Improvement of organic matter digestibility along with changes of physical properties of rice straw by chemical treatments *

This study was carried out to study the effects of different chemical treatments on physical properties and digestion of rice straw. Rice straw was treated with sodium hydroxide (NaOH) at 15, 30, 45, 60 or 75 g/kg dry matter, ammonia bicarbonate (NH4HCO3) at 30, 60, 90, 120 or 150 g/kg or their combinations (10 or 20 g NaOH/kg and 30, 60 or 90 g NH4HCO3/kg), respectively. The relationship between straw digestion and its physical properties was analysed. Treatment with NaOH increased the volumetric weight (VW), swelling capacity (SC), water-holding capacity (WHC), and in vitro organic matter digestibility (OMD), and decreased specific porosity (SP) of rice straw linearly. Treatment with NH4HCO3 did not significantly affect VW, SC, or SP, but increase OMD linearly and tended decrease WHC. For the combination treatment of NaOH and NH4HCO3, there existed significant interactions on SC, SP and OMD. A significant linear correlation was found between OMD and VW, SC, SP, or WHC, when the data for all treatments were analysed. The accuracy of regression equation could be improved by including multiple physical property parameters. It is inferred that change in all physical properties is indicative of improvement in straw digestion after treatment.


INTRODUCTION
In China, annual yield of rice straw totals currently about 188 million tonnes (Guo et al., 2002).Unfortunately, low nutrient content and poor digestibility of rice straw limit its efficient use in the diets for lactating and growing ruminants.Physical, chemical and biological pretreatments have been widely applied to upgrade the crop residues (Han and Garrett, 1986;Chaudhry, 1998).It has been found that alkali treatment may reduce the lignification of straw (Xiao et al., 2001), cleavage the ester and ether linkages between hemicellulose and lignin (Lawther et al., 1996), decrease the crystallinity of cellulose (Goto and Yokoe, 1996;Isogai and Usuda, 1996), and weaken the interlinkage of the silica-covered cuticle with underling tissues (Bae et al., 1997).These changes may result in modified physical properties of straw, reflected by increased swelling capacity and water retention of straw (Goto and Yokoe, 1996), and hence render the treated straw susceptible to attack by rumen cellulolytic microbes (Xu et al., 1993;Chaudhry, 2000;Selim et al., 2004;Liu et al., 2005).
The physical properties that strongly affect fermentation pattern in human colon are mainly the particle size or bulk volume, the surface area characteristics, and the hydration.For example, particle size influences transit time, fermentation, and faecal excretion (Guillon and Champ, 2000).These physical properties can be expressed by volumetric weight, water-holding capacity, etc. (Goto and Yokoe, 1996;Sangnark and Noomhorm, 2004).Measurement of the parameters for physical properties may quantitatively reveal the mechanism with which to upgrade the crop residues by chemical treatment, and simplify the assessment of treatment efficacy.However there are few systemic studies focused on physical properties and their relationship with nutritive value of cereal straws.
Besides its resourcefulness, ammonia bicarbonate (NH 4 HCO 3 ) is cheaper and decomposes more easily than urea, which is beneficial for inhibition of mould growth during the initial stage of treatment, especially in the rainy season (Liu et al., 1992).However, one may not expect strong effects on physical properties of straw due to its week alkali effect.Sodium hydroxide (NaOH) had strong alkalinity (Han and Garrett, 1986), so the treatment with combination of NH 4 HCO 3 plus NaOH may be expected have an optimal effect.Therefore, this study was conducted to investigate the effects of NaOH, NH 4 HCO 3 or their combination treatments on physical parameters of rice straw, such as volumetric weight, swelling capacity, specific porosity and water-holding capacity, and to analyse relationships between physical properties and digestion of straw.

Preparation and treatment of rice straw
The rice straw used was from the late season rice (va.Yiu64, V 35 ), cultivated in Zhejiang province.Following the harvest, rice straw was air-dried and manually chopped to 3-5 cm lengths.Chopped straw was put into polyethylene bags in three duplicates, and mixed with different dosages of NaOH, NH 4 HCO 3 , or their combination, respectively.Dosage of NaOH was at 15, 30, 45, 60 and 75 g/kg straw dry matter (DM), respectively.Amount of water used to dissolve NaOH was controlled to adjust the initial moisture content of the treated rice straw to about 40%.After rice straw was thoroughly mixed with NaOH solution, the sealed bags with straw were left in a thermostated container at 35°C for 3 d (Han and Garret, 1986).Dosage of NH 4 HCO 3 was at 30, 60, 90, 120 and 150 g/kg DM, respectively, with initial moisture content of the treated straw at about 50%.After rice straw was thoroughly mixed with NH 4 HCO 3 solution, the materials were left in a thermostated container at 35°C for 10 d (Liu et al., 1992).To evaluate combination treatments, chopped rice straw was treated with 10 or 20 g NaOH /kg DM at 35°C for 3 d, and then with 30, 60 or 90 g NH 4 HCO 3 /kg DM at 35°C for 7 d.Untreated rice straw was used as control.

Measurement of chemical compositions
Untreated and treated rice straw were oven-dried and ground to pass a 1mm sieve for chemical analysis.Contents of crude protein (CP) and ash were determined according to AOAC (1990).Neutral detergent fibre (NDF), acid detergent fibre (ADF), and acid detergent lignin (ADL) were determined following the procedures by Van Soest et al. (1991).Total silica content was measured directly starting from the ADF procedure of the sequential analysis and termed as acid-detergent insoluble silica (ADISi) after Van Soest et al. (1991).

Measurement of physical properties
The milled samples of untreated and treated rice straws were screened with a 0.45 mm screen, and only the samples on the screen were taken for later analysis to decrease discrepancy (Zhang, 1995;Sangnark and Noomhorm, 2004).
Volumetric weight (VW).Half gram of dry ground samples (W) was put into 10 ml-scaled glass tube.The tube was gently rapped to vent air space, and the volume (V) of samples was recorded.The VW was expressed as the dry weight per volume sample (g/ml).
Swelling capacity (SC).Eight ml of 0.9% NaCl (V 0 ) was added to a scaled glass tube containing 0.5 gram of dry samples (W) inside.The volume (V m ) of the mixture was recorded after it was stood for 24 h and the air bubbles were vented.The SC was calculated using the equation: SC (ml/g) = (V m -V 0 )/W.Specific porosity (SP).The SP was calculated from the VW and SC, using the following equation: SP (ml/g) = (V -(V m -V 0 ))/W.

Water-holding capacity (WHC).
The dry samples (1.5 g) were weighed into a nylon bag (40 μm mesh, 7 × 12 cm inner size).After being soaked in 0.9 % NaCl at 37°C for 2 h, the bags with samples were centrifugated in a dehydrator at 5000 r/min for 15 min.The weights before (W wet , g) and after drying (W dried , g) at 105°C for 4 h were recorded.Weights of the empty nylon bag before (W wet bag , g) and after drying (W dried bag , g) were obtained in the same way.The WHC was calculated as below:

Estimation of in vitro organic matter digestibility (OMD)
The OMD was estimated from the in vitro gas production and crude protein content, which was measured and calculated according to Menke and Steingass (1988).The details for gas test were described elsewhere (Liu et al., 2001).

Statistical analysis
Data for treatment with NaOH or NH 4 HCO 3 alone were analysed by one-way analysis of variance, and the results for combination treatment were analysed by two-way analysis of variance, respectively (SAS, 1996).The dosage of chemical was the source of variation in NaOH or NH 4 HCO 3 treatment, and orthogonal polynomial contrasts were used to test for linear, quadratic, cubic, and quartic effects of dosage level.Orthogonal polynomial contrasts were also used to test for linear, quadratic effects of NH 4 HCO 3 level in combination treatment.The interrelations between physical property parameters and OMD for all treatments were analysed using regression procedure (REG).

Change in chemical compositions of rice straw by different treatments
Changes in chemical compositions of rice straw by NaOH, NH 4 HCO 3 , and their combination treatment are presented in Tables 1, 2 and 3, respectively.Contents of NDF, ADF, ADL and ADISi were decreased (P<0.05), and ash was increased linearly (P<0.01) with increasing NaOH dosage.Dissimilar with NaOH treatment, ash was not changed (P>0.05), and CP was increased linearly (P<0.01) with increasing dosage of NH 4 HCO 3 .The effects of NaOH on NDF, ADL, CP, and ash, and of NH 4 HCO 3 on CP were significant (P<0.01) in combination treatment, and that interaction existed between NaOH and NH 4 HCO 3 on CP (P<0.01).The change in chemical properties of rice straw by different treatments in this study is consistent with other studies (Chaudhry and Miller, 1996;Shen et al., 1998).

Change in physical properties and digestion of rice straw by different treatments
The analysis of orthogonal polynomial showed that NaOH treatment increased the VW (P<0.01), and decreased SP (P<0.01) of rice straw linearly (Table 1).Treatment with NH 4 HCO 3 did not significantly affect VW, and SP (P>0.05;Table 2).The SP was decreased with increasing NH 4 HCO 3 (P<0.05) in the combination treatment, while the interaction between NaOH and NH 4 HCO 3 affected this change (P<0.01;Table 3).The effect of NaOH, NH 4 HCO 3 or the interaction between them on the VW was not significant (P > 0.05; Table 3).
The VW is the weight of material per unit volume, and the SP is indicative of the ratio of the lacuna volume out of cell wall substantiality to the total weight of straw.The change observed in VW or SP may reflect the modification of material structure.The increased VW or decreased SP indicates material contracted, while a decreased VW or increased SP value reflects material expanded (Zhang, 1995).In this study, the significant negative correlation between VW and SP (-0.932) was observed (Table 4).Increase of the VW in NaOH-treated straw may be caused by the high permeation of sodium ion.Although it was reported that liquid ammonia had the ability to form an ammonia-cellulose complex and to decrease the crystallinity of cellulose (Isogai and Usuda, 1992), Goto and Yokoe (1996) observed that gaseous ammonia-treated barley straw had 14% lower ratio of crystalline to amorphous regions than did the untreated straw.Lack of effect of NH 4 HCO 3 on VW or SP suggests the weakness of ammonia released from NH 4 HCO 3 on straw structure.The SC of rice straw was increased linearly (P<0.01) with increasing NaOH dosage (Table 1), but it was not significantly affected by NH 4 HCO 3 treatment (P>0.05;Table 2).In the combination treatment, the SC was increased with increasing NaOH or NH 4 HCO 3 (P<0.01),while the interaction existed between NaOH and NH 4 HCO 3 (P<0.01;Table 3).Treatment with NaOH, an alkali agent, may have increased SC by reducing the lignification of straw, and/or by destruction of the covalent bond between hemicellulose and cellulose (Spencer and Akin, 1980), and/or weakening of the interlinkage of the silica-covered cuticle with underling tissues (Bae et al., 1997).
The WHC was changed quadratically (P<0.05) with the dosage of NaOH (Table 1), a maximum value being estimated to be at the 39 g NaOH /kg DM treatment.However, it was decreased linearly (P<0.01) with increasing NH 4 HCO 3 (Table 2) .Alkali treatment opens the fibre structure, making available free hydroxyl groups of cellulose to bind with water.Therefore the WHC appears more likely to reflect the degree to which ester cross-linking between cell wall polymers is disrupted by treatment allowing the wall as a whole, rather than just the cellulose microfibrils to swell (Goto and Yokoe, 1996).The quadratic change in the WHC with NaOH may be dependent on the effect of NaOH on the ester or ether linkage.The WHC of NH 4 HCO 3 -treated straw obtained in this study was not consistent with results by Goto and Yoke (1996), who showed an increased effect of gaseous ammonia on the WHC of barley straw.This inconsistency may be due to the strict particle size used in this study, and also to the exclusion of leaf blade and leaf sheath materials from the straw samples in their study.
The in vitro OMD was increased linearly (P<0.01) with NaOH or NH 4 HCO 3 , either in the single treatment (Tables 1 and 2) or in their combination treatment (Table 3).The increased OMD in pretreated rice straw with NaOH or NH 4 HCO 3 are compared with others (Liu et al., 2002).

Correlation between in vitro OMD and physical properties of rice straw
Correlation coefficients between in vitro OMD and physical property parameters of straw are presented in Table 4.There was a significant correlation between the OMD and VW (r=0.614),SC (r=0.789),SP (r=-0.702),or WHC (r=0.364).The accuracy could be improved by including multiple physical property parameters in the equation (Table 5), as indicated by the decreased residual standard derivation.The VW, SP, and WHC or SC reflects particle size, surface area characteristic, and OMD = in vitro organic matter digestibility, g/kg: SC = swelling capacity, ml/g: SP = specific porosity, ml/g: VW = volumetric weight, g/ml: WHC = water-holding capacity, g/g DM; 2 The residual standard deviation, g/kg fibre hydration of straw, respectively (Guillon and Champ, 2000;Sangnark and Noomhorm, 2004).Relationship between OMD and these parameters indicates that physical properties are closely related to digestion of straw and may be used to evaluate treatment efficiency.
A close correlation also existed among VW, SP, WHC and SC of straw (Table 4).The increased VW was associated with a decreased SP and increased WHC or SC.The straw with decreased particle size, increased surface area and enhanced fiber hydration may be easily penetrated by the rumen fluid and attached by the rumen microbes (Xu et al., 1993;Goto and Yokoe, 1996;Selim et al., 2002), and hence improved digestion may be expected for the treated straw.

CONCLUSIONS
Treatment of rice straw with NaOH increases volumetric weight, swelling capacity and water-holding capacity, and decreases the specific porosity.Effects of NH 4 HCO 3 treatment on physical properties of rice straw are inferior to those with NaOH treatment.Significant correlation exists between in vitro organic matter digestibility and physical property parameters.It is concluded that change of physical properties is indicative of improvement in straw digestion after chemical treatments.

Table 1 .
Effects of sodium hydroxide treatment on chemical compositions, physical properties and in vitro organic matter digestibility of rice straw Item

Table 4 .
Correlation coefficients between physical property parameters and in vitro organic matter digestibility of rice straw 1

Table 5 .
Relationship between in vitro organic matter digestibility and physical properties parameters