The temperature of silage and how hot is too hot?

Many years ago I would sometimes use silage when I’d forgotten my socks; it’s a complicated story but a bare foot in a welly on a cold morning was made to feel much more comfortable by a handful of warm silage in the bottom. I was used to seeing the steam rise from the silage as it was torn from the face of the clamp. I used to think this was normal but now I know better.

For clarification, I always knew the silage for socks thing was not normal, it was the steaming silage I thought was normal. Now it’s common knowledge that hot silage is not great silage, but silage does heat up during fermentation so when does this heat become a problem?

When is silage too hot?

Silage is probably too hot when it’s any more than a few degrees above the ambient temperature. It’s far too hot when the temperature risks denaturing the forage proteins but to fully understand this we need to understand where this heat is coming from and why.

The heat ultimately comes from biological activity because almost every biological action is exothermic, in that it produces heat. In a field of fresh forage, the microbial activity produces tiny amounts of heat that collectively adds up to a warming of the forage itself. During the fermentation this might raise the temperature of the forage by 4-6°C above the ambient temperature. That will decline once the fermentation has stabilised but the insulation effect of several metres of silage means this decline is extremely slow for the silage in the middle of the clamp.

The amount the silage heats will depend on the type of biological activity that’s going on. More reactive microbes produce far more heat, and more reactive microbes are the ones that utilise oxygen. As we all know, oxygen is an extremely reactive molecule and so it’s no surprise that aerobic microbes are kicking out loads of heat as they munch your silage. So much heat that you can see temperatures as high as 70°C in the worst cases.

Why is hot silage a bad sign?

High silage temperatures indicate that you are losing forage feed value through this aerobic activity. This is a bad thing - but it gets worse. These high temperatures can also denature the proteins in the silage and make them unavailable when they reach the digestion system of the cow or AD tank. Not only this, but the high temperatures will also slow down - or possibly even cook - the lactic acid producing bacteria that you depend on to actually make the silage.

So what can you do about this? Well it comes down to that oxygen molecule, by eliminating as much oxygen as possible, as quickly as possible, you can reduce the amount of aerobic microbe activity. And remember that these microbe populations are increasing as well as eating your silage so a doubling in the time it takes to remove the oxygen won’t just double the aerobic losses, it might make these losses ten times greater!

This is particularly important for higher dry matter silage that is more difficult to compact and exclude the oxygen. This is just one more reason why choosing a shorter chop length for dryer silage is so important.

When does your silage get hot?

So far I have only really considered the fermentation process, but silage can also get hot once it’s exposed to the oxygen rich air. Those reactive aerobic microbes don’t need a second invitation and as soon as you open up the silage, they get to work converting your silage into ethanol, carbon dioxide and heat. It was that heat that kept my toes toasty but it comes at a massive cost; maybe as much as 40% of your silage feed value.

This heating when exposed to air is where most of silage losses occur. It can be managed by attention to detail, the chop length, compaction and shear grab are all vital to reduce the air permeability of the silage face, but the design of the clamp is the biggest factor. Get the progress of the silage face right, at least 2m per week, and you can control these losses.

What is heat doing to my silage?

The heat is an indication that all is not well, but it’s also a problem for the silage itself. As noted above proteins begin to breakdown and denature at 40°C and this applies to the proteins in the silage and the proteins in the microbes living in the clamp. In a compost heap, this heat is used to “sterilise” the material because the microbes are undesirable, but in silage many of the microbes are vitally important. We need these microbes to produce the acids to preserve the forage so we really don’t want to harm them.

Whilst this 40°C figure is important, it’s not a hard and fast limit. There is lots of variation across strains of bacteria and enzymes as well as within individual strains. This means that many of the microbes may suffer from high temperatures, there is also some data that shows some enzymes in particular that are still operating at temperatures close to 70°C. Unfortunately these are generally enzymes that you might want to minimise but it does demonstrate the range of activity across temperature scales.

A silage probe to measure temperature

How to measure silage temperature

What is really key to remember, is that high temperature is bad for your silage. If you think your silage is heating, you need to get an understanding as to the extent of the problem. This means measuring the silage temperature with a probe - or getting an expert to do it for you. If the heating is contained within the first few centimetres of the face then it’s probably because you are not using the silage fast enough. If the heating goes deeper into the silage, it indicates that it has not been compacted correctly, either because of excessive chop length or poor clamping techniques.

When measuring silage temperature deep within the clamp, you need to compare the silage temperature against the ambient temperature when it was ensiled. It’s important to have the ensiling date so local weather records can be used to check this. Whatever the reason for excessive heat in the silage, it’s probably too late to do anything about it this time. Get through the problem as quickly as possible now and work out what you need to do to avoid the issue next season.

If you want to discuss heating of silage, using silage as socks or would like to discuss any other aspects covered in this series, contact me at jeremy@silageconsultant.co.uk