Delaware County Cooperative Extension

Rain Damaged Hay?

Brian Pugh, NE Area Agronomy Specialist

Hay that has been cut and then rained on can lose quality in four ways. These include: 1) leaching
of soluble carbohydrates, vitamins and minerals. 2) Increased and prolonged plant respiration.
3) Leaf shattering. And 4) Microbial breakdown of plant tissue.
Leaching of carbohydrates, vitamins and minerals is usually at its highest when the hay has
dried somewhat and we then have a prolonged rain. Rainfall right after cutting usually results in
less leaching of nutrients and a quick splash and dash shower normally doesn’t result in large
losses of these nutrients on freshly cut hay.
Increased or prolonged respiration occurs when hay is not allowed to dry sufficiently to stop the
plants metabolic processes. Hay must reach moisture content of less than 30% for respiration
to be reduced to acceptable levels. Hay that is rained on when relatively green will continue to
respire for longer periods of time, resulting in the loss of forage nutrients and dry matter yield.
Likewise, partially dried hay that is rained on can continue to respire for longer periods resulting
in lower quality and yield of hay.
Increased leaf shatter is another problem associated with hay that has been rained on. Wet hay
usually means more mechanical handling of the hay in order to dry it. Since leaves tend to dry
quicker than stems, any increased raking or tedding tends to shatter leaves from stems. Since
more of the soluble nutrients are in the leaf tissue, the loss of leaf blades while raking and baling
can reduce hay quality substantially. Loss of leaf blades can also result in reduction of dry
matter yields.
Microbial breakdown of plant tissue occurs when fungi, molds and other microorganisms begin
to feed on the downed hay. These organisms develop rapidly in warm-moist conditions and
feed on the dead plant material. Hay that is lying on the ground and remains wet for long time
periods becomes a perfect environment for these organisms to live and breed. They can quickly
consume plant nutrients and destroy plant cell structure resulting in loss of dry matter yield, nutrient
content and given time, will completely rot the hay.
What are the consequences of hay being rained on? Research conducted at the University of
Kentucky by Michael Collins indicated that we can lose up to 5% of the dry matter per inch of
rain on cut hay. Digestibility can be reduced by 10% or more due to leaching of nutrients and
leaf shatter. A similar study done at Iowa State University reported protein loses of 3 % and total
digestible nutrient reduction of 4.6 %.
One fact seems to hold true, you still cannot
tell what the actual quality of the hay is until
you have it tested. Testing it is your best strategy
for determining the nutrient quality you will
get from the hay. Sampling rained-on hay will
give you the information you need to design a
supplementation program that will keep your
animals in good shape during the winter feeding
period. Your local county Extension Educator
can help you with sampling techniques, hay
probes, and testing. For a small investment of
time and money, testing will pay off big in the
health and nutrition of your herd.

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