The question people ask is whether two herbicides can go in one sprayer. Usually they can. The one that matters is a different question: what does the mixture cost you?
NDSU Extension’s answer for the most common home-and-plot combination:
“Most broadleaf herbicides tank mixed with POST grass herbicides will often reduce grass control compared to the grass herbicide applied alone.”
That effect is called antagonism, and you cannot see it. The tank looks fine. The spray goes on fine. The broadleaf weeds die on schedule. The grasses — the reason you added the second product — survive.
The Failure You Cannot See
Most spray problems announce themselves. This one does not, and that is why it persists.
What the reader experiences: a clover plot with both grassy weeds and broadleaf weeds. Two products go in one tank to save a pass. Three weeks later the broadleaf weeds are gone and the grasses are standing. The obvious conclusion is that the grass product was weak, or the rate was low, or the weeds were too big. All three are wrong, and the pattern that gives it away — broadleaf weeds dead, grasses standing — belongs to antagonism alone among the six things that commonly go wrong.
What happened: the broadleaf herbicide interfered with the grass herbicide’s activity inside the plant. NDSU’s phrasing is “will often reduce grass control” — not occasionally, and not only in extreme cases.
| What you observe | What it actually was |
|---|---|
| Broadleaf weeds died, grasses did not | Antagonism — the mix cost you the grass control |
| Grass browned then recovered | Could be antagonism, could be the wrong adjuvant |
| Tank separated or clogged the filter | Physical incompatibility, a different problem |
| Nothing died | Rate, timing, temperature or rain — not the mixture |
The second row is worth pausing on, because it has two possible causes and one is much more common on a home shelf. The grass herbicides want an oil adjuvant rather than a plain surfactant, and substituting one for the other produces exactly the same brown-then-recover result. Which of the two you are looking at is covered in surfactant vs crop oil.
The Fix Is Sequence And It Is Asymmetric
NDSU gives the interval in one sentence, and the asymmetry inside it is the practical part:
“Reduced grass control can be avoided by applying the grass herbicide at least 1 day before or 7 days after application of a broadleaf herbicide.”
One day if the grass herbicide goes first. Seven days if it goes second.
| Order | Wait |
|---|---|
| Grass herbicide first | 1 day before the broadleaf product |
| Broadleaf herbicide first | 7 days before the grass product |
NDSU gives the interval and stops there. The reason for the asymmetry is our reading rather than theirs, and it is worth saying which is which: a broadleaf herbicide slows nearby plants down, including the grasses, and a grass herbicide needs a growing plant to move within — the same requirement that makes hot weather and cold weather both reduce results. If that is right, the seven days is not a decontamination period so much as a recovery period, and a grass still visibly checked at day seven is telling you to wait longer. Treat the seven days as the published minimum, which is what NDSU actually says, and the rest as a reason to watch the plant.
The grass half of the pair
Whitetail Institute Arrest Max Selective Grass Herbicide, 1 Pint
★★★★☆ (4.3/5)
Clethodim kills grasses and leaves broadleaf plants standing — the mirror image of every lawn herbicide, and what a clover plot actually needs.
Best for: Grass weeds in a clover, alfalfa or brassica plot — spray it a day before your broadleaf product rather than with it, and give it the oil adjuvant its label asks for
$46–$56
View on Amazon
The broadleaf half, in a legume stand
Albaugh Butyrac 200 Herbicide, 1 Gallon
★★★★☆ (4.4/5)
Broadleaf weed control in alfalfa and clover — the one thing a normal broadleaf herbicide cannot do, because clover is itself a broadleaf.
Best for: Broadleaf weeds in clover or alfalfa. If this goes on first, NDSU's guidance is to wait seven days and until the grasses are growing again before the grass herbicide follows
$50–$62
View on AmazonAnd there is a second reason not to combine those two anyway: they want different adjuvants. Most clethodim labels specify an oil; a broadleaf partner is more likely to specify a non-ionic surfactant. One tank cannot satisfy both label instructions properly, and Penn State’s rule applies — if a label names a specific adjuvant, using another is a label violation.
What you can do in the tank
NDSU names two levers, and is careful about how much they achieve:
- Fertilizer. “Addition of fertilizer may partially overcome antagonism from broadleaf herbicides.” In practice a nitrogen solution or ammonium sulphate.
- A higher grass rate. Elsewhere in the same guide: antagonism “can be reduced or avoided by applying a higher rate of the grass herbicide.” Read that one in context — NDSU writes it in a paragraph about a specific crop and product pairing rather than as a general rule, so it is evidence the lever exists, not a licence to raise rates whenever you mix.
Read both as damage limitation. Partially is doing real work in the first sentence, and the second runs into the label’s maximum rate quickly — and the maximum rate is a ceiling, not a target.
Note also why agriculture tolerates this and you do not have to. On a field, a second pass costs fuel, time and compaction, so a partial antagonism offset with fertiliser can be the right trade. On a lawn or a half-acre plot, walking it twice costs an afternoon and nothing else. The sequence is available to you in a way it often is not to a farmer.
If You Do Mix: The Order, And The Test That Costs Nothing
Everything above is about whether two actives fight each other in the plant. This section is about whether they fight each other in the tank — a different problem with a cheap answer.
There is a published order, and it is not “whatever you pick up first.” Virginia Tech gives it as WALES (or DALES, for dry formulations):
- W / D — “wettable powders (D for dry formulations)”
- A — “agitate (in between each new product)”
- L — “liquid flowables”
- E — “emulsifiable concentrates”
- S — “surfactants or adjuvants”
The A is the part people skip, and it has a duration attached: “allow at least 3-5 minutes for a product to fully mix before incorporating additional products to the tank.” Three to five minutes per product. On a backpack sprayer that is the difference between a uniform tank and a gel at the bottom of it.
And the test. Before committing a full tank to a combination you have not used, Virginia Tech describes a jar test that takes fifteen minutes and one jar:
- “Fill a jar half full with water or other carrier”
- “Add products in proportion to the tank mix, one at a time”
- “Shake the jar between each addition and allow it to stand for 10-15 minutes”
- “Look for signs of incompatibility”
What counts as a failure is specific: “products that are incompatible will separate into layers, create clumping or foam, give off heat, or precipitate.” Heat is the one worth pausing on — a mix that warms up is telling you something is reacting, and that is not a sprayer problem you want to discover with a full tank on your back.
Incompatibility Is Not Antagonism
Some of that incompatibility is mechanical rather than chemical: suspended solids that settle or abrade. If the products you tank-mix are wettable granules, the pump in your sprayer is part of the question — see best backpack sprayer.
Two different failures, and only one is visible before you spray.
Physical incompatibility is a chemistry problem in the tank. NDSU’s example is specific: chelated zinc materials “may cause incompatibility problems resulting in zinc precipitation”, and the guide gives an order of operations to prevent it — fill the sprayer, agitate thoroughly, then add the other material.
Antagonism is a biological problem inside the plant. The tank is perfect. The result is not.
| Incompatibility | Antagonism | |
|---|---|---|
| When you notice | While mixing, or at the filter | Two to three weeks later |
| What you see | Curdling, separation, sludge | A perfect-looking spray that half worked |
| Fix | Mixing order, agitation, or do not mix | Separate the applications in time |
Mixing order matters for both, and the general principle from NDSU’s zinc example generalises: put water in first, agitate, add products one at a time with agitation running. A sprayer with no agitation — which is most backpack sprayers — is a reason to be more conservative about mixing, not less.
The Thirty Day Grass Crop Interval
This one is not about mixing at all, and it catches food plot owners specifically.
“‘Dim’ herbicides (Select and Poast) may leave a residue in the soil and injure corn and other grass crops if planted before 6 days after application. Grass crops may be planted 30 days after Poast or Select application.”
Clethodim and sethoxydim are the ‘dims’, and both are on this site. So:
- You spray a clover plot for grassy weeds with clethodim.
- You decide to add a cereal plot — wheat, oats, rye — on the same ground.
- Wheat is a grass. The residue that killed the grassy weeds is still there.
Thirty days is the figure to plan around. NDSU gives 6 days as a conditional floor with rate limits attached, which is not the number to build a planting date on. Neither interval is long, but both are longer than the same afternoon — and this is exactly the kind of restriction that turns a sensible two-crop plan into a failed seeding.
The crop-by-crop logic for those plots is in cereal food plots and brassica food plots, and the underlying point — that a grass-selective herbicide treats a cereal as a weed — is the same one that governs both.
The Short Version
Mixing is usually possible and sometimes expensive. The cost lands on the grass control, and it lands invisibly.
Sequence instead: grass herbicide one day before, or seven days after. That interval is NDSU’s. Our own suggestion, offered as such, is that if the broadleaf went first you watch the grass rather than only the calendar — seven days is the minimum, not a guarantee it has recovered.
Fertilizer and a higher grass rate help partially. Neither is a substitute for the interval, and the rate has a ceiling.
A clean tank that half worked is antagonism. A curdled tank is incompatibility. Different problems.
And after a clethodim or sethoxydim application, wait 30 days before seeding a grass crop on that ground.
Common questions
Frequently Asked Questions
Can you mix two herbicides in the same sprayer?
Often yes, and one specific combination reliably costs you half the result. NDSU Extension states it plainly: most broadleaf herbicides tank mixed with post-emergence grass herbicides will often reduce grass control compared to the grass herbicide applied alone. That effect has a name — antagonism — and it is not a compatibility problem you can see in the tank. The mixture looks fine, sprays fine, and the broadleaf weeds die on schedule. What fails is the grass control you also paid for. So the honest answer is that mixing is usually physically possible and sometimes agronomically expensive, and the two questions get confused constantly because one is visible and the other is not.
How do I avoid antagonism?
By separating the applications in time, and NDSU gives the interval in both directions: reduced grass control can be avoided by applying the grass herbicide at least 1 day before, or 7 days after, application of a broadleaf herbicide. Note the asymmetry, because it is the useful part. Grass herbicide first needs only a day's head start. Broadleaf herbicide first needs a full week before the grass herbicide follows. NDSU gives the interval and nothing more; the reason for the asymmetry is our own reading rather than theirs — a broadleaf herbicide slows the grass down, and a grass herbicide needs a growing plant to move within. If that is right, a grass still visibly checked on day seven is worth waiting on. But seven days is the published figure, and the rest is inference we are labelling as ours.
Is there any way to overcome it in the tank?
Partly, and NDSU names two levers rather than one. The addition of fertilizer may partially overcome antagonism from broadleaf herbicides — in agricultural practice that means a nitrogen solution or ammonium sulphate in the spray mix. And elsewhere in the same guide they note antagonism can be reduced or avoided by applying a higher rate of the grass herbicide — though they write that in a paragraph about one specific crop and product pairing, so treat it as evidence the lever exists rather than a general rule. Read both as damage limitation rather than a solution. Partially is doing real work in that first sentence, and the second option runs into the label's maximum rate quickly. If the sequence is available to you — and on a lawn or a food plot it almost always is, because nobody is paying for a second pass across a field — separating the applications is the better answer.
Does the label ever forbid a tank mix outright?
Yes, and the wording is worth recognising when you see it on your own bottle: labels of this kind tell the applicator to avoid tank mixtures with other herbicides that reduce the product's efficacy through antagonism, and to avoid tank-mix directions that would push application rates below the label rate. We are describing the shape of that clause rather than quoting a specific product, because we could not point you at which of ours carries it — check the Mixing or Compatibility section of the label in your hand. Penn State makes the related point about adjuvants: if a pesticide label lists a specific brand of adjuvant, that brand must be used, and any substitution would be a violation of the label. So a tank mix is not a free decision made in your shed. Where the label speaks to it, the label governs, and where two labels disagree the more restrictive one wins.
What about mixing a herbicide with fertilizer or insecticide?
Physical incompatibility is a separate failure from antagonism, and it is the one you can actually see. NDSU's guide notes chelated zinc materials may cause incompatibility problems resulting in zinc precipitation, and gives an order of operations to prevent it — fill the sprayer, agitate thoroughly, then add the other material. That is a mixing-order problem rather than a biological one. The distinction to hold: if the tank curdles, separates or leaves sludge in the filter, that is incompatibility. If the tank looks perfect and one of your two targets survives, that is antagonism. Different problems, different fixes, and only one of them is visible before you spray.
I sprayed clethodim and a broadleaf herbicide together in my food plot. What happens?
Expect the broadleaf weeds to die and the grasses to survive or recover, which is the classic outcome in a legume plot. Clethodim is the grass herbicide in that scenario and 2,4-DB or imazethapyr the broadleaf one, and NDSU's finding applies directly. The fix for next time is the sequence, not a stronger mix: clethodim first, then the broadleaf product a day later, or wait a full week if the broadleaf product went on first. There is a second reason not to combine them anyway — the two want different adjuvants. Most clethodim labels call for an oil adjuvant, and a broadleaf partner is more likely to specify a non-ionic surfactant, so a single tank cannot satisfy both label instructions properly.
Do grass herbicides leave anything in the soil?
The cyclohexanedione grass herbicides do, briefly, and it matters more to a food plot owner than to a lawn owner. NDSU: the dim herbicides — Select (clethodim) and Poast (sethoxydim) — may leave a residue in the soil and injure corn and other grass crops if planted before 6 days after application, and grass crops may be planted 30 days after a Poast or Select application. So if you spray a clover plot for grassy weeds and then decide to put in a cereal plot on the same ground, you have a waiting period. Thirty days is the figure to plan around, and 6 days is the absolute floor with rate conditions attached. Neither is long, but both are longer than the same afternoon.
Is it ever better to mix than to spray twice?
Yes, when the two products are not in an antagonistic pair and the second pass is genuinely impractical. On acreage, one pass across a field costs fuel, time and soil compaction, which is why agricultural practice tolerates a partial antagonism it can partly offset with fertilizer or a higher grass rate. On a lawn or a plot measured in thousands of square feet, walking the area twice costs an afternoon and no money, so the calculation is different. The reason to mix on a small area is usually convenience, and convenience is a poor trade against a treatment that half worked. If you are going to mix, mix things that target the same weed group rather than opposite ones.
Was this article helpful?
Verify our work
📚 Sources & Further Reading
This guide is written against published land-grant university extension research and USDA data. Verify local details with your own state's extension office — and always follow the product label.
- [1]Understanding Spray Tank Mixing Practices — Virginia Cooperative Extension, Virginia Tech (BSE-351P)(pubs.ext.vt.edu)
- [2]Crop Paragraph Supplement, North Dakota Weed Control Guide — NDSU Extension(ndsu.edu)
- [3]Spray Adjuvants — Penn State Extension(extension.psu.edu)
Spotted an error? Tell us — we correct verified mistakes and note the revision date. See our editorial policy.
