Quick answer: A hay preservative applicator is not a sprayer — it is a rate controller. The pump and nozzles are the cheap, boring part; what you are really buying is how the machine decides how much acid to put on. A manual unit gives you a dial and makes you guess. An automatic unit reads bale moisture continuously and varies the rate on the go. Automatic wins clearly on square balers, where the moisture sensor sits inches from the hay it is dosing. On round balers the case is weaker and owners split hard, because the sensor reads a bale that is already half-built. Budget roughly $2–$3 per bale in product at typical dry-hay rates — about what net wrap costs you — and expect to spend more time solving where the tank goes than anything else.
Everybody who bales hay in a humid climate eventually has the same week: the crop is fit, the forecast is not, and the windrows are sitting at 22% at four in the afternoon. A hay preservative applicator is the machine that lets you bale anyway. What almost nobody explains before you spend the money is that the applicators themselves are the easy part — pumps, hose, nozzles, a tank. The expensive difference between a $1,500 unit and a $6,000 unit is the control system, and whether that control system earns its keep depends almost entirely on what kind of baler it is bolted to.
This guide is for hay producers who have already decided that preservative makes sense for their operation and are now choosing hardware. If you are still deciding whether to use preservative at all — chemistry, product classes, whether it pays — start with the hay preservative guide instead. What follows is the equipment side: manual versus automatic, rates, tanks, sensors, and cleanup, with the arguments that actual owners have on both sides.
On this page
- What an applicator actually is
- Manual dial vs. automatic rate control
- Why round balers change the automatic argument
- Application rates: the numbers that decide whether it works
- What it costs per bale — and how that compares to net wrap
- Mounting the tank is the hard part
- Not every product needs rate control
- What your baler's moisture sensor can and cannot do
- Corrosion, cleanup, and what wears out
- Buying checklist
- Frequently asked questions
What an applicator actually is
Strip the marketing away and every on-baler system has four parts: a tank, a pump, a nozzle or manifold that sprays into the pickup or pre-chamber, and a controller that decides the flow rate. Three of those four are commodity plumbing. The controller is the product.
That framing matters because it tells you where to spend. A bigger tank does not preserve hay better. More nozzles do not preserve hay better. Getting the right pounds per ton onto hay whose moisture is changing under you is the entire job, and that is a control problem.
Manufacturers sell essentially three tiers. The entry tier is an on/off switch and a rheostat dial — you watch a moisture readout (or your own judgement), then set the dial. The middle tier adds a pressure or flow readout so you can confirm the pump is doing what you asked. The top tier adds moisture sensors and closes the loop: the controller reads moisture many times per bale and varies the pump output automatically. Harvest Tec's current automatic line, for example, uses disc sensors on round balers and star-wheel sensors on square balers, feeding a solenoid-pulsed bypass pump.
One practical note before you shop: you may not have to buy the hardware at all. Preservative is a consumable, and the companies that sell it compete hard for the recurring revenue.
"First, on the positive side, I doubt you'll have to pay for applicators. In most areas, the competition to sell preservative products is quite high. So a lot of vendors (all vendors in my area) let you borrow the applicators. The vendors in my area came out and actually did complete installs. The deal was, as long as I was using their product, the applicators stayed on my balers."
— NLC Farm · HayTalk thread 102737
Worth asking your supplier before you write a cheque for a system you might get on loan.
Manual dial vs. automatic rate control
This is the whole decision, and the forums are genuinely split — which is unusual. On most equipment questions experienced operators converge. Here they do not.
The case for automatic: your field is not one moisture
The strongest argument for automatic control has nothing to do with convenience. It is that the premise behind manual control — that you know the moisture of the hay going in — is often false.
"It's hard to believe until you actually witness it, but in our area and in our fields the actual moisture of the hay can vary up to 10%. Doesn't happen all the time but I've baled fields where the majority of it is getting close to or is at 16% while I have some areas go 26% or even higher. … Take in account rolling ground and the north side of a hill won't begin to get near the sunlight later in the year when the sun is lower in the sky as compared to the south facing side of the same hill."
— mlappin · HayTalk thread 14124
He did not take that on faith either — he checked it, and then stopped second-guessing the machine:
"After that I left the unit alone and let it do whatever it wanted as I quickly learned that since it takes 10's if not hundreds of samples per bale, it will be far more accurate than I can possibly be by grabbing a few hands full here and there. The few times I was absolutely certain something was defective and I tried running it manually as I thought it was reading wetter than it should, I lost the hay as it was as wet as the unit claimed it was and I didn't manually apply near enough acid."
— mlappin · HayTalk thread 14124
Two other things push the same direction. Manual operators tend to over-apply as insurance, which quietly costs money all season:
"We have just traded up from a manual liquid applicator to a fully automatic Harvestec unit. Cutting the cheque hurts at the time but I am figuring that I should save close to full tote of acid each season. I was always a bit trigger happy with the manual system to play on the safe side."
— MikeRF · HayTalk thread 14124
And on a big square baler running twenty years, one operator's summary is simply that it removes a variable:
"We have a Harvest Tec automatic system on a 2x3 big square baler. It would just be an over grown version of your baler. We have used it for almost 20 years now with zero problems. If you can swing the cost I would highly recommend the automated system. It takes the human element out of the equation."
— Gearclash · HayTalk thread 102737
The case for manual: you can see things the sensor cannot
The counter-argument is not that automatic control is a gimmick. It is a specific, physical objection about where the sensor sits relative to the nozzle, and it is the sharpest thing said on either side:
"We've been using the electronic systems (with the dial) for about 20 years. Right now we have 2 of them. I wouldn't even consider the auto. Besides spending thousands more for the applicator, I feel that I am much smarter than the applicator. I can see what's coming up in the roll. I can see the terrain - trees, low spots, etc. The applicator can't see that, or anticipate what's coming. Think about it - where are the star wheels vs where the spray is at? There's most of a bale between them, on a 3x3 that's a lot of hay."
— Rodney R · HayTalk thread 102737
That is a real control-theory problem, not a grumble. The nozzle sprays hay entering the chamber; the sensor measures hay that has already been compressed. Everything between them is dead time, and an automatic system is always dosing slightly in the past. A driver watching a green streak come up in the windrow genuinely is ahead of it.
The second manual argument is reliability. Automatic systems add sensors that can be fooled:
"The units were auto-rate applicators with a star wheel used to calculate the injection rates. The problem was that the star wheel mounts were of poor design and did not apply enough downward pressure on the bale chambers. The end result was the star wheel would bounce upward, lose contact with the bale, and momentarily spin freely ever time the bale would move reward from the forming of the flake. As a result, the computer would read the injection rate to be four to five times the needed rate."
— NLC Farm · HayTalk thread 102737
And plenty of small operators are simply happy with the base unit, for reasons that are hard to argue with:
"I have the 25 gallon tank, single nozzle and electronic controller. I had no trouble at all running it. It was very easy to switch, the pump on and off at the start and end of a Windrow. … It was very easy to keep an eye on the pressure from the cab. I really like its simplicity. No electronics to worry about, no Bluetooth to lose connection, etc."
— Saucymynx · HayTalk thread 102737
| Factor | Manual dial | Automatic rate control |
|---|---|---|
| Rate decision | Operator sets it from a readout or judgement | Controller varies it continuously from sensor data |
| Handles within-field moisture swing | Only if the operator notices and reacts | Yes — this is its core advantage |
| Product usage | Tends to run high; operators over-insure | Lower; owners report saving most of a tote per season |
| Failure modes | Pump, hose, nozzle plugging | All of those, plus sensor contact and calibration faults |
| Reacts to what is coming | Driver can anticipate a wet spot | No — always reacting to hay already in the chamber |
| Best fit | Small acreage, uniform fields, attentive owner-operator | High volume, variable ground, or hired drivers |
The honest synthesis: automatic control is worth most where the operator is worth least — long days, hired help, big acres, broken ground. If you bale 150 acres of flat, uniform alfalfa yourself and you are already watching a moisture readout, the dial is defensible.
Why round balers change the automatic argument
Here is the pattern that emerges once you sort the opinions by machine, and it is not something manufacturers advertise. The people happiest with automatic control are running square balers. The people who bought automatic and went back to manual are, disproportionately, running round balers. The same owner can hold both views:
"I had a Harvestec 470 automatic on my small square--worked really good. It seemed to apply acid appropriately. I put a similar automatic Harvestec system on my BR780 big round baler. I tried automatic---used a lot of acid. I now control it manually. Typically, I put acid on the inner 40" as a safeguard."
— rjmoses · HayTalk thread 102737
There is a mechanical reason for that, and it is worth understanding before you assume a unit is faulty.
A square baler makes a flake every stroke. The star wheel rides the bale a few feet behind the nozzle, so the loop from "measure" to "correct" closes in seconds, and every flake gets dosed for roughly its own moisture. A round baler forms one bale over a minute or more, in layers, from the outside in. The sensor is reading a rolling surface that is a blend of everything already in the chamber, while the nozzle is dosing fresh material at the pickup. The controller cannot dose a layer for its own moisture — only for the average of what came before it. Compounding that, extension guidance is explicit that large bales require higher application rates than small ones at the same moisture, because a big dense bale has far less surface area per ton to shed heat. So a round-baler system is being asked to hit a higher target with a blurrier signal.
None of that makes automatic control wrong on a round baler. It does mean two things: judge it on season-total product used and bales saved, not on whether any individual bale looked over-treated; and rjmoses's fallback — treating the core, where heat has the hardest time escaping — is a rational manual strategy rather than a failure to use the machine properly.
Application rates: the numbers that decide whether it works
Preservative failures are usually rate failures, not product failures. Two published anchors are worth having in the cab.
Mississippi State University Extension gives both a solution-strength guide and a rule of thumb that is easy to remember: square bales at 20–25% moisture take a 0.5–1.0% solution of propionic acid, bales at 25–30% take 1.0–1.5%, and an alternative shortcut is to subtract 10 from the moisture percentage to get pounds of product per ton of dry matter — so hay baled at 30% moisture needs about 20 lb per ton.
| Windrow moisture | Approx. rate (lb product / ton) | Notes |
|---|---|---|
| Under 18% | 0–4 | Not strictly needed; some owners run a low rate anyway to stop sweating |
| 20% | ~10 | 0.5% solution |
| 25% | ~15 | Upper end of routine use |
| 30% | ~20 | Practical ceiling |
| Over 30% | — | Inconsistent results; make baleage instead |
Both South Dakota State University Extension and Mississippi State are blunt that above roughly 30% moisture the research shows no consistent response. That is the line where the answer stops being acid and starts being baleage.
Now the interesting part: working farmers routinely run below those published rates. The same thread that produced the 4 lb/ton figure also produced the correction:
"4 lb to the ton sounds low. I think we run around double that on hay over 24%. BTW, the auto system will kick the rate up for you as the crop gets wetter."
— Gearclash · HayTalk thread 14124
Eight pounds per ton at 24% is still only about half the extension figure. That gap explains a lot of the preservative doesn't work reports — and it also explains why SDSU describes trial results as erratic. If you are going to use the stuff, use enough of it, and understand what it buys you:
"Keep in mind that even though this hay contains preservative, it needs to be allowed to dry down. Stack it with enough gap between bales to permit airflow around the bales. Preservative gives you about 2 months for the hay to dry without heating and damage."
— Gearclash · HayTalk thread 102737
That is the single most useful sentence in this guide. Preservative buys time, not permanence. It does not turn 25% hay into 15% hay; it stops the mold clock long enough for the bale to finish drying in the stack. Stack treated bales tight against each other and you have spent the money and thrown away the mechanism. Our notes on stacking net-wrapped bales in a barn apply directly here.
One more thing to expect, so you do not panic: treated bales often read wetter right after baling. Moisture meters measure conductivity rather than water, and a liquid acid product both adds water and conducts. As one grower found on his first run, bales baled at 22% read 25–26% two days later while staying cool — a rise that Gearclash calls normal: "Typical to see a measured moisture rise after baling, even with no preservative."
What it costs per bale — and how that compares to net wrap
Preservative is a per-bale consumable, which makes it directly comparable to the other per-bale consumable on the machine. One owner did the arithmetic before buying:
"4 lbs/ton = about 2-2.5 lbs/bale = about 1/3 gallon/bale. At $6-8/gallon, this is about $2-3/bale, about the same as net wrapping."
— rjmoses · HayTalk thread 14124
Two caveats. That is a dry-hay rate; at 24% and up you may be spending two to four times as much per bale. And the comparison is not either/or — you are paying for net wrap regardless. What preservative actually buys is captured in his next sentence: he could stack the bales immediately instead of letting them sweat for two weeks, so he "would only have to handle them once instead of twice." Halving your bale handling on a wet year is where the money is, not in the chemistry.
The hardware sits at roughly $1,500–$3,000 for a manual liquid system and $4,000–$6,000 and up for a full automatic with sensors; rjmoses was quoted about $4K for his automatic round-baler unit, plus another $500 for optional hay-presence electric eyes. If you want to sanity-check the per-bale side against your own numbers, the cost per bale calculator handles the wrap side of the ledger, and is net wrap worth it covers the storage-loss math that preservative is competing with.
Mounting the tank is the hard part
This is the complaint that shows up in every applicator thread and in no sales brochure. A 55-gallon tank of liquid weighs roughly 500 lb and has to live somewhere on a machine that was not designed around it.
"My concern with this unit is that the 55 gal. tank sits in front of the belts and I wouldn't be able to see the belts! I have to watch the belts pretty carefully when starting a bale going across a hillside--the belts will want to roll over one another."
— rjmoses · HayTalk thread 14124
On a round baler, the natural mounting point for a tank is exactly where you need to see belts starting a bale. The workarounds owners actually use:
- Drop to a 25-gallon tank and refill more often. The simplest fix, and the one most small operators land on.
- Have a low, flat tank fabricated — stainless or poly — that sits below the sightline.
- Split into two smaller tanks mounted in the twine boxes on either side, which is available on net-only machines since the twine boxes are empty.
- Fix the underlying belt problem instead. Several operators pointed out that consistent windrow width and an upper belt guide stop belts rolling in the first place — rjmoses ultimately bought the belt guide along with the applicator.
Resist the temptation to move the tank to the tractor. It is the obvious idea and it comes up every time, but as MikeRF found, the manufacturer's objection is that the extra lift and line length make the pumps work too hard. If you go that route, talk to the maker first.
Not every product needs rate control
Before you spend money on automatic rate control, check whether the product you intend to run actually varies its rate. Buffered propionic acid does. Some non-acid preservatives explicitly do not:
"Nice thing about the Silo-Guard / Hay-Guard that I use is that it doesn't matter what moisture the hay is, application rate is 2lbs/ton at 16% moisture or 22% moisture."
— JoshA · HayTalk thread 14124
If your product is flat-rate, the entire economic argument for an automatic system evaporates — you are paying thousands for a controller to hold one number. A simple pump-and-dial unit is the correct purchase.
The catch is the ceiling. Flat-rate products have a hard moisture limit and no way to tell you that you have crossed it:
"Does the dorhman unit have a moisture sensor of any kind? Just wondering as the Silo/Hayguard has an upper limit of 24-25%? Be nice to know if you have a few bales over the limit."
— mlappin · HayTalk thread 14124
That is the right way to think about it: with a flat-rate product you may not need automatic dosing, but you still need moisture information. A standalone sensor is far cheaper than a full automatic system — one owner noted the factory moisture-sensor option on his Vermeer baler was $308.
What your baler's moisture sensor can and cannot do
Applicator sensors and baler moisture monitors are the same technology, and they share the same two limits.
They measure conductivity, not water. Anything that changes conductivity moves the number — salt content, product residue, temperature, and how hard the hay is pressed against the contact. This is also why treated hay reads high.
They top out well below baleage moisture. This is the limitation that catches people converting to wet wrapping:
"My Harvest Tech, Delmhorst and Agritronics all limit out at 36-40% moisture. Using the wife's microwave out in the field isn't very efficient."
— r82230 · HayTalk thread 95374
Since baleage should be wrapped between 40% and 60% moisture, an on-baler sensor is essentially blind in that range — it will read "high" and stop being informative. Owners doing both dry hay and baleage on one machine end up with a second instrument, typically a microwave-type tester or a silage tester, for the wet side. Below that ceiling the on-baler readings hold up well: one operator cross-checked his big square baler's sensor against a lab sample on the same field and found them "very close."
For the handheld side of this, see the hay moisture tester guide; for what those numbers should be when you are wrapping in net, see baling moisture for net-wrapped bales.
Corrosion, cleanup, and what wears out
Propionic acid is corrosive — SDSU Extension notes it "can cause damage to machines and people," which is precisely why buffered ammonium propionate is the recommended form for on-baler use. Buffered is gentler, not inert.
The routine that owners describe is simple and non-negotiable: keep a drum of clean water beside the acid and flush the pump and lines after every use. rjmoses keeps a 15-gallon tank of water by his acid barrels for exactly this and reports "so far, so good" after more than a decade.
What actually fails over time, from an owner more than twelve years in:
"I had to replace all the plastic fittings after about 12-13 years--they get brittle and break. I've had to replace two of the three pumps over the years. While I don't like stuff failing on me, I consider this more or less normal wear and tear nowadays. I would prefer that they used metal fittings and neoprene tubes, but...."
— rjmoses · HayTalk thread 102737
Two pumps and a set of fittings across thirteen seasons is a reasonable service record for a machine that sprays acid for a living. Budget for a pump rebuild kit and a set of fittings as consumables, not as failures. It is also worth knowing that a second supplier, Dohrmann Enterprises, has a reputation among owners for hardware that holds up, and some Harvest Tec owners fit Dohrmann parts at replacement time.
Buying checklist
Run this before you commit:
- Ask your preservative supplier whether they loan applicators. In competitive markets many do, with the hardware staying as long as you buy their product.
- Check whether your product is flat-rate or moisture-variable. Flat-rate products do not justify automatic rate control.
- Decide by baler type, not brochure. Square baler and high volume — automatic pays. Round baler and modest acres — the dial is defensible, and treating the core is a legitimate strategy.
- Measure where the tank goes before you order. Sit in the seat, look at the belts or the pickup, and decide honestly whether a 55-gallon tank blocks something you need to see. Choose the 25-gallon option if it does.
- Confirm the moisture-sensor ceiling if you also make baleage. Most top out at 36–40%, below the 40–60% baleage window.
- Price the rate, not the jug. Compare cost per pound of active ingredient, not per gallon of product.
- Plan the flush. Water supply, a place to drain, and a routine — before the first wet night, not after.
- Plan the stack. Leave airflow gaps. Treated hay still has to dry.
The bottom line
A hay preservative applicator is a rate controller wearing a tank, and the right one depends on how much your own judgement is worth in the seat. Automatic control earns its price where moisture varies more than an operator can track — broken ground, long days, hired drivers, square balers where the sensor sits close to the nozzle. A manual dial remains a rational choice on uniform ground, with flat-rate products, or on a round baler where the feedback loop is long anyway. Whichever you buy, the two things that decide whether it works are unglamorous: apply enough product for the actual moisture, and stack the bales loose enough that they can still finish drying.
And whatever you spray on the hay, what holds the bale together still matters. Preservative controls what happens inside a bale; net wrap controls what the weather does to the outside of it. Factory-direct net wrap from XES Netting ships in 48, 51, 64 and 67 inch widths for every mainstream round baler.
Frequently asked questions
Is an automatic hay preservative applicator worth the extra cost?
On a square baler and decent acreage, usually yes — moisture can swing 10 percentage points within one field, and the automatic varies the rate faster than an operator can. On a round baler with modest acres the case is weaker, because the sensor reads a partly formed bale, and several owners have gone back to manual control.
How much hay preservative do you apply per ton?
Rate rises with moisture. Mississippi State Extension suggests subtracting 10 from the windrow moisture percentage to get pounds of product per ton of dry matter — so about 10 lb/ton at 20% and 20 lb/ton at 30%. Large round bales need higher rates than small squares at the same moisture. Above 30% moisture, results are inconsistent.
Can you put a preservative applicator on a round baler?
Yes. Kits exist for all mainstream round balers, using disc-type moisture sensors rather than the star wheels used on square balers. The practical problem is tank placement — a 55-gallon tank commonly blocks your view of the belts, so many round-baler owners fit a 25-gallon tank or a custom low-profile tank instead.
Does hay preservative let you bale wet hay?
It lets you bale wetter, not wet. Buffered propionic acid suppresses mold and heating for roughly two months while the bale finishes drying in the stack. It does not remove water, so treated bales must still be stacked with airflow gaps. Above about 30% moisture, make baleage instead.
Why does my hay read wetter after applying preservative?
Moisture meters measure electrical conductivity rather than water. Liquid preservatives add some water and conduct on their own, so readings rise. A moisture increase in the first days after baling is normal even on untreated hay. Watch internal bale temperature rather than the meter to judge whether a stack is safe.
Do baler moisture sensors work for baleage?
No. Common on-baler sensors top out around 36–40% moisture, while baleage should be wrapped at 40–60%. In that range the sensor simply reads high and stops being useful. Operators making both dry hay and baleage generally add a microwave-type or silage moisture tester for the wet side.
This guide is maintained by the XES Netting team — a bale net-wrap manufacturer. Every farmer quote in this post is verbatim with a thread link, so you can go read the originals. Preservatives are chemicals: follow the product label, wear the specified protective equipment, and follow your baler's operator manual for mounting, guarding and lockout before working around the pickup or chamber. Application rates here are general guidance, not a substitute for the label or your local extension service.
Featured photo: Baling hay in Vermont by Putneypics, licensed under CC BY 2.0, via Wikimedia Commons.
