Variation in the nutritional value of rice straw

Abstract

Ten rice varieties, five modern semi-dwarf (SD) and five traditional tall (TA), were studied for their chemical and morphological characteristics, over two seasons (wet 1987 and dry 1988) with two different levels of applied nitrogen fertiliser (0 and 90 kg N ha^-1). Harvested plants were fractionated into their component parts and physically and chemically analysed in order to demonstrate varietal differences in their characteristics and determine the environmental effects on these, as well as to attempt to identify predictors of straw quality. There were statistically significant differences (P<0.01) within the semi-dwarf and tall varieties in nutritive value, as measured by in vitro digestibility, but these differences were not consistent between the wet and dry seasons. Although a statistical comparison between the semi-dwarf and tall varietal groups was not possible, it was clear that there was no overall difference between them as the range of variety means was completely overlapping. This implies that breeding for increased grain yield, through the use of semi-dwarf varieties, does not necessarily increase or decrease straw quality. The effect on nutritional quality of added nitrogen to the soil varied with variety. In vitro dry matter cellulase solubility (IVDMS) and in vitro dry matter digestibility (IVDMD) using rumen liquor were significantly correlated (r=0.64^∗∗ for wet season and 0.72^∗∗ for dry season), although they did not rank the varieties in the same order of digestibility. Also, this order of ranking was inconsistent for the two seasons. IVDMS results demonstrated a difference between seasons which was not obvious in the IVDMD results. Although there were significant individual varietal effects (P<0.01) in both the crude protein and ash contents of the straw from the semi-dwarf and tall varieties, there appeared to be no consistent overall relationships between the two groups. No single plant characteristic was shown to be consistently reliable in predicting straw quality. However, multiple regression, using ash (silica) content of the harvested straw and days to maturity of the plant, produced the most promising coefficient of determination for all varieties (except for one traditional variety, Gam Pai) over two seasons (R²=0.74^∗∗ for wet season and 0.81^∗∗ for dry season). This suggests that other physical or chemical characteristics, not measured in this trial, must also affect straw digestibility. The results from this investigation suggest that although there are differences between varieties in the in vitro digestibility of straw, there is no consistent difference between the modern and traditional varieties overall. Fundamentally, though, the taller varieties have the advantage of providing more nutrients per hectare from their straw.