While dairy operators often have little difficulty putting up nice corn silage, it is not uncommon for the same techniques to yield foul-smelling, undesirable cereal or alfalfa silage. This is usually caused by the growth of clostridial bacteria in the silage. These organisms produce butyric acid, which smells like rancid butter.
Clostridial bacteria normally live in manure and soil, and often their spores are present on forage. They grow in the absence of oxygen and convert the desirable lactic acid, plant sugars, and/or amino acids to butyric acid and/or acetic acid and ammonia. This results in dry matter losses and a higher pH, which is more unfavorable to silage preservation.
Bad smelling, less palatable clostridial silage can be avoided. Most important is to make sure the silage is put up no wetter than 70% moisture. Clostridial bacteria cannot survive drier conditions and are rarely a problem in silage at 70% moisture or lower.
Low pH will also inhibit the growth of clostridial bacteria. The wetter the silage, the lower the pH has to be to inhibit the clostridial bacteria. Grasses need a lower pH than alfalfa for the same percent moisture. At 70% moisture, a pH of less than about 4.4 for grass and 4.7 for alfalfa is needed to inhibit clostridial bacteria, while at 75% moisture, a pH of 4.2 for grass and 4.4 for alfalfa is needed.
For any silage to come to a stable, preserved, condition, the pH must be lowered to a point where all bacterial activity is stopped. Since both clostridial and lactic acid bacteria survive better at higher moisture contents, a lower pH is needed to stop bacterial activity in high moisture silage than in lower moisture silage where the bacteria are under more stress and therefore easier to kill.
In order to get a rapid drop in pH to the necessary level, there have to be sufficient numbers of lactic acid producing bacteria and sufficient sugars for them to work on. If sugars are depleted before fermentation stops due to low pH, alfalfa and some cereal silages at a moisture content higher than 65% may never stabilize, setting the stage for clostridial bacteria and other organisms which break down the lactic acid to weaker acids and other undesirable by-products, which raise the pH even more.
Corn is high in sugars, and native lactic acid bacteria are usually abundant at harvest time which make it relatively easy to achieve the rapid drop in pH necessary to avoid rancid silage. Cereals usually have sufficient sugars if they are less than 80% moisture. Alfalfa even at 70% moisture may not have sufficient sugars to support enough of a pH drop. Wilting alfalfa to less than 70% moisture may help some, but packing and aerobic deterioration become problems at low moisture contents. Wilting alfalfa quickly conserves available sugars because there is less time for plant respiration to use them up. Rain on wilting forage will leach out sugars in addition to prolonging respiration. Some Wisconsin trials have shown good results with adding sugar to alfalfa silage along with the inoculate.
Adding a high-quality silage inoculate to cereal silage or alfalfa is often helpful to ensure enough lactic acid bacteria for a rapid pH drop. However, if the moisture content is higher than 70% for cereal and especially alfalfa, the lactic acid bacteria may still run out of sugars before the silage is preserved, and a poor quality silage may result even if inoculant was added.
November 5, 1999