Volatile fatty acids (VFA)

Volatile fatty acids are the main end products of fermentation in the rumen and supply most of the energy a cow uses. The three that matter most are acetate, propionate and butyrate. Acetate and butyrate are the building blocks for much of the fat in milk; propionate is converted to glucose in the liver and so drives lactose and milk volume.

Forage-rich diets favour acetate. Diets high in starch move fermentation toward propionate and produce acid faster, which is why the acetate to propionate ratio falls as rumen pH falls.

Written for the 2026 edition of the library.