Background: The hygienic quality of silage is important for animal health, animal production and food quality. The aim of this study was to evaluate the effect of five silage additive mixtures on yeast, mould, and clostridia growth and other silage quality criteria. The additives, composed of sodium nitrite, sodium benzoate and/or potassium sorbate, were used to treat a grass forage crop before ensiling. The silages were compared with untreated silage and silages treated with two commercial additives: Promyr NF (PNF), which is based on formic and propionic acid, and Kofasil Ultra (KU), a mixture of sodium nitrite, sodium benzoate, sodium propionate and hexamine. Results: All tested additives significantly (P < 0.001) reduced butyric acid and ammonia-N formation in low-dry-matter (DM) silages compared to the control. Only KU and the mixtures of 600 g sodium nitrite, 250 g sodium propionate, 750 g sodium benzoate (A1) and 250 g sodium nitrite with 1000 g sodium benzoate and 500 g potassium sorbate (A5) ton-1 of fresh forage were found to reduce (P < 0.001) the presence of clostridia spores in low-DM silages. In high-DM silages, treatments A1, A5 and A2 (250 g sodium nitrite and 1000 g sodium benzoate) contained less (P < 0.02) yeast than the control and PNF, but not KU. All treated silages were aerobically stable. The concentrations of nitrate-N in all silages and nitrite-N in low-DM were below the threshold level for feed at the end of storage. Conclusions: The additive mixtures A1 and particularly A5 improved silage quality at least to the same extent as commercial products, even though they contained no hexamine and less nitrite. © 2009 Society of Chemical Industry.