Silage is a type of high-moisture animal feed produced by a controlled process called ensiling. It preserves the nutritional content of freshly harvested forage for later use when pasture growth is limited. This preservation is achieved through fermentation, which rapidly converts plant sugars into organic acids. The resulting acidity effectively “pickles” the crop, stabilizing the material and preventing spoilage organisms from growing.
Primary Plant Materials
The selection of plant material for ensiling depends largely on the desired nutrient profile and the crop’s natural sugar content. Corn silage (maize) is widely used because of its high concentration of digestible starches and water-soluble carbohydrates (WSC). This high energy content makes corn an ideal substrate for fermentation.
Grass silages, often made from perennial ryegrass or fescue, are also common, though their natural sugar levels are typically lower than corn. Legume silages, such as those made from alfalfa or clover, are valued for their higher protein content. However, legumes possess a greater buffering capacity, which can slow down the critical acidification process.
Small grain crops like oats, wheat, and sorghum are also successfully ensiled. Sorghum is particularly useful in arid regions due to its drought tolerance, although its overall energy density may be lower than that of corn. Crops are harvested at specific maturities to balance high yield with maximum nutritional quality before being cut and compacted.
Essential Requirements for Successful Fermentation
The biological mechanism that converts fresh forage into stable silage relies on three interconnected physical and microbial requirements. The most important biological agents are Lactic Acid Bacteria (LAB), which are naturally present on the surface of the harvested plants. These bacteria consume the plant’s WSC and produce organic acids, primarily lactic acid.
Successful preservation depends on creating a strictly anaerobic environment by excluding oxygen from the forage mass. Oxygen allows undesirable aerobic microorganisms (like yeasts and molds) to grow and permits the plant material to respire, consuming the sugars needed by the LAB. Rapid and tight packing of the chopped material into a silo or bunker, followed by immediate sealing with plastic, is necessary to achieve this anaerobic state.
The crop’s moisture content at harvest influences the ability to exclude oxygen and control the fermentation pathway. For most silages, the optimal moisture range is between 60% and 70% (or 30% to 40% dry matter). Material that is too wet can lead to excessive seepage and undesirable fermentation, while overly dry material is difficult to pack tightly, allowing oxygen to remain trapped within the mass.
Additives Used to Enhance Preservation
Farmers often employ commercial additives to improve the speed and efficiency of the ensiling process, especially when conditions are not optimal. Microbial inoculants are the most common type, consisting of specific commercial strains of LAB, such as Lactobacillus plantarum, applied to the forage. These inoculants rapidly dominate the fermentation, ensuring a quick drop in pH to preserve nutrients.
Another type of inoculant uses heterofermentative bacteria, like Lactobacillus buchneri, which produce acetic acid in addition to lactic acid. Acetic acid suppresses the growth of yeasts and molds, significantly improving the aerobic stability of the silage when it is exposed to air during feeding. This helps prevent spoilage after the silo is opened.
Chemical inhibitors are also used, such as various organic acids. Propionic acid, for example, is primarily used to inhibit mold and yeast growth, while formic acid can quickly lower the pH to suppress protein-degrading bacteria like Clostridia. Additionally, nutrient enhancers such as anhydrous ammonia or urea are sometimes added to increase the crude protein content of low-quality forages.