This summer there's been a lot of
forage that got rain-washed before being baled or ensiled. Rain that occurs between the
time alfalfa is cut and harvested causes both yield and quality losses that reduce the
value of the crop as an animal feed and a marketable commodity. Weather-induced losses are
caused by:
Let's take a closer look at how rainfall during the hay curing process impacts the yield and quality of alfalfa. How much does rainfall reduce yield? Several researchers have studied the effects of rainfall on cut alfalfa. Wisconsin researcher Michael Collins measured dry matter losses of 22% when alfalfa was exposed to 1 inch of rain after 1 day of curing. Similar hay cured without rain damage lost only 6.3% of the initial potential yield. In this same study, hay exposed to 1.6 inches of rain over several days suffered a 44% loss in dry matter. In Michigan, Rotz and co-workers did several different studies to examine the effects of rainfall on field cured hay. The first study reported maximum dry matter losses of 34%. In a second study, rainfall intensity was kept constant at about 0.7 inches but spread over periods of 1 to 7 hours. Dry matter losses ranged from 4 to 13% with highest losses occurring when the rain was spread over a longer duration. Overall, dry matter losses were much lower in these experiments even though rainfall amounts were about 2 inches. How does rainfall reduce dry matter yield? Three primary factors are involved: leaching, respiration, and leaf loss. Leaching is the movement of cell solubles out of the plant. Components of the plant that are very water soluble are leached out of the forage and lost when rain occurs. Unfortunately, most of these compounds are those highly digested by the animal. They include such things as readily available carbohydrates and soluble nitrogen, minerals, and lipids. About one-half of the dry matter leached by rain is soluble carbohydrate. Excessive leaching of soluble carbohydrates by rainfall can cause problems if forage is ensiled. It is these same carbohydrates that serve as a substrate for bacteria involved in the fermentation process. In situations where carbohydrates are in low concentrations, silage additives that provide fermentable substrate might be needed to insure proper fermentation. It is difficult to measure dry matter losses from increased plant respiration as a result of rainfall. Even without rain, respiration occurs in the cut forage until the plant reaches near 30% moisture. Every time the forage is wetted by rain, respiration is either prolonged or begins again in cases where the cured forage was below 30% moisture. In either case, additional dry matter is lost. 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 the Wisconsin studies, leaf loss ranged from 8 to over 20% as a percent of the initial forage dry matter where rainfall amounts were from 1 to 2.5 inches. In the Michigan studies, direct leaf loss was much lower (0.5 to 4.2%). In both of these experiments the forage was physically picked-up and lost leaves underneath the swath were weighed. In a Guelph, Ontario study, rain-induced leaf loss was determined to be minimal (less than 0.5%). Here, the forage treatments were not hand manipulated. Perhaps the issue of leaf loss from rainfall is a mute point. Experience and common sense tell us that rain damaged alfalfa is more predisposed to leaf shatter after it dries and rainfall often means additional raking or tedding; hence, more lost leaves. How does rainfall intensity and forage moisture affect losses? Research is conclusive on these 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 the 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 cured. How does rainfall affect forage quality? Perhaps nothing is more frustrating than to see excellent quality alfalfa turn into cordwood with each passing rain storm and subsequent raking. Most rainfall studies are in agreement that wetting of field cured alfalfa has little impact on crude protein concentration. For rained-on hay, it is common to see relatively high protein values in comparison to fiber concentrations. Protein values alone will not be a good measure of forage quality in 1999! 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 previously discussed, the digestibility of rained-on hay can be extremely low and offer little nutritional value.
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