Acetic acid in the silage
Acetic acid (like butyric acid and propionic acid) is a volatile fatty acid formed during the beneficial preservation process of grass and corn silage. Acetic acid is not a palatable acid, but it is a very important factor in preventing heating: a grass silage with a low proportion of acetic acid is very susceptible to heating. Acetic acid, and indirectly the pH, can be influenced by harvesting a crop that is not too dry and high in sugar. Acetic acid is formed by the conversion of sugars by the bacteria present. A dry silage often has a relatively high sugar content, but due to a lack of moisture, the conversion processes cannot take place sufficiently. Because of the low acetic acid content, they can therefore still be very susceptible to heating.
The sum of acetic acid and propionic acid is part of Forage analysis, developed by Eurofins Agro. The optimum level of acetic acid + propionic acid in grass silage is between 20 and 35g per kilogram of dry matter. Levels that are too low stimulate heating as soon as the silage is opened, while levels that are too high are undesirable for taste reasons.
Acetic and propionic acids are weak acids, which means that at low pH they have an antibacterial effect on fungi, yeasts and unwanted bacteria. Therefore, in a well-preserved silage with low pH, the chance of heating is minimal.
An overly generous nitrogen fertilization, and therefore a high protein content, can also play a negative role in the stabilization of the silage. Protein is, after all, an important fuel for negative bacteria. In addition, large amounts of nitrogen have a buffering effect, which slows down acidification and acetic acid formation.
Acetic acid in the rumen
In the rumen, volatile fatty acids (acetic, propionic and butyric) are formed by rumen microbes. Acetic acid is produced by cellulolytic bacteria during the breakdown of cell wall constituents (particularly cellulose).
The acetic acid formed is absorbed by the rumen wall. It provides energy and is mainly used for the production of milk fat. When the structural value of a ration is low (e.g. grazing on young grass) relatively to propionic acid, less acetic acid is formed and the milk fat content will decrease.
The same happens with a ration consisting of an excessively high proportion of fast carbohydrates; the pH in the rumen can then drop rapidly (rumen acidification). Cellulolytic bacteria are very sensitive to a low pH, so the breakdown of cell wall components decreases and with it the production of acetic acid.