The objectives of this trial were to evaluate the quality and DM loss of wet distillers grain (WDGS) mixed or not with 2 different fibers sources during short and long storage times. To replicate a conventional Ag-bag silo, 24 bags of 90 cm long and 38 cm diameter were made using the same 150-μm UV treated polyethylene plastic. Eight laboratory bags were randomly assigned to one of the 3 treatments: (1) control (CON; WDGS 100%); (2) WDGS plus alfalfa hay (WDH; WDGS 87%+ alfalfa hay 17%); (3) WDGS plus corn silage (WDC; WDGS 70%+ corn silage 30%). The experiment had a completely randomized design with 3 treatments, 2 opening dates (30 and 180 d) and 4 replicates per treatment. Data were analyzed by ANOVA using the GLM procedure of SAS and the model included the treatment, time and treatment by time interaction. The DM content was similar between treatments (average 35.41%; SEM = 0.72). Protein (28.0 vs. 26.17 and 19.67%; SEM = 0.56), fat (12.93 vs. 11.88 and 10.22%; SEM = 0.17) and NDF (47.16 vs. 42.28 and 38.88%; SEM = 0.85) content were higher to WDGS in comparison with WDH and WDC respectively. ADF (22.60 vs. 18.41 vs. 17.46%, SEM = 0.78) and ash (6.49 vs. 4.97 and 4.31%; SEM = 0.08) content were higher for the WDH in comparison with WDGS and WDC. The lowest pH (4.08 vs. 4.34 and 4.30; SEM = 0.03) and deoxynivalenol concentration (1.52 vs. 2.11 and 2.08 µg/kg; SEM = 0.11) were obtained with the treatment WDC. Zearalenone (165.27, 180.61 and 115.50 µg/kg SEM = 33.36) and aflatoxin (1.03, 1.53 and 1.24 µg/kg, SEM = 0.248) concentrations were similar between treatments. Extended periods of storage (180 d) increased DM loss in all treatments, but in less extent in treatment WGC when compared with WDGS or WDH (10.86 vs. 13.85 and 19.35; SEM = 1.78). Based on DM loss, mycotoxin and pH data, treatment WDC is the most suitable to be used for extended period of storage. The use of by-products combined or not with fiber sources should always account for significant DM loss, as observed in this trial.