Researchers in the agricultural programs at University of Minnesota and University of Wisconsin have completed research on how varying amounts of rain affect hay quality and nutritional value. During summers with heavy rain, perhaps nothing is more frustrating than to see excellent quality hay turn into unsuitable feed with each passing rain and subsequent raking.
In their joint studies, Dan Undersander, University of Wisconsin, and Krishona Martinson of the University of Minnesota determined that rained on hay can be a suitable forage, but quality depends on several factors.
Forage quality tends to be retained if rain occurs soon after cutting when the forage has had minimal time to dry.
Effects of rain fall on hay quality
Rained on hay can be a suitable forage, but quality depends on several factors.
Research is conclusive on two points. Given the same amount of total rainfall, a low intensity rain will result in more leaching of soluble compounds than a high intensity rain. Also, as forage moisture content declines, it is more prone to dry matter loss from rain. In Wisconsin rainfall studies, the maximum loss in dry matter (54%) was a treatment where 2.5 inches of rain fell on hay that was nearly dried.
Most rainfall studies are in agreement that wetting of field dried alfalfa has little impact on protein concentration. For rained-on hay, it is common to see relatively high protein values in comparison to fiber concentrations, unless significant leaf loss occurs. With the leaching of soluble carbohydrates, structural fibers (acid and neutral detergent fibers) comprise a greater percent of the forage dry matter.
Depending on numerous factors, the digestibility of rained-on hay may decline from 6 to 40%. Changes in fiber components are thought to occur by indirect mechanisms, where the respiratory activity of microorganisms has a concentrating effect on fiber components by oxidizing carbohydrate components, The researchers noted that additional fiber is not made during the wetting process.
Unlike other livestock, losses of soluble carbohydrate can be beneficial for some horses. Laminitis is a painful and debilitating disease of the horse hoof. Laminitis typically occurs during periods of increased or rapid intake of water soluble and nonstructural carbohydrates. In order to manage laminitc horses and reduced amounts of carbohydrates in harvested forage, horse owners have resorted to soaking hay.
There is some disagreement in the research literature regarding the amount of leaf loss that occurs in cut alfalfa as a direct result of rainfall. In Wisconsin studies, leaf loss ranged from 8 to greater than 20% as a percent of the initial forage dry matter when rainfall amounts were from 1 to 2.5 inches. In Michigan studies, direct leaf loss was much lower (0.5 to 4.2%). Perhaps the issue of leaf loss from rainfall is a mute point.
Experience and common sense acknowledge that rain damaged alfalfa is more predisposed to leaf shatter after it dries, and rainfall often means additional raking and more lost leaves.