AbstractThe effects of a range of fatty acids (C1‐C18) on silage fermentation in laboratory silos have been examined. When acids were applied to ryegrass in equimolar concentrations (50 mM) in the C1‐C6 range, only the C6 acid (hexanoic) had a marked effect in inhibiting lactate production. When hexanoic acid was examined in a second experiment over the concentration range 9 mM to 50 mM, concentrations below 50 mM did not prevent acid production, and at the lower levels production of lactate was stimulated. When higher fatty acids hexadecanoic (16:0), octadecanoic (18:0) and octadecenoic (18:1) were added at similar concentrations (8 to 9 mM) and octadecadienoic (18:2) added at concentrations ranging from 8 mM to 29 mM to ryegrass samples, lactic acid fermentation was not inhibited.