Alfalfa from 2 consecutive harvests (H1, H2) was treated with Lactobacillus plantarum (LP, 10⁶ cfu/g alfalfa), formic acid (FA, 5.4 kg/Mg), and no additive (control), ensiled in mini-silos, fermented for 60-d at room temperature (22°C), and analyzed for nutritive value and fermentation profile. Silages were wet ground to a particle size of 2–3 mm using a food processor for in vitro true DM digestibility (IVTDMD). High (HE) and low efficient (LE) groups of 3 cannulated cows each were identified based on 2 lactations and had an average energy corrected milk/dry matter intake (ECM/DMI) ratio of 1.88 and 1.61, respectively. In each in vitro run, 12 silos from a harvest were analyzed simultaneously with 6 rumen inocula, one from each cow, for 24 h. Data analysis was conducted using the MIXED procedure of SAS, as a split plot design. For silage fermentation, harvest was the whole plot and alfalfa treatment as subplot. For the in vitro, HE and LE groups were main plot and treated alfalfa as sub-plot. In both cases mean differences were declared significant at P < 0.05. Across harvests, LP had lower pH (4.47) and greater lactic acid concentration (64.3 g/kg DM) than control (4.77, 52.1) and FA (4.70, 33.9). Moreover, ammonia-N was lower in LP (3.8% total N) and FA (4.2) than control (5.4) in both harvests. IVTDMD of the H1 silages was greater with the HE cows (79.6%) than LE cows (75.13%), but there was no difference at H2 (P = 0.310). The IVTDMD was not different among silage treatments at H1 (P = 0.078), but at H2 control had greater IVTDMD (79.8%) than did LP (78.1) and FA (77.6). Rumen fluid from HE cows produced greater acetate and butyrate than did LE on H1 silages, with no differences on H2. In vitro acetate and propionate were contrasting between H1 and H2 by silage treatment. At H1, LP and FA had greater acetate and propionate than control, but it was opposite at H2. It is concluded that LP and FA decrease ammonia-N during fermentation of alfalfa silage, and rumen inocula from HE cows displayed numerically higher IVTDMD, but the effect was only significant with the earlier harvested silage.