Studies on untreated and urea-treated rice straw from three cultivation seasons: 1. Physical and chemical measurements in straw and straw fractions

Seven rice varieties selected according to three cultivation seasons were examined with regard to their grain and straw production, and the chemical composition of straw. Weather conditions, fertiliser application and harvesting time were major factors affecting grain yield and straw composition. The following correlations were found: grain yield and straw length (r=0.82, p<0.05), grain yield and ear length (r=0.90, p<0.01), and grain yield and weight of thousand grains (r=0.85, p<0.05). Seasonal differences in chemical composition of straw were found for nitrogen (p<0.001), hemicellulose (p<0.001), cellulose (p<0.001), and content of K (p<0.01). The stem contained less hemicellulose (p<0.001), less acid detergent insoluble lignin (p<0.01), less total biogenic silica (Si) (p<0.001) but more cellulose (p<0.001) than sheath and blade. Urea treatment caused an increase from 35 to 171% in nitrogen (N) level as compared to untreated straw (p<0.001), and significantly reduced the hemicellulose content (p<0.001). The K content in straw was related to the efficiency of extracting biogenic Si from straw with neutral detergent solution (r=0.77, p<0.05). K and Si content in straw were identified as indicators of straw quality. The Si content of the stem was found low and the extractable portion was extremely high (>80%). The blade retains a higher level of Si and the Si was less extractable than that of sheath. Extractable Si seemed to increase after urea treatment and this response was greatest in leaf sheath. Moreover, supplement of K to the rice land, especially to the late season rice, appeared to have a beneficial effect on yield and straw quality.