So what is aerobic stability of silage and why is it so important?

It’s not a sexy title but this is perhaps the most important topic when it comes to making good clamp silage. Aerobic stability is the one single area where most farmers can reduce the losses and improve the physical and financial performance of their forage based enterprise. And just to grab your attention – if you get this bit wrong, you can lose 40% of the feed value of your silage!

Aerobic stability of silage is all about how the silage reacts once you open the clamp and begin to “feed-out” the forage. I think it’s fair to say everyone understands how important it is to seal silage against the air – or the oxygen within the air – whilst it is fermenting and being stored. But there comes a time when the seal has to be broken and the silage is opened up to the world. This is an issue for all but individually wrapped bale silage and how the silage reacts to the air is termed its aerobic stability.

What about the low pH – doesn’t this protect the silage?

The low pH that was created during fermentation has done a great job of halting the activity of all the yeasts, molds and bacteria that have been living on the forage. The low pH and absence of oxygen halted their activity, but it didn’t kill them off completely. So once the clamp is opened, then these micro-organisms emerge from lockdown and come back to life. This leads to the sugars and proteins so carefully stored in the silage, being gobbled up by these micro-organisms. This is where the invisible losses occur, as you can’t see the losses happening, the silage doesn’t change appearance but the performance of the livestock or the AD gas yield just drops away.

Micro toxins in silage

The activities of these microbes can produce significant levels of toxins in the silage. So not only are the feed nutrients being stolen from you, but these guys are also making the silage, at best unpalatable and, at worst something that can finish off your livestock for good!

What are the signs of aerobic instability?

The activity of these microbes will create heat – just like any other life form. So if your silage is heating up when it’s opened up, then you have some aerobic instability. The silage will generally be warmer than ambient temperature in the winter as it was clamped on a nice warm day (hopefully) and has been insulated by metres of silage on top of it. Excessive aerobic instability will generate temperatures more like a compost heap than a good silage clamp. So if your silage is hot, then it’s likely not as good as it could be.

How do I control aerobic stability?

Control is the key word here, as it’s impossible to completely eliminate aerobic instability but there lots of things you can do to control it. Let’s start at the fundamentals as this is the optimum solution. Limiting exposure to oxygen is the best tool at your disposal. Working through the silage quickly is the best solution as you keep the face fresh. The golden rule is to use at least a foot of silage per day – or 300mm per day or 2m per week. This comes down to clamp design and as we have looked at elsewhere, getting this correct is the best investment you can make in the silage making process. Unfortunately the 2m rule relates to a proper British winter, once the temperature rises, the activity of the microbes will increase and so silage will spoil more quickly in the summer. If you are feeding 365 days into an AD plant or for constantly housed cows, then the 2m rule is fine for the winter but you need to be at 2.5m or 3.0m per week for the warmer months.

The next thing is getting the face as impervious as possible so use a shear grab to give you a “closed” texture to the silage. Only uncover about 1m of silage at a time to keep the seal as close to the leading edge as possible. And remember, aerobic instability continues right up to the point where the silage is eaten by the animals or the AD tank. It’s therefore important to keep fresh forage in front of your customers, little and often and clean up what has not been eaten as its an ideal breeding ground for unwanted microbes.

Making silage to increase aerobic stability.

There are things you can do whilst making silage that can increase the aerobic stability of the silage. Shorter chop lengths and good compaction will create a silage face that’s much tighter and less permeable to air ingress. For silage that you know will be fed out during the higher risk months, then chop shorter, fill in 150mm layers and get some train wheels on the clamp to compact it really well.

Can additives and preservatives help aerobic stability in silage?

When faced with a problem, modern agriculture has become used to help in a bottle, so can additives solve the problem of aerobic instability? Well as stated above, nothing can solve the problem but you can use additives to help control the problem. There are a couple of schools of thought on this that have significantly different approaches.

Acetic acid - Some trials have shown silage with higher levels of acetic acid had shown greater aerobic stability, but there are also many trials proving no significant improvement so I remain sceptical on this point. What is not in doubt is the fact that silages with higher acetic acid levels smell “off” and reduces palatability to livestock.

Potassium Sorbate - This additive (sometimes known as salts) reacts in silage in a similar way to acetic acid but doesn’t have the negative effects on forage intakes. Many additive producers now offer products containing potassium sorbate for at risk silage. The test data I have seen would all seem to prove that these products can increase aerobic stability. There is one downside however which is that potassium sorbate can detrimentally impact the lactic acid producing bacterial population so some tests show an aerobically stable, but not great silage.

Lactobacillus Plantarum - The most commonly employed type of bacteria, these lactic acid producing bacteria are found in most inoculants.  But the pH and the level of lactic acid has no effect on the aerobic stability of the silage so how can this help? The idea is a slightly different approach to the acetic acid and potassium sorbate additives. When the forage is first harvested, it’s covered with a vast array of microbes that all begin to feast on the fermenting silage as their populations explode. As the pH drops, their activity gets curtailed until only lactic acid producing bacteria can operate. The theory is that by using an inoculate with really fast acting lactobacillus, the pH drops so fast that the yeast and mold populations don’t become established. When the clamp is opened up there are so few “bad guys” that the silage remains stable and protected just as if it were treated with potassium sorbate.

There is no real conclusive proof one way or another so I guess you read the information and make your choice. As far as the practical issues are concerned, then there is no debate, to control aerobic stability in silage:

-       Make sure the clamp isn’t too wide

-       Keep chop length short

-       Lay silage in 150mm layers and consolidate really well

-       Work through the silage at least 2m per week in winter and 2.5m in summer

-       Use a shear grab and keep the face tight and clean

-       Don’t uncover more than you need

-       Keep the trough clean and feed little and often

-       Select an additive with care

If you want to discuss how to improve the aerobic stability of your silage or want to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash at Jeremy@silageconsultant.co.uk