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Summary — Comparative studies were made at a laboratory level to assess the effectiveness of the treatment of straw with anhydrous ammonia (anhydrous NH 3 ) and urea. The influence of moisture, dosage, and temperature were also assessed. The influence of the addition of an urease source was studied in treatments with urea. Two experiments were performed: 1), Anhydrous-NH 3 and urea were used separately to treat a total of 13 samples of barley straw. Anhydrous NH 3 was applied to 3 samples at a dosage of 4% and at moisture levels of 9.2, 22.2 and 32.7%. Urea was applied to 9 batches at doses at 4.3, 6.5 and 8.0% and at 3 moisture levels: 22.2, 31.1 and 42.7%. One sample was left untreated. 2), Urea at a dosage of 3% was applied to 24 samples of wheat straw and 24 samples of maize stover at 4 moisture levels, 10, 20, 30 and 40% and 3 temperatures: 15, 25 and 35 °C for wheat straw and 5, 15 and 25 °C for maize stover. All these parameters were studied both with and without the addition of crude soya bean (CSB).In experiment 1 the increase in moisture content when NH 3 was applied reduced the neutral detergent fibre (NDF) content, but did not modify the in vitro dry matter digestibility (IVDMD) values, which were similar to those obtained on the application of urea at moisture levels of 31.1 and 42.7%. In experiment 2, the increase in moisture content to 30% produced a positive response in most of the chemical parameters analyzed. However, with an increase in humidity to 40% the results were less homogeneous. The temperatures used were not limiting, and only had a significant effect (P < 0.05) on the NDF and total nitrogen (N) contents. For maize stover the temperature did not influence (P > 0.05) either the degree of ureolysis or the IVDMD. The addition of CSB had no significant effect (P > 0.05) when applied to the maize stover, and when applied to wheat straw its only significant effects (P < 0.05) were on the NDF and IVDMD values. At low moisture levels the addition of CSB increased the degree of ureolysis in wheat straw. straw I chemical treatment I anhydrous ammonia / urea / moisture / temperature / urease