You’re ready to start cutting silage and Mother Nature decides to dampen your plans – literally. Rain can cause headaches at harvest, so here are a few guides to get you through a rainy harvest. 

A lot of rain, even an afternoon shower, inevitably creates a muddy mess in the field, so steer clear until the field dries enough to maneuver through.

“At harvest time if you have several days of rain in the forecast, you’ll need to make decisions based on the moisture level of the plants at that time,” says Luiz Ferraretto, assistant professor at the Department of Animal Sciences, University of Florida.

“Often, the stated moisture level I will have after the forecast is good enough for me to go back to the field and harvest. Normally, corn plants accumulate about 0.5% dry matter [DM] each day as it gets close to harvest. Because of the rain, the plants absorb some water and it doesn't dry out very fast, so you still have some time to avoid those issues. If corn is already too dry, this 1% or 2% in extra moisture may be a problem.”

The biggest problem with any weather-related issue is delay of harvest and the loss of silage quality that comes with it.  Forages with excess moisture, meaning greater than 70%, may not ensile well. Higher moisture forage concentrations may result in greater seepage losses. Clostridial fermentation can also be a problem with high-moisture silages, which can lead to high DM losses, protein degradation, high butyric acid concentrations and reduced cow palatability. 

“Every farmer will develop their own guidelines for when to harvest each field,” Ferraretto says. “So every day of delay, you're going farther from your guidelines and losing quality along the way. You also have to balance harvest with several fields and several hybrids. Sometimes, a field may already be late in maturity, and it may be an even bigger problem to delay the harvest.”

Harvesting amid rainy conditions can also cause another issue: extra water and mud going to the silo.

“If you have a bunker silo without concrete underneath, you can have a lot of mud in the first silage you cut,” he notes. “This could result in a lot of mud in your silo, and you don't want to feed a lot of dirt to the cows.”

For an alfalfa crop, leaf shatter, plant respiration and leaching by rain damage during harvest can significantly reduce forage quality. Even moderate rain damage has been shown to reduce alfalfa crude protein levels slightly and digestibility dramatically.

“Energy content is going to decline significantly because respiration will continue. The quality is in the leaves; we should remember that we're trying to harvest leaves, not total tonnage,” says Dan Undersander, professor at the Department of Agronomy, University of Wisconsin.

If rain occurs before the plant has dried below 60% moisture, respiration will continue to cause loss of starch and sugars. If the rain is heavy, it can knock the leaves off the plant. If a significant loss of leaves occurs, it will concentrate the stems which can reduce the potassium content. As a result, lightly rained-on hay can make good dry cow feed.

“Leaves are about 500 relative forage quality, while stems are about 80. We'd like to average a forage that’s about 150 to 180 relative forage quality,” Undersander says.