The effect of agronomic improvement and urea treatment on the nutritional value of Malaysian rice straw varieties

The objectives of the study were to estimate the effect of agronomic improvement on rice straw nutritional value and to compare the response of straw varieties to urea treatment. The agronomic characteristics (i.e. grain yield, days to maturity, culm height and morphological composition) and straw characteristics (i.e. chemical composition and in vitro dry matter digestibility (IVD)) before and after 4% urea treatment of two single-crop and six double-crop rice varieties were measured. The single-crop varieties were taller, 89 cm versus 78 cm, late maturing, 130 days versus 122 days, lower in grain yield, 1450 kg/ha versus 4382 kg/ha and contained more stem, 48% versus 28%, than the double-crop varieties. The total ash, neutral detergent fibre (NDF), insoluble ash, IVD and crude protein (CP) of the single-crop varieties averaged 18.0, 60.9, 6.2, 41.2 and 6.0%, respectively, in comparison with 17.8, 63.5, 5.2, 54.0 and 3.3%, respectively, for the double-crop varieties. Covariance-adjusted means of the single-crop varieties after urea treatment were 20.4, 64.5, 6.5, 64.4 and 11.6%, respectively, and corresponding estimates for the double-crop varieties were 19.5, 61.7, 9.2, 59.6 and 5.2%, respectively. The IVD and CP of straw, respectively, improved and declined (P<0.05) with improvements in agronomic characteristics. The relationship between IVD (y) and grain yield (x) was explained by the equation: y=35.9+0.004x (P<0.001, R2=0.91). Varietal response to urea treatment was not uniform with respect to IVD and CP. The relationship between IVD after (y) and before (x) urea treatment was explained by the equation: y=41.5+0.38x (P<0.01, R2=0.77) indicating that the single-crop varieties of lower initial IVD responded better to urea treatment. Principal component analysis ranked the single-crop varieties lower than the double-crop varieties based on their agronomic and untreated straw characteristics, but higher in their straw characteristics after urea treatment. Cluster analysis classified the single- and double-crop varieties separately. Consequently, the genetic distance between them was larger than within them. Results show that improving the agronomic qualities of rice may concurrently improve straw characteristics and that urea treatment may be suitable for improving straws of poor quality only.