AbstractFive experiments were carried out in the years 1980‐1983 and 1986 to study the effect of treating grass at ensiling with sulphuric acid (850 g kg−1) and formic acid (850 g kg−1) additives alone, and in mixtures with or without formalin on the preservation of grass,in vivodigestibility in sheep, in‐silo loss, intake and performance of finishing cattle.Primary growth grass was ensiled in experiments 1 (3–4 June 1980), 2 (12‐15 June 1981) and 3 (31 May‐2 June 1982), primary regrowth grass in experiment 4 (1‐2 August 1983) and secondary regrowth grass in experiment 5 (7‐10 October 1986). During the ensiling period within each experiment, approximately 60 t of unwilted, double‐chopped, additive‐treated or untreated grass was packed into covered concrete‐walled 60‐t capacity silos. The dry matter (DM) contents of the ensiled grass in experiments 1, 2 and 5 ranged from 155‐180 g kg−1and were lower than those recorded in experiments 3 and 4 (214 g kg−1). With the exception of grass ensiled in experiment 2, where water‐soluble carbohydrate (WSC) contents were low, at 104 g kg−1DM, grass in all other experiments contained relatively high WSC contents ranging from 140‐154 g kg−1DM.In experiments 1, 3 and 4 all silages were well‐preserved. However, in experiment 2 the 450 g kg−1sulphuric acid‐treated and formic acid‐treated silages displayed significantly lower pH, buffering capacity (Bc) and ammonia nitrogen contents than the untreated silage. In experiment 5, the sulphuric acid‐treated and formic acid‐treated silages displayed significantly lower pH, Bc, ammonia nitrogen, butyrate and volatile fatty acid (VFA) contents than the untreated silage.Each of the silages was offered daily with various levels of a supplementary concentrate for approximately 70‐d periods to twelve animals of mixed breed in experiments 1, 3, 4 and 5 and to fifteen animals in experiment 2. All animals weighed between 380‐470 kg at the start of the experiments.In experiments 1, 2 and 3 there were no significant differences between silages for any of the intake or animal performance parameters. In experiment 4, cattle fed the formic acid‐treated silage displayed significantly higher silage DM intakes and daily liveweight gains than those fed the sulphuric acid‐treated and untreated silages; in experiment 5, cattle fed the formic acid‐treated silage displayed significantly higher silage DM intakes than those fed the untreated silage. It was concluded that formic acid was a more effective silage additive than sulphuric acid.Increasing the level of supplementation significantly decreased silage DM intakes in cattle in experiments 3 and 4, and significantly increased daily liveweigh