The objective of this study was to evaluate a microbial inoculant and molasses effects on DM losses, dry matter recovered, pH and buffering capacity of whole-plant soybean silage (SS) harvested in different reproductive growth stages. Two hundred mini-silos were used in a completely randomized experiment with treatments in a 5 × 2 × 2 factorial arrangement. Treatments consisted of 5 harvest stages of soybean plant (R3, R4, R5, R6, and R7), 2 levels of molasses inclusion (0 and 40 g/kg of fresh forage), and 2 levels of microbial inoculant inclusion Silobac (Chr. Hansen Industry and Trade Ltda, Valinhos, São Paulo; 0 and 2.5 × 10¹⁰ cfu/g of a mixture of Lactobacillus plantarum and Pediococcus pentosaceus). Soybean plants were harvested at 5 cm from the ground, chopped with cut length of 20 mm and ensiled into polyvinyl chloride pipes with 0.5 m length and 0.1 m diameter. Bulk density was set at 650 kg of natural matter/m3. The total DM losses were obtained by the difference between DM ensiled and DM recovered when the silos were opened after 90 d from ensiled. Effluent losses were quantified by weight difference pipe, after silage removal. Gas losses was quantified by the difference in weight of the whole silo after silage and immediately before opening. Silage samples were collected for pH and buffering capacity expressed as miliequivalent of alkali required to change the pH from 4 to 6 per 100 g of dry matter. Data were analyzed using PROC MIXED of SAS 9.3 including the fixed effects of molasses, microbial inoculant and reproductive growth stages. There were 3 ways interactions for gas losses, where molasses increase (P < 0.001) dry matter recovered and reduced (P < 0.001) pH without microbial inoculant addition. The molasses decrease (P < 0.05) total dry matter losses regardless reproductive growth stage except in R4. Molasses addition decrease (P < 0.03) effluent losses in R4, R5, and R6 when associated with microbial inoculant. In general the use of molasses as a fermentative additive improved the quality of soybean silage.