In the silage
Propionic acid is a volatile fatty acid (like acetic acid and butyric acid) and has an unpleasant sharp acidic odor and taste. During the preservation process of grass and corn silage, under favorable conditions, lactic acid, acetic acid and propionic acid are formed from the sugars present. This results in a rapid drop in pH, which ensures the silage is well preserved and that overheating and mold formation are inhibited.
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 lies between 20 and 35 g/kg dry matter. Levels that are too low stimulate overheating as soon as the silage is opened, and levels that are too high produce an undesirable taste.
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 well-preserved silage with a low pH, the chance of scalding is minimal. Propionic acid is often an important component of silage additives.
In the rumen
In the rumen, volatile fatty acids are formed by rumen microbes (acetic acid, propionic acid and butyric acid). Propionic acid is formed during the breakdown of non-resistant starch (e.g. from cereals).
Propionic acid provides energy and is mainly used for the formation of lactose in milk. Lactose is a determining factor for the quantity of milk. Too high a level 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.