Bacterial Inoculants can Improve the
Quality of Silages
by Limin Kung, Jr., Ph.D. Department of Animal and Food Sciences University of Delaware Newark, Delaware
Introduction
Silage fermentation can be roughly divided into four phases. The first phase of fermentation is
characterized by the presence of oxygen in the silo. Plant respiration continues for several hours
(and perhaps days if silage is poorly packed) and plant enzymes (e.g., proteases) are active until
oxygen in the silo is used up. During this phase, excess oxygen can lead to unwanted protein
breakdown, excessive heating, and the growth of undesirable yeasts and molds. Oxygen can be
eliminated by quick packing, even distribution of forage in the storage structure, chopping to a
correct length, and ensiling at recommended dry matters (DM) for specific storage structures and
type of silage/haylage. Oxygen must be eliminated before optimal fermentation can take place.
Under anaerobic conditions (lack of oxygen), the second phase of silage fermentation is dominated
by microbial activity. Fermentation is controlled primarily by:
 Type of microorganisms that dominate the fermentation.
 Available substrate (water soluble carbohydrates) for microbial growth.
 Moisture content of the crop.
During this phase, lactic acid-producing bacteria (LAB) should utilize watersoluble carbohydrates to
produce lactic acid, the primary acid responsible for decreasing the pH in silage. Undesirable
fermentations from microorganisms such as Enterobacteria and Clostridia can dominate if the pH
does not drop rapidly.
When weather permits, wilting forages to obtain dry matter greater than 30-35% prior to ensiling can
help eliminate Clostridia.
Lack of oxygen prevents the growth of yeast and molds, and low pH prevents the growth of most
bacteria during the third phase of fermentation. Silage can be kept for prolonged periods of time if
these conditions prevail. The last, and fourth, stage of silage fermentation is during feed out and the
subsequent exposure to oxygen. Good silage will remain stable and not change in composition or
heat during the third and fourth stages of fermentation. Airtight silos and removal of sufficient silage
during feed out can prevent aerobic spoilage. Some good silage management practices are listed in
Table 1.
The end products of silage fermentation are often monitored to assess silage quality. The
composition of "normal silages" is presented in Table 2. Several commercial laboratories now offer
analytical services for silage end products. Readers should be aware that numerous factors can
affect silage composition.
Of the many factors that can affect silage fermentation, the type of microorganisms that dominate the
process often dictate the final quality of the silage. As shown in Table 3, homolactic fermentation is
more desirable than other types of fermentations because the theoretical recoveries of DM and
energy are greatest. In contrast, other fermentations are less efficient.
What Good-Quality Silage Smells Like
Theoretically, high-quality silage which has undergone a fermentation that was predominately
homolactic in nature should have very little smell. The reason is that lactic acid has relatively no
smell when compared to the other major fermentation acids. In fact, you may be able to tell what type
of fermentation dominated your ensiling process by smelling your silage. Silage that has a strong
vinegar smell has most likely undergone a heterolactic acid fermentation. Such silages are high in
acetic acid concentration and are usually very stable when exposed to air. In contrast, silages that
smell very sweet, like alcohol, have undergone a yeast fermentation and are very unstable when
exposed to air. A burnt, but sweet smelling, tobacco-like odor is often commonly found in very dry
alfalfa silages and is a result of heat damaged protein. Finally, very wet silages that are slimy and
smell fishy and putrid, have undergone clostridial fermentations and are low in energy and usually
have excessive amounts of soluble nitrogen.
Microbial Inoculation
Natural populations of lactic acid bacteria (LAB) on plant material are often low in number and
heterofermentative (fermentation yields various end products). Thus, the concept of adding a
microbial inoculant to silage was to add fast growing homofermentative (fermentation yields one
consistent end product) lactic acid bacteria (hoLAB) to dominate the fermentation, producing a
higher quality silage.
Some of the more common h0LAB used in silage inoculants include Lactobacillus plantarum, L.
acidophilus, and Pediococcus acidilactici. Commercially available microbial inoculants contain one or
more of these bacteria that have been selected for their ability to dominate the fermentation. Some
companies rationalize the need for multiple organisms for potential synergistic actions. For example,
growth rate is faster in Streptococcus > Pediococcus > Lactobacillus. Some Pediococcus strains are
more tolerant of high DM conditions than are Lactobacillus and have a wider range of optimal
temperature and pH for growth (they grow better in cool conditions found in late fall and early spring).
However, organisms from different companies often have the same name, but are different. Thus,
multiple strain inoculants are not necessarily better than an inoculant with a single strain of
homolactic acid bacteria.
When compared to untreated silages, those silages treated with adequate numbers of a viable
h0LAB should be lower in pH, acetic acid, butyric acid, and ammonia-N, but higher in lactic acid
content. In a review of the literature between 1990-95, Dr. Richard Muck (U.S. Dairy Forage
Research Center) and I reported that microbial inoculation lowered pH, improved the lactic:acetic
ratio, and lowered ammonia nitrogen content in more than 60% of studies. Dry matter recovery was
improved in more than 35% of the studies and bunk life improved in only about 30% of the studies.
Dry matter digestibility was also improved in about one-third of the cases.
Microbial inoculation usually has little or no effect on the fiber content of silages because most lactic
acid bacteria contain little or no ability to degrade plant cell walls. Decreases in fiber content may be
due to partial acid hydrolysis of hemicellulose. Bunk life or aerobic stability has not been consistently
improved by inoculation and, in some instances, inoculation has made aerobic stability worse. This
was probably due to a lower acetic acid content.
Relative to animal responses, Dr. Muck and I also found positive responses to microbial inoculants
on intake, gain, and milk production (see Table 4). The average response in milk production was a
+3.1 lb per day in studies where milk production was statistically improved. Although literature
summaries are encouraging, caution should be used when interpreting such data because all
inoculants are not equal and the conditions (e.g. rate of application, inoculant viability, species of
bacteria, crop, moisture levels) varied markedly among the studies. As many have pointed out in the
past, products with organisms with the same name are not necessarily the same organism and may
not have the same effectiveness.
An impressive number of animal experiments have been conducted using a single silage inoculant
containing Lactobacillus plantarum MTD/1. A summary of 14 lactation studies conducted in
university and government research institutes in North America and Europe using MTD/1 is shown in
Table 5. Statistical analyses revealed DM intake was numerically increased by 4.8% and milk
production was significantly increased by 4.6%. Improvements in milk yield were obtained with a
variety of crops (grass, corn, and alfalfa) across a wide spectrum of DM contents (15 to 46% DM).
Body weight gain also tended to be better in cows fed silage treated with MTD/1. Similarly, 19
comparisons among untreated silages and silages treated with MTD/1 were summarized by U.K.
researchers for beef cattle. Across all studies and types of forage, cattle fed inoculated silage ate
7.5% more DM and gained 11.1 % more weight.
The organisms) from microbial inoculants must be present in sufficient numbers to effectively
dominate the fermentation. Most research suggests that an inoculation rate of 100,000 (or 1 x 105)
homolactic acid producing bacteria per gram of wet forage is a good compromise between adequate
numbers and cost effectiveness of the additive. Purchasing an inoculant from a reputable company
with good manufacturing and quality controls is a key to obtaining a product with viable organisms.
Most microbial inoculants are available in powder or granular form. Inoculants applied in the dry form
are often mixed with calcium carbonate (limestone), dried skim milk, sucrose, or other carriers.
These products can be applied by hand or by solid metering devices as per manufacturer's
recommendations. Inoculants to be applied in the liquid form come as dried powders and are mixed
with water just prior to use. (Caution: use of chlorinated water is not recommended because it may
be detrimental to the inoculant.)
Microbial inoculants can be applied to the forage at a variety of locations. However, inoculant
application to forage at the chopper is highly recommended to maximize the time microorganisms
have in contact with fermentable substrates. Inoculants can also be applied at the blower of an
upright silo or sprinkled over the forage mass between loads in a bunk silo. Application can be with a
simple watering can by weighing the incoming forage load and adjusting application based on the
average unloading time.
A better method is to use a metered liquid sprayer to evenly disperse the inoculant onto the forage.
Unused liquids should be discarded after a period of 24 to 48 hours because bacterial numbers
begin to decline and there is potential for growth of contaminating organisms. Proper distribution
cannot be overlooked and is important for the inoculant to be effective. Throwing dry inoculant onto a
wagonload of forage and hoping for even distribution is not an acceptable practice!
Storage is an important aspect of a high-quality inoculant that contains live microorganisms.
Inoculants should be kept in cool, dry areas away from direct sunlight. Moisture, oxygen, and
sunlight will decrease stability of inoculants.
Opened bags of inoculants should be used as soon as possible.
Conclusion
Silage inoculants can be useful tools to improve silage quality and animal performance; however,
they are not replacements for good management practices. Care should be taken when choosing a
silage inoculant. Producers should ask for proof of claims that are usually in the form of published
scientific articles that have undergone peer review.