Silage making preserves green or wet crops. Scientists now understand the chemistry of silage making and how these chemical reactions hurt or help the quality of silage.

Scientists have described five phases of making silage.

Phase one: This phase begins when silage is chopped and put in a silo or pile. Activity begins as bacteria feed on the contents of damaged (chopped) plant cells. The final respiration of plant cells produces heat and carbon dioxide. During respiration, the oxygen supply is reduced and carbon dioxide is produced. This establishes an anaerobic condition for the optimal growth of lactic-acid producing bacteria. The duration of phase one greatly influences silage quality.

Phase two: Plant cell respiration ends. Acid production begins. As lactic acid and volatile fatty acids form, the pH decreases and helps prevent the growth of undesirable bacteria and fungi. The number of microbes producing exclusively acetic and butyric acid (volatile fatty acids) decreases rapidly as the level of lactic-acid-forming bacteria rises.

Phase three: Lactic acid is produced. During the first few days, settling of the forage occurs and the seepage rate can increase rapidly. Peak seepage occurs on the fourth or fifth day. Seepage occurs when plant cells rupture from pressure and heating due to plant and fungal respiration.

Phase four: Lactic-acid-producing bacteria dominate this phase and the silage is fermented. Phase four starts three to five days after ensiling and takes up to 20 days before completion. Phase Four determines the success of silage making. The lactic-acid-producing bacteria dominate the silo�s bacterial population. As the lactic acid content of the silage increases so does the level of acidity which slows and stops further bacterial and fungal action. By the end of Phase Four, the silage is fermented.

Phase five: This phase lasts from the end of phase four through the feed out period. When properly ensiled, the silage remains in good condition. Its low pH prevents further microbial activity. If insufficient acid production occurred during the first four phases, however, the silage may be subjected to breakdown and attack by undesirable microbes. Should this occur, the silage biochemistry becomes somewhat unpredictable.

Butyric-acid-producing bacteria can use not only carbohydrates but also lactic acid as food for growth. In addition, these butyric producing clostridia species can breakdown proteins to amino acids which are degraded to undesirable compounds including nonprotein nitrogen (NPN). The leads to a reduction of digestible protein. If this occurs, the silage pH increases and permits undesirable fermentations to continue until much of available energy is gone.

Like clostridia, fungi (molds and yeast) also can degrade the forage palatability, nutrient content, and availability to the animal. These are many of the reasons why it is important to inoculate the forage with a proven treatment that will help to preserve critical nutrients meant for the animal.

COMMENTS BY DR. WHITMORE:

Scientists now understand much more of the chemistry of silage making. This new information has resulted in the development of two new tests to evaluate the quality of silage.

Test number one is an in vitro digestibility profile. This test collects rumen fluid from live cows and mixes rumen fluid with a sample of corn silage. Tests are then performed to determine the net energy of the silage.

Test number two is a fermentation profile test. This test evaluates lactic, acetic, and butyric-acid concentration. It also tests the pH and certain chemicals that indicate mold concentration.

These two new tests are predicted to become very popular in the U.S. during the next few years. Knowing the quality of the new silage will help nutritionists balance the ration based on the net energy and acid profile tests.

(Part of this article was published in Western Dairy Business, June, 2001, page 44-47.)