Silage Fermentation


2. The Ensiling Process

At first glance the principles of making silage appear simple. Grass is ensiled (clamp or bale) and a microbial fermentation converts sugars into lactic and acetic acid and the silage gets pickled and stabilised.

Silage making is a lot more complicated. The outcome is subject to a microbial ‘’foot race’’ between the desirable (good) microbes and the undesirable (bad) microbes which makes consistency in silage quality difficult. The ‘undesirables’ are trying to compete for food with the ‘desirables’ . They are responsible for:

  • degrading nutrients and good fermentation products.
  • degrading protein and increasing fermentation dry matter losses.
  • reducing silage quality and intakes.
  • producing heat and possibly toxins.


Stages of Fermentation


A. Aerobic Phase

This lasts a few hours (or longer if your compaction isn’t good) until the oxygen is reduced. At this stage plant cells respire (use oxygen), aerobic microorganisms and plant enzymes use oxygen and water soluble carbohydrates (sugars) which are converted to water, carbon dioxide and heat. Undesirable microbes also undertake proteolysis which is the breakdown of protein to amino acids and ammonia. Your fresh grass analysis may have 22-24% crude protein but your silage analysis is 14% CP. This is the stage in combination with the ‘fermentation’ phase where your crude protein is lost. Unfortunately this is a phase where nutrients are lost .Potential loss of 2-12% Dry Matter.


B. Fermentation Phase

The types of bacteria that establish in the anaerobic phase are crucial to ensuring the silage is nutritious, safe and palatable. Relying on chance that there is sufficient lactic acid producing bacteria present in the forage is risky. The numbers of these bacteria are highly variable and the efficiency of the bacteria differs hugely between strains. This is why a scientifically proven silage inoculant is such an important investment.

Early phase – Anaerobic (non oxygen using) micro-organisms grow and compete for food. The bad microbes - Clostridia, Enterobacteriaceae, Yeasts and Moulds compete with the desirable microbes - homofermentative lactic acid bacteria and heterofermentative lactic and acetic acid bacteria. Again there is a protein and energy loss.

Lactic phase – Lactic acid bacteria (LAB) produce lactic acid which inhibits spoilage bacteria. This is not a fast process and the silage pH ( an indication of lactic acid levels) may reach 5.5 in 3 days and possibly 3.8 -4.2 after 3 weeks. Potential loss of 5-20% Dry Matter.

The fermentation rate depends on:

  • Types of crop ensiled - high protein forages are difficult to ensile as they resist a pH drop.
  • Dry matter – LAB need moisture and their action declines with higher D M’s.
  • Amount of air and compaction in a clamp. LAB work at their best in anaerobic conditions.
  • Amount of sugar in the forage which fuels the LAB fermentation
  • The variable number and strains of LAB present in the grass or introduced by means of an additive

Stable Phase - Lactic acid inhibits the growth of spoilage bacteria but they are still present in the forage.


C. Feed out Phase

Oxygen has unrestricted access to the face which allows aerobic spoilage bacteria to use nutrients such as sugar and lactic acid as a food source. This can cause heating and possibly raise pH which causes more instability.

Often a slightly warm clamp face is wrongly maligned as aerobic instability. Clamps that are created in warm ambient summer conditions often hold heat. In addition the natural desired fermentation can raise the clamp temperature by up to 10 degrees. This type of warm silage will cool down when removed from the clamp. An aerobically unstable silage (in which warmth is mostly caused by yeast fermenting nutrients) will heat even more on removal from the clamp with greater exposure to oxygen.

Losses at the feed out phase are largely due to silage management practises such as compaction density, bad face management and contamination. Potential loss of 1-10% Dry Matter.


A few of the bad bugs in silage

Enterobacteria- Grow with or without air and flourish in high pH but die below pH 5.5. They ferment sugar to alcohol and other products (such as endotoxins) . They degrade protein and increase ammonia which slows rate of pH drop.

Clostridia- Grow without air and are present in soil and manure. They use sugar, lactic acid and protein to produce butyric acid which makes the silage smell sour and reduces palatability. They flourish in low DM silages.

Yeast- Grow with or without air. Use sugar and lactic acid as a feed source. They produce heat, alcohol and carbon dioxide. They raise pH and are present in early fermentation and on clamp opening. Aerobic spoilage is primarily caused by yeast.

Bacillus- Grow with or without air. They ferment a wide range of carbohydrates and enhance later stages of aerobic deterioration and can cause bitter milk.

Moulds- Use sugars and produce mycelia which results in slimy silage. They cause heating and can produce mycotoxins .


The good bugs in silage

Homofermentative- These are bacteria that produce lactic acid which acidifies silage and prevents breakdown of nutrients by spoilage bacteria. They need moisture to work and their action diminishes in higher dry matter silages.

Heterofermentative- These are bacteria that produce a mixture of chemicals including acetic,lactic and propionic acids. They inhibit yeast and moulds that cause aerobic instability.


3. When to use an additive?

There are many farmers that do not understand the benefits of silage additives and see the purchase of an additive as an unnecessary expense. They leave their silage preservation to chance by not controlling the fermentation. With varying climatic conditions the natural good and bad bacteria levels that ferment their silage will vary from year to year. You will never make the same silage year on year! Others see an additive as insurance in bad conditions. It might help but won’t turn a bad silage good. Time and time I have had the words ‘All I need is sunshine’ quoted at me with a smile. The truth is yes sunshine is good..... perhaps by raising sugar levels to ease the fermentation. Your best return on your investment in purchasing an additive is to use it in ideal conditions......simply because there are more nutrients to be lost to spoilage bacteria and it will help make a good silage into an excellent silage. By using an additive you can:

    • Reduce clamp dry matter losses by 10-15%
    • Reduce protein breakdown by 15-20%
    • Increase daily live weight gain & milk yield by 3-5%


4. Which additive?

There are many additives on the market designed for different functions. Some will restrict fermentation (acids and food preservatives) and others stimulate fermentation (biological). The majority of additives on the market are biological additives which are designed to direct , control and dominate the fermentation. These contain strains of the good fermentation bacteria and produce a faster and better fermentation to preserve more nutrients, improve palatability and consistency in a forage clamp. They are living organisms which increase in number after application ,are safe to handle, flexible in application and generally cheaper than chemicals.

Some additives are solely homofermentative with the aim of rapidly driving down pH by a lactic fermentation. Others are heterofermentative and produce primarily acetic acid for anti-fungal properties which is important if aerobic stability is required on opening. Many of the market leading additives will have a combination of homo- and heterofermentative bacteria to reduce pH and provide aerobic stability.

Choosing the best additive is a difficult choice. There is a big difference betwen individual strains of bacteria. Although they might have the same name such as Lactobacillus plantarum , different strains have different modes of action. Some maybe slow multipliers ...others fast. An analogy would be describing the strain as a car. Is it a Porsche or a Morris minor? Also beware of the salesman selling on numbers of bacteria. The older additives often claim 1-2million bacteria (colony forming units-cfu ) inoculation per gram of silage in an attempt in making you think you’re getting more for your money. I have heard somebody describe this type of bacteria a bit like Granny running the first part of a 4 x100 metre race. Science has stated that 100,000 cfu/g is enough to dominate a fermentation. More modern strains are chosen for being fast replicators and will outcompete some of the older strains in a matter of hours. Choosing an additive is not about numbers of bacteria!

In general you should ask your additive salesman for technical production trials on the additive and also for other farmer referrals before choosing. Ask for what crop type and what dry matter range that the additive will work on– beware many straight lactic acid bacteria will struggle to work in dry silages and not protect you from heating. If you’re organic please check the additive has a non-genetically modified statement which is needed for annual inspections.