A genetic basis for the feeding value of rice straw / J Vadiveloo

The botanical composition, chemical composition and in vitro digestibility (IVD) of two rice straw varieties, MR 84 and MR 77, from three locations, Bumbung Lima, Tanjung Karang and Kota Baru was investigated over two seasons, the main (October/November 1987) and off (July/August 1988) cultivation seasons, to determine the environmental factors affecting the nutritive value of rice straw. Although the effects of season and location were small, the main season produced straw of better quality due probably to the higher availability of rainfall. Locational differences were attributed to differences in the proportion and quality of botanical fractions. Stem was more digestible than leaf and contained less ash and silica. Varietal differences were small. The in vivo feeding value of whole straw of varieties MR 84, MR 71 and MR 1, and the leaf and stem fractions of MR 84 and MR 71 were studied with goats who also received supplements of either a 3:1 and 1:1 pelleted mixture of dehydrated palm oil mill effluent (POME) and palm kernel cake (PKC). There were no significant differences (P>0.05) in chemical composition, straw intake and dry matter digestibility (DMD) between straw varieties. The voluntary intake of leaves was higher than stems despite a lower DMD of the leaf diets. A 52-week growth study to compare the 3:1 and 1:1 supplement ratios showed that both were equally good supplements for rice straw. Exploratory studies on the composition and IVD of one indeterminate and four released varieties (MR 84, MR 27, MR 1 and MR 71) before and after chemical treatment were carried out to identify indicators of nutritive value, to assemble the statistical procedures required to screen a large number of straw varieties and to ascertain if varietal selection could be an alternative method to chemical treatment for improving straw quality. The indicators identified were IVD, neutral detergent fibre (NDF), crude protein (CP) and insoluble ash; multivariate statistical procedures were very effective in discriminating between varieties and botanical fractions. Sodium hydroxide treatment substantially increased straw quality, the increase being greatest for the variety of poorest quality. Urea treatment made only marginal increases in quality and did not alter the ranking of varieties before and after treatment. The mean proportion of leaf blade, leaf sheath and stem (including inflorescence) of 32 varieties of rice straw harvested from the same location; year and season, was 32.4%, 25.5% and 41.0%, respectively in whole straw. Stem had a significantly higher IVD (58.9%), lower insoluble ash (6.9%) and lower total ash (18.9%) but a higher NDF (62.7%) and lower crude protein, CP (3.4%) than blade or sheath. Canonical correlation analysis showed that varietal differences in whole straw were significantly correlated (P<0.001) with differences in the composition and IVD of the blade, sheath and stem fractions but not with differences in their relative proportions. Varietal differences were also independent of agronomic characteristics. Total ash content and IVD were important indicators of varietal differences in rice straw. The genetic distance between the best and poorest varieties, ranked on the basis of their principal component scores, was small. The best varieties, Y 818, YTK 34 and YTK 108, may be selected for feeding to ruminants. There is potential for breeding and selecting rice varieties combining good straw and agronomic characteristics.