Studies on untreated and urea-treated rice straw from three cultivation seasons: 2. Evaluation of straw quality through in vitro gas production and in sacco degradation measurements

Untreated and urea-treated straw and straw fractions of seven rice varieties from three cultivation seasons have been evaluated on their DM, OM loss and degradation characteristics from in sacco disappearance and in vitro gas production measurements. Drying temperatures from 45°C to 100°C did not seem to influence the degradability of urea-treated rice straw, whereas urea-treated straw dried at freezing temperatures (−35°C) gave slightly higher degradability than higher temperatures. Untreated early season rice straw showed higher degradability than straw of middle and later season rice. There was a significant increase in the degradation of straw after urea treatment, and greatest for late and middle season rice straw. On average, urea-treatment of rice straw increased the DM and OM in sacco losses after 48 h of incubation (48 h) by 24.0% and 30.7%, respectively. In order to study the kinetics of the degradation of fibre fractions, the disappearance in sacco was also estimated for the loss of hemicellulose, cellulose and extractable biogenic silica (EBSi). There was a great variation in the content of silica between varieties. Rice straw degradation seemed to be related to the biogenic silica content (acid detergent insoluble silica (ADISi)). Urea treatment increased the extraction of biogenic silica and hence increased the degradation of hemicellulose and cellulose. The improvement in sacco disappearance of cellulose due to urea treatment was 36.8%, 19.5% and 5.3% for late, middle, and early rice straw, respectively. The degradability was higher for the stem than for the leaf blades and leaf sheaths. The response to urea treatment, however, was higher for leaf sheaths and leaf blades than for the stems, evening out differences in degradability. Urea treatment tended to increase the production of acetic acid whereas there was no effect on propionic and butyric acid production.