What Really Happens Inside the Silage Pit?

Understanding the Science of Silage Fermentation in Ireland

Silage season in Ireland is one of the most critical times on every livestock farm every year. Quality silage plays a huge role in driving animal performance through the winter, reducing the need for expensive concentrates, and improving overall farm efficiency. But what actually happens inside that pit or wrapped bale once the grass is harvested and stored?

In this first edition of Precision Microbes Silage Sessions, Ryan Duffy dives into the science of silage fermentation — and why understanding what’s going on behind the scenes is essential for making top-class silage.

Ensiling Isn’t Just Storage – It’s a Biological Process

When you fill a silage pit or wrap a bale, you’re not just storing grass. You’re kicking off a complex biological process called microbial fermentation. This process does two key things:

1. Preserves your harvested forage for safe long-term feeding.
2. Retains the nutritional value of the original crop by protecting key nutrients such as protein.

The aim is to manipulate this fermentation process to lock in feed quality and avoid spoilage.

The First Fundamental: Excluding Oxygen (O₂)

The first and most important step in forage conservation post-harvest is producing anaerobic conditions (i.e., create an environment devoid of oxygen). This is why good compaction, effective sealing and continued management of clamps and effective wrapping of bales is critical when filling your pit or making round bales.

By excluding oxygen, you create what’s called an anaerobic environment — and that’s exactly what your beneficial microbes need to start fermenting the grass correctly. This is key in the conservation of home-grown forages. Once oxygen has been eliminated it is now time for the next phase of ensiling: microbial fermentation.

Meet the Good Guys: Lactic Acid Bacteria

Once oxygen is excluded, we want a group of beneficial microbes called lactic acid bacteria (LAB) to become active. In most circumstances these bacteria are naturally present on the grass when we are harvesting — and their job is to:

• Consume plant sugars (water-soluble carbohydrates such as glucose and fructose).
• Produce lactic acid in return (necessary for nutrient preservation).

This acidification of the crop driven by lactic acid is key. As lactic acid is produced, the pH level drops rapidly, which stops the activity of unfavourable microbes which degrade silage. In high-quality grass silage, we aim for a pH of around 4.0 (ideal range 3.8-4.2).

There are two main varieties of LAB: homofermentative LAB and heterofermentative LAB.

As you can see from the above table homofermentative LAB are much more efficient at producing lactic acid from hexose sugars (such as glucose and fructose): they are twice as efficient at utilising glucose to produce lactic acid and six times more efficient at utilising fructose in producing lactic acid compared to their heterofermentative counterparts. This means they are best when it comes to causing a rapid drop in pH (to the desirable range of 3.8-4.2) and saving as much of the nutrients within your grass from the negative effects of undesirable microbes.

On top of this, you will notice that homofermentative LAB only produce lactic acid, meaning that some 99% of the energy from the original crop is retained within your clamp and bales (i.e., significant retention of dry matter). In contrast heterofermentative LAB do cause a loss in dry matter in the form of further end products they create, such as CO₂.

If we achieve a low pH we see:

• Deactivation of bad bacteria which compromise silage fermentation like clostridial species (found as spores in soil) and enterobacterial species (which originate from slurry) while also inactivating pathogenic organisms such as listeria species.
• Preservation of the nutrients found within the harvested crop.
• A safe and palatable feedstuff for livestock.

Precision Microbes Silage Additive is a liquid, ready-to-use (RTU) formulation containing live bacterial agents, specifically lactic acid forming bacteria (LAB) and homofermentative bacterial populations. Designed for efficient crop preservation, it promotes a rapid pH drop in both clamps and bales, ensuring high-quality silage. The product is organic certified and can be applied using either ultra-low dosing or standard dosing methods, offering flexibility and ease of use for farmers aiming to enhance silage fermentation and nutrient retention.

What Happens If Fermentation Goes Wrong?

The microbial element of grass conservation can go wrong in two ways: undesirable bacterial fermentation during storage (i.e., strict and facultative anaerobic contaminants such as clostridial and enterobacterial species) or due to unwanted aerobic spoilage when oxygen has not been completely eliminated during compaction, is permitted entry to clamps and bales during storage, or when clamps and bales are opened for feeding (i.e., yeasts and moulds).

Undesirable bacterial species, such as Clostridial species and Enterobacterial species, can be incorporated into your crop during harvesting, especially when there are wet underfoot conditions but there are further risks which include:

• Leaving A Very Low Residual (i.e., mowing close to the ground)
• Poorly Positioned Tethers
• Poorly Positioned Rakes
• Poorly Positioned Blades
• Blunt Knives
• Soft/Flat Tyres
• Applying Slurry With High Residual Grass (i.e., no exposure to UV light)

Concerningly, you can see that Saccharolytic Clostridial species use lactic acid as a substrate to produce butyric acid and the gases of CO₂ and H₂. This utilisation of lactic acid contributes to a slower drop in pH. Furthermore, we should all aim to have minimal butyric acid in our clamps and bales as this is foul-smelling and makes silage unpalatable for stock.

The Proteolytic Clostridial species utilise the crop’s amino acids, the building blocks of proteins, during their metabolism, and produce a variety of ‘weaker’ acids such as acetic acid, butyric acid, and pyruvic acid, which delays a drop in pH. This means the valuable nutrients within our crops are exposed to the activity of undesirable bacteria for longer. They are also responsible for the production of ammonia (NH₃) from this degradation of valuable amino acids through the actions of deamination, decarboxylation, and oxidation. This is one of the most unwanted reactions to see on any silage analysis report. The goal is to have an ammonia-nitrogen percentage (NH₃-N %) of less than 5% of your crude protein levels – at the most <10%. Adding to this list of unfavourable outcomes, the breakdown of amino acids also produces biogenic amines (such as cadaverine and putrescine) which have been shown to have direct detrimental effects on gut health.

In contrast, Enterobacterial species (such as coliforms like Escherichia Coli) utilise the same fuel as the beneficial LAB – simple sugars such as glucose. This competition of the same energy source means that these microbes can slow the drop in pH by reducing the amount of lactic acid produced. On top of this they produce the ‘weaker’ acetic acid in its place. They are also responsible for haemorrhaging dry matter in the form of CO₂ and H₂.

 If oxygen isn’t removed properly, the desired fermentation process does not proceed and unwanted yeasts and filamentous moulds can take over. This is a process known as aerobic spoilage, and the valuable nutrients which we have spent months cultivating are now subject to degradation. These microbes consume simple sugars (such as glucose and fructose) and lactic acid, and therefore they directly compete for food with favourable microbes (i.e., the LAB) and they breakdown the good work carried out by these favourable microbes (i.e., by degrading lactic acid they raise the pH). This leads to:

• Aerobic fermentation, where moulds and yeasts thrive.
• Nutrient losses, particularly sugars and proteins.
• Poor feed quality, with reduced animal performance.
• Spoilage, heating, and visibly mouldy silage when pits and bales are opened.

Aerobic spoilage is an avoidable phenomenon. However, 10% of silage can be compromised during storage and 15% can be compromised during unloading if the management of clamps and bales is not adequate. The exclusion of oxygen from silage is non-negotiable.

4 Keys to Good Silage Fermentation

Here’s a quick recap of what needs to happen to make great silage:

1. Exclude Oxygen

Quickly packing and compacting your pit well and sealing it promptly with a good cover, weights, and oxygen barrier film or wrapping your bales tightly using several layers of quality wrapping plastics is essential. Remember: no air, no spoilage.

2. Encourage Lactic Acid Bacteria

These are your microbial workers who you rely on once clamps are sealed and bales are wrapped. Giving them the right conditions (anaerobic and sugar-rich) means they’ll dominate the fermentation and outcompete harmful microbes looking to degrade your silage. Microbial silage additives allow us to manipulate this outcome.

3. Lower the pH Rapidly

The faster the pH drops, the better. It’s this acidity that preserves the forage and protects it from the detrimental effects of harmful organisms.

4. Maintain Anaerobic Conditions

Keep pits sealed and bales unpunctured. Oxygen exposure at any stage after fermentation can restart spoilage activity by yeasts and moulds.

Should You Use a Silage Additive?

While lactic acid bacteria occur naturally on grass, their numbers — and how active they are — can vary. That’s why many Irish farmers are now using biological silage additives.

A good silage additive:

• Increases the number of homofermentative lactic acid bacteria.
• Swiftens and promotes an appropriate microbial fermentation process.
• Helps achieve lower pH faster.
• Improves preservation and maintains feed quality.

Products like those developed by Precision Microbes are designed to support and optimise this fermentation process, helping you get more from every bale or pit.

Silage and Animal Health Go Hand-in-Hand

Well-preserved silage isn’t just about energy and protein. It also has a big impact on:

• Rumen function and microbial balance in cattle and sheep.
• Silage intake, which affects overall productivity.
• Health issues, with poor silage linked to conditions like acidosis, lameness, and poor fertility.

Getting the fermentation right means feeding your animals safer, more palatable, and more consistent forage.

Silage and Animal Health Go Hand-in-Hand

Well-preserved silage isn’t just about energy and protein. It also has a big impact on:

• Rumen function and microbial balance in cattle and sheep.
• Silage intake, which affects overall productivity.
• Health issues, with poor silage linked to conditions like acidosis, lameness, and poor fertility.

Getting the fermentation right means feeding your animals safer, more palatable, and more consistent forage.