The response of rice straw varieties to urea treatment

Sixteen rice straw varieties comprising 6 single-crop and 10 double-crop varieties were treated with 40 g/l urea solution (50 g straw per 200 ml of solution) for 21 days at 27 °C and total and insoluble ash, crude protein, neutral detergent fibre and in vitro digestibility (IVD) were measured after samples were dried at 60 °C to constant weight. The mean IVD values across treatments for the single-crop varieties were 0.500, 0.480, 0.418 and 0.499 for all fractions (whole straw, stem, leaf sheath, and leaf blade), corresponding values for the double-crop varieties were higher, 0.560, 0.598, 0.542 and 0.627, respectively. The untreated IVD values across varieties were 0.496, 0.497, 0.404 and 0.462 for whole straw, stem, leaf sheath, and leaf blade; and for urea treated, 0.564, 0.581, 0.556 and 0.665, respectively. Urea treatment caused a decrease in neutral detergent fibre exclusive of residual ash (NDFom) and in increase in IVD of all fractions (P<0.001). Single-crop fractions were all lower in IVD (P<0.001), and all fractions except whole straw were higher in NDFom (P<0.05) than double-crop varieties. Based on criterion variables selected from step-wise discriminant analysis, single- and double-crop varieties clustered separately before but together after urea treatment which may be important in selecting genetic varieties. Regression analysis showed, in general, fractions with a lower IVD intercept responded better to urea treatment evaluated by relatively higher regression coefficients of 0.39, 0.67, 0.79 and 0.80, respectively for whole straw, stem, leaf sheath, and leaf blade. The effect of the untreated stem, leaf sheath, and leaf blade IVD on the IVD of untreated and urea-treated whole straw was described by single and multiple regressions where the most important fraction was the leaf blade IVD for urea-treated whole straw IVD and the leaf sheath IVD for the untreated whole straw IVD. Multiple regressions, although they had higher R2, did not show any fraction to be more important than found in the simple regressions. It is concluded that although urea-treatment may result in a better response in poorer quality straws, rice straw varieties with a high leaf sheath IVD would improve the quality and utilisation of whole straw without urea treatment whereas high leaf blade IVD would improve the quality and utilisation of whole straw with urea treatment.