Nutritional properties of the leaf
and stem of rice straw
J. Vadiveloo
*Faculty of Applied Sciences, MARA University of Technology, 40450 Shah Alam, Malaysia
Received 13 January 1999; received in revised form 26 July 1999; accepted 14 September 1999
Abstract
The proportion of the leaf blade, leaf sheath and stem fractions in six rice straw varieties averaged 30, 40 and 30%, respectively. In vitro dry matter digestibility (IVD) of these fractions, estimated by the cellulase-neutral detergent solution procedure, was 503, 513 and 610 g/kg, respectively. The lower IVD of the blade and sheath was due to lower degradation parameters estimated by a simple technique of incubating samples in a cellulase-buffer solution at 388C for 3, 6, 12, 24, 48 and 72 h. Treatment with 4% urea solution for 21 days increased the IVD and the degradation characteristics of the leaf fraction more than the stem but treatment with a 4% sodium hydroxide (NaOH) solution for 21 days improved the IVD and degradability characteristics of all straw fractions to the same extent.
The leaf had a lower neutral detergent fibre (NDF), higher total and insoluble ash than the stem. Urea treatment which increased total ash but decreased insoluble ash in the leaf were increased in the stem. NaOH treatment increased total and insoluble ash in all fractions. The NDF content, lower in the untreated leaf compared to the stem, was unaffected by but decreased after urea and NaOH treatment, respectively. Crude protein (CP) content, similar between untreated straw fractions, increased after urea but decreased after NaOH treatment .Principal component scores of the leaf and stem fractions derived from a principal component analysis of the analysed variables showed that the leaf ranked higher in nutritive value than the stem before and after chemical treatment. The high leaf content of modern rice straw varieties should therefore promote the utilisation of rice straw as a ruminant feed.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Nutritional value; Leaf; Stem; Rice straw 83 (2000) 57±65
*Tel.:60-35564330; fax:60-35597681.
E-mail address: [email protected] (J. Vadiveloo).
1. Introduction
The most widely used measure of the nutritive value of a feed is its digestibility. The digestibility of the leaf of rice straw determined in vivo (Phang and Vadiveloo, 1992), in sacco (Walli et al., 1988; Nakashima and Orskov, 1990) and in vitro (Bainton et al., 1991; Vadiveloo, 1992) is characteristically lower than the stem. As leaf comprises some 70% of whole straw (Vadiveloo, 1995), low leaf digestibility reduces the overall quality of rice straw and limits its use as an animal feed. Strategies to improve feeding value, such as varietal selection (Capper, 1988; Vadiveloo, 1995) and chemical treatment (Tuen et al., 1991; Mgheni et al., 1993; Vadiveloo, 1996) have consequently focussed on securing improvements in digestibility.
Although digestibility is important, it is not the sole determinant of the nutritive value of a feed (Vadiveloo and Fadel, 1992). With low quality feeds, feed intake and compositional characteristics are also important (Madsen et al., 1997). An objective of the present study was therefore to compare the nutritive value of leaf and stem based on several measures of nutritive value. This assessment was made before and after chemical treatment as chemical treatment still remains a popular method of upgrading rice straw (Tuen et al., 1991; Mgheni et al., 1993; Vadiveloo, 1996).
Quantitative estimates of the degradation of a feed provide important information on fermentation characteristics and improve our understanding of feed utilisation. Studies on degradation kinetics have been conventionally carried out using the nylon bag (Mehrez and Orskov, 1977) or the in vitro gas production technique (Menke and Steingass, 1988; Blummel and Orskov, 1993). Both techniques require fistulated animals and a high degree of standardisation. The use of a prepared cellulase solution, which avoids these problems, has been employed to study the degradation kinetics of hays (Lopez et al., 1998). However, there are no reports in the literature of using an enzymatic procedure to determine the degradation kinetics of rice straw. In this study, a cellulase-buffer solution was used to compare the degradation of the leaf and stem fractions before and after chemical treatment.
2. Materials and methods
2.1. Rice straw
Six straw varieties, MR 84, MR 151, MR 162, MR 164, MR 166 and MR 167 hand-harvested at maturity 12 cm above the ground in June 1994 from the Tanjung Karang district of Selangor were field-dried. The relative proportion by weight of leaf blade, leaf sheath and stem (including inflorescence) in approximately 500 g of whole straw was estimated in triplicate for each variety. Straw fractions were ground to pass through a 1 mm sieve and the ground fractions hand-mixed (using a glass rod) with 4% urea or 4% sodium hydroxide (NaOH) solution in the ratio of 50 g straw to 200 ml of solution (equal to 160 g chemical/kg straw in 4000 ml of water). The treated samples were kept in screw-top plastic jars at room temperature for 21 days after which they were oven-dried at 608C to constant weight.
2.2. Chemical composition and degradability
Treated and untreated samples were analysed for total ash, crude protein (AOAC, 1984), neutral detergent fibre (NDF) and ash insoluble in neutral detergent solution (Van Soest et al., 1991). In vitro dry matter digestibility (IVD) was estimated by the cellulase-neutral detergent solution procedure of Bughrara and Sleper (1986) using the Onozuka 3S enzyme (Yakult Biochemicals).Treated and untreated straw fractions of weight 0.5 g were incubated in cellulase-buffer solution for 3, 6, 12, 24, 48 and 72 h at 388C. After incubation, the residues were washed with water, oven-dried at 1008C and weighed. Time of incubation on the cellulase digestion of the straw fractions was compared by an analysis of variance. Degradation characteristics were described by the exponential equation pab(1 ± eÿct) where p is degradation at time t(h) and a, b and c are
constants (Orskov and McDonald, 1979). Constants a, b and c represent the rapidly degradable fraction, slowly degradable fraction and fractional degradation rate, respectively.
2.3. Statistical analyses
Differences between straw fractions in chemical composition, IVD and degradation kinetics before and after chemical treatment were compared by an analysis of variance. Principal component analysis of the data on NDF, total ash, insoluble ash, IVD and CP was carried out with the following objectives:
(a) To quantify the contribution of each variable to the variation in nutritive value between the straw fractions. The sign and magnitude of the eigenvectors (linear combinations of the original variables) were examined for their relevance in explaining the nutritive values of the straw fractions, negative signs for NDF, total ash and insoluble ash contributing negatively, and positive signs for IVD and CP contributing positively, to nutritive value.
(b) To compute a single variable (principal component score) for each straw fraction that best summarised all the original variables. The principal component scores were standardised to unit variance and were used to rank the nutritive value of the straw fractions (Vadiveloo, 1995). Principal component analysis is a multivariate statistical procedure which allows several criterion variables to be used simultaneously in evaluating mean differences. Statistical analyses were run on a SAS Version 6 statistical package (Statistical Analysis Systems Institute Inc., 1987).
3. Results
The mean proportion of the straw fractions in the six varieties, their chemical composition and IVD before and after chemical treatment are shown in Table 1. Untreated leaf blade and leaf sheath were lower than stem in IVD and NDF but higher in ash content. The CP contents were similar. Urea treatment markedly increased the IVD of the leaf blade and leaf sheath but did not alter the IVD of the stem. NDF values were not
affected but insoluble ash contents were reduced. The CP was increased for all fractions following urea treatment. NaOH treatment increased the IVD and ash content, reduced NDF and CP for all fractions. Varietal differences in the composition and IVD of straw fractions were small and hence are not reported.
Statistics from the principal component analysis of the straw fractions are shown in Table 2. An examination of the eigenvectors for the first principal component for untreated and NaOH-treated straw showed that the sign and magnitude for total ash and NDF, insoluble ash and IVD were of the same magnitude but of opposite signs. Numerically therefore only the positive eigenvector for CP would make a net contribution to the component scores which effectively represented differences between straw fractions in their CP content. Leaf blade with the highest positive score was thus ranked first and stem with the highest negative score was ranked third (Table 2). Eigenvectors with negative signs for total ash, NDF and insoluble ash and positive signs for IVD and CP were found in the second principal component for untreated and NaOH treated straw. High positive scores associated with these components would mean high nutritive value Table 1
Proportion (% in whole straw), chemical composition (g/kg DM) and in vitro dry matter digestibility (IVD, g/kg DM) of untreated and chemically-treated straw fractionsa
Fraction Proportion
Leaf blade 31.1 Untreated 185 581 64 503 46
(2.10) Urea 192 584 63 704 62
NaOH 234 522 71 823 36
Leaf sheath 40.3 Untreated 191 644 59 513 29
(1.70) Urea 218 634 57 576 40
NaOH 250 554 80 821 28
Stem 27.8 Untreated 140 681 22 610 35
(1.40) Urea 155 676 32 610 46
NaOH 187 593 28 863 29
SE diff. 3.0*** 5.3*** 2.5*** 8.1*** 1.9***
a*** ±P< 0.001.
Table 2
Variance (%), eigenvectors, scores and ranks on the first and second principal components (PC)
Treatment PC % Eigenvectors Scores Rank
Total ash
NDF Insoluble ash
IVD CP Blade Sheath Stem 1 2 3
Untreated 1 77 0.47ÿ0.47 0.50 ÿ0.50 0.23 0.84 0.26 ÿ1.11 blade sheath stem 2 23 ÿ0.36ÿ0.34ÿ0.17 0.19 0.83 0.79 ÿ1.12 0.33 blade stem sheath Urea 1 65 0.24ÿ0.55 0.48 0.45 0.45 1.08 ÿ0.20 ÿ0.89 blade sheath stem
2 35 0.68ÿ0.09 0.37 ÿ0.44ÿ0.44ÿ0.40 1.14 ÿ0.74 sheath blade stem NaOH 1 78 0.47ÿ0.48 0.49 ÿ0.50 0.25 0.69 0.46 ÿ1.15 blade sheath stem 2 22 ÿ0.34ÿ0.33ÿ0.26 0.15 0.83 0.93 ÿ1.06 0.14 blade stem sheath
and high negative scores, poor nutritive value. For both these components leaf blade was ranked highest in nutritive value. The sign and magnitude of the eigenvectors on the first component of urea treated straw showed that the component scores were influenced by the net positive loading on IVD and CP. Thus, the main effect of urea treatment was to increase the IVD and CP of the straw fractions; the component scores indicate that this increase was in the order blade > sheath > stem. The magnitude of the eigenvector for NDF on the second component for urea treated straw was small and can be ignored. The positive score for leaf sheath indicates that urea treatment had a greater effect on its ash values (positive eigenvectors) than on the ash values of the stem fraction (negative score).
The mean effect of time of incubation on the cellulase degradation of straw fractions before and after chemical treatment are shown in Table 3 and Fig. 1. Treatment effects were significant (P< 0.001) for all incubation times and except for urea-treated stem, were in the order NaOH > urea > untreated. Degradation of all fractions increased cumulatively with time for all treatments.
Mean degradation characteristics of straw fractions before and after chemical treatment are shown in Table 4. Degradation characteristics (constants a,b andc) for untreated straw fractions were in the order stem > sheath > blade. Urea treatment decreased but NaOH treatment increased the rapidly degradable fraction of blade, stem and sheath. Urea treatment increased the slowly degradable fractions in the order blade > sheath > stem but the increase was the same for all fractions after NaOH treatment. Consequently the increase in potential degradability (constantsab) for urea-treated straw was in the order blade > sheath > stem, the reverse of untreated straw. The NaOH treatment affected similar increases in the slowly degradable fractions of leaf and stem. Degradation rate was significantly reduced by urea and NaOH treatment for stem and for NaOH treatment only for leaf sheath. The degradation rate for leaf blade increased after urea and NaOH treatment but was not significant (P> 0.05).
Table 3
Mean effect of time of incubation (h) on the cellulase degradation (% in DM) of straw fractions before and after chemical treatmenta
Fraction Treatment Time of incubation (h)
3 6 12 24 48 72
Leaf blade Untreated 15.3 17.8 19.8 26.1 25.1 28.4
Urea 18.0 24.7 30.6 40.3 44.3 42.7
NaOH 37.9 46.6 53.3 58.2 65.6 65.4
Leaf sheath Untreated 19.4 22.9 25.5 32.5 31.3 33.7
Urea 19.2 26.2 29.9 35.9 38.3 37.5
NaOH 40.6 45.8 54.5 59.6 69.1 69.7
Stem Untreated 27.2 32.7 36.6 44.4 43.3 45.6
Urea 24.5 30.2 34.3 41.4 43.4 44.5
NaOH 46.4 52.3 61.8 72.0 79.9 80.5
SE diff. 0.53*** 1.05*** 0.72*** 0.88*** 0.82*** 0.96***
a*** ±P< 0.001.
Fig. 1. Cellulase degradation (% in DM) of straw fractions (control^; urea&, NaOH~).
Table 4
Degradation characteristics of the straw fractions before and after chemical treatmenta
Fraction Treatment Constants Asymptote RSD
a(%) b(%) c(hÿ1) ab(%)
Leaf blade Untreated 12.4 15.8 0.0657 27.6 2.26
Urea 10.7 33.1 0.0849 43.8 1.61
NaOH 32.2 33.2 0.0795 65.0 1.73
SE diff. 0.88*** 0.87*** 0.01048ns 1.02***
Leaf sheath Untreated 15.4 17.8 0.0853 33.1 1.55
Urea 13.2 24.7 0.1061 37.9 1.70
NaOH 35.6 35.0 0.0574 68.9 2.84
SE diff. 0.83*** 1.32*** 0.00695*** 1.05***
Stem Untreated 21.0 24.0 0.1033 44.6 2.10
Urea 19.3 25.0 0.0841 44.3 2.12
NaOH 38.9 42.5 0.0647 81.3 2.27
SE diff. 1.17*** 1.09*** 0.00071*** 1.39***
a*** ±P< 0.001; ns ±P> 0.05.
4. Discussion
The IVD of the untreated straw fractions estimated by the cellulase-neutral detergent solution procedure ranged from 50% for leaf blade to 61% for stem (Table 1), similar to earlier results with 32 straw varieties (Vadiveloo, 1995) and in sacco results by Shen et al. (1998b). These data are also comparable to the dry matter digestibility of 57% and 69% obtained with supplemented rice straw leaf and stem diets fed to goats, respectively (Phang and Vadiveloo, 1992).
The lower IVD of the leaf compared to that of stem is in agreement with Malaysian (Vadiveloo, 1992, 1995) and other studies (Shen et al., 1998b; Nakashima and Orskov, 1990; Bainton et al., 1991). This was ascribed to lower values for all degradation parameters (Table 4). Urea treatment increased the IVD of leaf blade and leaf sheath through increases in the proportion of their slowly degradable fractions. Hence, only the leaf fraction recorded an improvement in potential degradability. These results obtained by a simpler cellulase technique, are corroborated by Nakashima and Orskov (1990) using the nylon bag technique on ammonia-treated rice straw and Shen et al. (1998b) using the nylon bag and in vitro gas production techniques on urea-treated straw. It is interesting that with barley straw, the results are reversed ± untreated leaf is more degradable than stem but is less improved by ammonia treatment (Ramanzin et al., 1986). Urea treatment reduced the insoluble ash content of the leaf fraction probably by an increased extraction of biogenic silica (Shen et al., 1998a). Urea treatment may also weaken the bonds between the silica-covered cuticle and underlying tissues (Bae et al., 1997) thus removing an important barrier to digestion and exposing the underlying silica-free tissues to enzymatic action.
The NaOH treatment increased the IVD of both leaf and stem fractions by increasing their rapidly and slowly degradable fractions. In contrast to urea treatment, the magnitude of the increase was similar for all straw fractions. Given the absence of similar studies in the literature, more studies need to be carried out to corroborate these findings. The enhanced degradability has been ascribed to a solubilisation of total phenolics (Chesson, 1981), arabinoxylans and cellulose (Lindberg et al., 1984) arising from the cleavage of alkali-labile lignin-carbohydrate linkages (Alexander et al., 1987; Ternrud et al., 1988).
Apart from IVD, the leaf and stem also differed in total ash, insoluble ash and NDF contents. Principal components analysis was therefore used to evaluate the relative nutritional value of the leaf and stem fractions by accounting for all parameters instead of only digestibility. Principal components analysis is a versatile though exploratory technique which has been used to rank straw varieties (Vadiveloo, 1995), interpret near infra-red spectroscopic data (Bertrand et al., 1987; Grant et al., 1988) and characterise analytical data (Martin-Alvarez et al., 1988; Martin-Alvarez and Herranz, 1991).
Principal components analysis ranked the leaf fraction higher in nutritive value than the stem before and after chemical treatment. The practical value of this result is that the increasingly high leaf content of modern rice straw varieties arising from breeding for higher grain yield and quality should provide an impetus for the utilisation of rice straw as a ruminant feed.
Acknowledgements
Technical assistance by Nik Nazri Nik Yusof, Ku Aishah Ku Din and Astifarini Buang is recorded with thanks. Financial assistance was provided by MARA University of Technology. Rice straw was provided by the Malaysian Agricultural Research and Development Institute, Tanjung Karang.
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