Four experiments were carried out to compare the effects of formic acid and a partially neutralised blend of aliphatic organic acids on fermentation, effluent production and aerobic stability of silage. Precision-chopped unwilted grass was harvested during October and November and ensiled in laboratory silos (6 kg grass/silo) with either no additive, formic acid (850 g/kg) applied at 3 ml/kg or an acid-complex mixture (680 g ammonium hexamethanoate, 100 g ammonium hexapropanoate and 20 g octanoic acid/kg; Maxgrass, BP Chemicals Ltd.) applied at 6 ml/kg. Additives were applied manually after harvesting, there were four replicates per treatment and the storage period was > 100 days. The mean (s.d.) dry matter (DM) concentrations for grass in Experiments 1, 2, 3 and 4 were 139 (2.1), 109 (0.8), 163 (3.8) and 162 (5.8) g/kg, respectively. Corresponding water-soluble carbohydrate (WSC) concentrations were 13 (0.6), 9 (0.5), 18 (0.6) and 17 (1.9) g/l aqueous extract. The main effect of the formic acid treatment was to significantly restrict fermentation, and fermentation was further restricted by the acid mixture treatment. There were additive x experiment interactions. In Experiments 1 and 2, where the silages made without additive treatment were poorly preserved and had high pH (5.0 and 5.0), acetic acid (62 and 42 g/kg DM) and ammonia-N (161 and 125 g/kg N) values, formic acid reduced (P < 0.001) pH (4.4 and 4.4), acetic acid (31 and 19 g/kg DM) and ammonia-N (91 and 48 g/kg N) concentrations. The acid mixture produced similar effects to formic acid but in addition further reduced acetic acid (P < 0.01) and lactic acid (P < 0.001) and increased (P < 0.001) residual WSC concentrations. In Experiments 3 and 4, where the untreated silages underwent lactic-acid-dominant fermentations, and were consequently well preserved, both formic acid and the acid mixture restricted (P < 0.05) fermentation. All silages treated with the acid mixture had higher (P < 0.05) propionic acid concentrations than the formic-acid-treated silages. The acid mixture also improved aerobic stability. Both additives, but particularly the acid mixture, increased (P < 0.05) the initial flow-rate of effluent and increased total effluent production. It is concluded that both additives were effective silage preservatives at the rates applied, but the acid mixture treatment was superior to the formic acid treatment in restricting fermentation and improving aerobic stability.
