Silage Fermentation and Preservation

AS-1254, June 2004
J.W. Schroeder, Extension Dairy Specialist

Quality silage is achieved when lactic acid is the predominant acid produced, as it is the most efficient fermentation acid and will drop the pH of the silage the fastest. The faster the fermentation is completed, the more nutrients will be retained in the silage.

The Fermentation Process

Harvesting forages as silage is a compromise between reducing labor requirements and field losses versus losses in the fermentation process that will eventually preserve the crop. Ideal fermentation is dependent upon decisions and management practices implemented before and during the ensiling process. The primary management factors that are under the control of the producer are:

1. Stage of maturity of the forage at harvest.

2. The type of fermentation that occurs in the silo or bunker.

3. Type of storage structure used and methods of harvesting and feeding.

During the ensiling process, some bacteria are able to break down cellulose and hemicellulose to various simple sugars. Other bacteria break down simple sugars to smaller end products (acetic, lactic and butyric acids). The most desirable end products are acetic and lactic acid. As the bacteria degrade starches and sugars to acidic and lactic acids, dry matter is lost.

Attention to details such as speed of harvesting, moisture content, length of chop, silage distribution and compaction can greatly influence the fermentation process and storage losses. Efficient fermentation ensures a more palatable and digestible feed, which encourages optimal dry matter intake that translates into improved animal performance. Making consistent, high-quality silage requires sound management decisions and attention to details.

It is important that bacteria responsible for production of acetic and lactic acid grow and multiply immediately after storing the forage for maximum quality haylage. Proper packing of the hay and voiding of air (oxygen) provides the environment needed by bacteria to break down fiber components and sugars. Oxygen must be removed from the haylage to maximize reproduction of acetic and lactic acid-producing bacteria. Microbes (bacteria) responsible for fermentation need anaerobic (in the absence of air) conditions. As bacteria consume sugars, end products produced (acetic and lactic acid) cause the pH to drop.

Quality silage is achieved when lactic acid is the predominant acid produced, as it is the most efficient fermentation acid and will drop the pH of the silage the fastest. The faster the fermentation is completed, the more nutrients will be retained in the silage.

A critical time during the ensiling process occurs after the initial three to five days and requires some 15 to 20 days for completion. The success of the ensiling process is determined during these two weeks. There is a gradual increase in lactic acid as lactic acid-producing bacteria break down simple sugars. The pH drops to between 3.8 to 4.2. At such acidic conditions, further bacterial action is topped. The critical difference between silage and haylage is the effect of moisture content of the forage during this two-week fermentation process. If the forage is too dry, fermentation is restricted and the pH cannot drop sufficiently. If pH of the haylage does not drop sufficiently, spoilage will occur.

The following six phases describe what occurs during ensiling, storage and feed-out of fermented forages:

Phase I

As the forage is harvested, aerobic organisms predominate on the forage surface. During the initial ensiling process, the freshly cut plant material, and more importantly the aerobic bacteria, continue to respire within the silo structure. The oxygen utilized in the respiration processes is contained within and between the forage particles at the time of ensiling. This phase is undesirable since the aerobic bacteria consume soluble carbohydrates that might otherwise be available for the beneficial lactic acid bacteria or the animal consuming the forage. Although this phase reduces the oxygen to create the desired anaerobic conditions, the respiration process produces water and heat in the silage mass. Excessive heat build-up resulting from an extended Phase I period can greatly reduce the digestibility of nutrients such as proteins.

Another important chemical change that occurs during this early phase is the breakdown of plant proteins. Proteins are first reduced to amino acids and then to ammonia and amines. Up to 50 percent of the total plant protein may be broken down during this process. The extent of protein breakdown (proteolysis) is dependent on the rate of pH decline in the silage. The acid environment of the silage eventually reduces the activity of the enzymes that break down proteins.

Phase I ends once the oxygen has been eliminated from the silage mass. Under ideal crop and storage conditions, this phase will last only a few hours. With improper management, this phase could continue for several weeks. The primary objective at ensiling time is to manage the crop so air infiltration is minimized; thereby shortening the time required to achieve an anaerobic environment. Key management practices are proper maturity, moisture, chop length and rapid filling with adequate packing and proper sealing of the storage structure.

Phase II

After the oxygen in the ensiled forage has been utilized by the aerobic bacteria, Phase II begins. This is an anaerobic fermentation where the growth and development of acetic acid-producing bacteria occurs. These bacteria ferment soluble carbohydrates and produce acetic acid as an end product. Acetic acid production is desirable as it can be utilized by ruminants in addition to initiating the pH drop necessary to set up the following fermentation phases. As the pH of the ensiled mass falls below 5.0, the acetic bacteria decline in numbers as this pH level inhibits their growth. This signals the end of Phase II. In forage fermentation, Phase II lasts no longer than 24 to 72 hours.

Phase III

The increasing acid inhibits acetic bacteria and brings Phase II to an end. The lower pH enhances the growth and development of another anaerobic group of bacteria, those producing lactic acid.

Phase IV

This is a continuation of Phase III as the lactic-acid bacteria begin to increase, ferment soluble carbohydrates and produce lactic acid. Lactic acid is the most desirable of the fermentation acids and for efficient preservation, should comprise greater than 60 percent of the total silage organic acids produced. When silage is consumed, lactic acid will also be utilized by cattle as an energy source. Phase IV is the longest phase in the ensiling process as it continues until the pH of the forage is sufficiently low enough to inhibit the growth of all bacteria. When this pH is reached, the forage is in a preserved state. No further destructive processes will occur as long as oxygen is kept from the silage.

Phase V

The final pH of the ensiled forage depends largely on the type of forage being ensiled and the condition at the time of ensiling. Haylage should reach a final pH of around 4.5 and corn silage near 4.0. The pH of the forage alone is not a good indicator of the quality of the silage or the type of fermentation that occurred. Forages ensiled at moisture levels greater than 70 percent may undergo a different version of Phase IV. Instead of lactic acid producing bacteria developing, large populations of clostridia bacteria may grow in the silage. These anaerobic bacteria produce butyric acid rather than lactic acid, which results in sour silage. With this type of fermentation, the pH may be 5.0 or above.

Phase VI

This phase refers to the silage as it is being fed out from the storage structure. This phase is important because research shows that nearly 50 percent of the silage dry matter losses occur from secondary aerobic decomposition. Phase VI occurs on any surface of the silage that is exposed to oxygen while in storage and in the feedbunk. High populations of yeast and mold or the mishandling of stressed crops can lead to significant losses due to aerobic deterioration of the silage. Proper management is vital to reduce these losses and improve the bunk life (aerobic stability of the silage.

Table 1. Six phases of silage fermentation and storage.

*Temperature dependent on ambient. Ensiling temperature generally is 15 higher than ambient.
Adapted from McCullough.

General Silage Management Factors

1. Maturity and moisture - Recommendations vary with different silage crops (Table 2). Proper maturity assures adequate fermentable sugars for silage bacteria and maximum nutritional value for livestock. Maturity also has a tremendous impact on moisture with unwilted forage crops such as corn silage. Adequate moisture for bacterial fermentation is essential and aids in packing to help exclude oxygen from the silage.

Table 2. Harvest and moisture recommendations.

2. Length of cut - The most desirable length of cut is between ¼ to ½ inch depending upon the crop, storage structure and amount of silage in the ration. This will give ideal compaction of the silage and yet allow for ease of unloading from an upright silo. Setting the chopper to cut any finer could have a negative impact upon milk fat production and the incidence of displaced abomasums in dairy cattle due to inadequate scratch factor.

Chopping forages too long makes compaction difficult and air will remain trapped in the silage resulting in heating and spoilage (Phase I). A recutter screen is not generally recommended unless the silage is too dry, as it increases the power requirement and will slow harvesting.

3. Filling, packing, sealing - The crop should be harvested and the silo filled as rapidly as possible. Filling delays will result in excessive respiration and increased silage losses. Packing should begin immediately when storing silage in bunker silos. A wheeled tractor is preferred as a packing vehicle, as it will supply greater weight per surface unit than a tracked vehicle. The silo should be sealed with an air-tight cover once it is filled to prevent penetration of air and rainfall into the silage. A good grade of plastic weighted down with discarded tires will generally provide an adequate seal.

4. Additives - Most silage additives marketed in the United States today are designed to aid fermentation by providing fermentation bacteria, enzymes or fermentable substrate. Although not a replacement for good management, they are tools to help ensure that the ensiling process stays within acceptable boundaries.

Manage the feeding face

Face management requires skill and is a real problem in the United States. Many bunker, trench and drive-over pile silos are too large to manage correctly. The silage face should be maintained as a smooth surface, perpendicular to the floor and side walls in bunker and trench silos. This technique will minimize the square footage of surface exposed to air. An average daily removal rate of 6 to 12 inches from the face should prevent heating and spoiling of exposed silage. Well-packed silage reduces the distance that oxygen can penetrate in an exposed silage face.

Dispose of the spoilage

Even the best-managed silage will always have some spoilage. The temptation is to feed spoiled silage, but it is always a mistake and can destroy the forage mat in a cow's rumen. In tests at Kansas State University, fistulated cows fed spoiled silage didn't ruminate, reducing chewing and saliva production. Instead, always throw out the spoilage. You can't afford to use it.

Additions to silage

Various additions to silage have been suggested as methods to improve or alter the fermentation process. These materials may be referred to as additives, conditioners and preservatives.

• Additive - A material that adds nutrients to silage.

• Conditioner - A material that absorbs excess moisture from chopped forage or which increases the moisture content of excessively dry forage.

• Preservative - A material that stimulates the fermentation process or a material that inhibits fermentation.

The benefits obtained from silage additives, conditioners and preservatives depend upon their influence on the silage fermentation process. These benefits are usually measured by the reduction in fermentation losses and/or improvement in silage quality and feeding value.

Silage additives, conditioners and preservatives function in the following ways:

• Add dry matter to reduce moisture content

• Add water to increase moisture content

• Alter the rate, amount and kind of acid production

• Acidify the silage

• Inhibit bacterial and mold growth

• Culture silage (inoculants) to stimulate acid production

• Increase nutrient content of the silage

Add dry matter to reduce moisture content

The objectives of adding dry matter are to reduce seepage losses and provide a more suitable medium for the fermentation process. The goal of producers should be to harvest corn and sorghum for silage at the proper growth stage or at physiological maturity when plant moisture content is ideal for silage. Grasses and legumes should be wilted or dried in the swath to an average of about 65 percent moisture or less depending on the type of storage used.

If forage crops must be harvested too wet for silage, the following guideline for dry matter addition may be used.

• Cereal grains, coarsely ground and chopped, air dry alfalfa or grass forage will decrease the moisture content of wet forage approximately 5 percentage units for each 150 to 200 pounds of material added per ton of wet forage weight.

Add water to increase moisture content

If forage crops to be stored as silage become too dry, packing to exclude air is difficult. Under such conditions, water must be added to raise the moisture content to the desired level or severe dry matter losses will result.

• The amount of water required to increase forage moisture content 1 percentage unit is approximately 5 to 6 gallons per ton of ensiled material.

Altering the rate, amount and kind of acid production

Acid production is essential in the keeping qualities of silage. The rate, amount and kind of acid produced is influenced by the moisture content of the chopped forage and the readily available carbohydrate content of the forage.

Corn silage harvested at the proper growth stage or at physiological maturity has a high level of readily available carbohydrates for lactic acid production. If legumes and grass crops are not wilted in the field to an average of 65 percent moisture or less depending on the type of storage, then the addition of a carbohydrate-rich feedstuff will enhance fermentation.

Molasses is an excellent carbohydrate or sugar source for legumes and grasses containing 75 to 80 percent moisture. For legumes, about 80 pounds of molasses per ton is required, and for grasses 40 pounds is generally used.

Cereal grains are another source of carbohydrates. These materials, when added at 150 to 200 pounds per ton of wet forage, will also reduce the moisture content of the chopped forage approximately 5 percentage unit.

Acidifying the silage

Acidifying silage, using a strong acid, has been practiced in Europe on high-moisture grass silage. The purpose was to produce an immediate acid condition rather than waiting for the silage to produce its own acid. This practice is not recommended in the United States because of its high cost, the corrosive nature of the acids and low forage palatability.

Inhibit bacteria and mold growth

While acid formation and the exclusion of oxygen stop bacterial growth, several chemicals used as silage preservatives also inhibit undesirable bacterial and mold growth. Acids, such as formic and propionic, enhance the preservation of forage. The major benefit of adding weak acids to silage appears to be in reducing spoilage in open storage structures. Formic acid is added to hay crop silages at 0.45 percent of the wet weight or 2.25 percent of the dry matter weight. Propionic acid is added at the rate of 0.5 to 1.0 percent of the wet forage weight.

"Culturing" silage (inoculants)

A number of commercial products, referred to as fermentation aids and/or inoculants, are available for adding to silage at the time of ensiling. Since silage is a product resulting from the action of bacterial enzymes on the material stored, attempts have been made to alter or regulate silage fermentation through the addition of materials containing bacteria, yeasts and molds. The primary purpose for adding bacterial inoculants is to increase the number of lactic acid-producing bacteria, thus encouraging more lactic acid production and
a well-preserved forage mass.

Research using various bacterial inoculants indicates highly variable results. Products showing consistent, positive results indicate about a 5 percent increase in dry matter preservation. Therefore, cost of the inoculant per ton compared to the dollar value of the dry matter saved will determine the profitability of using a silage inoculant.

The addition of bacterial inoculants to corn silage harvested at the proper growth stage and moisture content has not shown consistently positive results. Results have been more positive when inoculants have been added to alfalfa, a low carbohydrate forage, and sorghum silages.

Increasing the nutrient content of silage

Various materials added to silage will increase nutritive value to the extent that they themselves contain nutrients. Increasing the nutrient content of silage will greatly affect the final cost of the product produced.

Materials such as cereal grains, molasses, dry forages, limestone, urea and anhydrous ammonia are examples of nutrient additions to silage. Commercial products are also available that contain one or more of the above materials.

Limestone (calcium carbonate) is sometimes added to corn silage to increase the calcium content and extend the fermentation process. The rate frequently added is 10 to 20 pounds per ton of corn silage.

Urea and anhydrous ammonia, non-protein nitrogen (NPN) sources, are sometimes added to corn silage to increase the protein content. The various NPN sources listed in Table 3, when applied at the suggested rate per ton of stored forage, usually will increase forage protein content about 4 percentage units on a dry matter basis.

Table 3. Non-protein nitrogen sources for adding to corn silage and suggested application rates.¹

¹ Commercial products should be applied at a rate to provide 5 pounds of actual nitrogen/ton
of forage.
² Add 5 lbs. dry urea to provide 5 lbs. of nitrogen.

Adding Urea to Corn Silage

Following are some points to keep in mind before you add urea to corn silage to increase its protein content:

• It is critical that ensiled material be evenly distributed.

• Only about 30 percent of the urea is hydrolyzed in the silo, the "preservative" effects are minimal.

• Use of urea in the silo "locks" the producer into a feeding system that cannot be altered.

• Animal performance trials, for the most part, have been disappointing.

If you need NPN (urea) in the ration, the best way to get it there is to add feed grade urea to the Total Mixed Ration. The only time to put it into the silo is if there is no other way to mix it into the ration.

Ammoniating Corn Silage

Ammoniating feed can be easy, pleasant, safe and profitable if you take a few simple steps to ensure that the job is done properly. Here are some tips:

• Test feeds before you ammoniate as a way to check results. If ammoniation doesn't boost protein by at least three points, you haven't ammoniated properly, and you won't get the benefits.

• To apply the 7 pounds per ton recommended for corn silage, you must start with a 10- to 12-pound application rate to cover losses. Be sure the float gauge on your supply tank works and check it frequently to ensure that you are using enough ammonia for the silage processed.

• Ammonia weighs 5 pounds per gallon, so you need just over 2 gallons per ton of corn silage. Be sure you use enough and adjust the regulator setting if you're not.

• Use a cold flow converter which you can buy or make. It helps greatly to put ammonia on at the chopper. But if you put it on at the silo, try to use a blower that augers feed from the hopper back to the thrower. Augers help blend the feed and ammonia together, and augers are more efficient ammoniators than spinning pans that "throw" feed.

• Adding water at the blower to help retain the ammonia is almost essential, especially with dryer silage. If you ammoniate at the blower, add water.

• Be sure that hoses "downstream" from the regulator are short and drop straight down without kinks or loops to trap ammonia. It is very important that ammonia evacuates quickly from the system when the regulator is cut off to minimize wasted material and unnecessary odors.

When properly rigged, ammonia is safe and relatively pleasant to handle. If it is improperly set up, you will not be able to understand how anyone could stand the stuff, much less use ammonia.

Precautions

There are several precautions that must be observed when ammoniating corn silage. Keep equipment in top shape, especially hoses.

Hoses should not be left under pressure for extended periods. They should be stored inside, out of sunlight. Replace hoses every three to five years, and remember that 90 percent of ammonia accidents on farms occur from broken hoses.

Most other ammonia accidents occur when ammonia equipment is hooked up, moved or unhooked.

Corrosion to equipment is a problem too, but a manageable one. Ammonia is not corrosive to most steel, though it does remove paint, and so it exposes unloaders more quickly to the corrosive effect of the silage itself. Ammonia is harmless in choppers, but is rough on electrical equipment, and you should take precautions to protect the equipment.

Keep spare parts on hand, such as collector rings, rollers, plugs and switches. Use stainless steel screws to hold the collector ring in place. Seal motors that are in a silo with silicone sealer, especially where the cord enters the motor housing and around the capacitor box. If an ammonia user prepares for electrical problems, he will have less trouble than the non-user who is prepared.

Significantly, ammonia is only corrosive to copper, brass and zinc, and only when being applied. Once silage has been treated with ammonia, it is less corrosive than untreated silage.

References

In addition to sources cited, materials were adapted with permission from Pioneer Forage Manual, no longer in print.

Other publications in the Quality Forage series

• AS-1250 Forage Nutrition for Ruminants
• AS-1251 Intrepreting Composition and Determining Market Value
• AS-1252 Haylage and Other Fermented Forages
• AS-1253 Corn Silage Management
• AS-1254 Silage Fermentation and Preservation
• AS-1255 Storage, Sampling and Measuring
• AS-1256 Stressed-Damaged Crops

For more information on this and other topics see: www.ag.ndsu.edu 

AS-1254, June 2004