By Martin Mangual

Controlled fermentation is the metabolic process that allows the preservation of the nutritional value of the corn silage. In this process, carbohydrates from the plant are broken down by bacteria that produce acids. The acidic environment is what inhibits the growth of mold and unwanted microorganisms, preserving the quality of the silage. However, careful attention should be given to each step of the preparation and storage of the silage to guarantee the best on-farm results.

The ensiling process is composed of three main phases. The first is aerobic respiration, which starts at harvesting and continues while oxygen is present in the chopped plant. This occurs at the field, during transport, and should last only for the first few hours of storage.

During the aerobic respiration phase, sugars (mostly glucose) are broken down by the bacteria existing in the plant, to produce carbon dioxide, water and energy. This energy, however, is consumed by the bacteria, and not by the cow, therefore decreasing the nutritional value of the silage.

This phase is important, because it consumes the oxygen trapped within the particles of the plant and creates an anaerobic environment for the next phase. However, because it doesn't provide any useful nutritional products to the animal, Michigan State University Extension recommends to limit the length of oxygen exposure of the plant after harvest as much as possible.

This could be achieved by harvesting efficiently, packing the silage properly to the desired density and covering the silo in less than 48 hours.  

At the end of Phase 1, when most of the oxygen is consumed, there is a switch in the dominant bacterial population, and Phase 2 begins. The second phase is the fermentation phase and consists of two main steps. In step one, anaerobic bacteria that tolerated the high temperatures from the previous phase start to thrive in the reduced oxygen environment and to metabolize the soluble carbohydrates, producing acetic acid. The production and accumulation of acetic acid reduces the pH of the silage. This step usually lasts between two to three days.

Once pH reaches about 5, the bacterial population shifts again. The acetic acid producers cannot tolerate the new pH, and then the lactic-acid-producing bacteria start to thrive. These bacteria can tolerate highly acidic conditions, and therefore they keep fermenting soluble carbohydrates from the silage. Glucose is converted to lactic acid that accumulates and continues to decrease the pH until it reaches a level that limits even the acid-resistant microbial growth. Typically, this step lasts 14 days. The two steps of the second phase together should last between 16 and 18 days after storage.

Finally, the third phase, known as the stable phase, occurs when silage reaches a stable pH and temperature. At this point, nutritional preservation has occurred, as no further bacterial breakdown of nutritional material is occurring. With proper coverage and monitoring to prevent air infiltration, the stable phase will last until the silage is opened for feeding. It is crucial to monitor the storage and check for holes or rips in the covering to ensure the oxygen barrier is intact. Often with bags or piles, the plastic film can be broken by equipment, animals and other environmental hazards, allowing oxygen to enter. This reinitiates Phase 1, causing spoilage and nutritional loss if unattended.

Always monitor silage storing facilities to make sure that the fermentation process occurs without disruption, ensuring optimal nutritional preservation of the feed.

Mangual is an MSU dairy Extension educator.