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 plant’s metabolic processes. Hay must reach moisture content of less than 30 percent 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 percent of the dry matter per inch of rain on cut hay. Digestibility can be reduced by 10 percent or more due to leaching of nutrients and leaf shatter. A similar study done at Iowa State University reported protein loses of 3 percent and total digestible nutrient reduction of 4.6 percent.

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.