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Hay curing weather
You need 3–5 clear days in a row from cutting to baling — counted from the day you drop the mower, not the day you check the forecast. One sunny Monday isn't a hay window. Here's what actually cures a windrow, what moisture to bale at, and why the wet bale you sneak into the shed can burn it down a month later.
The run matters more than the day
Hay is the one job you can't do on a good day — it takes a good run. In southern Australia, plan on 3–5 consecutive dry days between cutting and baling: about three for a conditioned pasture cut in warm spring weather, four to five for heavier cereal hay or lucerne, and add a day or two for early-spring or autumn cuts with their shorter days and heavier dews.
That changes how you read a forecast. The question is never "is tomorrow nice" — it's "where does the run start and does it hold to the end". A mostly-sunny week with a change due Thursday is not a five-day window; it's a three-day window with a deadline, and if your crop needs four, it's not a window at all. Most rained-on hay was cut into a run that was never there.
What actually cures hay
Sun, warm dry air and a stirring breeze cure hay — not heat on its own. A hot, still, humid day is a worse curing day than a mild one with low humidity and a steady northerly. The crop comes off the mower at roughly 75–80% water and has to get down near 15%, and four things move it:
- Direct sun. Solar radiation is the engine. Cloud cover halves your day even when the temperature holds.
- Low humidity. Dry air pulls moisture out of the windrow; humid air hands it back.
- A breeze. A steady 10–20 km/h keeps swapping the damp air inside the windrow for dry air. Still days stall the cure even in full sun.
- Your gear. A mower conditioner cracks the stems so moisture can escape — the single biggest lever you have on curing time. Tedding soon after mowing can cut curing time by roughly a third, and wide windrows put more crop in the sun instead of its own shade. Rake in the morning while a little moisture keeps the leaf on the stem.
Dew: the overnight tax
Hay barely dries at night. Once the sun's off, humidity climbs, the crop keeps respiring, and dew re-wets the top of the windrow — so overnight the moisture line holds at best and creeps backwards at worst. A heavy dew can hand back a decent slice of the previous day's drying, which is why the count is consecutive dry days and why humid, still nights stretch a three-day cure into five.
Dew has its uses at the very end: a light dew on properly cured hay softens the leaf, which is why lucerne growers bale in the evening or at first light so the leaf bends instead of shattering. But that only works on hay that has already reached target underneath. Surface dew on cured stems and stem moisture that never cured are different things wearing the same feel.
Baling moisture: the numbers that keep sheds standing
Bale by the meter, to the target for your bale type — the denser the package, the drier it has to be, because a dense bale can't breathe its moisture out.
| Bale type | Target moisture | Above target |
|---|---|---|
| Small squares | 15–18%, and 18% is the ceiling | Mould and heating — feed quality first, fire risk close behind |
| Large rounds | 14–16% | Core heating you can't see from the outside |
| Large squares | 12–14% | Densest bale, least forgiving — the classic haystack-fire package |
No bale of any shape is safe above 18%. Test with a moisture meter — several bales, several spots — and act on the highest reading, not the average. Fires start at the wet spot, not the mean, and a hand on the bale can't tell 17 from 20.
How a haystack catches fire
Bale damp hay and the microbes in it go to work, and their work makes heat. In a stack that heat can't escape, so the temperature climbs for days and weeks — quietly — until somewhere in the middle it's hot enough to char, and the stack starts smouldering from the inside. That's why haystack fires so often turn up weeks after baling, in a shed, at night. Spontaneous combustion is a leading cause of haystack fires in Victoria, and one wet bale can take the stack, the shed and the machinery beside it.
If a stack is heating, don't pull it apart. Opening a hot stack feeds it air. Probe new stacks for warmth over the first weeks — a temperature probe, or a crowbar left in and checked by hand — keep stacks separated and away from sheds and fuel, and if one is hot and steaming, call the CFA before you touch it.
How CropWindow finds the run
CropWindow treats hay the way you do — as a sequence, not a day. It scans the hour-by-hour forecast for the full run of curing weather your crop needs before it green-lights the cut: sun, humidity, wind and overnight dew across every day from mower to baler, not just a promising Monday. If the change speeds up mid-week, it tells you Tuesday morning — while the hay can still come in early — not after the windrow's wet. You get the start day, red, amber or green confidence, and the reasons, and every threshold is yours to adjust.
Fair questions
Hay weather, asked and answered
How many dry days do I need to make hay?
In southern Australia, typically three to five clear days between cutting and baling depending on the crop, the weather and your gear — heavier crops and cool, humid weather stretch it. The forecast run matters more than any single day: a perfect cutting day means nothing if rain lands on the windrow two days later.
What moisture should hay be baled at?
Small squares are commonly baled up to around 18% moisture; large squares and rounds want lower, and anything above 18% risks heating. Wet hay doesn't just moulder — microbial heating can smoulder into a haystack fire weeks after baling. When in doubt, test with a moisture meter, not by feel alone.
Does dew matter when curing hay?
Yes — heavy overnight dew rewets the top of the windrow and can undo a day of curing. Warm nights with low humidity cure fastest. It's why a run of textbook sunny days with dewy nights can still leave you short of baling moisture.
Find the run before you drop the mower
CropWindow reads the whole week — sun, humidity, dew and wind, cut to bale — and tells you which day to start, with the reasons. Free in early access.
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