Rice straw, the role of silica and treatments to improve quality

Rice straw is unique relative to other cereal straws in being low in lignin and high in silica. Unlike other cereal straws taller varieties of rice straws tend to be leafy while the leaves are less digested than stems. This may contribute to higher straw value with rice yield. There is genetic variation in straw quality but has not been exploited and tends to be smaller than environmental variation. Effort in plant breeding has been to develop short varieties with higher grain yield. This development has reduced straw quantity but not nutritive value. The relationship between plant genetics and silica metabolism is virtually uninvestigated, although reviews from plant physiology indicate it is a major factor.

Silica and lignin in that order are the primary limiting factors in rice straw quality. Silicon is a nutrient element which has been overlooked largely because of its assumed inertness, but also because of its geochemical abundance that so greatly exceeds its metabolic use by plants and animals. Silicon is involved in several major roles in rice: carbohydrate synthesis, grain yield, phenolic synthesis and plant cell wall protection. These vectors interact with each other to eliminate statistical association of silica and lignin with straw digestibility when varieties are compared. Yield of grain is highly related to silica content of straw, which reflects soil availability. There are no detailed studies on rice straw lignin. Most papers reporting lignin contents in rice straw have used acid-detergent lignin by either the sulfuric acid or permanganate versions. There are undoubtedly soluble phenolics in rice straw that need investigation. The effects of ammonia and urea on silica is to crack the silicified cuticular layer. Silica is not dissolved by these reagents in contrast to the action of sodium hydroxide.

Treatments on rice straw follow those applied to other lignified materials. In order of frequency of reports, urea followed by ammonia with comparatively fewer papers on sodium hydroxide, steam and pressure treatments or exploded by pressure release, and only one or two papers on acid treatments and white rot fungi. There are reports on animal supplementation and a few growth studies with young animals. Field surveys in India and the southeast Asian countries only report the use of urea, although it appears less efficient than ammonia. Farmer acceptance is related to their perceptions on costs, labor, equipment, health, safety, i.e. the exposure to ammonia vapor, economic and other social factors. The various papers reporting treatments have used animal digestion trials; a variety of in sacco, in vitro digestions with rumen organisms or cellulase, some in combination with pepsin digestion or neutral-detergent extraction. Gas production from in vitro rumen fermentation has also been used. Results are expressed mainly on dry matter basis and fewer reports on organic matter. Results are difficult to compare and standardization of procedures is badly needed. However, most treatments with ammonia and urea show some increase in digestibility and intake where measured in in vivo trials. In vitro and in sacco evaluations tend to overestimate improvement in digestibility.     

Wheat as a dual crop for biorefining: Straw quality parameters and their interactions with nitrogen supply in modern elite cultivars

Agricultural residues, such as straw, offer an opportunity to produce biofuels and chemicals in biorefineries without compromising food production. The ideal “dual‐purpose cultivar” would have high yield of grain and straw. In addition, the straw should be easy to process in a biorefinery: It should have good degradability, high concentration of carbohydrates, and low concentration of ash. Nitrogen (N) is an essential nutrient important for plant growth, crop yield and grain quality. However, N production and application comes with a high cost and high environmental footprint. The N application should consequently be based on an economical optimum. Limited knowledge exists on how N application affects the potential of straw for biorefining, for example, straw yield and quality. This study, conducted over three cropping seasons, investigated the effect of N supply on the biorefining potential and included 14 wheat cultivars and one triticale cultivar. The N supply directly affected the yield of straw and grain. In addition, the protein concentration in grain and straw increased, but the composition of the straw with respect to carbohydrates and lignin was largely unaffected by N supply. The only significant change was a lower silicon concentration at increasing N application rate, which could be beneficial for lignin valorization in biorefineries. Likely due to the negligible changes in cell wall composition, the effect of N application rate on straw degradability was not significant. N application should therefore primarily be optimized with respect to grain quality and overall yield of grain and straw. Differences between cultivars were also minor with respect to their performance in a biorefinery process. From a breeding and agronomic perspective, focus should therefore be put on maximizing the biomass output from the field, that is, selecting the cultivar with highest grain and straw yield and optimizing the application of fertilizer to get optimum N use efficiency.     

Purified lignin: Nutritional and health impacts on farm animals—A review

Lignin, the second most abundant natural compound after cellulose (Boudet and Grima-Pettenati, 1996), is a high-molecular weight polymer of phenolic compounds that occurs naturally in plants. It is mostly present in the cell wall, conferring structural support, impermeability and resistance to microbial attack. Commercial purified lignin is produced as a by-product of the paper industry, separated from wood by chemical pulping processes. These purified lignins are low molecular weight mono-phenolic fragments that have biological characteristics that differ from those of native lignin. Different chemical treatments during wood-pulping processes yield diverse types of purified lignin, such as Alcell lignin and Kraft lignin. Although these phenolic fragments may potentially have important applications in animal agriculture, research with purified lignin has not received much attention and there are few published results. In contrast to native lignin, purified lignin does not represent a barrier to digestion in monogastric or ruminant animals. Several in vitro and in vivo studies have demonstrated antimicrobial properties of the phenolic fragments in purified lignin. Recently, purified Alcell lignin has been shown to exhibit prebiotic effects in chickens, favouring growth of beneficial bacteria and improving the morphological structures of the intestines, as measured by increased villi height and goblet cell number. These findings suggest that purified lignin may exert health benefits in monogastric animals and could potentially be considered as a natural feed additive. Based on the few published studies, animal responses to purified lignin seem dependent on dosage, animal species and type and source of the lignin product. More research is required before establishing conclusive benefits of purified lignin on animal performance and health.     

Biodelignification of wheat straw and its effect on in vitro digestibility and antioxidant properties

A variety of methods for feed development have been introduced during last few years. Bioprocessed agricultural residues may prove a better alternative to provide animal feed. For the purpose, some white rot fungi were allowed to degrade wheat straw up to 30 days under solid state conditions. Several parameters including loss in total organic matter, ligninolysis, in vitro digestibility of wheat straw and estimation of different antioxidant activities were studied. All the fungi were able to degrade lignin and enhance the in vitro digestibility. Among all the tested fungi, Phlebia brevispora degraded maximum lignin (30.6%) and enhanced the digestibility from 172 to 287 g/kg. Different antioxidant properties of fungal degraded wheat straw were higher as compared to the uninoculated control straw. Phlebia floridensis found to be more efficient organism in terms of higher antioxidant activity (70.8%) and total phenolic content (9.8 mg/ml). Thus, bioprocessing of the wheat straw with the help of these organisms seems to be a better approach for providing the animal feed in terms of enhanced digestibility, higher protein content, higher antioxidant activity and availability of biomass.     

Towards understanding the nature of algal inhibitors from barley straw

The algal inhibitors released from barley straw decomposing in water and providing the basis for its use in algal control could be either of microbial origin or derived from straw components. We report here that unrotted straw releases algal inhibitors if finely chopped or autoclaved, providing further support for the view that straw, and not microbial colonists, is the primary source of inhibitors. Further support is also provided for the suggestion that inhibitors are or derive from oxidised lignin. Comparisons of lignin-enriched wood (brown-rotted) with lign-indepleted wood (white-rotted) from various deciduous trees show high antialgal activity of the former and little or no activity of the latter. Preliminary studies have shown that solubilised lignin is present in the liquor from rotted barley straw and brown-rotted wood. Since, however, the antialgal effects of deciduous leaf litter appear to depend initially on release of tannins and given that alkaline, oxidising conditions are usually essential for antialgal activity, it is proposed that oxidised polyphenolics, derived from lignin or tannins, are a source of algal inhibitors from plant litter.     

Progress in biopulping of non-woody materials: Chemical, enzymatic and ultrastructural aspects of wheat straw delignification with ligninolytic fungi from the genus Pleurotus

Abstract: During screening of basidiomycetes for wheat straw delignification, considerable lignin degradation with a limited attack to cellulose was attained with Pleurotus eryngii . Straw solid-state fermentation (SSF) was optimized, and the enzymatic mechanisms for lignin degradation were investigated. No lignin peroxidase was detected under liquid or SSF conditions, but high laccase and aryl-alcohol oxidase levels were found. The latter enzyme has been fully characterized in PI. eryngii and it seems to be involved in a cyclic redox system for H₂0₂ generation from aromatic compounds. Results obtained using homoveratric acid suggest that Pleurotus laccase could be involved in degradation of phenolic and non-phenolic lignin moieties. Histological and ultrastructural studies provided some general morphological characteristics of the fungal attack on wheat straw. Whereas a simultaneous degradation pattern was observed in straw treated with Phanerochaete chrysosporium , PI. eryngii caused partial degradation of middle lamella and separation of individual sclerenchymatic fibers. When these straw samples were subjected to refining tests, energy saving after biological treatment was the highest in the case of straw treated with PI. eryngii , which also produced the lowest substrate loss. From these results, a correlation between preferential removal of lignin, separation of sclerenchymatic fibers and pulping properties was provided during fungal treatment of wheat straw.     

Use of 'natural' products as alternatives to antibiotic feed additives in ruminant production

The banning in 2006 of the use of antibiotics as animal growth promoters in the European Union has increased demand from producers for alternative feed additives that can be used to improve animal production. This review gives an overview of the most common non-antibiotic feed additives already being used or that could potentially be used in ruminant nutrition. Probiotics, dicarboxylic acids, enzymes and plant-derived products including saponins, tannins and essential oils are presented. The known modes of action and effects of these additives on feed digestion and more especially on rumen fermentations are described. Their utility and limitations in field conditions for modern ruminant production systems and their compliance with the current legislation are also discussed.     

Negative effects of tropospheric ozone on the feed value of rice straw are mitigated by an ozone tolerance QTL

High surface ozone concentrations are recognized as an emerging threat to food security in Asia. This study aimed at determining the effects of ozone on the nutritive quality of rice straw, a by‐product of rice grain production and a major feed resource for ruminant livestock. Further, the question was addressed whether negative effects of ozone can be mitigated through molecular breeding. Rice plants from three different genotypes were exposed to four different ozone treatments in fumigation chambers from transplanting to maturity. These genotypes were (i) IR64, one of the most wide spread indica varieties in the world, (ii) Nipponbare, a typical japonica variety, and (iii) SL41, an ozone tolerant breeding line that carried chromosomal inserts at the ozone tolerance QTL OzT9 in the genetic background of Nipponbare. The treatments consisted of (i) charcoal filtered air, (ii) simulated ambient ozone concentration, (iii) 2 × ambient ozone concentration, and (iv) 2.5 × ambient ozone concentration. The effects of ozone on the chemical composition of straw were clearly dependent of the ozone level, and were significant even at ambient ozone concentration. Increases in crude ash, lignin and phenolics concentration adversely affected the digestibility as demonstrated in incubation experiments simulating rumen digestion in vitro. Negative ozone effects included reductions in the rate and extent of gas production due to inhibition of microbial fermentation, reduced formation of short chain fatty acids (SCFA), and a decrease in the true organic matter digestibility. The ozone tolerant genotype SL41 was less responsive to ozone than its more susceptible recurrent parent Nipponbare in terms of lignin and phenolics formation, organic matter digestibility and SCFA production. These data demonstrate that the feed quality of rice straw is affected by ozone even at ambient concentration, and that these negative effects are mitigated by the ozone tolerance QTL OzT9.     

RNAi‐suppression of barley caffeic acid O‐methyltransferase modifies lignin despite redundancy in the gene family

Caffeic acid O‐methyltransferase (COMT), the lignin biosynthesis gene modified in many brown‐midrib high‐digestibility mutants of maize and sorghum, was targeted for downregulation in the small grain temperate cereal, barley (Hordeum vulgare), to improve straw properties. Phylogenetic and expression analyses identified the barley COMT orthologue(s) expressed in stems, defining a larger gene family than in brachypodium or rice with three COMT genes expressed in lignifying tissues. RNAi significantly reduced stem COMT protein and enzyme activity, and modestly reduced stem lignin content while dramatically changing lignin structure. Lignin syringyl‐to‐guaiacyl ratio was reduced by ~50%, the 5‐hydroxyguaiacyl (5‐OH‐G) unit incorporated into lignin at 10‐–15‐fold higher levels than normal, and the amount of p‐coumaric acid ester‐linked to cell walls was reduced by ~50%. No brown‐midrib phenotype was observed in any RNAi line despite significant COMT suppression and altered lignin. The novel COMT gene family structure in barley highlights the dynamic nature of grass genomes. Redundancy in barley COMTs may explain the absence of brown‐midrib mutants in barley and wheat. The barley COMT RNAi lines nevertheless have the potential to be exploited for bioenergy applications and as animal feed.     

Delignification of straw with ozone to enhance biodegradability

Summary Wheat straw was treated with ozone to remove the lignin and increase its biodegradability. The attack of ozone on straw is not selective. Lignin and carbohydrates are oxidized concurrently though the rate of reaction with the latter is slower. A 50% reduction of the original lignin content is optimal for enzymatic hydrolysis. After treatment, 75% of the cellulose in straw is degraded within 24 h as compared to 20% in untreated straw. During ozonation lignin is converted to soluble products which to a great extent are biodegradable and thus yield a useful byproduct. At the moment, ozonation ranks among the more expensive methods of treatment. However, the economics may be improved by reducing the cost of ozone production; this is likely to take place in the near future due to technological improvements, and by reducing the ozone consumption by optimizing the process of ozonation.     

Solid-state fermentations for biological delignification

Biological delignification, using white-rot fungi to liberate cellulose and hemicellulose from their complex with lignin, can aid the use of lignocellulose as ruminant animal feed, a source of sugars, and pulp. Solid-state fermentation is the method of choice for biological delignification. Lignin-degrading fungi, their ecophysiological requirements, and the design of solid-state fermenters for delignification are discussed. Capital and operating costs for solid-state fermentation can be kept low, and the lignocellulosic substrate is likely to be the major component of the cost of the delignified product. Experience in the operation of biological delignification processes at pilot plant or larger scale is needed to establish realistic process costs.     

The evolution of phylogenetic differences in the efficiency of digestion in ruminants

This study investigates, for the first time (to our knowledge) for any animal group, the evolution of phylogenetic differences in fibre digestibility across a wide range of feeds that differ in potential fibre digestibility (fibre to lignin ratio) in ruminants. Data, collated from the literature, were analysed using a linear mixed model that allows for different sources of random variability, covariates and fixed effects, as well as controlling for phylogenetic relatedness. This approach overcomes the problem of defining boundaries to separate different ruminant feeding styles (browsers, mixed feeders and grazers) by using two covariates that describe the browser-grazer continuum (proportion of grass and proportion of browse in the natural diet of a species). The results indicate that closely related species are more likely to have similar values of fibre digestibility than species that are more distant in the phylogenetic tree. Body mass did not have any significant effect on fibre digestibility. Fibre digestibility is estimated to increase with the proportion of grass and to decrease with the proportion of browse in the natural diet that characterizes the species. We applied an evolutionary model to infer rates of evolution and ancestral states of fibre digestibility; the model indicates that the rate of evolution of fibre digestibility accelerated across time. We suggest that this could be caused by a combination of increasing competition among ruminant species and adaptation to diets rich in fibre, both related to climatically driven environmental changes in the past few million years.     

Pretreatment and enzymatic hydrolysis of wheat straw (Triticum aestivum L.)--the impact of lignin relocation and plant tissues on enzymatic accessibility

Wheat straw is a potential feedstock for bioethanol production. This paper investigates tissues from whole internode sections subjected to hydrothermal pretreatment at 185 °C and subsequent enzymatic hydrolysis up to 144 h. Analyses revealed an increase in surface lignin as hydrolysis progressed, which could be coupled to the gradual decrease in hydrolysis rate over time. The data support the hypothesis of lignin extraction from the cell wall matrix during pretreatment and deposition as droplets upon cooling. These droplets are assumed to accumulate during enzymatic hydrolysis. Additionally, after 144 h of enzymatic hydrolysis the cortex had vanished, exposing the heavier lignified vascular tissue. Accumulation of lignin droplets and exposure of residual lignin could be part of the explanation for the decreasing hydrolysis rate. Flattening of macrofibrils after pretreatment together with more indentations on the surfaces was also observed, possibly caused by a proposed synergistic effect of cellobiohydrolases and endoglucanases.     

白腐菌及黑曲霉所产的纤维素复合酶对稻草秸秆的生物降解

研究了白腐菌及纤维素复合酶对稻草秸秆的协同生物降解。结果表明,利用黄孢原毛平革菌固态发酵稻草秸秆的过程中,LiP和MnP的最大活力可以达到28.3U/g和12.6U/g,同时,秸秆中的木质素能被有效降解,但纤维素、半纤维素降解率较低。添加黑曲霉所产的纤维素复合酶能有效地促进秸秆腐熟程度。在接入白腐菌培养10天后,每克稻草添加3 IU纤维素酶液并酶解48h可以使稻草秸秆中纤维素降解53.8%,半纤维素降解57.8%,木质素降解44.5%,干物质损失46.3%。此时细胞壁出现大范围破损,整个组织变得松散,秸秆完全腐熟。     

Characterization of enzymatic saccharification for acid-pretreated lignocellulosic materials with different lignin composition

The enzymatic saccharification of three different feedstocks, rice straw, bagasse and silvergrass, which had been pretreated with different dilute acid concentrations, was studied to verify how enzymatic saccharification was affected by the lignin composition of the raw materials. There was a quantitatively inverse correlation between lignin content and enzymatic digestibility after pretreatment with 1%, 2% and 4% sulfuric acid. The lignin accounted for about 18.8–21.8% of pretreated rice straw, which was less than the 23.1–26.5% of pretreated bagasse and the 21.5–24.1% of pretreated silvergrass. The maximum glucose yield achieved, under an enzyme loading 6.5 FPU g^?1 DM for 72 h, was close to 0.8 g glucose/g glucan from the enzymatic hydrolysis of the pretreated rice straw; this was twice that from bagasse and silvergrass. A decrease in initial rate of glucose production was observed in all cases when the raw materials underwent enzymatic saccharification with 4% sulfuric acid pretreatment. It is suggested that the higher acid concentration led to an inhibition of β-glucosidase activity. Fourier transform infrared (FTIR) spectroscopy further indicated the chemical properties of the rice straw and silvergrass become more hydrophilic after pretreatment using 2% of sulfuric acid, but the pretreated bagasse tended to become more hydrophobic. The hydrophilic nature of the pretreated solid residues may increase the inhibitive effects of lignin on the cellulase and this could become very important for raw materials such as silvergrass that contain more lignin.     

Solid-state production of biopulp by Phanerochaete chrysosporium using steam-exploded wheat straw as substrate.

A novel material for biopulp-making, steam-exploded wheat straw (SEWS), was studied. During the steam explosion process, the hemicellulose was partly degraded and became water-soluble sugar as the carbon resource of the chosen microbe growth, and compared with non-SEWS, the degradation amount of cellulose decreased and the degradation amount of lignin increased for the fermented steam-exploded wheat straw (FSEWS) cultured with Phanerochaete chrysosporium ME-446. Under the optimum conditions of solid-state ferrmentation (SSF), the degradation amount of lignin reached 60% on the 5th day and the fermented straw residue could be used directly as the material for pulp making.     

Rapid Degradation of Isolated Lignins by Phanerochaete chrysosporium

Phanerochaete chrysosporium degraded purified Kraft lignin, alkali-extracted and dioxane-extracted straw lignin, and lignosulfonates at a similar rate, producing small-molecular-weight (~1,000) soluble products which comprised 25 to 35% of the original lignins. At concentrations of 1 g of lignin liter⁻¹, 90 to 100% of the acid-insoluble Kraft, alkali straw, and dioxane straw lignins were degraded by 1 g of fungal mycelium liter⁻¹ within an active ligninolytic period of 2 to 3 days. Cultures with biomass concentrations as low as 0.16 g liter⁻¹ could also completely degrade 1 g of lignin liter⁻¹ during an active period of 6 to 8 days. The absorbance at 280 nm of 2 g of lignosulfonate liter⁻¹ increased during the first 3 days of incubation and decreased to 35% of the original value during the next 7 days. The capacity of 1 g of cells to degrade alkali-extracted straw lignin under optimized conditions was estimated to be as high as 1.0 g day⁻¹. This degradation occurred with a simultaneous glucose consumption rate of 1.0 g day⁻¹. When glucose or cellular energy resources were depleted, lignin degradation ceased. The ability of P. chrysosporium to degrade the various lignins in a similar manner and at very low biomass concentrations indicates that the enzymes responsible for lignin degradation are nonspecific.     

Competition of wild oat with wheat in comparison to the wheat itself

In a glasshouse experiment, an increase of the number of wheat plants per pot caused the plants to became taller, have more ears and a greater grain yield per pot, while the number of tillers decreased and the straw mass did not change. The N and P contents in straw and N in grain also trended to decrease, while the translocation of these nutrients to the grain increased. The increase of wild oat plants (Avena sterilis spp.macrocarpa Mo.) per pot, produced a decrease of the growth attributes, grain yield and N accumulation in grain of wheat per pot. Wild oat competition with wheat was higher than the wheat competition with itself. Such competition affected the height, number of tillers and ears, the fertility index of the shoots, the straw and grain mass, and the total accumulation of N, P and K nutrients per wheat plant.     

采自武夷山高温木质素降解菌优良菌株筛选及其木质素降解效果测定

本文旨在筛选能够高效降解秸秆木质素的高温真菌。对来自福建武夷山的农田土壤进行富集,采用苯胺蓝、愈创木酚和α-萘酚3种筛选平板结合木质素磺酸钙降解试验筛选木质素高温降解菌,采用范氏洗涤剂法测定一株高效降解菌对秸秆木质素的降解效果;最后以经典形态学和多基因分子系统学相结合的方法对该菌株进行鉴定。结果表明：经钓饵法,分离获得8株高温菌;通过初筛和复筛,获得了1株较好的木质素高温降解菌A12638H;将其用于降解水稻秸秆和玉米秸秆,发现木质素降解率分别达到41.7%和48.3%;该菌株经鉴定为大孢戴氏霉Taifanglania major。菌株A12638H具有很好的应用价值,值得在秸秆资源的开发利用中开展更深入的研究。