The effect of urease enzyme on treatment time and the nutritive value of straw treated with ammonia as urea

Abstract

Three experiments were conducted with hammer-milled (10-mm screen) rice and barley straw treated with urea solution, alone or with urease enzyme. In Experiment 1 rice straw was treated with 10, 20, 40, 60 or 80 g urea dissolved in 1 l of water/kg straw, alone or with pure urease, in sealed polythene bags for 1, 2, 6, 14 or 21 days. In Experiment 2 rice straw was treated with 40 or 80 g urea/kg straw and ground soya bean as the source of urease, for 0, 2, 4 or 6 days. Both experiments showed that urea significantly ( P < 0.001) increased organic matter digestibility in vitro and the nitrogen content of the straw. The addition of urease reduced the treatment time to 2–4 days. Ground soya bean appeared to be a satisfactory source of urease for treated straw. In Experiment 3 barley straw was treated with 0, 40 or 80 g urea/kg straw, as in Experiment 1, for 0, 2, 4, 6 or 14 days. Half the treated material was further subjected to a spray treatment with 1 M acetic acid at the time of opening of the bags. Acetic acid treatment significantly ( P < 0.001) depressed organic matter digestibility in vitro of treated straw, but retained 10–15% of the added urea which otherwise would have been lost to the atmosphere as ammonia. It appeared that the urea treatment was inefficient as a means of increasing the nitrogen content of straw. However, the process improved the digestibility of straw and, in the presence of urease, the treatment process could be reduced to 2–4 days.