A swath is the row the mower leaves. Its width is the only variable the operator still controls at that moment, and it sets the drying rate for everything that happens afterwards.
Swath: the row a mower leaves behind
window HAY WINDOWchecked 2026-09-16responsible Windrow Field Manual (a named masthead, not a person)
[['Heavy first-cut grass', 'As wide as the mower conditioner will spread', 'Dries fastest; tedding may be unnecessary'], ['Thin alfalfa, hot and dry', 'Narrower, and keep the leaves', 'Below about 70% leaf retention the loss outweighs the speed'], ['Humid, low wind, rain due', 'Widest the machine allows', 'Surface area is the only lever left']]
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The swath is a decision, not a by-product
A mower-conditioner does not simply drop the crop. It crimps or rolls the stems and lays the material in a band whose width the operator sets. Spread wide, the swath catches more sun and wind and dries faster. Left narrow, it dries more slowly and the leaves stay on the stem, which matters because the leaf is where the protein and the digestible energy sit.
The 70% leaf retention rule
For alfalfa the practical rule is that once leaf retention falls to roughly 70% of the standing crop, further handling costs more feed value than the extra drying speed is worth. Leaf shatter happens when the leaves are dry and the stems are not, so the damage is done by moving the crop in the wrong window, not by the mower.
| Moisture at handling | What the leaf does | What it costs |
|---|---|---|
| Above 40% | Leaf stays intact and bends | Almost nothing |
| 25–40% | Leaf starts to shatter on impact | Measurable, and it is the usual range for tedding |
| Below 20% | Leaf shatters readily | The loss is already made; stop moving it |
Boundary: swath, windrow, and the row in between
The swath is what the mower leaves. Rake it and it becomes a windrow, and the width you set at mowing stops describing anything. The near kin is the merge — two swaths brought into one row — which raises mass without raising surface, and is the most common way to slow a crop down while believing you are speeding it up.
The misreading the manual sees most often: a grower reads a windrow moisture table, checks the unmerged swath, and gets a number that is three to five points lower than the row the baler will actually pick up. The measurement is right and the object is wrong.
A sentence to disagree with
Several bulletins present a wide swath as free. It is not free in a humid climate: a swath spread wide over soil that is still wet at the surface can pick moisture back up overnight, and the dew it collects is greater than the dew a narrow swath collects. The manual holds that in a dew-heavy climate the widest setting is a bet on tomorrow's weather, not a free improvement.
Where the numbers come from
- S1Moisture Content and the Hay Drying Curve, Field Crop News, Ontario Ministry of Agriculture, Food and Agribusinesssupports the drying curve a swath follows from cutting to baling moisture
- S2Moisture Content and the Hay Drying Curve (2021), Field Crop News, Ontariosupports the relationship between swath width and the rate moisture leaves the crop
- S3Hay Crops: Legumes and Pulses (Chapter VI), FAO Agricultural Services Bulletin 154supports the leaf-to-stem proportion that makes leaf loss expensive in legumes
One link, one document. No home pages, no search results, no bare domains.
Put this page down Put this page down when you have chosen a swath width for the weather you expect, not the weather you want.