Using Acetic Acid or Potassium Sorbate to aid stability in silage

When you’re feeling really low, you might be tempted to think the answer to your problems is at the bottom of a bottle, or a packet, or a plastic bucket – or whatever they have packed the silage additive into.   That is if your problem is aerobic stability. And that problem is something everyone who uses ensiled forage as a feedstock is faced with. It’s also a problem that is costing your business thousands of Pounds, Euros or Dollars a year – so you do need to do something about it.

If you have read a few of these blogs, you will be aware of the concept of aerobic stability in silage. It’s the spoiling of silage exposed to oxygen, at the exposed clamp face, in the diet feeder and in the trough. Due to the time silage is in each location, the clamp face is the critical one here. Once oxygen is re-introduced, microbes (that were previously dormant) spring back into action consuming your silage, growing and reproducing. To make matters worse, not only are your nutrients being “stolen” but the culprits are also producing toxins that add insult to injury.

What can I do to control stability?

There are some simple things you can do to reduce the risks of aerobic instability in the silage. Getting the dry matter, chop length and consolidation right will reduce the porosity of the silage clamp face. This means oxygen can get at less of the silage. Using a neat finish grab or shear also helps to reduce the exposed surface area – as does choosing the correct size clamp to suit your feed out rates.

Now that lot doesn’t look too simple, and you might well be advised to choose a silage additive that can aid stability if the silage is at high risk. So the crops that are at high risk tend to be the ones at higher dry matter – maybe late cut wholecrop - that might be fed out at a rate less than 2m per week.

Trioliet self loading diet feeder

What are the preservatives in silage additives

It’s a common belief that if you can get a really low pH in the fermentation, then this will “kill off” all the bad stuff and you will have good stable silage. Unfortunately the activity of aerobic microbes seems to be unaffected by the pH of the stored silage. Sure they are held in a state of inactivity locked up in low pH anaerobic conditions, but as soon as the air gets in, they get up and running in no time.

Manufactures of silage additives have the choice of two different substances to fight aerobic moulds and yeasts, acetic acid and potassium sorbate – as these two act in a very similar manner within a silage clamp. It’s a widely accepted view that high levels of acetic acid (or potassium sorbate) will reduce the activity of aerobic moulds and yeasts in the clamp. High levels of butyric acid can also produce similar results but nobody wants butyric fermentation so acetic acids are the only real option. There are lots of scientific papers that refer to the effects of acetic acid and many papers have trial data that proves this.

You will never get unanimous agreement within the science community and some of the data is inconclusive. So a Swiss trial was set up to prove that acetic acid improved aerobic stability in silage. Several farms were selected and the trial was run for 7 years to try and eliminate as many seasonal variations as possible. And the results conclusively prove – that they couldn’t prove one way or another….!

The difference between acetic acid and potassium sorbate

Assuming that both of these products are effective in aiding silage preservation, the key differences come down to the limitations of each product. Acetic acid will usually occur naturally within the fermentation or an inoculant may be applied to the crop that can include a starter culture bacteria to promote this type of fermentation. Lactic acid will (should) always be the predominant, and it will be the acid that dominates since the bacteria that produce it can tolerate a lower pH than the acetic acid producing bacteria. The main down side of a higher level of acetic acid in a silage is the effect it has on forage intakes.

Higher levels of acetic acid give the silage a smell that is not particularly pleasant to us or the livestock – obviously this is not an issue for an AD plant as the digester will get what it is given. But the cow is different, she is a selective feeder and she will eat less of what she doesn’t like. 

This is a key benefit of potassium sorbate over acetic acid bacterial inoculant; it does not seem to reduce forage intakes. Actually applying potassium sorbate is another issue however. On its own it is not a problem, but if you are using a lactic acid bacterial inoculant such as lactobacillus planetarium then it’s a little more tricky.  The problem is that potassium sorbate in solution will kill the bacteria in the tank before it ever reaches the crop. 

How to apply potassium sorbate to silage

To make the most of potassium sorbate, you need to either have two applicators on the harvester or apply the sorbate at the clamp and the inoculate on the chopper. I am currently not aware of a factory fit forage harvester that has the option of two additive applicators that run at the same time. This means you might have to set up some sort of dosing system at the pit.

If all this is looking a bit complex, then I should remind you that doing the basics well is the first tool to fight against aerobic instability. I would also add that there are significant improvements that lactic acid bacteria can make. Now a bit further up the page it’s stated that the pH has no effect and that still remains true.

We need to go back to the silage first hitting the clamp. As soon as it is ensiled all the microbes tuck into the all you can eat buffet. As time goes on lactic acid builds up, pH drops, CO² increases and one by one the other microbes activity halts. Once the clamp is opened the oxygen comes back and they get going again. Now we need to consider two different clamps, one that hits low pH very quickly and another that takes for weeks. The one that had a very rapid pH drop gave the moulds and yeasts little time to grow and populate. In the slow clamp there is a large population of dormant microbes just waiting to get going again. So using a rapid action lactic acid inoculant can also help with aerobic stability.

If you want to discuss the options for improving aerobic stability of silage and what tools you can use to try and control it or just want to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash at Jeremy@silageconsultant.co.uk

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