Silage is the plant material preservation process guided by homofermentative and heterofermentative lactic acid bacteria fermentation of carbohydrates soluble in water. Production of organic acids, mostly lactic acid, lowers the pH which inhibits undesirable microorganisms and preserves ensiled material. The latest research shows that, despite low pH value, microbial activity can persists in a stable phase, which can lead to changes in the quality of the silage mass. The focus of this research was to determine the effect of ensiling time and two types of additives (inoculant and lactic acid additive) on main corn silage quality parameters (dry matter, lactic acid, water soluble sugars, pH value and buffering capacity) during eight sampling periods of extended ensiling up to one year. Added inoculant Bio-Sil contained lyophilized cultures of Lactobacillus plantarum 8862 and 8866 and added lactic acid was in the linear range from 1% to 7% of DM silage mass. Water-soluble sugars were determined by modification of the Luff-Schoorl and Nelson- Somogy methods. The content of lactic acid in the silage samples was determined by the spectrophotometric method. During 364 days of ensiling all tested quality parameters of whole plant corn silage, except dry matter, recorded change. Silages with added inoculant had a sudden increase in lactic acid (from 3.99 to 51.23 g/kg DM) and decrease in pH value (from 5.47 to 3.82) in the first days of ensiling while the same results were recorded in the later stages of ensiling in silages with the added lactic acid. The controlled silages had the lowest values of lactic acid, slightly higher amounts of water-soluble sugars and higher pH values throughout the whole ensiling period. In all silages during examined period we recorded decrease in water soluble sugars and increase of the buffer capacity which is directly related to synthesis of lactic acid. The results imply that the addition of the inoculant and lactic acid had favourable impact on the silage quality when compared to controlled corn silage and that in whole plant corn silage quality parameters are continuously changing during the whole ensiling period although in stable phase the changes are minimal.