Rain is always a wild card when deciding the right time to mow hay.
“Pop-up showers and storms are a risk this time of the year,” University of Missouri Extension Agronomist Hunter Lovewell said in a news release.
This unpredictability makes it important to keep an eye on weather forecasts and understand how rainfall affects forage that is drying down but not yet baled, he said.
And not all rains are created equal, Lovewell said. Quick, short rains cause less damage than prolonged, slow rains.
If the total amount of rain is the same, a lower-intensity rain will result in worse quality and lower yield compared to higher-intensity rainfall.
Rained-on hay likely is less digestible with lower yields, respiration and microbial breakdown of the plant tissue.
Wet hay requires more mechanical handling to dry, which results in leaf shatter. This reduces digestibility when leaves fall and tougher stems remain. Fewer leaves also mean less mass and lower yields, according to research.
When this happens, livestock owners should consider supplementing hay with nutrients, Lovewell said.
Since rain affects grasses and legumes differently, growers should also consider what type of hay they have. Forage legumes such as bird’s-foot trefoil and red clover do not lose as much dry matter as alfalfa does. Most studies agree that crude protein in alfalfa is not greatly affected by rainfall on a mowed crop.
Rained-on hay also allows microorganisms such as fungi and molds to develop.
Respiration is the process of plant enzymes breaking down soluble carbohydrates. This process occurs whether the forage is rained on or not. It is normal for a small percentage of the total dry matter to be lost through respiration until moisture dries down to around 30%.
Some research shows leaching of carbohydrates, vitamins and minerals is usually highest when hay has partially dried and there is a prolonged rain.
Rainfall right after cutting usually results in less leaching of nutrients.
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