Silage parameters (Part 4): The role of pH
As Published in Silage talk of Stockfarm Magazine December 2020
The amount of silage-related problems that I encounter daily, compelled me to write a short series on silage parameters that require adherence. This is the last article in the series and deals with decreased levels of acidity (pH) triggered by fermentation.
Aspects of preservation
The effective preservation of silage for longterm storage is not determined by only one step, but by the successful amalgamation of all the key steps. This includes working within suitable moisture limits, creating an anaerobic (oxygen-free) environment as well as achieving a low-enough pH, noting that the ideal pH of each crop differs.
Mechanics behind a drop in pH
Anaerobic, homofermentative lactic acid organisms produce lactic acid from simple sugars, causing the pH to drop during primary fermentation. These organisms can either occur naturally or can be administered by adding bacterial silage inoculants. If no inoculant is added, it is up to naturally occurring organisms to drive fermentation. This poses several disadvantages, including producers not knowing how many suitable lactic acid organisms are present, how active they are and how many are present compared to spoilage bacteria. Spoilage organisms often compete for the same sugars but do not convert these sugars to lactic acid. Among others, these organisms form too much acetic or even butyric acid. At certain levels both these volatile fatty acids have a detrimental effect on intake. Silage inoculants are designed to add enough selected organisms to counteract the effects of spoilage organisms, leading to guaranteed fermentation. Yet this alone does not guarantee success – the other factors, as well as the silage’s limitations or parameters, must be considered.
What happens during preservation?
All the aforementioned organisms, as well as the yeasts and moulds that might be added, exhibit varying tolerance levels for fermentation factors. Some are eliminated when oxygen is absent and others when at a low pH, while all have a certain moisture level at which they’ll thrive. However, most spoilage organisms are only eliminated when all factors work together. The lactic acid bacteria we use are relatively oxygen tolerant, meaning that high oxygen levels will initially not harm them. However, they are not working at full speed yet! This only occurs a few days later when the storage format becomes highly anaerobic. This means that aerobic organisms such as enterobacteria are the first to start working, forming unnecessary levels of, for example, acetic acid before the necessary lactic acid is formed. This is one good reason why rapid oxygen removal is so crucial. If this step takes too long, producers are literally growing future problems for themselves! Other bacteria, for example certain Clostridia species, may be more active at a lower dry matter (DM) content. Although low oxygen levels after compaction limit their activity, the pH must drop even lower than usual to fully inhibit their negative impact in these moisture-rich conditions. This is rarely a problem in drier silage. Organisms such as naturally occurring yeasts are inactivated upon removal of oxygen, but the lowest pH achieved with high-sugar crops (approximately 3,6) is not low enough to kill them. The yeasts simply become dormant, waiting to do damage once the silage is opened.
Aerobic instability
If all the steps initially followed to achieve preservation come undone when opening the silage, then unstable silage is the result. Oxygen will reactivate the dormant organisms and heat will be generated in the process – this is the heat detected when removing silage. These organisms continue to utilise available nutrients – such as some yeasts that live on lactic acid – and this in turn raises the pH. A rise in the pH will compromise preservation and efficiency. The duration of preservation after opening is called aerobic stability and producers should always strive to extend this period for as long as possible to allow animals to efficiently ingest the silage before any silage-associated problems occur.
