Forage plant cells respire. Until plant cells are dead or have no oxygen available, they respire to generate the energy they need for normal activity.
Respiration is the conversion of carbohydrates into energy. See Figure 1.
Respiration is necessary to deplete the forage mass of oxygen shortly after it has been put in a sealed structure. However, respiration is burning up carbohydrates (forage quality) and producing heat so its duration should be minimized. See Figure 2.
Prolonged respiration in the forage mass can reduce forage carbohydrates levels which can jeopardize the fermentation process. In addition, prolonged respiration allows rapid mold growth in the forage and forage heating. Minimizing the amount of oxygen in the forage mass when it is in a sealed structure is the best way to shorten the respiration period and avoid unnecessary mold growth, quality loss or a potential silo fire.
Once oxygen is depleted in the forage mass, fermentation begins. Fermentation is the lowering of the pH in the forage to a point where no organism (mold or bacteria) can function.
The pH is lowered by lactic acid which is a byproduct of lactobacillus bacteria. The lactobacillus bacteria are on the forage when it is mowed and multiply rapidly until the forage is fermented. Lactobacillus bacteria consume forage carbohydrates for their energy source and excrete lactic acid. The lactobacillus bacteria continue to produce lactic acid and lower the forage pH until they can not function anymore. At this pH level, the forage is fermented and can exist unchanged for many years as long it is not exposed to oxygen.
Clostridia bacteria are also on the forage when it is mowed and are put in the silo with the forage.
Clostridia bacteria consume forage carbohydrates, forage proteins, and lactic acid as their energy source and excrete butyric acid. Butyric acid is associated with rotten or putrefied silage. Situations that might benefit clostridia growth are insufficient forage carbohydrate levels (rain while forage is wilting, extended respiration period due to poor packing, seepage due to excessive forage moisture) to complete the fermentation process and/or low lactobacillus bacteria levels.