High-moisture ear corn (HMEC) was untreated, treated with propionic acid (PA), or inoculated with a mixture of Lactobacillus plantarum and Enterococcus faecium and allowed to ensile in laboratory silos for 0, 7, 21, 42, 138, or 202 d. The silages were evaluated for fermentation quality, microbial populations, and aerobic stability. In all treatments, silage pH declined rapidly within 7 d, but the rate of decline seemed greatest with the inoculum. The lactic acid content of inoculated HMEC was higher (P < .05) than that of control of PA-treated HMEC. Regardless of treatment, the population of lactic acid bacteria (LAB) increased (P < .1) up to 7 to 21 d of fermentation then declined; LAB counts decreased (P < .05) up to 42 d in control and PA-treated silage but continued to decline until 138 d for inoculated silage. Yeast and mold counts tended to decrease up to 42 d of ensiling then decreased (P < .05) as fermentation progressed. Between 138 and 202 d of ensiling, the control silage showed a marked increase (P < .10) in pH and yeast and mold populations, providing evidence of secondary fermentation; PA treatment and bacterial inoculation prevented secondary fermentation. Inoculation tended to reduce estimates of sample temperature for silage stored for 138 d and exposed to air, but not for the corresponding silage stored for 202 d. Treatment with PA prevented the loss (P > .05) of acetic acid and the rise (P > .05) in pH during air exposure of the 138-d silage; both control and PA-treated silage showed an increase (P < .05) in yeast and mold populations, but the increments were 38% and 23%, respectively. Compared with PA, the relative efficacy of inoculation in improving aerobic spoilage of HMEC depended on the period of silo storage and the criterion used to assess aerobic stability.