Three experiments are described in which formic acid was added at different levels to low dry matter herbages. In the first and second experiments, using timothy/meadow fescue, the levels of acid used were 0·22% and 0·34% respectively, whereas in the third experiment, using cocksfoot, the concentration of acid was 0·51%. The immediate effect of the formic acid was to lower the pH values to 4·75, 3·81 and 4·11 in experiments 1, 2 and 3 respectively. Formic acid preserved water‐soluble carbohydrates in the early stages of harvesting and ensiling but did not prevent the formation of lactic acid in experiments 1 and 3. Ethanol concentrations in the formic acid‐treated materials in the first two experiments were higher than in the control silages. In all three experiments, the addition of formic acid did not reduce loss of dry matter and, in experiment 2, losses from the formic acid‐treated herbage were higher than those from the control materials. This can be attributed mainly to the higher effluent production when formic acid was used. Changes in carbohydrates, nitrogenous components, organic acids and digestible nutrients are described.