Damage occurs when your hay is cut and rained on before you are able to bale it. Dr. Brian Pugh of Oklahoma State Extension indicates at cattlenetwork.com that hay that has been cut and rained on can lose quality in four ways. The first is 1) leaching of soluble carbohydrates, vitamins and minerals, 2) increased plant respiration, 3) leaf shattering and finally 4) microbial breakdown of plant tissue.

Leaching of carbohydrates, vitamins and minerals is usually at its highest when the hay has already dried some and then a prolonged rain occurs. Rainfall right after cutting usually results in less leaching of nutrients and a small shower has very little effect on leaching of nutrients at all. Increased or prolonged respiration occurs when hay is not allowed to dry sufficiently to stop the plant’s metabolic processes. The hay must be at less than 30 percent moisture for respiration to be reduced to acceptable levels and hay that is green when rained on will continue to respire for long periods, decreasing the quality and yield of the hay. Increasing leaf shatter occurs with hay that has been rained on because more mechanical handling of the hay is required to dry it. Increased raking tends to shatter leaves from the stems and since more of the nutrient is in the leaves, raking and baling can reduce the quality of the hay substantially. Hay lying on the ground can be consumed by organisms that lead to loss of dry matter and yield. One study indicated cut hay can lose 5 percent of dry matter per inch of rain, so if you have some hay that received substantial rain after cutting, it is a good idea to send the hay off to a lab for analysis so you can determine its quality.