Rain leaches the more soluble, highly digestible nutrients from hay. It leaches some of the soluble protein and reduces the digestibility of the remaining protein. As a result, rain damage decreases digestibility and increases fiber concentration. Rainfall can cause additional losses by prolonging respiration. After it is cut, hay continues to respire until its moisture content drops to less than 40 percent.
Rain rewets the forage and allows respiration to continue.
The effect of rain on hay quality depends on the amount, intensity, and duration of the rain as well the moisture content of the hay at the time of rainfall. Leaching losses increase as the amount and duration of rainfall increase. An intense rain for a short time has less effect or forage quality than the same amount of rain over a longer duration. Both leaching and leaf loss are greater with drier hay than with that which is freshly cut. Because of these variable, it is difficult to predict the quality of rain-damaged hay.