Late harvest of whole-plant corn silage (WPCS) into late fall and winter months during 2014/2015 raised concerns among central and northern Wisconsin dairy farmers and their nutritionists about fermentation of frozen WPCS. The objective of the present study was to evaluate fermentation profile of defrosted WPCS after several months stored frozen. An unfermented WPCS sample that had been obtained from the University of Wisconsin–Madison Agricultural Research Station (Arlington, WI) on September 23, 2014, was immediately frozen and stored at −20°C until January 26, 2015. Sample was defrosted, homogenized, and divided into 24 sub-samples of 250 g each. Sub-samples were vacuum-sealed in plastic bags and randomly assigned to 8 treatments so that each treatment had 3 replications. Treatments were 0, 0.5, 1, 2, 3, 7, 14, and 28 d fermentations. Bags were stored in the dark at room temperature (approximately 20°C) until reaching the targeted ensiling time. All samples were analyzed for DM, pH, organic acids and ammonia-N (%DM). Data were analyzed using Proc Mixed of SAS with the Fixed effect of ensiling time and the random effect of bag. Content of DM did not differ (P = 0.31; 36.1% on average). Measurements of pH were affected by ensiling time (P = 0.001) with a decline observed after only 1 d (5.23) of fermentation and a gradual decrease until 28 d (3.84). This is related to the gradual increase (P = 0.001) in lactate and acetate concentrations from 1 (0.88% and 0.39%, respectively) to 14 d of fermentation (5.48% and 1.22%, respectively). Total acid concentrations follow the same pattern (P = 0.001). Propionate concentration did not differ (P = 0.77) whereas butyrate was not detected. Concentration of succinate increased (P = 0.001) after 1 d of fermentation, peaked on 3 d and decreased on 14 d. Ethanol concentration was greater (P = 0.001) for 2, 3, 7, 14 and 28 d (0.32% on average) compared with 0, 0.5 and 1 d (0.03% on average). Ammonia-N increased (P = 0.001) 3-fold from 0 to 28 d of fermentation (0.02% vs. 0.06%, respectively). These findings suggest that WPCS maintains fermentation capacity upon defrosting even after frozen for a prolonged period in storage.