E. CHARMLEY
Agriculture and Agri-Food Canada, Research Farm, Nappan, Nova Scotia
Corn silage is a high energy feed source for ruminants. Being part forage and part grain, it has characteristics of both feed types and is a valuable component of dairy rations in regions where corn can be grown. Nutritionally, corn silage is lower in crude protein (CP) and higher in digestible energy (DE) than other forages. It also differs from other forages in that quality does not decline with advancing maturity. This is because the increasing amount of grain in the crop offsets the decline in digestibility normally associated with structural tissues (in the case of corn, stem). Compared to many crops corn is relatively easy to ensile. It is however a high-cost crop to grow, ensile and feed.
i) Moisture
One of the most critical factors affecting the process of fermentation is the amount of water present in the crop at ensiling (Fig. 1). Silage microbes need water in order to thrive and multiply. That is why drying hay, for example, is effective at preventing most microbial growth. However, the amount of water is important in determining which microbes grow best. In silage, we normally quantify water in terms of dry matter (DM) content of the crop. Thus a low DM silage contains more water than a high DM silage.
Figure 1. Moisture and silage fermentation. The drier the silage
the higher the pH can be for successful preservation.
The wetter a silage, the more biological activity, of all kinds, there will be. This is not necessarily a good thing. Managing the ensilage process means creating an environment that favours desirable microbes over undesirable microbes. However if a silage is too dry, there will not be enough moisture to support sufficient microbial growth to produce the acids which reduce the pH and preserve the crop. Optimum crop DM content is between 25 and 50%, depending on the type of crop and storage system. In most forage crops, optimum DM content is reached by wilting prior to ensiling, a sometimes tricky business. However, in corn, the crop loses moisture as it matures and an optimum DM content of between 33 and 36% can be reached by waiting for the crop to mature (provided there are enough heat units). This feature, makes achieving the desirable DM content relatively simple in many silage growing areas. As a general guide, corn silage should be harvested when the milk line has descended 1/3 to 1/2 of the way from kernel crown to the base (see Corn Growth and Development section).
So why is wet silage undesirable? The wetter the crop the more active all bacteria become and the more food (substrate) is needed to sustain them. Since there is a limited amount of food reserves (substrate) in the harvested corn, beneficial bacteria have to compete with all the other types of bacteria. As crops dry, some types of bacteria are more affected than others. A major group of undesirable bacteria (the clostridia) are particularly susceptible to a scarcity of moisture. So by increasing the DM content we can weed out the bad bacteria from the good.
Wet silage is also problematic, because when it is piled in a bunker or, even worse, a tower, then pressure squeezes out water and many soluble sugars, proteins and minerals (Fig. 2). This silage effluent or run-off, not only represents a loss of nutrients, it is also a potent pollutant, having a very high biological oxygen demand (BOD). Upon entering waterways it causes eutrophication (rapid growth of biomass) and takes the dissolved oxygen out of the water. This will render a stream dead quite effectively.
Figure 2. Losses in a bunker silo.
So why is dry silage undesirable? Silage that is too dry will not ferment enough to reduce pH to a level that will kill spoilage-causing yeasts, moulds and aerobic bacteria (Fig. 3). In wet silage, the spaces between the plant material are filled with water, but in dry silages they are filled with air. Also, dry material tends to be more springy and resistant to compaction. Under these conditions aerobic microbes predominate, particularly yeasts and moulds. These consume valuable nutrients, produce heat and thus cause spoilage. Heating is a particular problem, because as the silage heats up, then the yeasts and moulds proliferate even faster. Heating, which starts in the silo, tends to continue in the feed bunk, reducing feed intake and leaving large amounts of rejected feed.
Figure 3. pH decline and silage stability.
ii) Substrate
Silage micro-organisms need a supply of soluble carbohydrate or sugars. Most crops, including corn, contain between 3 and 10% of their DM as sugar. Corn usually has close to 10% of DM as sugar, which makes it a reasonably easy crop to ferment. These sugars are used up by the microbes to produce acids. Since the role of acids is to reduce pH, strong acids are needed; the strongest acid produced by bacteria is lactic acid. In a good fermentation, the goal is to have as much of the sugars converted to lactic acid as possible. Since the sugars are a finite resource, efficient use is paramount. Manipulating DM content will help to ensure that the desirable or homolactic bacteria will predominate in the microflora, producing lots of lactic acid to effect preservation.
Unlike most other silages, corn silage can actually have too much sugar! The unfermented sugars can remain at quite high concentrations even months after the crop has been ensiled. These sugars can be used by aerobic bacteria once the silo has been opened for feed-out. In this case the so-called (but misnamed) secondary fermentation can take place. It is not fermentation at all but aerobic respiration and produces a lot of heat and ultimately leads to spoilage of the silage.
The fact that corn silages can have too much substrate for the traditional fermentation, has led researchers to look at the possibility of using some of that sugar to produce acetic (vinegar) or propionic acid. (Manufactured propionic acid is often used as an additive help preserve high moisture grass hay.) Although these are weak acids and not very effective at reducing pH, they are quite effective at inhibiting aerobic microbes, particularly yeasts. Recent research has looked at the possibility of adding specific strains of bacteria to corn silage to produce these acids (See below).
iii) Silage bacteria
When corn is harvested, plant surfaces are covered by a wide range of micro-organisms. These are known as epiphytic bacteria because they live on the host plant in a natural symbiosis. They are present in the thousands per gram of crop, but only a small fraction, namely the lactic acid bacteria (LAB), are of value in silage fermentation (Table 1). The numbers of lactic acid bacteria on corn at ensiling is low compared to the numbers needed for ensiling. After the crop is chopped and placed in the silo, the plant cells and aerobic bacteria continue to respire for a day or two and conditions in the silo become increasingly anaerobic. Eventually this aerobic respiration by the crop and microbes will use up all the oxygen in the silo. Then under these anaerobic conditions, the anaerobic bacteria, including the lactic acid bacteria, begin to proliferate. The anaerobic epiphytic bacteria now produce lactic acid, as well as a range of other acids, alcohols and related compounds from sugar present in the crop.
In the initial phase of ensiling, competition for substrate by the various types of bacteria is intense. Eventually, however, conditions will begin to favour one or another group. Under conditions of optimum moisture and substrate availability, the lactic acid bacteria will predominate (Fig. 4). The more rapidly they can proliferate, the more rapidly the pH drops and under these conditions lactic acid will be the dominant organic compound produced. Good silage fermentation will ensue.
Figure 4. Change in population over time of 4 bacterial groups in good fermentation (top), clostridial fermentation (middle), aerobic fermentation (bottom).
When conditions are sub-optimal for LAB, they may never dominate the fermentation. For example, in wetter silages the Clostridia may predominate (Fig. 4). These bacteria are always present on the standing crop but are particularly numerous in soil and manure. Although manure is a potential source of contamination in corn, the long interval between manure application and harvest and the fact that corn does not normally come into contact with the ground during harvest means that clostridial contamination from manure is unlikely. Another group of bacteria which sometimes come to dominate silage fermentation are the ‘coliform’ bacteria, or enterobacteria. These tend to dominate in silage when the rate of pH decline is slow or where the final pH is high. These bacteria produce a range of organic acids and alcohols and are not effective at preserving silage. They are also associated with the production of endo-toxins and ammonia. High levels of ammonia in drier silages are indicative of fermentation dominated by coliform bacteria.
i) Management
No matter what crop or system of ensiling is used, the single most important determinant for making successful silage is good management at the time of ensiling. The various aids to making good silage do not substitute for good management practices, and in fact are wasted if not used in association with good management.
Understanding the principles of ensiling helps the farmer understand the critical control points for success. Since ensiling is an anaerobic process, rapid and effective filling and sealing of the silo is critical. It is essential to minimize the amount of air that gets into the silo. How this is done will depend on the type of silo used.
Silos come in many forms. The ‘Cadillac’ of silos is the gastight, glass-lined upright silo, which became very popular 20 to 30 years ago. These are expensive, but effective. The silo walls are totally airtight, and the silo is filled from the top. The weight of the crop provides the pressure for packing. Without any ingress of air to the system, the silage quickly becomes anaerobic and good fermentation usually results.
This system is generally best used in silages of 40% DM or higher. Corn silages, which are usually ensiled below this threshold, may have too much moisture. The pressures exerted in such a silo will produce silage effluent (also known as seepage or run-off ), which will either collect at the base or leak out of the silo. In either case, this is bad. The higher moisture will sour the silage at the base of the tower if it remains in the silo and the effluent poses an environmental threat. To maintain an anaerobic atmosphere during unloading, these silos typically empty from the bottom.
The concrete stave silo, is a less costly and less elaborate version of the upright tower. The principle behind achieving anaerobic conditions is the same, although the system is slightly less effective. These silos are unloaded from the top. It is critical to remove 10 –15 cm (4 to 6 in) of silage a day so that the silage is fed before it begins to deteriorate. These silos are relatively common, and those filled with corn silage are often seen weeping effluent from the sides in the lower portion of the silo — a clear sign the silage was too wet.
Bunker silos are effective. Although corn silage can be successfully stored in towers, the crop is eminently suited to bunker silos because there is less pressure to cause seepage in a bunker silo. To be effective, bunker silos have to be filled quickly and sealed effectively because of the large surface area.
Sealing is particularly crucial for corn silage because it contains a lot of sugar for microbes to grow on. If air is present, the sugars are used by aerobic organisms causing heating. Plant and aerobic microbial respiration using up the sugars produce heat warming the air and causing it to rise while bringing down cool air filled with fresh oxygen (called flue effect). This stimulates more aerobic activity and the cycle continues (Fig. 5).
Figure 5. Transverse section of a silo during filling, showing “flue effect”.
Compared to grass silages, corn silage is easy to pack, especially when well chopped or ‘processed’ (see Processing Corn Silage section). It tends to pack well, up to a point, beyond which further packing has little added benefit. Forage harvesters should be set for a theoretical chop length of 6–9 mm (¼ to 3/8 inch). About half the silage should be as particles 13 mm (½ in) long, with the rest somewhat longer. Excessive long particles indicate that the chopper needs sharpening and re-setting.
ii) Silage inoculants
Conventional microbial inoculants work by adding a fairly large amount of lactic acid producing bacteria—usually 100,000 organisms (or cfu) per g of crop in North America and 1,000,000 cfu per g of crop in Europe. The hope is, that these relatively large numbers of homolactic bacteria will be able to quickly dominate the natural, less efficient, lactic acid bacteria, and so produce a better silage. In many cases this does happen.
Research has shown that on average silage inoculants will result in small improvements in milk or beef production. Table 2 shows the results of a survey the author conducted in 1994. This survey considered all silages, not just corn silage. We found a small benefit in digestibility and intake. Combined, these resulted in a 2 to 10% improvement in animal performance. Since that time, it is reasonable to conclude that silage inoculants have improved, however more recent reviews of the literature still reach similar conclusions. Nevertheless, products are priced to ensure an economical return, on the average. However, in about . of cases, the additive will have no discernable effect on the silage fermentation, and about half the time they will have no effect on animal output (Table 3).
Table 2. Animal response to silage inoculants. Results of a survey by the author (1994).
Table. 3. Animal response to silage inoculants. Results of a survey by Kung and Muck (1990 to 1995)
More recent commercial additives usually contain a cocktail of microbial strains and types. By providing a complex mixture, manufacturers claim better success. Some bacteria, such as Streptococcus or Pediococcus, will be active very early on in the fermentation, when the pH is still quite high and there is still some oxygen in the system. Others, like the Lactobacilli, will play a longer term role, continuing to reduce the pH once the “starter bacteria” have died off. However, single strain inoculants can still be very effective. In choosing an inoculant, the buyer should look for products that are from a recognizable company and have a proven track record. Generally, these have been vigorously researched and newer, more effective strains are continually being developed. Products with complicated formulations may not be any better and even if the bacterial name is the same in two different products, the strain will probably not be. Just as all Holstein cows are not created equal, neither are all strains of Lactobacillus.
Additives manufactured specifically for corn silages often contain enzymes as well as microbial inoculants. Corn silage contains a large proportion of starch, as a result of having a high grain component. By adding amylase enzyme that can convert starch to sugar, the expectation is that this will improve fermentation. While this approach may give some added benefit, the evidence for this is not strong. Given that corn silage already contains adequate sugars for fermentation, production of more may actually be detrimental. Additional sugar may serve as fuel for yeasts and moulds, particularly during feed-out.
i) Bunker management
As already mentioned, corn silage is susceptible to heating and spoilage during feed-out. So one of the main goals of bunk management is to work to reduce the opportunity for heating at every stage of the operation. The silo, whether tower or bunker, should be designed such that enough silage is removed each day to keep the face moving back at least 4 to 6 inches a day, on average. The more susceptible a silage is to deterioration, the faster it should be removed from the silo. The following steps are recommended when emptying the silo:
ii) Bunk management
Cleanliness is again crucial to minimize heating in the feed bunk. Old silage should be removed daily if possible. Heating silage left in the bunk, will cause the new feed to heat all the faster. These tips are particularly critical in hot weather.
Figure 8. Top: smaller bunker silo (12 ft face) designed to ensure face moves back 1 ft per week during summer feeding — minimal signs of spoilage. Bottom: surface spoilage on the same silo — close up view.
Corn silage is an excellent feed for high producing ruminants. Although making silage is always a potentially risky business, the risks with corn silage are often less than with grass and legume silages. The crop is naturally at the correct DM content for ensiling and almost always has ample sugars to ensure a satisfactory fermentation. Perhaps the biggest challenge with corn silage is controlling heating and aerobic deterioration. However, careful management both when filling and emptying the silo can reduce the risks. New silage inoculants just coming onto the market may one day prove to be very beneficial in controlling spoilage, thus reducing the risk still further.
Processing Silage Corn on Particle Size, Packed Density, and Silage Fermentation
J. Harrison and L. Van Wieringen
Department of Animal Sciences, Washington State University, Puyallup, WA
Processing silage corn results in forage with smaller particle size. Using the Nasco or Penn State forage particle separator we found that processing reduces the percentage of particles remaining on the top sieve (greater than 18 mm or ¾ in) and increases the percentage of particles remaining on the middle (between 5–18 mm or 3...–¾ in) and bottom sieves (Fig. 1). The decrease in particle size increases how densely corn silage is packed in silos, hence the porosity and rate of air infiltration, which ultimately determines the amount of spoilage that occurs at the time of feedout. Many studies have shown that processing corn silage increases wet pack density in the silo over a range of maturity (one-third milk-line to physiological maturity) and theoretical chop lengths (6–13 mm or ¼–½ in) (Fig. 2). The only exceptions were 2 experiments where the chop of the corn silage was long (10–38 mm or 2..–1½ in). Based on this, we expected that processed silage would undergo faster silage fermentation and result in more lactic acid (Fig. 3) and a lower pH (Fig. 4). However, in the majority of cases, we observed that mechanical processing did not enhance lactic acid production or pH decline at first. It appears that some of the mechanically processed corn silages had increased buffering due to the exposure of cell contents to the forage mix. This likely resulted in the higher pH of processed silage during fermentation in the silo. The processed silage did have lower pH and more lactic acid after several weeks in the silo.
Figure 1. Particle size distribution of processed and unprocessed corn silage (theoretical chop length 12 mm or 1/2 in).
Figure 2. Effect of processing on wet pack density (for kg/m3 multiply lb/ft3 X 16.1).
Figure 3. Effect of processing on increase of lactate during ensiling of corn silage harvested at blackline (greater than 40% dry matter).
Figure 4. Effect of processing on change of pH during ensiling of corn silage harvested at blackline (greater than 40% dry matter).
Aerobic Stability – Processed Corn Silage
J. Harrison and L. Van Wieringen
Department of Animal Sciences, Washington State University, Puyallup, WA
The greater wet pack density for processed corn silage tends to improve aerobic stability at feedout. Aerobic deterioration occurs as a result of microbial activity. The factors that influence deterioration include: oxygen (exposure time), composition of the microbial population, substrate type, and temperature. Yeasts are usually the initial cause of aerobic deterioration. As lactic acid (the major end product of silage fermentation) and other residual sugars are combusted and used by yeast, the temperature starts to rise. Aerobic stability can be measured as: 1) number of hours until temperature of corn silage increases 2°C (3°F) above ambient, 2) number of hours until corn silage reaches peak temperature, and 3) maximal temperature rise above ambient.
In our studies, processing of corn silage enhanced aerobic stability. While it took longer for processed corn silage to heat by 1.5°C (3°F) (Fig. 1), both silages reached maximum temperature in about the same amount of time. This indicates that the processed corn silage was more stable in an aerobic environment early on (i.e. took longer to start heating) due to greater pack density that limited exposure of processed corn silage to oxygen during the storage phase. However, once the silages were exposed to air there was nothing particular about the processed corn silage that inhibited growth of aerobic microorganisms.
Therefore, once they began to grow, the microorganisms in both silages multiplied causing both silages to heat at the same rate and the same amount (ranging from 6.4° to 13.1°C or 11.5° to 23.6° F).
Figure 1. Aerobic stability of processed and unprocessed corn silage.
J. Harrison and L. Van Wieringen
Department of Animal Sciences, Washington State University, Puyallup, WA
The amount of packing time and the thickness of the silage as it is layered in to the silo interact to affect silage density. It is desirable to layer the forage into a bunker silo at depths of 15 cm (6 in) or less and pack to a rate of 2-3 minutes per tonne (or Ton) of forage or 300–600 hrkg per tonne (600–800 hr-lb per T) of wet forage. When forage delivery to the silo is in the range of 40 wet tones (or Tons) per hour these rates are achievable and realistic. Delivery rates greater than this will require large packing tractors and likely multiple packing tractors. Common recommended feed-out rates are 15 cm (6 in) per day, but more is recommended in hotter weather. It is best not to “buck” into the silage mass when removing silage as this allows channels for the entry of air back into the silage mass.
Tools for Estimating Bunker Characteristics A number of publications and computer software based tools for silage storage are available at website: http://www.uwex.edu/ces/crops/uwforage/storage.htm. In particular look for two spreadsheets entitled “Bunker Silo Density Calculator” and “Bunker Silo Sizing Spreadsheet”. The software tool, DAFOSYM, is a whole farm economic model with particular emphasis on forage management. A free copy of DAFOSYM is available at: http:/ /pswmru.arsup.psu.
Figure 6. Cross sectional diagram of a bunker silo during filling.
Corn silage is susceptible to aerobic deterioration during feed-out. This is attributed to three factors:
1.High levels of residual sugars
2. Potentially high yeast populations
3. Friable, open consistency of the silage heap
A new microbial inoculant has just been developed which is proving to be very effective at reducing aerobic spoilage during feed-out — Lactobacillus buchneri. This bacterium has a heterofermentative fermentation and produces acetic acid from lactic acid. Normally, this would be undesirable in silage. Acetic acid is weaker than lactic acid and therefore not as good at reducing the pH of silage. However, in corn silage, there is usually ample lactic acid produced to ensure preservation. The trouble is, if all the acid is lactic acid, then corn silages frequently heat and are prone to aerobic spoilage. Why is this?
Lactic acid reduces pH effectively, but has no direct anti-fungal properties. Acetic acid on the other hand, is less effective at reducing pH, but has anti-fungal properties, not related to pH. This phenomenon has come to light in recent years following the widespread use of lactic acid-producing inoculants. Corn silages have increasingly become more homofermentative (i.e. a higher proportion of their acids are as lactic acid). At the same time corn silages have also become less stable when the silos are opened up for feed-out.
Research has now shown that L. buchneri is quite specific in producing acetic acid and 1,2 propanediol (another anti-fungal compound). However, it produces these products not from the sugars initially present on the crop, but from lactic acid produced in the course of the normal lactic acid fermentation. Thus it is not in direct competition with the lactic acid bacteria, but comes into its own later on during the fermentation, producing the acetic acid when it is needed.
A recent example of its effectiveness is demonstrated by the work of Ranjit et al (2002) from the University of Delaware (Table 4). In that study they added a fairly high number of L. buchneri (400,000 cfu/g) and succeeded in markedly improving the aerobic stability of the silage by increasing the concentration of acetic and propionic acids. When fed to sheep, this resulted in a 70% increase in rate of gain. In the same work, the authors looked at different levels of application of L. buchneri. It is clear that high application rates are essential if this approach is to be effective. When lower rates of the L. buchneri were added, there was no effect on stability in laboratory-scale silos.
Another similar approach being developed is to add strains of propionic acid producing bacteria to silage. In the same way as the acetic acid-producing bacteria, these ‘Propionibacteria’ produce propionic acid. Initial research suggests that this approach also holds possibilities, but these bacteria appear to be less aggressive in the silo and unrealistically high numbers need to be added in order to improve aerobic stability. With further development, however, the propionic acid bacteria may well become another tool to increase aerobic stability of corn silage.
Table 4. Effect of Lactobacillus buchneri on the composition and aerobic stability of corn silages and on animal performance in sheep¹
¹ Ranjit, N.K., Taylor, C.C. and Kung, L. (2002). Grass and Forage Science, 57:73-81.
