The Impact of Technical Excellence in Microbiology on the results obtained with Silage Inoculants and Bacterial Biopesticides

How Bacteria work in Silage Inoculants

Using products containing bacteria is different from working with plants, animals or chemicals.

Bacteria are not visible without a microscope and we normally only see the effects of what bacteria do when after they grow. We normally don’t know which bacteria have been growing and why it matters if one bacterial species grows instead of another.

Here is why it matters which bacteria are present in silage.

1. Firstly, bacteria grow really fast .....

Bacteria in the genus Bacillus are used in bacterial biopesticides.
They are also amongst those bacteria which destroy silage if you get air into it. Bacillus doubles in about 0.5 to 1 hour.
[Bacillus is a rod-shaped bacterium.]

The species, Lactobacillus plantarum, is the main species in silage inoculants. L. plantarum will double in about 2 hours.
[L. plantarum is also a rod-shaped bacterium, smaller than Bacillus.]

Lactobacillus plantarum

A 450 gram bag of SI-LAC treats 50 tonne of silage.
There is only a small amount of that 450 grams that is actually the bacteria, since a lot of the weight is the sugar and other nutrients to allow it to grow.
In 24 hours, the L. plantarum cell weight in a 50 tonne silage pit is 4 kg.
In 40 hours, the L. plantarum cell weight is 1 tonne.

Unless they run out of food,
In 48 hours, the L. plantarum cell weight is 16.7 tonnes.
In 72 hours, the L. plantarum cell weight is 68,000 million tonnes.
In another couple of days, they cover the earth a meter deep.
They actually slow down and then stop in 30 - 40 hours.

The key fact is that L. plantarum can reach 1 tonne in 40 hours.
However Bacillus, which can destroy silage, can reach 1 tonne in 10 hours.

2. The fact that bacteria grow so fast matters because of the chemicals bacteria produce ....

L. plantarum is useful because it converts sugar into lactic acid, a palatable food acid which makes the silage acidic, thereby preserving it. The amount of lactic acid is exactly proportional to the amount of bacteria.

No adverse chemicals are producted by L. plantarum and it does not have the ability to destroy the main nutritional value of the silage, just some of its free sugar.

Bacillus, unlike L. plantarum, is capable of digesting the entire crop, generating huge amounts of heat and producing large amounts of unpalatable fatty acids with great rapidity.

As is often said in human health, there are good bacteria and bad bacteria in silage. So the question becomes, how does the silage process work at all?

3. Which bacteria win at different stages of the silage process ... and why

When the pit is first laid down, there is always some air present.
The biomass of aerobic bacteria doubles every 30 minutes until the air is used up or the acidity as measured by pH gets below pH 6.

What happens to the pit depends on just how much air is present and which inoculum species are present.

(a) Huge amounts of air present will lead to the destruction of all the protein, fats and other nutrients in the silage, and heat it so much it may burst into flame.

(b) Moderate amounts of air will lead to the period of pit heating to be extended and the nutritional value of the silage will be reduced. If Bacillus reaches a high enough biomass, then, when the air runs out, they will start producing butyric acid, which can make the silage unpalatable to animals.

(c) Normal, well-made silage pits still have some air. Bacillus and similar bacteria always heat up a pit and some of the nutritional value is always destroyed.

With a good inoculant present, it is possible for L. plantarum to lower the pH so fast that Bacillus is stopped by the acidity even before the air runs out. pH values of 6 and below drastically slow down Bacillus so that L. plantarum can take over the silage pit.

Some poorly made pits can succeed if the inoculant contains Enterococcus faecium. This species resembles L. plantarum in that it does not damage the nutrient value of the silage and rapidly lowers the pH. Unlike L. plantarum, E. faecium can use air to generate acetic acid, which inhibits the Bacillus and similar bacteria that heat the pit.
[E. faecium is a spherical-shaped bacterium, often in chains.]

Enterococcus faecium

L. plantarum continues to be the key component of silage inocula because it is the only species which can overrun the pit and dominate all other bacteria in it.

4. Why is an Inoculant needed at all and what makes an Inoculant a good quality inoculant.

The quality of silage is the result of a speed race between the harmful bacteria and the good bacteria. The winner of that race is determined by several factors.

(a) The quality of the crop affects the amount of sugar for L. plantarum to grow on. Too little sugar and the pH cannot go low enough to inhibit the bad bacteria.

(b) The quality of the pit preparation affects the amount of air available for the bad bacteria. Too much air and no inoculant can save the pit.

(c) The nature of the crop affects whether L. plantarum is already present. For instance, although maize always has some L. plantarum present on the foliage there is none on the grain. With the silaging of grains, there is no choice about adding an inoculant.

(d) When we have a good quality crop of maize which already has some of its own L. plantarum present and a well prepared silage pit, what does using an inoculant do?

  • Silage quality will come down to the speed race between good and bad bacteria and the total biomass of L. plantarum will depend on exactly how much is present to start with and how healthy it actually is. Variations in numbers which cause a delay of just a few extra hours before L. plantarum dominates can affect silage quality.
  • A similar situation occurs with 10 bottles of milk on the window-sill; not all of them will make good cheese.
  • Using an inoculant makes sure that the pit starts with an adequate number of L. plantarum cells.

(e) As long as an inoculant adds enough bacteria, is there any real difference between one inoculant and another?

The bacterial speed race which governs silage quality is affected by how fast the bacteria actually grow and to grow really fast, they need to be both in sufficient numbers and healthy.

Most reputable inoculants have sufficient numbers. The focus of our technological innovation has been on the benefits of the bacteria being healthy.

The speed at which bacteria grow is directly related to the number of little molecular machines called "ribosomes" in each of them, like a whole lot of little motors.

When bacteria have finished growing rapidly, they enter a condition called the Stationary Phase, during which they break most of these down to their nuts and bolts. This happens when the bacteria are processed to make an inoculant.

Before they can grow rapidly again, the bacteria need to be in good growing conditions so that they can re-assemble all these little motors. This period is called the Lag Phase during which no actual growth occurs. This can be several hours or longer if conditions are poor.

The formulation of SI-LAC products includes the nutrients which enable the inoculant bacteria to prepare themselves for really rapid growth. These conditions are better than the conditions in the silage so the pre-incubated SI-LAC is in the best possible condition to give the fastest possible benefit to the silage process.

5. How does Lactobacillus buchneri fit in to this picture ?

E. faecium helps a silage pit with too much air by its production of acetic acid and initial reduction of pH to under pH 6.

L. plantarum drives out all other bacteria by its rapid production of large amounts of lactic acid and pH reduction to under pH 4.5.

What does L. buchneri do?

Lactobacillus buchneri

L. buchneri grows with a doubling time of around 5 hours, which is a lot slower than L. plantarum or E. faecium so L. buchneri does not contribute to the initial making of silage.

However L. buchneri can do two things that the other bacteria cannot:
(1) it can grow at pH 3.5 when all the other bacteria in the pit have stopped
(2) it can convert some of the lactic acid in the pit to acetic acid.
This process starts as soon as the pH reaches a minimum, which in maize would be within 20 - 30 hours or so.

This is useful when the pit is opened. A low pH and lots of lactic acid will preserve an untouched silage pit for a very long time. However in the presence of air, some yeasts can grow rapidly on the lactic acid despite the low pH, and cause heating on feedout.

These yeasts are not always present in every silage pit, but if they are present, they can be inhibited by the acetic acid produced by L. buchneri.

This works far more efficiently that just trying to spray the whole of the silage with acetic acid. The reason is that these yeasts are trying to grow in just those spots within the silage where there is the most lactic acid. These are the same spots when L. buchneri grows so those spots are where the most acetic acid accumulates.

6. Conclusions

There are a lot of types of bacteria out there.

  • Some of them are helpful.
  • Some of them are very destructive.
  • Many of them are very fast.

Top quality silage is a speed race between different types of bacteria.

The bad guys are
  1. the air-using bacteria like Bacillus which attack the silage at the start of the silage process
  2. the yeasts which can cause heating on feedout

An inoculant is needed to be certain that the good types of bacteria are there in enough numbers.

E. faecium growth is the first step in the silage process. Natural bacteria of a similar type occur in most silage and are needed in the inoculant if the pit has more than normal amounts of air.

L. plantarum takes over from E. faecium and finishes off the initial silage process, after which it runs cool. Again, L. plantarum is present in most silage but is essential to all inoculants to ensure reliable silage quality.

L. buchneri takes over from L. plantarum as soon as the silage reaches it's lowest pH and converts some of the lactic acid to acetic acid, which inhibits the yeasts that may be there to cause heating on feedout.

Dr John L. Reichelt
Director and Chief Microbiologist
Bacterial Fermentation Pty Ltd
(subsidiary of Genesearch Pty Ltd)