Silage pH - when is it low enough?
The principal of making silage is well understood, pile it up, cover it up and let the bugs make some acid to get the pH down – that’s it isn’t it? And yes that’s about it, but what pH are we really looking for and what pH is “low enough”?
Anything with a really low pH is nasty to handle, sulphuric acid in the school lab was always treated with some reverence (do they still allow kids to play with this?) yet we happily handle thousands of tonnes of low pH silage every year without much of a thought. To put silage into some sort of context a “good” silage fermentation might produce an analysis at pH 3.8-4.2. Depending on concentration, hydrochloric acid is about pH 3.0 and sulphuric acid is about pH 2.75 but before you start to panic about the pH of your silage, the vinegar you might put on your chips is about pH 2.5 and fresh lemon juice can be as low as pH 2.1.
What is pH?
We live in a world where everything has an effective pH but we only really consider the measure in terms of soil and silage. Whilst trying not to make this an A level chemistry lecture - pH is really just a measure of the “reactivity” of a substance; basically how desperate a substance is to gain or get rid of electrons. Acidic materials are greedy and want extra electrons, and alkaline (or base material) are generous and want to give them away. Neutral products are really happy as they are but are always vulnerable to being attacked by the other two. If you mix up really greedy and really generous things together, you get one hell of a party, just like students in a free bar or baking soda and vinegar – although the end results are a bit different!
So why does low pH preserve silage?
It comes down to that reactivity; the aggressive nature of the acid keeps the activity of all the bad guy microbes in the clamp at bay. If you get the pH down low enough and exclude any oxygen, all the living things you collected along with the silage are sent into lockdown. They can’t feed, grow or reproduce and as such, they can’t be eating up your silage doing those things.
When is the pH low enough to make silage?
The generally accepted rule is around pH 4.0, get the levels down to this or below and the silage should remain stable. However this very much depends on the crop species, stage of growth and so there is no one specific pH figure for a stable silage. The absolute level of pH is only one thing, it also depends how fast you can get there. Lactic acid is the key to this as we have already discussed and lactic acid is pH 3.5 but that’s unlikely to be the pH of your silage. This is due in part to the buffering of the rest of the forage but also due to the actions of other bacteria in the clamp all producing other acids. Buffering is really a measure of just how much acid the crop needs to be exposed to before the pH drops. Acetic acid is also readily produced during fermentation and this has a pH of around 3.9. You only need a small volume of acetic acid to “dilute” the lactic acid and you also get a fermentation that can produce a very unappetising smell that reduces forage intake in cattle.
In a “poor” or butyric acid fermentation, you can get volumes of butyric acid with a pH of 4.8 or so and this will never produce a stable silage. So lactic acid is the solution to producing good quality stable silage but unless you can provide the ideal conditions for the lactobacillus bacteria that produce it, the silage will struggle to get the required pH.
Does it matter how long it takes to get to pH 4.0?
To be blunt, yet it does, it really does. That’s because the longer it takes, the more time all the other microbes are burning up your valuable sugars, the sugars you took time to grow and harvest. It also gives the moulds time to produce loads of unhelpful mycotoxins that have all sorts of negative effects on the livestock feeding on the silage. That together with the dilution of the overall acid blend means it’s vital to get the lactic acid production in dominance as soon as possible.
Can silage be too acidic?
Yes it can and it can become unpalatable if the pH is too low – bit like raw lemons doused in malt vinegar – just too much. Silage with really low pH can also eat through whatever you’re trying to store it in too, walls and floors are all vulnerable to acid attack. A silage that’s too acidic isn’t just due to really low pH in the clamp. The real issue with very low pH silage is the difficulty in maintaining a stable pH in the cow’s rumen (at about pH 6.5). The silage pH will contribute to this acid level, but the fermentation that takes place inside the animals the rumen is also creating acids that need to neutralize. This is more likely in high D-Value silage or highly digestible silage and can be offset with other feedstuff in the diet. If the conditions become really acidic – less than pH 5.5 it can cause acidosis in cattle and the overall diet needs to be formulated with this in mind. Analysis of the silage really helps here, as the Potential Acid Load, or PAL, of the silage will be quoted. This is, as said above, not just down to the pH level but the PAL value will also incorporate the rumen fermentation potential of the silage. And whilst it’s a life and death issue for livestock farmers, it’s also really important to AD plant managers, as a stable pH in the tank is just as vital to success of the operation.
If you want to discuss the importance of silage pH 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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