CHAPTER 2 - COMPOSITION, NUTRITIVE VALUE AND UPGRADING OF CROP RESIDUES

Meng Qingxiang
China Agricultural University

Introduction

Crop residues usually consist of the aboveground part of cereal plants after grain removal. They are potentially rich sources of energy because up to 80 percent of their dry matter (DM) consists of polysaccharides. Due to the prevalence and intensity of agriculture in most regions of China, crop residues represent a high proportion of total feed for herbivores. It is estimated that 550 million tonne of these resources are available annually in China (Feng Yanglian, 1996). However, they are not all well utilized as energy sources at present, since their digestibility is often low. They partly resist rumen microbial action so their digestion is far from complete. Due to their rigid structure and poor palatability, intake of crop residues is low. These constraints are mostly related to their specific cell wall structure and chemical composition, but there are also deficiencies of nutrients essential to ruminal micro-organisms, such as nitrogen, sulphur, phosphorus and cobalt.

In this section, the current understanding of crop residue structure is reviewed, with particular attention to the chemical and physico-chemical characteristics that influence their digestibility in ruminants. Subsequently, treatments for improving feeding value are discussed.

Botanical structure and cell wall chemical composition

Botanical structure and digestion differences

As parts of plants, crop residues contain five different tissue types: (a) vascular bundles containing phloem and xylem cells; (b) parenchyma bundle sheaths surrounding the vascular tissue; (c) sclerenchyma patches connecting the vascular bundles to the epidermis; (d) mesophyll cells between the vascular bundles and epidermal layers; and (e) a single layer of epidermal cells covered by a protective cuticle on the outside. These tissues are digested to different degrees in the rumen. In general, the extent of tissue digestion by ruminal bacteria is as follows: mesophyll and phloem > epidermis and parenchyma sheath > sclerenchyma > lignified vascular tissue. These differences in tissue digestion explain the wide range in nutritive value of crop residues compared to conventional feeds (Minson, 1990).

The cells have two major components: contents and walls. The cell content fraction contains most of the organic acids, soluble carbohydrates, CP, fats and soluble ash. The cell wall fraction includes hemicellulose, cellulose, lignin, cutin and silica. In most crop residues, the cell wall fraction accounts for 60-80 percent of dry matter (DM) (Xiong Yiqiang, 1986).

Chemical composition of cell walls

Cell walls of crop residues consist mainly of polysaccharides, protein and lignin. These substances, with small amounts of other components, like acetyl groups and phenols, are organized in a complex three-dimensional structure. Other wall components include suberin, cutin, tannins, waxes and minerals.

Polysaccharides

Major polysaccharides in primary cell walls of most higher plants include cellulose, xyloglucan and pectic polysaccharides, while secondary cell walls contain mainly cellulose and xylans.

Cellulose is a highly ordered linear homopolymer of glucose linked by b-1,4-bonds. In all higher plants, cellulose in primary and secondary walls exists in the form of microfibrils. The crystallinity of cellulose microfibrils is highly variable depending on the source and age of the tissue. In primary walls, the crystallinity has been estimated to be 20-30 percent, while in secondary walls it is 40-70 percent. The crystallinity of the cellulose in wheat internode walls is 30-50 percent and does not change markedly with maturity. Cellulose in mesophyll cell walls from ryegrass leaves, although fibrillar in appearance, had a low crystallinity index (ª1) compared with >12 for cotton mesophyll cells and 4-5 for non-mesophyll cells. Cellulose molecules in primary walls are heterogeneous in their degree of polymerization (DP), between 2-6 000, but in secondary walls they are longer and more homogeneous (DP = 14 000). Cellulose from maize stalk and wheat straw has DP values of 6-7 000 (Lam et al., 1990).

Hemicelluloses are a wide group of polysaccharides that basically share only the property of being soluble in dilute alkali and being able to bind to cellulose by multiple hydrogen bonds and to bind to lignin by covalent bonds. In grasses, the main fraction of hemicellulose is xylans, with a backbone of 4-linked xylose residues and short side chains of arabinose, glucuronic acid and 4-O-methyl-glucuronic acid residues. Most of xylose residues in higher plants are acetylated, mainly on the C-2 hydroxyl groups, but also on C-3. Hemicellulose polysaccharide concentrations in grasses can range anywhere from 150 to 400 g/kg DM, whereas in legumes, the concentration is much lower, generally between 80-150 g/kg DM. For both grasses and legumes, xylose usually comprises half or more of total sugars of hemicellulosic fraction. Furthermore, rhamnose only exists in the hemicellulosic polysaccharides of legumes.

Pectic polysaccharides are present in the primary cell walls of all seed bearing plants and are located particularly in the middle lamella. They are the major components of the primary cell walls of dicotyledons (e.g. legumes) but account for relatively less of the primary walls in monocotyledons (grasses). Three pectic polysaccharides have been structurally characterized from the primary walls of both monocotyledons and dicotyledons: rhamnogalacturonan I, rhamnogalacturonan II, and homogalacturonan (O'Neill et al., 1990). Pectic polysaccharide concentration is quite low in grasses (monocotyledons), generally <10 to 40 g/kg DM, while fairly high in legumes (dicotyledons) ranging from 50 to 100 g/kg (Van Soest, 1994). The distribution of the different pectic polysaccharides within the cell walls is largely unknown.

Proteins

Proteins make up 2 to 10 percent of the primary cell wall of many dicotyledons and some monocotyledons, and may become cross-linked by the formation of isodityrosine or dityrosine. Cell wall proteins may also be involved in covalent bonding with polysaccharides. Glycoproteins seem to be invariably found in primary cell walls. Apparent covalent protein-lignin linkages have also been observed in wheat internodes. Of the several types of structural proteins known, the best-characterized are the family of hydroxyproline-rich proteins, or extensin. These glycoproteins with rod-like conformations are components of the wall matrix in dicotyledons and in grass walls (e.g. maize pericarp). Other wall proteins, e.g. glycine-rich proteins, have been found in walls of herbaceous dicotyledons (Lamport, 1977).

Lignin

Lignin represents between 5-20 percent of crop residue DM. Lignin is described as three-dimensional networks of phenylpropane units. It is generally recognized that the precursors of these building stones are coniferyl, sinapyl, and p-coumaryl alcohols, which are transformed into lignin by a complex dehydrogenative polymerization process. These three aromatic monomers in lignin are referred to as p-hydroxyphenyl, guaiacyl and syringyl residues, respectively. Depending upon the number and type of functional groups on the aromatic rings and propane side chains, lignin has variable solubilities. Wheat straw lignin has higher alkali solubility than wood lignin. When wheat straw lignin is methylated with diazomethane, the number of free phenolic hydroxyl groups in the guaiacyl monomeric units resembled that in pine lignin (Lapierre et al., 1988). Grass lignin is esterified by cinnamic acids, chiefly p-coumaric acid through hydroxyls on its monomers. In addition, ether-linked ferulic acids have been observed in lignin from maize stalks, wheat straw, rice straw and bagasse (Lam et al., 1990).

Lignin in plant cell walls is physically and chemically associated with wall polysaccharides and proteins. The association between lignin and polysaccharides includes glycosidic linkages, ether cross-linkages, ester cross-linkages and cinnamic acid bridges. On the association between lignin and proteins in straws, limited information is available. Only a covalent protein-lignin linkage was reported in wheat internodes (Iiyama et al., 1993). The strong linkage between lignin and polysaccharides or proteins would definitely prevent cell wall components from enzymatic hydrolysis by ruminal micro-organisms, and thus limit the digestion of cell walls.

Others

Other components - including cutin, suberin, tannins, waxes and minerals - are also found in the cell walls. Cutin and waxes are attached to the epidermal walls on plants surface. Cutin is a three-dimensional polyester composed of w-hydroxy and mid-chain hydroxy fatty acids. It is often esterified with phenolic acids, and maintains a close association with pectin in the epidermal cell walls. Cutin appears to be embedded in wax and pectin; these components serve as diffusional barriers that impede ruminal digestion of the intact tissue. Suberin is a functional component of cell walls. The polyesters that appear in suberized tissue can be esterified with phenolic monomers, oligomers or lignin. Silicon is an important inorganic element in plant cell walls and mainly present in the form of silica in the walls of epidermal cells and leaf hairs. The presence of silica in the cell wall of rice straw can limit the rumen digestibility of polysaccharides. Tannins are phenolic compounds synthesized by some plants as a defence. They may inhibit the activity of specific enzymes, such as cellulases. Since tannins are often insoluble, they can contaminate the crude lignin, resulting in higher analytical value. As a result of complexes with protein, tannins would depress its utilization, but may not affect cell wall carbohydrates.

Nutritive value of crop residues

Nutritive value is generally determined by feed composition, intake and utilization efficiency of digested matter. Thus, the value of a feed depends on chemical composition, digestibility, intake and efficiency.

Table 2-1 contains the nutrient content of some cereal crop residues in China. All of these residues, with the exception of peanut hay, have insufficient CP for efficient rumen fermentation (<9 percent of DM). Apart from sweet potato vines, all have a high crude fibre (CF) or cell wall content (in terms of neutral detergent fibre (NDF)) and low available energy (e.g. NEL). Such high fibre contents are believed to be negatively correlated with voluntary intake, rate of organic matter fermentation, microbial cell yield per unit organic matter fermented, and propionate: acetate ratio in fermentation end products. Crop residues also have a low mineral content, especially P, and are deficient in vitamins. Therefore, supplementation of crop residues before feeding is necessary, in addition to various treatments. In rice straw, another unwanted constituent is oxalic acid, which can cause rumen disorders. However, oxalic acid can be eliminated by alkali treatment, washing or ensiling.

Various crop residues have their own nutritional values and are used for different animal species. Sweet potato vines and peanut hay are relatively rich in protein, available energy and vitamins, and are mainly fed to pigs in most rural areas. According to a survey conducted by Zhou Meiqing (1994), partially feeding pigs with fibrous feeds, including peanut hay and/or sweet potato vines, fresh, dried or ensiled, is a popular practice in Sichuan province, where close to 100 million swine are marketed each year. In this feeding system, fibrous feeds can meet a large percentage of the nutrient requirements: 40 percent of digestible energy, 70 percent of proteins and 80 percent of minerals and vitamins. Wheat straw and rice straw have high contents of cell walls, and are basically used for feeding ruminants. Millet straw and soybean straw, in contrast, are fairly palatable for herbivores, and are mostly used as feed sources for horses, donkeys, mules and rabbits.

Table 2-1. Nutrient content of some crop residues

SOURCES: Chinese Feeding Standard for Dairy Cattle (Anon., 2000); Xing, 1995; Xiong, 1986.
KEY TO ABBREVIATIONS: DM = dry matter; NE_L = net energy for lactation; CP = crude protein; EE = ether extract; CF = crude fibre; CW = cell wall or NDF; Ca = calcium; P= phosphorus.

Table 2-2. In situ DM and NDF digestibility (%; 48 hour) of some crop residues

SOURCES: (1) Xing Tingxian, 1995; (2) Feng Yanglian, 1996.
KEY: ISDMD = in situ dry matter digestibility; ISNDFD = in situ NDF digestibility.

Considerable information on DM digestibility of crop residues was obtained from universities and research institutes in China. From the data presented in Table 2-2, it is clear that in situ DM and NDF digestibilities of wheat straw and rice straw are lower than in other residues.

The relatively high lignin content in those residues is probably responsible, at least to some extent, for the limited cell wall digestibility.

Since the 1950s, many in vivo digestibility experiments and feeding trials have been conducted in China to determine nutritive values of crop residues. Some results have been summarized elsewhere (Feng Yanglian, 1996; Meng Qingxian, 1990; Xiong Yiqiang, 1986; Bian Sibei et al., 1999). It has been concluded that without treatment or nutrient supplementation, feeding most crop residues can just, or barely, meet maintenance energy requirements.

Factors affecting the nutritive value of crop residues

A variety of factors have been identified that may influence nutritive value of crop residues. From literature reports and our experience, factors can be divided into three categories: plant, animal and environmental.

Plant factors

The lignin fraction and associated phenolic compounds are factors most consistently associated with the rigid structure of plants and limited accessibility. The association of lignin with cell wall polysaccharides is also believed to be responsible for resistance of plant cell walls to microbial digestion in the rumen. Table 2-3 shows the main chemical composition and in vitro DM digestibility of three major crop residues widely used in China.

Table 2-3. Composition and in vitro DM digestibility of three major crop residues

SOURCE: Xing Tingxian, 1995.
KEY: DM = dry matter; CP = crude protein; NDF = neutral detergent fibre; NDS = neutral detergent soluble; ADF = acid detergent fibre; CEL = cellulose; HC = hemicellulose; ADL = acid detergent lignin; IVDMD = in vitro dry matter digestibility.

Wheat straw has higher lignin, and therefore lower DM digestibility, compared with maize stover. Although rice straw has a medium lignin content, its DM digestibility is rather low, which may be caused by its relatively high silica concentration.

Other plant factors include species, stage of maturity at harvest, cultivar, and proportions of leaf, sheath and stem. All these are believed to influence the nutritive value of crop residues. As plants mature, nutrient digestibility generally declines, linked to a decrease in the digestibility of cell wall components. Xing Tingxian (1995) reported that, at an early growth stage, in vitro DM digestibility (IVDMD) of wheat straw is pretty high. As wheat matures, however, the IVDMD of straw progressively decreases. When the grain is completely mature at harvest, the straw has its lowest IVDMD value, resulting from decreased nitrogen content and increased lignification. Xing Tingxian (1995) has also reported a variation in chemical composition and digestibility of crop residues among cultivars. Table 2-4 clearly shows differences in chemical composition, especially in CP, among different rice cultivars, that result in large variations in IVDMD values as crop residues.

Table 2-4. Composition and in vitro DM digestibility of straw of various rice cultivars

SOURCE: Xing Tingxian, 1995.
KEY: DM = dry matter; CP = crude protein; NDF = neutral detergent fibre; NDS = neutral detergent soluble; ADF = acid detergent fibre; CEL = cellulose; HC = hemicellulose; ADL = acid detergent lignin; IVDMD = in vitro dry matter digestibility.

Another large variation in chemical composition and digestibility is found among straw fractions. Research data from Xing Tingxian (1995) showed that stems from wheat straw have a much lower organic matter (OM) digestibility, compared with leaf blades and sheathes (Table 2-5). This low digestibility can be attributed to their high content of NDF and lignin. In rice straw, the OM digestibility of stems is much higher than that of leaf sheathes and blades (Table 2-5). Similar results were also reported by Aman and Nordvist (1983).

Table 2-5. Composition and in vitro DM digestibility of wheat and rice straw fractions

SOURCE: Xing Tingxian (1995).
KEY: n = number of replicates; NDF = neutral detergent fibre; ADF = acid detergent fibre; CEL = cellulose; HC = hemicellulose; ADL = acid detergent lignin; IVOMD = in vitro organic matter digestibility.
NOTE: (1) Without grain.

Animal factors

Little information is available about animal factors that influence the nutritive value of crop residues. Farmers in China have long known that different breeds and types of animals use crop residues with various efficiencies. Cattle, which retain fibrous matter in the rumen slightly longer than sheep or goats, presumably have an advantage with lower quality crop residues.

Cross-bred Brahman (Bos indicus) steers, when fed hay with 730 g/kg NDF, digested more NDF in the rumen and had longer ruminal retention time for lignin than did Hereford (B. taurus) steers (Kennedy, 1982). With increasing popularization of cross-breeding techniques in China, farmers noted that, on a high concentrate diet basis, hybrid beef steers have much better growth performance than do native breeds; in contrast, on a low-quality fibrous feed (e.g. crop residues), the contrary was observed. Although the exact mechanism of this difference in animal performance between hybrid and native breeds is unclear, an inherent difference in food intake and digestion capacity may be responsible.

Environmental factors

Some environmental factors, including location, climate, soil fertility and soil type, seem to influence the nutritive value of crop residues. Recent studies (Xing Tingxian, 1995) have indicated that there can be significant differences in chemical composition and digestibility of crop residues grown on different soil types (Table 2-6). Irrespective of crop cultivar, straw from wheat grown in the so-called tide soils (alluvial soils with diurnal variation in groundwater level) had considerably higher CP content and lower fibre (NDF, ADF and ADL) content than straw from drab soils (cinammon soils of forest origin). These are probably the cause of digestibility differences.

Table 2-6. Composition and in vitro DM digestibility of wheat straw by soil type

SOURCE: Xing Tingxian, 1995.
KEY: CP = crude protein; NDF = neutral detergent fibre; ADF = acid detergent fibre; ADL = acid detergent lignin; IVDMD = in vitro DM digestibility.

Improving feed value by processing or treatment

Ruminants despite their unique and highly efficient digestive system, are not able to extract sufficient energy to grow and produce milk from low quality or highly lignified residues. These must be properly processed or treated in some way to make them useful for production.

Traditional processing and feeding methods

Historically, many fibrous crop by-products have been used as energy sources for ruminants in China. More than 1 000 years ago, during the Song Dynasty, Chen Fu in his Nong Shu Book [Agriculture Encyclopaedia] described the following method of processing and feeding crop residues:

"Mix finely chopped straw with wheat or millet bran and beans, slightly soak them with water, and then feed animals ad libitum."

and there is a farmers' proverb:

"chopping hay to one inch, fattening can be done without concentrate".

These ancient processing and feeding methods include particle reduction and reconstitution of roughage, and are still included in university textbooks and scientific publications. Chopping and water soaking are popular practices for crop residue feeding throughout the country. Although they do not always result in consistent improvements in animal performance, they definitely result in reduced diet wastage and diet selection (Xiong, 1986).

A method called Jiegan Nian Qing (crushing freshly cut alfalfa with straw) has been widely used in the southern part of Shanxi Province. A thick layer (about 30 cm) of wheat or rice straw is spread on a flat threshing ground. A layer of freshly cut alfalfa (about 30 cm thick) is put on top, followed by another layer of straw. A heavy stone roller is passed over the layers, squeezing out the alfalfa juice, which is absorbed by the straw. The alfalfa treated in this way can be dried much faster, with minimum leaf loss, and at the same time, the alfalfa juice absorbed by the straw enhances the straw feed value. This method is very useful in those areas where alfalfa drying is difficult.

Physical treatment

Numerous physical processing techniques to enhance the utilization of crop residues by ruminants have been used, with varying degrees of success. In this section, the more common methods - including grinding and pelleting, irradiation and steam treatment -will be briefly reviewed as they relate to crop residue utilization in China.

Grinding and pelleting

The most studied physical treatments for enhancing crop residue use by ruminants are grinding and pelleting. Grinding, or fine chopping, decreases particle size, increases surface area and bulk density of both leaf and stem fractions, and hence raises rumen microbial accessibility or feed intake. The increase in intake due to grinding is generally higher with low quality than with high quality residues, and with small and young animals rather than with older and larger animals. The critical feed particle size to exit the rumen is smaller in sheep than in adult cattle and therefore a greater degree of grinding is necessary before they leave the rumen. Screen sizes for hammer mill grinding in China range from 2.5 mm to 25 mm. Considering differences in intake between animal species and the energy expenditure for grinding, Xiong (1986) recommended 6 mm for sheep and 12 mm for cattle as the appropriate screen sizes for hammer mills.

Ground crop residues are often pelleted or cubed before feeding. Benefits derived from pelleting include a further increase in density, decreased dustiness and easier handling. However, DM digestibility of pelleted straws is depressed relative to the long or chopped forms, primarily due to faster passage rate. Pelleting usually augments straw intake due to quicker passage, which can offset the negative effect from decreased digestibility. Therefore, the net benefit of feeding pelleted crop residues in practice is increased energy intake and animal performance. In dairy cows, fine grinding and pelleting of forages can dramatically reduce rumination and rumen digestion times. Consequently, saliva production is reduced and the rumen fermentation pattern is altered, together with reduction in acetate/propionate ratio. This is believed to be the reason for the milk fat reduction with ground and pelleted forages.

Few studies have been conducted to assess the feeding value of ground and pelleted crop residues in China. Fu et al. (1991) studied the response of lamb growth performance to ground and pelleted maize stover. Compared with coarse grinding of maize stover (through a 25-mm screen), processing with fine grinding (through an 8-mm screen) followed by pelleting increased feed intake by half (1 098 g vs 728 g DM) and daily gain by 129 percent (148 g vs 65 g), and reduced the feed/gain ratio by 34.1 percent (7.4 vs 11.3).

Kneading

As milk fat can be lowered with finely ground or pelleted straws, the development of another physical processing method was necessary. Recently, a novel method for processing crop residue using a kneading machine has been reported (Gao Zhenjiang et al., 1994). When fibrous crop residues enter the machine, they are kneaded into threadlike fibres or hairs with no apparent stem internode structure. Kneading extensively destroys the rigid structure and thus significantly increases voluntary intake. Unlike other physical processing, such as grinding or pelleting, rubbing of crop residues produces long threadlike fibres (usually 8-12 cm long), and therefore should not affect milk fat content. Compared to chopping, kneading requires higher energy expenditure.

Several studies have been conducted to compare kneading with traditional chopping. Sun Zhongyin et al. (1991) reported that dairy cows fed with scrubbed soybean residue had higher dietary DM intake and milk production than with untreated residue. A similar result with kneaded maize stover fed to dairy cows was reported by Zhao Xiyou and Sun Qinglin (1992). Kneading treatment is becoming popular in China.

Irradiation

Irradiation treatment of lignocellulosic materials to improve the utilization of cell wall polysaccharides dates back to the work of Lawton et al. (1951). They found that when basswood was irradiated with high velocity electrons, rumen bacteria fermentation was increased. Electron irradiation of straw can also increase polysaccharide digestibility by ruminal micro-organisms. Based on volatile fatty acid (VFA) production during fermentation, the optimum dose appears to be at 2.5 ¥ 108 rad. In vitro DM disappearance increased with irradiation dosage up to 108 rad (Pritchard et al., 1962).

Several studies on irradiation of crop residues for increasing their nutritive value have been conducted in China. Meng Qingxiang and Xiong Yiqiang (1990) treated wheat straw with a combination of g-rays from a cobalt-60 source and NH₃ (3 percent of DM) or NH₃ (1 percent) plus CaOH (5 percent of DM) at different moisture levels. They found that irradiation doses (2 ¥ 105, 2 ¥ 106 or 2 ¥ 107 rad) had a significant interaction with the moisture level (20, 40 or 60 percent for ammoniation, and 40, 50 or 60 percent for NH₃ + CaOH). On either chemical treatment, as irradiation and moisture level increased, in situ DM disappearance (ISDMD) increased and NDF content diminished dramatically. These results suggest that responses to irradiation at a lower dosage can be compensated by higher moisture levels. In another study, Gu Chuipeng et al. (1988b) found lower contents of fibrous fractions (NDF, ADF and lignin) and elevated ISDMD with irradiation of rice straw. When straw was irradiated at dosage of 0, 5 ¥ 106, 107, 5 ¥ 107 and 108 rad, the ISDMD were 54.0, 54.7, 57.5, 71.0 and 75.5 percent, respectively. Treatment of rice straw with a combination of electron irradiation and NaOH also resulted in a considerable higher glucose release (Lu Zhaoxin and Xiong Changren, 1991).

Although irradiation is very effective in improving the nutritional value of fibrous crop residues, it remains unfeasible at the farm level.

High pressure steaming

High pressure steaming (also called "Heat spurt" by the inventor) to improve the nutritive value of low quality feeds was closely studied at the Inner Mongolian Academy of Animal Sciences (He Jian et al., 1989). Ground wheat straw or other crop residues are placed in a closed steel tank and saturated with high pressure steam. When the expected temperature (or pressure) and time are reached, a tank valve is suddenly opened allowing materials to enter a pressure-release tank through a specially designed tube. This high pressure steaming and explosion result in a brown straw with looser structure. High pressure steaming markedly decreases straw CF (Table 2-7) and therefore increases the in vitro DM digestibility (Table 2-8). Results from an in situ study (He Jian et al., 1989) showed that NDF digestibility (48 hour incubation) of the treated wheat straw was increased by 68 percent (38 to 69) in rumen-fistulated sheep and by 233 percent (19 to 62) with caecum-fistulated pigs. Rumen VFA concentration was also increased 9.9 percent (55.3 vs 50.6 mM/litre) in sheep fed diets based on the high pressure steamed wheat straw, compared to untreated straw. In lamb feeding trials (Hou Guizhi et al., 1997), animals were fed equal amounts of mixed concentrate and wheat straw per day per animal (230 g dry weight). Lambs fed high pressure steamed straw ate more of it (433-595 g DM) and gained faster (44.8-50.3 g) than lambs with untreated straw (intake of 413-535 g DM and gains of 18.6-18.8 g).

Compared with chemical treatment, high-pressure steam does not require reagents and thus minimizes potential environmental pollution. In relation to other physical treatments, high-pressure steam is more effective in improving crop residue nutritive value. However, it implies high investment for equipment and a steam generator, and it has not been developed for wider utilization throughout the country.

Table 2-7. Composition (% of DM) of wheat straw after high-pressure steaming

SOURCE: He Jian et al,. 1989; Lu Dexun et al,. 1990.
KEY: DM = dry matter, CP = crude protein, CF = crude fibre.

Table 2-8. In vitro digestibility (%) of fibrous feeds after high-pressure steaming

SOURCE: He Jian et al. (1989).
KEY: WS = wheat straw; CS = corn [maize] stover; RS(1) & RS(2) = rice straw samples; SD = sawdust.

Chemical treatment

Since the beginning of the 19th century, attempts have been made to improve the digestibility and nutritive value of crop residues. A major breakthrough was chemical treatment to remove encrusting substances (cellulose, hemicellulose and lignin). Many chemicals have been screened in laboratory experiments for their potential to enhance digestibility. However, only three are being routinely used in animal research: sodium hydroxide (NaOH), ammonia (NH₃), and calcium hydroxide (CaOH).

The modes of action of chemical treatment on crop residues, especially with alkalis, have been described by Klopfenstein (1981):

(1) hemicellulose solubilization,
(2) increases in cellulose and hemicellulose digestion, and
(3) increases in digestion rate for cellulose and hemicellulose.

The data on ammoniation of maize stover and rice straw from many studies (Mao Huaming and Feng Yanglian, 1991; Meng Qingxiang, 1988; Xing Tingxian, 1995; Ji Yilun et al., 1988; Gu Chuipeng et al., 1988a; Liu Jianxin et al., 1992) support these modes of action for the chemical treatment.

Sodium hydroxide treatment

Sodium hydroxide treatment of crop residues has been investigated and used in some areas of the country since the late 1970s. The procedure basically followed the "dry method," where NaOH is applied at 3-5 percent and the moisture content is 20-30 percent of DM. Alkali treatment may saponify the ester bonds between lignin and carbohydrates or the phenolic acid-carbohydrate complexes in plant cell wall. Through these effects, structural carbohydrates in both lignified and unlignified plant tissues become more digestible, with consequent increases in rate and digestibility.

The treatment with NaOH results in increases in crop residue palatability and digestibility, and in animal performance (Xiong Yiqiang, 1986). Steers fed rations based on NaOH-treated wheat straw gained 20 percent faster than did the control group when concentrate was half of total ration (Sun Qinghai, 1985). Ye Risong et al. (1999) reported that dairy cows fed NaOH-treated rice straw diets ingested 1.9 kg (86.4 percent) more straw and produced 1.4 kg (7.9 percent) more milk per day than those on untreated-straw diets.

Although NaOH treatment works effectively in improving the nutritive value of crop residues, NaOH is expensive, corrosive and its use may result in significant excretion of sodium ions in animal excreta. Long-term accumulation of sodium may lead to soil fertility problems and environmental pollution. Thus, application of NaOH treatment of crop residues is not popular with the farmers at present.

Ammoniation

Since the middle of 1980s, ammoniation of crop residues has drawn a great deal of attention in China due to several advantages: effectiveness in improving digestibility, addition of non-protein nitrogen to treated residues, and absence of sodium accumulation in soils. Ammoniation is dealt with in Chapter 3.

Most data have shown a decreased NDF content, but little change in ADF and ADL contents of crop residues due to ammoniation (Meng Qingxiang, 1988; Wu Keqian, 1996; Xing Tingxian, 1995). The results suggest that ammoniation can break the linkage between hemicellulose and lignin and make the hemicellulose fraction partially soluble to NDF solution. The soluble hemicellulose would be highly digestible by ruminal micro-organisms. After being ammoniated, treated crop residues have an increased N content relative to untreated residues (Table 2-9).

Many in vitro and in vivo digestion trials have been conducted to evaluate the effect of ammoniation on digestibility of different crop residues. Ten studies with ammoniated crop residues indicated an average increase in digestibility of 24.3 percent (12.4-44.6) or 11.2 (6.4-17.8) percentage units. Animals ingested ammoniated residues faster than the untreated (Table 2-10).

Improvements in the feeding value of crop residues due to ammoniation have been observed in many feeding trials. Some of the results are summarized in Table 2-11.

Table 2-9. Effect of ammoniation on composition (% of DM) of crop residues

KEY: UNT = untreated; AMM = ammoniated; CP = crude protein; NDF = neutral detergent fibre; ADF = acid detergent fibre; ADL = acid detergent lignin.

Computer simulation results on beef production under different practical conditions were always in favour of ammoniated wheat straw over untreated straw when comparison was made on the basis of maximum benefit per unit of body weight gain (Meng Qingxiang et al., 1990b).

Table 2-10. Effect of ammoniation on ingestion rate of crop residues

NOTE:(1) Intake rate is expressed as time (minutes) spent in ingestion of one kg of roughage.

Table 2-11. Effect on herbivore growth of ammoniation of crop residues

SOURCES: (1) Zhang Tianzeng et al,. 1 982; (2) Wu Keqian, 1996; (3) Meng Qingxiang, 1990a; (4) Cao Yufeng et al,. 2000; (5) Yuan Zhizhao et al,. 1986; (6) Jiang Zhijie et al,. 1986; (7) Lu Donglin et al., 1984; (8) Chen Ruirong and Li Yongfu, 1998.
KEY: BW = body weight; UNT = untreated; AMM = ammoniated.

The maximum benefit is expressed as minimum concentrate consumption per unit gain, or minimum feed cost per unit gain. Based on the beef market situation at that time, a two-stage feeding optimized system was proposed (Meng Qingxiang et al., 1990b). During the first period (from birth to 250 kg) with minimum concentrate consumption per unit gain, cattle should be fed a largish amount of ammoniated crop residues to maintain a relatively low rate of daily gain (300-500 g). During the finishing period (250 to 450 kg) with minimum feed cost per unit gain, cattle were to be fed on low crop residue and high concentrate diets to allow faster gain rates (> 1 000 g/day). The computer simulation results from this two-stage feeding system compare well to actual feeding results from Beijing, Shandong, Shanxi and Henan (Table 2-12).

Table 2-12. Beef cattle growth from computer simulation and actual feeding studies

Source: Meng Qingxiang et al., 1990b. Note: (1) Number of cattle used in the study

In another study, Meng and Xiong (1993) found that lambs fed ammoniated wheat straw had increased dietary intake, body weight gain and better concentrate conversion efficiency compared with animals fed untreated wheat straw. The magnitude of the improvement gradually declined with increasing proportion of mixed concentrates in the diet. Regression showed that feeding ammoniated straw diets to lambs could benefit either by increased daily gain at similar concentrate level, or by less concentrate feed consumption at the same rate of gain. Based on the results, it was calculated that each tonne of ammoniated wheat straw replacing untreated straw could produce 105.2 kg (37-159.2 kg) more of liveweight gain at concentrate levels from 22 to 72 percent. When lambs gained at equal rates, each tonne of ammoniated wheat straw could save about 285.4 kg (71.9-593.1 kg) of mixed concentrates or grains at the above range of concentrate levels. This conclusion agrees well with experience from commercial animal production: each tonne of ammoniated crop residues when replacing untreated residues could save 250-300 kg of grain in cattle or sheep (Guo Tingshuang, 1996).

Many studies have also demonstrated that feeding ammoniated crop residues greatly improved lactating performance of dairy cows. Table 2-13 summarizes the results of 4 trials and shows that ammoniation of crop residues increased actual yield on average by 1.7 kg (20.1 vs 18.4 kg) without changes in milk composition, including fat percentage.

Table 2-13. Effect of ammoniation on lactation performance of dairy cows

NOTE: (1) Number of cows in each study.
KEY: FCM = fat-corrected milk yield

Other treatments

Since limestone is available cheaply in China, the use of Ca(OH)2 to treat crop residues attracted a great deal of interest from the 1950s. Calcium hydroxide is generally less effective in treating crop residues than other alkaline sources, such as NaOH or NH₃. Combining Ca(OH)₂ with urea or other alkalis seems to solve this problem. Combining Ca(OH)₂ with urea, Mao Huaming and Feng Yanglian (1991) showed that rice and wheat straw treatment increased the CP content by 3.5 times (8.3 vs 3.1 percent) and in situ DM digestibility by 69.8 percent (65.9 vs 38.8). In a feeding trial (Feng Yanglian, 1996), dairy heifers fed such treated rice straw showed significant increases in dietary DM intake (from 6.56 to 6.89 kg), weight gains (from 829 to 898 g/day), feed conversion (7.9:1 to 7.6:1) as compared with those fed the untreated straw.

Cao Yufeng et al. (2000) reported significant improvements in the nutritive value of wheat and rice straws as a result of combination treatment with urea, calcium hydroxide and common salt. Table 2-14 shows the changes in NDF, ADF, ADL, cellulose and hemicellulose content and in vitro DM digestibility before and after treatment. Combined treatment reduced the content of NDF, ADF and hemicellulose, but did not change the content of cellulose and lignin over the untreated straw. The in vitro DM digestibility of treated straws was enhanced relative to untreated straws. Growth performance data with cross-bred beef cattle fed the combined, ammoniated or untreated rice straw diets are presented in Table 2-15. Cattle fed the combined diets had somewhat more dietary DM intake, better daily gain, improved feed conversion and considerably reduced feed cost per kg of weight gain than cattle on either untreated or ammoniated straw diet.

Table 2-14. Effect of combined treatment on composition (as % of DM) and digestibility (%) of wheat and rice straw

SOURCE: Cao Yufeng et al., 2000.
KEY: NDF = neutral detergent fibre; ADF = acid detergent fibre; CEL = cellulose; HC = hemicellulose; IVDMD = in vitro dry matter digestibility.

Table 2-15. Effect of combined treatment of rice straw on growth of cross-bred beef cattle

SOURCE: Cao Yufeng et al., 2000.

Other combination methods for treatment of crop residues with sodium hydroxide and urea (Shi Chuanlin, 1998), ammoniation and enzyme (Chen Sanyou et al., 1998), and ammoniation and ensilage (Wang Xiaochun et al., 1996) have also been reported elsewhere. In each case, the nutritive value was improved, but these methods have not so far been taken into practice.

Biological approach

Regular ensilage

This popular method is described in Chapter 4.

Microbial ensilage

Ensilage of whole fresh maize plants is only practised for large-scale feedlots and dairy farms. For small-scale family farms, ensiling dry crop residues after reconstitution of moisture is usually the best way for preserving feeds, since farmers do not have suitable equipment to quickly harvest their cereal plants. Another reason is that they have to sow promptly the next crop in most regions with a double-cropping system. Ensiling dry crop residues involves actions such as chopping, reconstitution of moisture, pressing and mixing with certain additives, including micro-organisms such as lactic acid producing bacteria, cellulolytic bacteria, for proper fermentation and nutrient preservation.

A large number of dry crop residues have been successfully ensiled with addition of microbial products in China in recent years. This method is commonly called "microbial ensilage," or Weizhu in Chinese. Some bacterial products with specialized functions and warranted quality have been developed and approved for practical use by the government. Wu Keqian (1996) and Meng Qingxiang et al. (1999) ensiled wheat straw with addition of a specific microbial product containing bacteria that function as lactic acid and propionic acid producers and cellulose degraders, and fed it to cross-bred steers. The results showed that microbial ensiling resulted in reduction of NDF, ADF, cellulose and hemicellulose, and an increase in in situ DM digestibility (Table 2-16). In some feeding studies, it was shown that microbial ensilage of crop residues such as wheat straw, rice straw, maize stover or soybean straw caused increased daily gains, feed intake and feed conversion, and decreased feed cost per unit gain in growing ruminants (Table 2-17). Several studies (Zhang Yang and Meng Dongli, 1995; Chen Xiling et al., 1995; Ma Yusheng and Zhu Guosheng, 1997) also indicated that lactating cows fed diets based on microbial ensiled straw had increased milk and fat-corrected yield, and slightly higher milk fat percentages, compared with diets based on untreated straw.

Table 2-16. Composition of wheat straw before and after microbial ensiling

KEY: NDF = neutral detergent fibre; ADF = acid detergent fibre; CEL = cellulose; HC = hemi-cellulose; ISDMD = in situ dry matter digestibility.

Table 2-17. Effect of microbial ensiliage of crop residues on animal growth

SOURCES:(1) Chen Ruirong and Li Yongfu, 1998; (2) Meng Qingxiang et al,. 1999; (3) Wu Keqian, 1996.
KEY: ADG = average daily gain.

Another significant effect of microbial ensilage of dry crop residues is probably to hydrate and weaken plant structures so that less energy is expended on rumination. Ensiled crop residues usually have good palatability for ruminants, and thus high intake. In comparison with ammoniated straw, microbial ensiled residues give higher intake, faster rate of passage and therefore better performance. Other advantage of microbial ensilage is its low input cost for acquiring microbial products and accessories, e.g. plastic sheets, and therefore microbial ensilage is considered a better method to enhance the feeding value of dry crop residues. However, microbial ensilage generally results in lower digestibility than ammoniation (Wu Keqian, 1996). Another disadvantage of microbial ensilage includes substantial loss (usually 5-10 percent of DM) of organic material that would otherwise be rapidly fermented in the rumen. As a result, it is still argued academically whether the anaerobic ensilage of such ready digestible materials within crop residues is economically beneficial to the animal. Further in-depth research is required to select bacteria strains that selectively degrade cell wall fractions, especially lignocelluloses.

Treatment with White Rot fungi

Because White Rot fungi can effectively attack lignin and cellulose, their use to treat lignocellulosic material to increase digestibility has been studied quite extensively in other countries, but little in China. Xiao Xunjun (1998) and Peng Jun (1998) at China Agricultural University treated wheat straw with strains of Cyathus stercoreus, Bjerkandera adusta, Dichomitus squalens, Pleurotus spp. and Pleurotus ostreatus for 30 days, and showed that treatment decreased NDF from 71.4 (control) to 67.4, 59.2, 62.7, 65.0 and 67.9 percent, and ADF from 53.1 to 50.3, 45.1, 46.0, 50.0 and 51.3 percent, respectively. After fermentation by the five fungus strains, a considerable loss was found in lignin, from 23 to 44 percent, and in DM, from 11 to 17 percent. There was no apparent loss in cellulose and hemicellulose (Xiao Xunjun, 1998). When wheat straw was incubated in vitro with mixed rumen micro-organisms for 24 hours, DM digestibility was increased 11 and 8 percent for the treatment with Bjerkandera adusta and Phleurtus spp., respectively, compared with the untreated control. Straw digestibility with the other three strains did not change. When activity of polysaccharide-degrading enzymes (FPase, avicelase, CMCase, xylanase) and ligninase (Mn-dependent peroxidase) was measured, Peng Jun (1998) found that enzyme activity varied considerably with different fungus strains. It was also noted that most White Rot fungi grew slowly on common crop residues and could not effectively compete against other microbes. These observations suggest that effective breakdown of crop residue cell walls by White Rot fungi in practice will require selection or creation of better strains, and also further refinement of the current treatment techniques.

Use of mushroom-substrate residues

Crop residues have been used as a substrate to grow mushrooms. This practice is a very profitable business in some areas of the country. The substrate residue after mushroom harvest can be used to feed animals. The most commonly used crop residues are cottonseed hulls, wheat straw, rice straw and maize stover. The residues usually have higher CP and lower CF contents compared with the original substrate. Yang Xunyi et al. (1986) reported that after the 2nd, 3rd and 4th harvest of mushrooms, the CP content of the residual substrate increased by 32.5, 44.2 and 60.9 percent, while its CF content reduced by 42.4, 48.1 and 50.4 percent, respectively. When the substrate residue was included in growing pig diets at level of 5 percent (replacing half of the wheat bran), there was no significant difference in average daily gain and feed conversion (Liu Jianchang et al., 1998). However, growth performance of pigs decreased with increased substrate residue inclusion (Zhou Zongwang, 1991). The only benefit from inclusion of the substrate residues at a low rate in pig diet is the decreased consumption of concentrate or feed cost per unit of body weight gain (Lu Zuozhou et al., 1995b).

Undoubtedly, the use of crop residues for mushroom production is a very good approach in China's agro-ecosystem. Research data have also indicated that some species or strains of mushrooms have strong enzymatic activities digesting cellulose and lignin. Regarding the feeding value of this residue, however, more work remains to be done before any overall recommendations can be given.

Enzymatic treatment

The use of enzymes to attack the lignocellulose structure of crop residues for enhancing their feeding value has been attractive. Crude enzyme products, with cellulolytic and hemicellulolytic capability, are usually added to fibrous feeds in attempts to improve their digestibility. Wang An (1998) observed that treatment of maize stover with an enzyme product, prepared from Trichoderma viride, reduced the contents of some cell wall components and enhanced the ruminal digestibility in sheep (Table 2-18). Huang Jianhua (1998) and Huang Jianhua et al. (1998) treated maize stover and spent grain from malting (60:40) with an enzyme mixture containing cellulase, proteinase and amylase, and measured the effects on the performance of finishing pigs and laying hens. Inclusion of 10 percent of the treated maize stover and spent grain mixture in the diet did not affect gain rate of pigs or egg production of hens, but reduced by ¥ 0.25 the feed cost per kg of liveweight gain with the pigs (Huang Jianhua, 1998) and by ¥ 0.49 per kg of egg production with the laying hens (Huang Jianhua et al., 1998).

Table 2-18. Effect of cellulase addition to maize stover on fibre fraction content and in situ digestibility in sheep

SOURCE: Wang An, 1998.
KEY: NDF = neutral detergent fibre; ADF = acid detergent fibre; ADL = acid detergent lignin; CEL = cellulose; HEM = hemicellulose; DM = dry matter.

Commercial cellulase products were also added to diets to increase the supply of readily available carbohydrate. When the enzyme products were included at 0.1-0.2 percent of the diet of pigs, cattle and geese, animal performance was considerably improved (Table 2-19). Chen Xiafu et al. (1986) also reported the use of crude enzyme products prepared from Trichoderma viride as feed additives for growing rabbits. In eight growth trials, rabbits fed on a diet with addition of the cellulolytic enzymes gained 17.5-39.3 percent faster than the control. The difference was consistent and highly significant (< 0.05).

Table 2-19. Effects of cellulase addition on animal performance

KEY: DMI = dry matter intake.

Although there is a tendency toward increased use of cellulolytic enzymes in animal feeds, at present the cost of suitable enzymes is too high for commercial use. Obviously, advances in biotechnology and increased production of effective enzymes would be expected to lower the cost of enzymatic treatment.