Microorganisms in Silage Production: The Role of Saccharolytic and Proteolytic Bacteria

Silage – fermented forage stored under anaerobic conditions – is one of the most efficient ways to preserve green plant material as livestock feed throughout the year. At the heart of the ensiling process is a complex microbial community. Some bacteria are essential allies that drive preservation, while others are destructive agents that degrade feed quality and threaten animal health. Understanding the roles of saccharolytic (sugar-fermenting) and proteolytic (protein-degrading) organisms in silage is key to producing safe, nutritious feed and preventing costly spoilage.

Lactic acid bacteria: the backbone of good silage
Homofermentative vs. heterofermentative LAB
Undesirable microorganisms in silage
Saccharolytic and proteolytic Clostridia
Listeria monocytogenes
Other spoilage organisms
Microbial competition and silage quality
Preventing spoilage with additives
Microbial inoculants
Chemical preservatives
Management practices

Lactic acid bacteria: the backbone of good silage

The desirable microorganisms in silage are the lactic acid bacteria (LAB) – a group of saccharolytic organisms that ferment water-soluble carbohydrates in the forage crop into lactic acid. According to a comprehensive review published in PMC, in conventional silage fermentation, water-soluble carbohydrates are primarily converted to organic acid mixtures by epiphytic LAB, which lowers the pH and preserves the forage under anaerobic conditions. This acid-driven drop in pH is what gives silage its characteristic preservation – effectively pickling the forage and halting the activity of harmful microbes.

The key LAB species involved include Lactobacillus, Pediococcus, Leuconostoc, Weissella, and Enterococcus. These naturally occur on the surface of plant material at harvest. As oxygen is depleted in the sealed silo, LAB become the dominant microbial group. According to a review in Biotechnology Advances, homofermentative LAB such as Pediococcus and Lactobacillus plantarum are widely used in silage production because of their ability to produce high lactic acid concentrations during fermentation.

Homofermentative vs. heterofermentative LAB

Not all LAB behave the same way. Homofermentative LAB convert glucose almost entirely into lactic acid, making them highly efficient at acidifying the silo. This efficiency results in minimal dry matter loss – University of Wisconsin Extension notes that homofermentation can improve dry matter recovery by 2-3% compared to heterofermentative fermentation, retaining nearly 99% of the energy from the original crop. In contrast, heterofermentative LAB produce a mix of lactic acid, acetic acid, ethanol, and carbon dioxide, resulting in some dry matter loss but also producing acetic acid – a compound that improves aerobic stability by inhibiting yeasts and moulds when the silo is opened.

The target pH for quality grass silage is approximately 3.8-4.2. Achieving this range quickly is critical. A fast pH drop suppresses all competing and pathogenic microorganisms before they can cause damage to the feed.

Undesirable microorganisms in silage

While LAB are the heroes of silage fermentation, a range of harmful microorganisms can take hold when conditions are not optimal. The silage microflora is broadly divided into desirable and undesirable organisms, and the balance between these groups determines whether the silage is preserved or spoiled.

Saccharolytic and proteolytic Clostridia

Clostridia are among the most damaging undesirable organisms in silage. They are obligate anaerobes that thrive when pH remains too high. According to Precision Microbes, saccharolytic Clostridial species use lactic acid as a substrate to generate butyric acid along with carbon dioxide and hydrogen gas. This utilisation of lactic acid slows the pH drop and makes the silage unpalatable and foul-smelling for livestock. Proteolytic Clostridial species, on the other hand, metabolise the crop’s amino acids and produce weaker acids such as acetic and butyric acid, as well as ammonia through deamination, decarboxylation, and oxidation. This results in the destruction of valuable protein in the forage and a delay in pH reduction, which exposes the silage to further microbial damage.

A review in the Journal of Food Microbiology confirms that Clostridium tyrobutyricum is of particular importance because of its ability to use lactic acid as a substrate. Its spores can also contaminate dairy milk from silage-fed cows, causing problems in cheese production.

Listeria monocytogenes

Listeria monocytogenes is another serious concern in silage. It is a psychrophilic gram-positive bacterium found widely in soil and animal faeces. According to Purdue University Extension, Listeria thrives in silage that does not reach an appropriate pH during fermentation – a pH below 4.5 is needed to inhibit its growth. When silage pH remains too high, this pathogen can proliferate and cause listeriosis in livestock. The Journal of Dairy Science reports that symptoms in dairy cows range from mild diarrhoea and reduced feed intake caused by Clostridium spp., all the way to abortion and death from Listeria infection.

Listeriosis typically presents as one of three disease syndromes in ruminants: encephalitis, abortion, or septicaemia. Because Listeria is also a zoonotic pathogen – meaning it can transfer to humans through contaminated milk – the consequences of poorly preserved silage extend beyond the farm.

Other spoilage organisms

Yeasts, moulds, Bacillus spp., and enterobacteria also play negative roles in silage quality. According to a review in the Journal of Food Microbiology, lactate-oxidising yeasts are generally responsible for initiating aerobic spoilage, with moulds, bacilli, listeria, and enterobacteria forming the secondary aerobic spoilage flora. The Northern Ireland Department of Agriculture (DAERA) notes that soil contamination during harvesting is a major route by which these organisms enter the silo. Clostridium spp. break down proteins and produce ammonia, directly reducing feed value for livestock.

Microbial competition and silage quality

The outcome of silage fermentation depends heavily on which microbial group gains the upper hand in the early stages of ensiling. Under optimal conditions – rapid oxygen exclusion, sufficient water-soluble carbohydrates, and correct moisture levels – LAB multiply quickly and produce lactic acid faster than harmful organisms can respond. This competitive dominance is the principle behind successful silage making.

As research published in Applied Sciences (MDPI) notes, the higher the LAB population, the faster pH drops, and the more effectively deleterious microbial growth is controlled. In contrast, when LAB are slow to establish – due to poor compaction, insufficient sugar content in the crop, or air infiltration – enterobacteria and clostridia seize the opportunity and begin their destructive activity. Once proteolytic organisms begin breaking down proteins into ammonia, or saccharolytic spoilers convert lactic acid into butyric acid, the quality of the silage declines rapidly and may be difficult to reverse.

Temperature and dry matter content also influence the microbial competition. Research from PMC on proteolytic microorganisms in TMR silage found that aerobic proteolytic bacteria are progressively replaced by proteolytic LAB as fermentation advances – a natural succession that reinforces the importance of managing fermentation conditions to favour LAB dominance from the start.

Preventing spoilage with additives

Given the stakes involved in silage quality – both for feed nutrition and animal health – farmers and feed producers increasingly rely on microbial inoculants and chemical preservatives to control the fermentation environment.

Microbial inoculants

Microbial inoculants are products that introduce selected LAB strains directly onto the forage at the time of ensiling. Their purpose is to ensure a fast, reliable pH drop by boosting the number of beneficial saccharolytic organisms from the outset. According to the University of Wisconsin Extension, adding homofermentative microbial inoculants helps to drop pH quickly, inhibiting other bacteria and preserving plant proteins, while also inhibiting the clostridial bacteria that produce butyric acid.

Two types of inoculants are used in practice. Homofermentative inoculants – typically containing strains such as Lactobacillus plantarum and Pediococcus acidilactici – maximise lactic acid production and dry matter recovery. Heterofermentative inoculants, such as those based on Lactobacillus buchneri, produce acetic acid in addition to lactic acid. The acetic acid generated by these organisms acts as an antifungal agent, effectively inhibiting yeasts and moulds when the silo is opened and the silage is exposed to air again. Research published in PMC found that silage inoculated with a blend of homo- and heterofermentative LAB showed significantly lower butyric acid, ethanol, and ammonia-N concentrations compared to untreated controls, alongside improved dry matter intake in beef cattle.

According to a review in PMC on current approaches to LAB in crop silage, inoculants are selected for their ability to rapidly lower silage pH through the fermentation of water-soluble carbohydrates to lactic acid, which further inhibits proteolytic activity and preserves nutrients. Some newer “functional inoculants” are also being developed to positively affect animal health, stress tolerance, and digestibility beyond simply improving fermentation quality.

Chemical preservatives

Beyond microbial inoculants, chemical preservatives such as organic acids (formic acid, propionic acid), sodium benzoate, and potassium sorbate are used either as standalone additives or in combination with LAB inoculants. These compounds work by directly acidifying the silage or by inhibiting the growth of yeasts, moulds, and clostridia. When combined with microbial inoculants, they can produce a synergistic effect: the inoculants drive lactic acid fermentation while the chemical additives provide a safety net against aerobic spoilage at feed-out. The MDPI Applied Sciences review confirms that LAB additives, including both inoculants and preservatives, effectively support silage safety and fermentation quality while decreasing pH.

Management practices

No additive, however effective, can compensate for poor silage management. Proper compaction to exclude air, correct moisture levels at harvest, rapid sealing of the silo, and minimising soil contamination are all essential steps. The Northern Ireland DAERA guidance emphasises that organisms like Clostridia and Listeria are associated with poorly compacted or poorly sealed silage, and that additives promoting rapid pH reduction are the most reliable tool for keeping these organisms at bay.

What do you think? Given that both saccharolytic and proteolytic organisms are naturally present in forage crops at harvest, how feasible is it to rely entirely on natural LAB fermentation without inoculants – and at what point does the risk of spoilage outweigh the cost savings? Do you think the development of “functional inoculants” that improve animal health beyond just fermentation quality represents the future of silage production?

References

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Table of Contents

Lactic acid bacteria: the backbone of good silage 🔗

Homofermentative vs. heterofermentative LAB 🔗

Undesirable microorganisms in silage 🔗

Saccharolytic and proteolytic Clostridia 🔗

Listeria monocytogenes 🔗

Other spoilage organisms 🔗

Microbial competition and silage quality 🔗

Preventing spoilage with additives 🔗

Microbial inoculants 🔗

Chemical preservatives 🔗

Management practices 🔗