Lactic acid bacteria (LAB) are used as silage additives to ensure the conservation of the nutritive value of forage by reducing the pH through acid production and inhibition of undesirable microorganisms in the epiphytic flora. The objective of this work was to evaluate the effect of a commercial inoculant and 2 strains of LAB with potential bacteriocinogenic activity on the microbial populations and the fermentation of alfalfa silage. The alfalfa was harvested 82 d after sowing and was wilted for 6 h. After, it was chopped into a 1.5 cm particle size and packed in bags, with 3 replications. A 4 × 6 factorial arrangement (4 inoculants and 6 fermentation periods) was used in a completely randomized design. The inoculants (I) were: 1) Control; 2) Sil All (CI; Alltech, Brazil); 3) Strain 10.4 (S-10.4; Pediococcus acidilactici); and 4) Strain 6.16 (S-6.16; P. pentosaceus). These strains have potential bacteriocinogenic activity and were isolated from Stylosanthes silage. The fermentation periods (P) were: 1, 3, 7, 14, 28 and 56 d after ensiling. The concentration of lactic acid and propionic acid were not affected by the interaction I × P (P > 0.05). There was effect of I on the DM, pH, ammonia, lactic acid and acetic acid. Silages treated with S-10.4 and S-6.16 resulted in the lowest pH. After 56 d, all silages treated with inoculants had lower (P < 0.05) pH compared with the control. The silage treated with S-6.16 showed the highest (P < 0.05) concentration of lactic acid (3.07% of DM) at 56 d. There was no effect (P > 0.05) of I on the concentrations of CP, NDF, ADIN, and lignin at 56 d. Only the population of enterobacteria was affected (P < 0.05) by P, showing a decrease overtime and the lowest number on d 14 (1.05 cfu/g). The strains evaluated improved the fermentation of alfalfa silage in tropical conditions by reducing the concentration of ammonia nitrogen, and increasing the concentration of lactic acid, leading to a faster pH drop and control of undesirable microorganisms.