The effect of physical form, moisture level and prolonged treatment period on the ammoniation of wheat straw using 75 9 urea per kg wheat straw was investigated in a 2 x 3 x 6 factorial experiment. Physical form was represented by ground (12 mm screen) and long wheat straw, moisture levels were 250, 375 and 500 g/kg air·dry wheat straw and treatment periods were 0, 4, 8, 12, 24 and 48 weeks. Total nitrogen content, urea content, free ammonia nitrogen, in vitro organic matter digestibility (IVOMD), and cell wall constituents (CWe) were determined. Physical form, moisture level and treatment period significantly (P ≤0,01) affected ammonia release from urea. The efficiency of ammoniation was affected accordingly. Ammonia release was slower in long wheat straw, resulting in a slower and smaller increase in IVOMD and a slower and smaller decrease in CWC content with an increased treatment period, when compared to ground wheat straw. Ammoniation was accelerated at the two higher moisture levels compared to the lower moisture level. The IVOMD of wheat straw increased substantially up to a treatment period of 8 -12 weeks, whereafter a slower increase was obtained up to a treatment period of 48 weeks. Wheat straw appeared to be adequately preserved by treatment with ammonia. No evident sign of deterioration was observed following prolonged treatment -periods of up to 48 weeks.