The three most important things that must occur to make good silage are
1) the rapid removal of air.
2) the rapid production of lactic acid that results in a rapid drop in pH.
3) continued exclusion of air from the silage mass during storage and feedout.
Rapid removal of air is important because it prevents the growth of unwanted aerobic bacteria, yeasts, and molds that compete with beneficial bacteria for substrate. Air can be eliminated by wilting plant material to the recommended dry matter (DM) level for the specific crop and storage structure; chopping forage to a correct length; packing quickly; compacting well; ensuring even distribution of forage in the storage structure; and immediately sealing the silo.
After chopping, plant respiration continues for several hours and plant enzymes (e.g., proteases) are active until air is used up. Air must be removed before optimal fermentation can take place. If air is not removed quickly, high temperatures and prolonged heating are commonly observed in the silage mass.
Once air is removed, fermentation can begin. Lactic acid bacteria (LAB) utilize water-soluble carbohydrates to produce lactic acid, the primary acid responsible for increasing the acidity and decreasing the pH in silage. The strength of silage acids can be determined by measuring silage pH. A pH above 7 is considered basic whereas a pH below 7 is acidic. A pH of 7 is neutral and means that a product is neither acidic nor basic. Depending on the crop, plant material in the field can range from a pH of about 5 to 6 and decrease to a pH of 3.6 to 4.5 after acid is produced.
A quick reduction in silage pH will help to limit the breakdown of protein in the silo by inactivating plant proteases. In addition, a rapid decrease in pH will inhibit the growth of undesirable anaerobic microorganisms such as enterobacteria and clostridia. Eventually, continued production of lactic acid and a decrease in pH inhibits growth of all bacteria.
Once fermentation is complete, good silage will remain stable and not change in composition or heat. This is why filling silos quickly and sealing silos immediately after filling is so important. However, depending on the mixture of fermentation end products, silage can spoil rapidly if exposed to air during storage and feedout.
Although the ensiling process appears quite simple, many factors can affect what type of fermentation takes place in a silo and, thus, the mixture of end products. The DM content of the forage can also have major effects on the ensiling process via several different mechanisms.
- Drier silages do not pack well and, thus, it is difficult to exclude all the air from the forage mass.
- As the DM content increases, growth of lactic acid bacteria is curtailed and the rate and extent of fermentation is reduced.
- Undesirable bacteria called clostridia tend to thrive in very wet silages and can result in excessive protein degradation, DM loss and the production of toxins.
Where weather permits, wilting forage above 30% to 35% DM prior to ensiling can reduce the incidence of clostridia because these organisms are not very osmotolerant (they do not like dry conditions). Delayed filling, which results in excessive amounts of air trapped in the forage mass, can have detrimental effects on the ensiling process. Another factor that can affect the ensiling process is the amount of water-soluble carbohydrates present, necessary for good fermentation to take place.
To learn more about the fermentation process, click here.