In the rumen
In the rumen, volatile fatty acids (acetic acid, propionic acid and butyric acid) are formed by rumen microbes. Propionic acid is typically formed during the degradation of unstable starch (for example from grains). Propionic acid provides energy and is mainly used for the formation of lactose in the milk. Lactose is a determining factor for the amount of milk. A too high content of unstable starch in the ration increases the risk of rumen acidification due to the formation of large amounts of propionic acid and other volatile fatty acids.
In the pit
Propionic acid is a volatile fatty acid (as well as acetic and butyric acid) and has an unpleasantly sharp sour smell and taste. During the preservation process in grass and maize silage, lactic acid, acetic acid and propionic acid are formed from the sugars present under favorable conditions. This results in a rapid drop in the pH drop, which ensures a well-conserved pit in which there is no chance of heating and fungi. The sum of acetic acid and propionic acid is part of the Conservation Index, developed by Blgg. The optimum content of acetic acid + propionic acid in grass silage is between 20 and 35 g / kgds. Too low levels stimulate heating as soon as the pit is opened and too high levels are undesirable because of the taste. Vinegar and propionic acid are weak acids, which means that they have an antibacterial effect on fungi, yeasts and unwanted bacteria at low pH. With a well-conserved pit with low pH, the chance of heating is also minimal. Propionic acid is often an important component of silage additives.