Food Technology Information Center

Technology of Forage and By-Products Preservation

10. Enrichment

Enrichment is not, in itself, a link in the chain of ensiling processes; if it has to be done it is combined with the chopping process. The chopping stage provides a good opportunity to incorporate any kind of additive – chemical or biological – that is added in small amounts and should be evenly distributed. Instruction brochures are published in the USA and Europe that contain lists of commercially available additives whose use is permitted; these brochures include descriptions of the products, their quality, and methods of use. The additives may be in liquid or powder form, and can be sprayed onto the chopped forage via a nozzle. In tower silos the spraying can also be carried out via the blower that elevates the forage to the top of the silo (Fig. 19 , Fig. 20 , Fig. 21 and Fig. 22.

Types of additives

Additives can be separated into three broad types:

  1. Stimulants or biological additives – mainly LAB inoculants and enzymes.
  2. Inhibitors or chemical additives – mainly organic acids that restrict undesirable bacterial activity or retard aerobic deterioration.
  3. Nutrient sources – mainly grains and molasses that increase the nutritional value and enhance fermentation.

The first two types of additives are used in very small amounts, whereas the nutrient sources are added in larger quantities, usually directly into the silo.

Biological additives.

The use of inoculants containing LAB has been reported since the early 20th century. In the early 1970s the use of acids diminished, particularly in light of the continually increasing attention to health and safety, including corrosion of machinery, etc., and the concept of biological additives was reincarnated, particularly with reference to selected strains of LAB and sources of more refined saccharolytic enzymes. Many early experiments on the addition of pure LAB did not give the expected results, mainly because of the poor adaptability and viability of the bacteria in the silage. Once these obstacles had been overcome, microbial inoculants began to gain popularity in the late 1970s, and have been widely used since the early 1980s Fig. 23 and Fig. 24. Apart from technological factors, the production of good silage necessitates appropriate combinations of three biological factors:

  1. Adequate fermentable sugars.
  2. Correct bacteria to ferment them.
  3. Appropriate enzymes to break down structural carbohydrates to WSCs in case of lack of sufficient sugars that the bacteria can digest.

The following table summarizes the possibilities of obtaining good or poor silage according to the combinations of the above three factors.

Probability of good silage fermentation

G = Good P = Poor
Ingredientssufficient present (+)absent (-)
Fermentable sugars++-++---
Lactic acid bacteria+++-++--
Silage qualityGGGPPPPP
Silage qualityGGGPPPPP

The table shows that of the eight possible combinations of presence or absence of enzymes, LAB and WSC, only three (37.5%) will yield good silage. However, the addition of LAB will increase this expectation to six (75%) cases. Spraying the inoculants or the enzymes onto the silage is an easy operation, and, since the materials are not expensive and have no harmful effects, the use of biological additives is extending very rapidly and is already in practical use in many places.

There are controversial views regarding the effects of adding enzymes. The enzymes remain effective for only a short time – until the pH declines – and then they are inactivated. Enzyme additives have been more effective in increasing the breakdown of fiber and in improving fermentation when applied to direct-cut or slightly wilted grasses than when applied to alfalfa or corn. Enzymes have not consistently improved DM digestibility, aerobic stability or animal performance, and they have been found to increase effluent production when forage containing less than 28-30% DM is ensiled. However, commercial additives usually contain the LAB inoculants and the enzymes packaged together. The addition of inoculants is intended to increase the initial LAB population in the forage in order to kick-start the correct fermentation. Such a start will lead to faster and deeper pH reduction, and the expected end result will be better silage with less losses. In such a case the performance of the animal (in milk or meat production) should also be better.

The exact type of inoculants in the package is usually not specified on the label, for commercial reasons. Usually the inoculants include bacteria from the following strains: Lactobacillus plantarum, Lactobacillus acidophilus, Pedioccus cerviseae, Pediococcus pentosaceus, Enterococcus faecium, and others. The number of added bacteria is usually calculated to reach 105-106 CFU/g in wet forage. It is important to note that the inoculants must be stored and used correctly, in accordance with the producer’s directions. Only living bacteria can be effective.

Usually the activity of bacteria depends on the environmental conditions (substrate, pH, temperature, oxygen level, etc.). In the initial phase of the ensiling process, conditions in the forage change very dramatically, therefore, to ensure the continuing effectiveness of the added bacteria on the forage, from the first stage to the achievement of a steady state (low pH), inoculants usually include a “cocktail” of different types of bacteria to ensure sequential activity.

The bacteria added in the inoculants are mainly homofermentative bacteria (that ferment sugar to lactic acid only), and there may be a lack of other organic fatty acids such as butyric, propionic and acetic, which are usually produced (even in small amounts) during fermentation. These fatty acids inhibit yeasts, molds and aerobic bacteria, and therefore play an important protective role in the “feed-out” phase, when the silage is exposed to air and the aerobic microorganisms start to act, and spoil the silage. Thus, in hot places or under conditions in which the aerobic stability of the silage is poor and losses are high, the addition of inoculants may have negative results, although in the initial phases the fermentation looks better. The most important consideration is the quality of the silage that is fed to the animal.

Many factors related to the bacteria affect their influence on the silage end product. Such factors include: relative doubling times; the effect of falling pH on the bacterial growth rate; the effect of pH on bacterial activity; and probably others. Usually the added inoculants contain more than one kind of bacteria to ensure extended effectiveness. However, the user of the inoculants (the farmer) lacks the knowledge and has no indicator to judge the quality or the effectiveness of the inoculants. The question then remains: is it recommended to add inoculants?

The answer is very simple: each inoculant must be tested on the particular forage crop, and under the local conditions, climate and husbandry, and its use should depend on its effect on the animal performance.

Chemical additives:

Formic acid (FA) (HCOOH)In areas where it is difficult to wilt forages, acids, especially formic acid (FA), are commonly used as additives. Formic acid is a relatively strong acid that acts by immediate reduction of the pH in the forage. The use of FA as a silage additive started in 1929, and led to its subsequent designation as the “silo acid”. The commercial product has an FA concentration of 85%, and it is usually applied undiluted to herbage at a rate of 2.3 l (2.8 kg)/ton, which is equivalent to 2 l of pure acid per ton. For legumes and poorly fermenting crops, a higher level is recommended. Formic acid in silage prevents the activity of clostridial bacteria, inhibits protein solubilization, and has selective antibacterial action. However, yeasts have been shown to be partially tolerant to FA.

Formic acid is extremely dangerous to handle, since it is caustic to skin and eyes, and corrosive to farm equipment. It is volatile and its fumes can be extremely damaging to the respiratory system. Strong regulations in Europe (where this acid is still in use) govern the handling and use of this material and minimize its hazards, but salts (calcium or sodium) of the acid can be used as a safe alternative; however, they are more expensive than the pure acid. Among the homologous fatty acids, the effect of the hydrogen ion concentration decreases with increasing molecular weight. However, the antibacterial effect increases at least up to the C12 acid.

Mineral acids, such as sulfuric or hydrochloric acid, act solely by reducing the pH and have no specific antimicrobial properties.

Propionic acid (PA) (CH₃CH₂COOH)Propionic acid is a weak acid, with minimal effect on the pH; its main effect is to inhibit the activity of yeasts and molds, especially in the aerobic phase, when silage is being fed out. Propionic acid acts by disrupting the enzymatic process associated with plant microbial respiration: it reduces losses associated with infiltration of oxygen, reduces the silo temperature, which in turn reduces protein solubilization, and increases aerobic stability. Propionic acid acts immediately upon application; its effectiveness does not depend on temperature or DM content, but on the level of addition. Propionic acid is caustic to skin and eyes, and corrosive to farm equipment, and therefore must be handled and stored carefully. Sodium propionate is less caustic, but also less effective.

Ammonia and urea [NH₃ and CO(NH₂)₂]Ammonia and urea are added to silage mainly to increase the crude protein content. However, ammonia also has antibacterial properties and can act as a preservative. Ammonia addition is most often associated with whole-crop-corn silage, since this crop has a low crude protein content, with a high level of WSC. Ammonia has a strong alkaline reaction, and can immediately raise the pH of the forage to pH 9, thus prolonging the fermentation period; even the final pH of the silage tends to be raised. Anhydrous ammonia (when added) is rapidly assimilated by the water in the forage, and may release some heat as it goes into solution. Therefore, the moisture content in the forage is an important factor in determining the success or failure of this treatment; in dry material, only little ammonia will be assimilated (most will evaporate), and the effect on the silage will be marginal. Ammonia mainly increases the crude protein fraction, but the high initial pH also inactivates the plant proteases, so that the degradation of the protein is reduced. Ammonia also breaks some of the linkages between the hemicellulose and the other cell-wall components, which should increase digestibility. Ammonia can also increase aerobic stability, probably by inhibiting yeast and mold activities, and it increases the lactic acid content. However, if the ammonia level is too high, it will suppress fermentation, decrease aerobic stability and reduce the animals’ DM intake. This technology of adding ammonia to silage (developed by Prof. Tal Huber in Michigan State University) is based on high-pressure ammonia in liquid form, which is sprayed directly into the chopper during the forage chopping operation. The recovery of the ammonia absorbed into the silage is only around 50% in dry material and in hot locations recovery will be even lower. Since ammonia is a poisonous gas, proper care must be taken while handling it. In fact, the high risks and many resulting restrictions involved in the use of ammonia have led to the abolition of its use.

Urea has a similar effect on silage to that of ammonia, but it must first be broken down to ammonia by the enzyme urease, which is present in plants: CO(NH)₂ + H₂O → 2NH₃ + CO₂

Urea is easy to handle, but its ammonia release is not immediate, therefore its inhibitory effect on aerobic microorganisms is delayed. Moreover, the price per unit of nitrogen is higher in urea than in ammonia. In practice both additives are unpopular Fig. 25.