The effect of treatment on the chemical composition and in vitro organic matter digestibility (IVOMD) of wheat, oat and barley straw and oat hay at 25 °C was investigated in a 4x3x2x5 factorial experiment. Apart from the four roughages, urea levels of 30, 50 and 75 g kg- roughage, moisture levels of 250 and 400 g kg^-1 roughage and treatment periods of 4, 6, 8, 12 and 18 weeks were included as independent variables. The dependent variables included crude protein (CP) content, urea content, cell-wall constituents (CWe) and IVOMD. Hydrolysis of urea to ammonia was significantly (P ≤ 0,05) improved at the higher moisture level and longer treatment periods. IVOMD values of all treated roughages were increased at all treatment periods investigated. The interaction between treatment period and moisture level as well as urea level indicated that the level of urea ammoniation at low moisture and urea levels may be improved by increasing the treatment period. It is, however, evident that 30 g kg-! urea was not sufficient for effective treatment, and that extending the treatment period provided no further advantage. CWC values of the cereal straws and oat hay decreased during each of the treatment periods investigated. In the case of the 3% urea treatment, CWC remained constant from 12 to 18 weeks of treatment.