Definition and characteristics

Butyric acid is a volatile fatty acid, such as acetic acid and propionic acid, with a very pungent odor, likened to sweaty feet or strong cheese-like smell, and an unpleasant taste. Butyric acid is produced by butyric acid bacteria in poorly preserved grass and corn silage. The smell of butyric acid is therefore characteristic of poor, wet grass silage. The formation of butyric acid can be largely prevented by driving the silage in well, chopping the grass, pre-drying it, covering it quickly and using silage additives, for example.

Butyric acid and the conservation index

Butyric acid is an important component of the Conservation Index developed by Eurofins Agro. The standard for butyric acid content is < 3 g/kg dry matter. The more the content in a grass silage exceeds the standard, the lower the Conservation Index.

Butyric acid bacteria and spores

Butyric acid bacteria are anaerobic bacteria, meaning they grow without oxygen, of the species Clostridium (tyro)butyricum, and are naturally abundant in water, soil and manure. Butyric acid bacteria are spore-forming bacteria. Spores are bacteria that are in a so-called resting state and are very resistant to drought, heat and other extreme conditions. In this way, they can survive for years and germinate again into harmful bacteria under favorable conditions.

Through litter, manure, and especially bad silage, butyric acid bacteria and spores can get into milk. Unlike butyric acid bacteria, the spores survive the milk’s pasteurization process and can cause feed problems in cheese making, such as a rancid taste. When levels are too high, milk producers issue warnings and discounts on the milk price.

Butyric acid and butyric acid bacteria in the rumen

In the rumen, volatile fatty acids (acetic acid, propionic acid and butyric acid) are formed by rumen microbes. Butyric acid is formed particularly during the breakdown of sugars and unstable starch.

Butyric acid and, to a lesser extent, propionic acid stimulate the growth of rumen papillae. These rumen papillae increase the absorption surface and are essential for the absorption of the formed volatile fatty acids through the rumen wall. In a ration with a very high structural value (for example, during dry periods), the rumen papillae can degenerate, which can endanger rumen stability and cause rumen acidification.

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