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Effect of environment and quality of fibre on the nutritive value of crop residues

P.J. Van Soest
Cornell University, Ithaca, NY, USA

Analytical systems
Tropical forages, straws and stovers


Straws and stovers are major feed resources for ruminants in Africa and other parts of the developing world and are more important than cultivated forages because of competition for land for human food production. These crop residues are high in fibre and the fibrous carbohydrates are their most important nutrients. Hence the nature and quality of fibre is of special interest. The environmental conditions under which the crops are grown and L post-harvest storage conditions have a large effect on straw and stover quality.

Fibre is often used as a negative index of nutritive value in the prediction of total digestible nutrients (TDN) and net energy. Prediction equations assume that higher fibre means lower digestibility. The association between fibre and digestibility is strong in temperate forages because of the strong association between lignin and cellulose in first cuttings in temperate climates. However, this association is weak in tropical forages, straws and stovers and fibre cannot be used to predict digestibility of these feeds.

The role of fibre analysis in the evaluation of feed quality is complex: the total amount of fibre (plant cell wall) is a negative index, while the amount of available fibrous carbohydrates is a positive index. Coarse fibre is required for normal rumen function and metabolism and is a positive dietary factor. The physical characteristics of fibre (particularly particle size) are also important in regulating rate of passage, rumination, insalivation and the pH of the rumen (Van Soest, 1982).

The total fibre or cell-wall fraction of plants comprises cellulose, hemicellulose, lignin, cutin, silica and a variety of minor substances. The proportions of these components vary among parts of the same plant and also change as plants mature.

Analytical systems

Forage dry matter can be divided by means of the detergent system into a readily available soluble fraction and a fibrous residue of limited availability (Van Soest, 1982). The nutritive value of the fibrous fraction is determined by the degree of lignification, while that of the soluble non-fibrous portion is completely available to digestion. Utilisation of starches and other non-fibrous carbohydrates is limited only by the extent to which they escape the digestive process by passing rapidly through the digestive tract (Van Soest, 1982).

The value of chemical analysis for evaluating forages has been called into question (Preston and Leng, 1987; Ørskov, 1987) on the basis that composition does not predict nutritive value, and that such analyses are too expensive relative to the amount of information provided. The critics would replace chemical analyses with nylon-bag degradability tests, and perhaps an analysis for nitrogen by Kjeldahl. They also charge that the originators of new chemical methods would convert all ruminant nutrition laboratories in the world to their procedures. This view ignores the purpose of detergent analyses which is to understand factors that limit the availability of energy and protein, and to correct the errors of proximate analysis and crude fibre by providing an accurate partitioning of cell-wall components into hemicellulose, cellulose and lignin (Van Soest, 1982).

Chemical analysis of feedstuffs is aimed at understanding why a given feedstuff exhibits its nutritional peculiarities. When this is understood, evaluation for causative and controlling factors can become routine, and not very expensive, since only the relevant factors are analyzed. Unfortunately, the particular limiting nutritional factor varies among individual forages and feedstuffs, such that a single, universal analysis is not possible. Most of the misunderstanding of the use of chemical analyses for estimating nutritive value has arisen from incorrect application of methods. Proper application of analyses is directed toward solving specific problems, and thus the set of particular analyses will vary with the experiment al situation.

The primary purpose of laboratory analysis is to characterise forages and feedstuffs so that nutritive value and performance in livestock can be related to chemical composition. This relationship has been the teas is of compositional feeding tables for some time. Compositional data should include those components most pertinent to nutritive value. These vary among feeds but, for many feeds, determination of cell wall, lignification and nitrogen or crude-protein content will be sufficient. Ash, lipid and available carbohydrate contents are the next most useful factors to determine.

Proximate analyses including crude fibre and nitrogen-free extract (NFE) are inadequate for estimating nutritive value because they do not represent the components that limit biodegradability in the digestive tract. Nylon bag degradability is also unsatisfactory, because it measures digestibility and is not a chemical entity. Nylon bag measurements reflect rumen environment involving diet and animal differences and are thus inherently more variable than in vitro measurements of digestion. The latter are recommended for plant breeding studies.

Conceptual problems

The detergent system of analysis is intended to provide a biologically realistic description of forages and fibre. The use of fibre values for predicting digestibility is called into question by the chemical and nutritional non-uniformity of these fractions and thus the problem of crude fibre and NFE is not only one of erroneous analytical fractionation, but also of application in the form of empirical regression equations that attempt to estimate digestibility from fibre content.

The use of regressions of nutritive value on chemical composition assumes that nutritive value is determined by the chemical fraction measured. Applied to fibre, the assumption is that fibre limits nutritive value. However, the digestibility of fibre is limited by lignification and fibre is only secondarily related to digestibility through its association with lignin. Since lignification is greatly affected by environmental conditions, such as temperature, daylength, light and plant stress, the association of fibre with lignin content is highly variable. This is reflected in a positive association between fibre and lignin in forages growing in the spring (when temperature and daylength are positively related) and a negative association in the autumn. There is little or no association between fibre and lignin in forages growing in midsummer or under tropical conditions (Butterworth and Diaz, 1970; Van Soest, 1982). Despite this, acid-detergent fibre (ADF) or modified ADF (MADF) are used as principal criteria for estimating digestibility in both America and Europe.

The interactions of environment and climate with plant physiology and growth are sufficient to render associations between fibre components and nutritive value unreliable, and thus demolish the model that fibre regulates digestibility. The continued use of such regress: ions does not constitute a valid scientific operation and is an abuse of the respective methods of analysis.

Chemical entities

Those who would justify empirical regression systems argue that ADF and NDF do not represent chemical entities and can therefore be treated like crude fibre. This overlooks the biological criteria for uniform chemical factors used in the establishment of detergent analyses, i.e. by the methods of Lucas (Van Soest, 1982). Crude fibre was intended as a determination of cellulose and, although imperfect by modern standards, is closely correlated with cellulose content.

Acid-detergent fibre is intended to represent lignocellulose and provide a preliminary step in the preparation of lignin, free from interference from protein. It contains small amounts of other cell wall components, viz bound protein and nitrogen, cutin, biogenic silica and micellular pentosans. However, these are of interest as unique fractions of very low digestibility. The determination of acid-detergent insoluble nitrogen (ADIN) for unavailable protein and the preparation of ADF for lignin and cellulose measurements are its main uses (Van Soest and Robertson, 1980).

Neutral-detergent fibre (NDF) includes hemicellulose and represents the insoluble plant cell wall matrix that is cross-linked with lignin. The digestibility of the cell wall matrix depends on the extent of lignification. Pectin is not covalently bound to the cell wall matrix and is largely extracted by neutral detergent and is completely fermentable (Van Soest, 1982).

NDF contains the indigestible lignified matrix and associated components of the cell wall and represents the skeletal structure and volume of the plant. It is the only plant fraction that can account for rumen fill and voluntary intake of forage and that is highly correlated with both rumination and chewing time among a wide range of forages (Van Soest, 1982).

Tropical forages, straws and stovers

The TDN of tropical forages averages 15 units lower than that of temperate forages (McDowell, 1972) due to effects of climate and management. However, reported maximum and minimum digestibilities from extreme immaturity to full maturity of forage grasses vary with latitude (Figure 1). The relationship between straw digestibility and latitude is probably similar to the minimum values in Figure 1.

Figure 1. The relationship between digestibilities of perennial grass and latitude. Digestibility of first cuttings declines below 30° latitude. Vertical bar at 30° latitude is the approximate upper limit of the semitropics. Digestibilities of mature forages decline progressively with decrease in latitude.

Source: Van Soest (1982).

The tropics are the geographical regions that are free from frost and forages can grow continuously in this region if sufficient moisture is available. In temperate latitudes growth begins with the cessation of frost. Thus, maximum digestibilities are high and show no change with latitude above 30 degrees. However, in tropical areas growth begins at higher temperatures, usually after cutting or when rains end a dry spell. Survival of exposure to frost demands the accumulation of reserves to provide cold-tolerance, thereby increasing digestibility. The same effects are seen at high altitudes in the tropics and in arid areas (Van Soest et al, 1978).

The digestibility of plant material at maturity is determined by the cumulative environmental effects during growth and maturation. Level of digestibility is related to latitude, reflecting an inverse relationship with temperature.

Lower digestibility at higher temperatures is the result of the combination of two main effects: lignin synthesis and elevated metabolism. Higher temperatures increase lignification of the plant cell wall and promote more rapid metabolic activity, which decreases the pool of metabolites in the cell. Photosynthates are more rapidly converted to structural components, which reduces nitrate, protein and soluble carbohydrate contents and increases cell wall content (Deinum et al, 1968; Van Soest et al, 1978). Higher temperature increases the rate of enzymatic processes associated with lignin biosynthesis. Tropical plants are subjected to long nights during which soluble sugars and other highly digestible intermediates are respired, which lowers quality.

The general effects of environmental temperature upon plant growth and composition appear uniform in all plant species studied (Wilson, 1982). However, the quantitative effects of temperature upon forage quality vary with plant parts and with plant species. Temperature has its greatest overall effect on plant development in promoting the accumulation of lignified cell wall. This may be modified by growing conditions and species. For example, plant species that remain vegetative, whether by reason of too low environmental temperature during growth or by genetic character, are almost always Less lignified that those plants that develop to the flower stage under similar environmental conditions (e.g. pangola grass). A physiological reason is that lack of flowering and seed development allows the required resources to remain: in the leaves and stems promoting higher nutritive value. This effect is very important in cereals grown under conditions of poor grain production leading to a straw of higher nutritive value.

Another characteristic of tropical forages is the wide range in quality within the same standing plant (Van Soest, 1982). Animals tend to select better quality material and the difference in composition between what is eaten and what is refused may be considerable. In straws and stovers the nutritional quality of the leaf may be considerably higher than that of the stem. Thus a major factor affecting quality of straw and stover is the recovery of leaves. An exception to this is the case of rice straw where leaves are of lower quality than stems.

Straws and stovers are often chopped in order to reduce bulk, increase consumption and reduce wastage. This practice forces the animal to eat more of the low quality parts and reduces the nutritive value of that actually consumed. More selective feeding on the part of the animal is a recognised feeding strategy of goats and sheep which are more limited by their smaller metabolic size. These animals can be adversely affected by chopping forages. High producing dairy cattle are also sensitive to the physical form of forage.

Soil effects

Many cereals and grasses metabolise silica and deposit it in opaline form in the cell walls of leaves, reducing their digestibility (Jones and Handreck, 1967). Thus availability of soil silica can affect straw quality. This effect is particularly striking in rice which contains up to 20% silica, which is selectively distributed in leaves and seed hulls. Rice straws are low in lignin and silica is the prime factor limiting digestibility (Jackson, 1977; Van Soest, 1981).

C3 and C4 plants

The first stable products of photosynthesis in many tropical grasses are four carbon compounds, while those of dicots and most temperate grasses are three carbon compounds, hence the designation as C3 and C4 species. C4 plants are photosynthetically more efficient than C3 plants. Tropical C4 plants tend to accumulate large amounts of low-quality dry matter. These plants have fewer mesophyll cells between vascular bundles than C3 plants and, since mesophyll cells are comparatively unlignified and highly digestible, their proportion influences quality (Akin, 1980).

Not all C4 plants have lower nutritive value than C3 plants. Maize, for example, is a C4 plant developed from tropical ancestors. When grown in temperate regions its nutritive value is high. However, Deinum (1976) noted t hat maize grown under warm conditions reverts to many of its tropical ancestral characteristics and is of lower nutritive value (Table 1).

Table 1. Effect of environmental temperature on the in vitro digestibility of mature leaves at final her vest in maize.

Temperature1 (°C)





OM digestibility (%)





Cell wall content (%)





Cell wall digestibility (%)





Source: Deinum (1976).

1. Day/night controlled temperature.

C3 and C4 plants coexist in the tropics but C4 forage plants (mainly grasses) dominate favourable environments because they are more aggressive and higher yielding. The C3 tropical forages include the legumes and grasses adapted to less favoured conditions. As a group they are higher in nutritive value, but yield less and are less responsive to fertilization. Agronomists in the tropics have favoured using the higher yielding C4 grasses, despite their lower quality.

Fibre quality

Fibre quality is defined as the ability to promote efficient rumen fermentation, and includes the potential digestibility and rate of fermentation of cellulosic carbohydrates, particle size and strength and cation exchange capacity. Graminaceous straws tend to be poor in these factors, although considerable variation exists. Legume fibre is superior in cation exchange capacity and rate of hydration to the average grass fibre, causing it to have shorter lag times after feeding and faster rates of fermentation. This occurs even in legumes of higher lignification. The shorter lag and faster fermentation are associated with higher consumption.

Grasses are characterized by high NDF and hemicellulose contents. As a result, intake of grass is lower than that of legume at a given digestibility. Grass fibre is also lower in lignin content than legume fibre. Thus, grass fibre is a better energy source for cellulolytic organisms than legume fibre, but its rate of fermentation and buffering capacity are lower. Lignin protects cellulosic carbohydrates from digestion but is responsible for much of the cation exchange capacity (McBurney et al, 1986). Thus lignin may have both a positive and a negative effect on fibre quality.


Treatment of low-quality forage to improve its nutritional value has usually employed alkali (Jackson, 1977; Sundstøl et al, 1978). Removal of the limitation of lignification on digestion depends on either cleavage of the bond between lignin and carbohydrate or hydrolysis of the polysaccharide away from the lignified matrix. Most studies of the effects of roughage treatment have measured animal responses but not chemical changes in the forage, and thus most procedural evaluations used at the present are quite empirical.

Analysis of lignin is the most obvious means of determining the efficiency of delignification. However, the current practice not washing the straw (for economic reasons) allows the cleaved lignin to remain in the product. Also NH3-treatment with heat can elevate apparent lignin content through polymerisation of carbohydrates and proteins in the Maillard: reaction. Unfortunately, lignin analyses do not distinguish cleaved from uncleaved or synthetic lignin and neither lignin nor fibre content reflects the improvement in in vitro digestibility of treated straws (Rexen and Vestergaard Thomsen, 1976; Ørskov, 1987). Subsequent studies indicate that the relationship between lignin and digestibility is considerably different in treated straws (Van Soest et al, 1984a). As a result, alkali- treated straws are often evaluated via rumen in vitro or cellulase digestion techniques.

A chemical method to evaluate alkali-treated straw must distinguish cleaved lignin from uncleaved lignin. One procedure uses saponification of the isolated neutral-detergent fibre prepared without the use of sulphite (Lau and Van Soest, 1981). This measures the ester bonds left unhydrolysed after treatment with alkali.

The solution from neutral-detergent extraction can also be used to assess the quality of treated straws because cleaved lignin is soluble in neutral buffers and can be measured by UV absorption at 280 and 314 nm. Saponification with sodium hydroxide provides a solution which contains the residual lignin. Absorption at 280 nm results largely from phenolics, whereas ester linkages absorb at 315-340 nm; however, the latter wavelengths probably also include a component that is not ester related (Hartley, 1983). A third possible analysis is to determine residual lignin on an acid-detergent residue that has been prepared by sequential extraction with neutral detergent followed by acid detergent.

The cleavage of lignin-carbohydrate ester bonds in graminaceous straws results in the release of phenolic compounds into the soluble fraction (Hartley, 1983). The digestibility of the soluble fraction is depressed because the cleaved phenolics are not digestible (Neilson and Richards, 1978). This may partly explain the difference between rumen in vitro and in vivo digestibility coefficients in treated straws (Berger et al, 1979). This difference has not been explained by any laboratory measurements, although it has been attributed to faster passage of the treated forages due to changed physical structure (Berger et al, 1979).

McBurney (1985) examined 30 treated and 15 untreated samples of straws. Sixteen forages were treated with ammonia and 14 were treated with sodium hydroxide. Treatment tended to decrease NDF, but increased ADF, causing a drop in hemicellulose content. The effect was more pronounced with NaOH. The availability of additional nitrogen supplied by ammoniation varied widely: 0 to 66% of the additional nitrogen was in the acid-detergent indigestible nitrogen (ADIN) fraction which is unavailable to the animal and to the microbes. The relationship between in vivo and in vitro OM digestibilities is shown in Figure 2. In vitro measurements can both overestimate and underestimate in vivo values. The inverse correlation between lignin and digestibility was stronger in untreated than treated forages (Table 2). The correlation between digestibility and optical density of neutral-detergent extracts, which measures cleaved lignin carbohydrate bonds, was positive in the treated samples. The solubles obtained after saponification of the treated forages contained intact lignin-carbohydrate bonds that were still susceptible to cleavage by alkali, and represent a measure of the inefficiency of treatment.

Figure 2. The relationship between in vivo and in vitro digestibility of organic matter.

Source: McBurney (1985).

Table 2. Correlations between predictors of in vivo apparent organic matter (OM) digestion and in vitro apparent OM digestion.


In vivo apparent OM digestion

In vitro apparent OM digestion














Lignin %







NDS1 OD280







NDS OD314 2







Sap. Meq.







SS3 OD280







SS OD314







Source: McBurney (1985); Van Soest and McBurney (1985).

1. Optical density of neutral-detergent extract (OD units meq-1 OM).
2. Saponification value of neutral-detergent fibre (meq. base g-1 OM).
3. Optical density of solubles obtained from laboratory saponification of NDF (OD units mg-1 OM).

The improvement in digestibility due to treatment is highly variable. In some cases digestibility decreased, which appeared to result from moulding or other fermentation and an increase in net lignin and phenolic absorbance from the formation of Maillard products from heating. However, much of the variation in efficiency of treatment may be due to buffering capacity, which differed widely among the straws (Table 3). No current recommendation for straw treatment considers buffering capacity. Buffering capacity is related to cation exchange capacity, one of the criteria of fibre quality. Ironically, much of the exchange capacity is lost upon alkali treatment of fibre.

Table 3. Buffering capacity means (X) and standard errors (SE) of a subset of control feeds.





Coefficient of variation (%)

Cereal straw















Source: McBurney (1985)

Generally, alkali treatments lack quality control and are expensive relative to the increase in nutritive value obtained. This severely limits their application, and it may be more realistic to supplement nutrient-deficient: straws than to treat them.

Buffering capacity and cation exchange

Exchangeable groups in the plant cell wall include carboxyl, amino, nonhydrogen-bonded hydroxyls, and phenolic hydroxyls, all of which may bind metal ions (McBurney et al, 1986). Thus the surface properties of fibre, i.e. hydration and cation exchange, are correlated (r about 0.7) and influence cell wall fermentation. The lag between ingestion and fermentation is related to how fast the fibre becomes hydrated and subsequent attachment of microbes. The amount of fibre digested after 6-12 hours of incubation is highly related to feed intake. Microbes have negatively-charged cell walls (Stotzky, 1980) and recognize and attach to fibrous particles by their exchangeable surface. This attachment requires formation of ligands between the microbial cell wall and fibre by divalent cations (probably magnesium). The cation exchange capacity of the fibre is its ability to bind and hold metal ions on its surface in much the same way that clay minerals hold cations in soil. The exchange serves as a bank exchanging K, Ca, Na, Mg for hydrogen when the pH drops and recharging as cations become available when saliva and ingesta are mixed. An advantage of this regenerable bank is that ruminated fibre passing down the digestive tract contributes buffering action further down the gut.

The buffering capacity of feedstuffs derives in part from the physical effects they elicit in the rumen and during rumination and ensalivation. Eating and rumination promote salivary flow containing much buffer that neutralizes acids produced in fermentation. Fermentation of carbohydrates results in production of large amounts of organic acids, which must be removed by absorption to maintain pH and the normal rumen environment. Recycling of mineral ions is also important in maintaining rumen pH. The more slowly digested solid matter-fibre-contributes most to the maintenance of normal rumen environment. Tropical grasses and straws have low exchange and buffering capacities, and supplementation with starchy concentrates renders the rumen sensitive to acidotic conditions, which reduce rumen efficiency and net feed intake. Tropical legumes and citrus are high in exchange and buffering capacity and are useful supplements.

Microbial ecology

Quality forage fibre is also associated with microbial efficiency. Fibre-digesting bacteria manufacture more cellular protein than do starch-digesting bacteria because of their lower maintenance costs, the relatively high ATP yield of acetate fermentation and evolutionary selection for cellular storage. Rumen organisms have substrate preferences and can divided into competitive consortia of mutually symbiotic species. For the purposes of this discussion they can be conveniently divided into: (1) fibre digesters and (2) those that specialize in starch (such as Streptococcus bovis) and more soluble carbohydrates. The second group tends to be adventitious, producing lactic acid at the expense of cellular efficiency. Rapid production of lactic acid reduces pH and thus renders the environment more favourable for their growth because low pH is more inhibitory to the slower digesting organisms dependent on cellulose and hemicellulose. The rate of carbohydrate digestion is set by the physicochemical limitations of the substrate, which in turn limit bacterial efficiency. The combination of slow fermentation rate and competitive passage leads to inefficient use of the potentially digestible carbohydrates, with low microbial output and increased faecal losses.

There are important relationships between fermentation rates of carbohydrates and microbial efficiencies, i.e. production of microbial protein per unit of feed digested in the rumen. The fermentation rate sets the amount of feed energy available to rumen bacteria per unit time. Faster digestion provides more food, which dilutes the energy costs of maintenance and leaves more energy for growth and production (Sniffen et al, 1983).

Bacteria have a maintenance requirement that must be met before growth can occur. The maintenance requirement of cellulolytic bacteria is one sixth that of bacteria that ferment soluble starch and sugar (Sniffen et al, 1983). The type and quality of carbohydrate can have considerable impact on rumen microbial yield because carbohydrates are degraded at different rates. Cellulolytic bacteria grow slowly and a decrease in fibre quality can dramatically reduce yield by reducing their rate of digestion and growth. This limits the utilisation of straw, stovers and tropical grasses, which tend to be slowly digested.


Straws and stovers contain large amounts of carbohydrates, mainly in the fibre, but availability may be low. Long lag times and slow fermentation are probably the main factors that limit intake and utilisation of straws and stovers. Tropical forages, straws and stovers are of lower quality than those from temperate regions because of environmental effects (temperature and daylength) on plant growth. Relationships between chemical composition and measures of nutritive value are poor in tropical forages, straws and stovers. Chemical treatments are expensive in developing countries and it may be more practical to supplement straws and stovers to optimise their utilisation. Alternatively, the introduction of varieties of superior nutritive value in straw or stover may provide a solution. Legumes may provide useful supplements to cover the nutrient deficiencies of many straws.

Legume fibre is superior to grass fibre: because of intrinsic compositional and structural factors it is consumed more readily and thus gives greater feed efficiency (Van Soest et al, 1984b). These intrinsic factors include rate of fermentation and buffering capacity (cation exchange) and, paradoxically, greater lignification. Tropical legumes are higher in protein and lower in fibre than their grass counterparts and thus can serve as valuable supplements to straw- or stover-based rations.


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Thomson: Is it possible to adjust the amount of alkali added to crop residues to obtain optimal improvements in digestibility?

Van Soest: One approach would be to conduct a laboratory titration.

Jenkins: Could you explain the terminology associated with tannin effects on bacteria?

Van Soest: Tannins are defence compounds which react with protein, as in the leather reaction. Degraded tannins may have analogous effects on bacteria in the rumen.

Pearce: The role of silica in influencing nutritive value may not be so clear cut. Dr Juliano of IRRI has grown rice under hydroponic conditions with silica solutions of varying concentration. This did not influence rice straw digestibility.

Van Soest: The lignin content of straws may be important as it has been suggested that higher lignin contents occur in rice straw with lower silica contents, providing a compensatory effect. However there have been reports that silica does not limit straw digestibility under field conditions.

Ørskov: I have not found that silica affects rice straw feeding value.

Van Soest: Lignin is chemically different in legumes and grasses but degradability is the same. These differences disappear when results are expressed on a neutral-detergent fibre basis.

McAllan: You have suggested that phenolic compounds have tanning reactions with rumen bacteria. Could you explain that further?

Van Soest: The tannins react with nitrogenous compounds and for m a polymer which becomes physically attached to bacteria. Condensed tannins are not digestible.

Uden: Can you explain how you separated out these indigestible compounds in the course of metabolic experiments?

Van Soest: This research was conducted in vitro and the optical density of the compounds was measured.

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