An inch of rain on a curing crop costs dry matter and leaches soluble nutrients, and the loss depends on how dry the crop already was. The drier it was, the more the rain takes.
What an inch of rain costs a curing crop
window HAY WINDOWwritten 2026-07-21responsible Windrow Field Manual (a named masthead, not a person)
[['Freshly cut, above 70% moisture', 'Small, mostly mechanical', 'Little; the crop is still wet anyway'], ['Half cured, 40–50% moisture', 'Moderate', 'Sugars and soluble protein begin to leach'], ['Nearly dry, below 25% moisture', 'Largest per unit of rain', 'Soluble carbohydrate leaches readily']]
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The three losses and how they scale
Rain on curing hay does three things. It adds water that then has to be dried again, which costs time and fuel. It leaches soluble carbohydrate and soluble protein out of the leaf, which costs feed value directly. And it causes the crop to be handled more, which costs leaf through shatter. The three scale differently with the amount of rain and the state of the crop.
| Rain amount | Rewetting cost | Leaching cost | Handling cost |
|---|---|---|---|
| A shower, under 5 mm | Small | Negligible | One extra pass |
| About 25 mm, one inch | Moderate: about a day of drying | Measurable, in the soluble fraction | One extra pass, at higher moisture |
| Over 50 mm | Large: the crop may need turning to dry at all | Large, and the colour changes | Several passes |
Why the driest crop loses the most
A nearly dry crop has already lost most of its water, so the rain that lands on it is a large fraction of its mass and it re-wets quickly. A wet crop absorbs the same rain into a mass that is already mostly water, so the proportional change is small. The intuition that rain hurts a wet crop most is backwards.
The manual's contestable claim is that a single inch of rain on a nearly dry windrow costs more than a day's delay in cutting, and that the common practice of cutting ahead of a marginal forecast is a bet against this arithmetic. The bet sometimes wins. Over a season it usually does not.
What the manual would measure
- Dry-matter yield from paired plots, one rained on and one covered, for the same crop.
- Soluble carbohydrate before and after a measured rain event.
- The extra drying days each rain amount added, from a moisture series.
Where the numbers come from
- S1Moisture Content and the Hay Drying Curve (2021), Field Crop News, Ontariosupports the moisture-loss curve used to convert rain into drying days
- S2Preventing and Extinguishing Hay Fires, Field Crop News, Ontariosupports the rain-damage sequence and the losses it causes in curing hay
- S3Modelling the field drying of hay, The Journal of Agricultural Science, Cambridge University Presssupports the modelled drying behaviour that sets the rewetting cost
One link, one document. No home pages, no search results, no bare domains.
Put this page down Put this page down when you have decided how much rain you are willing to risk. That number is your cutting rule.