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Notes: Pure perennial ryegrass or perennial ryegrass/white clover mixtures (70:30 and 40:60 on a fresh-matter basis) were ensiled in laboratory silos either untreated or alter treatment with freshly cultured Lactobacillus (Lb.) plantarun or freshly cultured Lb. plantarum plus Lactococcus (Lc.) lactis. freeze-dried Lb. plantarum or freeze-dried Lb. Plantarum plus sodium formate, sodium formate or formic acid. The effect of these additives on silage fermentation characteristics and quality of the resultant silages was examined. There were significant interactions between treatments and herbages for all silage quality parameters measured, except for acetic acid concentration. The influence of additives on the final pH of all silages was small but statistically significant. Lactic acid concentration was not directly related to herbage mixture, overall mean values ranging from 118 to 120 ± 1.5 g kg−1 dry matter (DM), but wider variation was seen between treatments for individual herbage mixtures. Acetic acid concentrations were significantly (P〈0·001) affected by herbage mixture ensiled, increasing linearly as clover content increased from zero to 60%. Untreated control and formic acid-treated silages contained significantly (p〈0·001) higher acetic acid concentrations than those treated with other additives. Silage ammonia N concentrations were significantly (p〈0 001) influenced by herbage mixture. Lowest ammonia N concentrations (〈 50 g kg−1 DM) were observed in silages that had been treated with formic acid, freshly cultured Lb. plantarum or Lb. plantarum plus Lc. lactis. The fraction 1 leaf protein (FILP) contents of silages were significantly (P 〈0·001) affected by both treatment and herbage mixture, with consistently and significantly higher values found in freshly cultured inoculant-treated silages. A poor correlation (r2= 0·12) existed between ammonia N and FILP in all silages. The inclusion of up to 60% white clover in the ensiled herbage did not adversely affect final silage quality. However, additive treatment markedly influenced the residual FILP content of silages, those treated with freshly cultured inoculants having the highest values.