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1.1. Anatomy of the digestive tract

Ruminants (cattle, sheep, goats, buffalo) are able to use cellulosic biomass and simple forms of nitrogen due to possessing a digestive tract which has the particular feature of comprising three compartments known as “prestomachs”. These are located before the abomasum, which is the equivalent of the mono-gastric stomach (Figures 1a and 1b). Their size represents between 70 and 75 % of the total size of the digestive tract.

The rumen (paunch or belly) is by far the most voluminous of the prestomachs (some 100 litres for adult cattle weighing between 500 and 600 kg); this represents more than 90 % of their total volume. The other prestomachs are known as the reticulum (or honeycomb stomach) and the omasum. The ensemble of the rumen and the reticulum, often referred to only as the rumen, show all the essential characteristics of a “fermentation unit”. The environmental conditions show:

These conditions are favourable for the development of a population of microorganisms (also known as the rumen microbes), which are characterised by their variety and their density.

One finds there:

Figure 1a

Figure 1a: Schematic representation of the digestive tract of a ruminant, as compared to that of a monogastric.

Figure 1b

Figure 1b: Schematic cross-sectional representation of the rumen and the reservoir. The arrows indicate the movement of the contents.

1.2. The importance of microbes to ruminants

They provide energy to the host animal (Figure 2)

The ingested feed is first fermented by the microbes in the rumen. This microbial fermentation is very important as between 60 and 90 % of the carbohydrates of the ration, including those from the cell walls, are fermented there. These cell walls, which are the essential components of low quality forages, are partially degraded by the microbes, aided by a cellulolytic enzyme (cellulase) which is secreted by them but which is not possessed by the host animal. The fermentation of the carbohydrates leads to energy production in the form of adenosine triphosphate (ATP) which is used by the microbes for their own maintenance requirements and for self multiplication.

The end products of this fermentation are,

The volatile fatty acids, produced during the fermentation process in the rumen, are absorbed into the blood stream through the walls of the rumen. They constitute the main energy source for the host animal as they supply between 70 and 80 % of the total energy absorbed by the ruminant (VERMOREL, 1978). In contrast, it should be appreciated that a monogastric draws its energy mainly from glucose and lipids supplied by the diet and absorbed by the small intestine.

They provide proteins to the host animal (Figures 2, 3, 4)

Nitrogen compounds ingested by the animal (both proteic and non proteic) are subjected to the proteolytic action of the rumen microbes (bacteria, protozoa and fungi). They are partially degraded in variable proportions depending upon numerous factors, in particular their solubility (INRA, 1988). Non protein nitrogen sources in the diet, such as urea, which might have been added to the ration, are completely dissolved and hydrolysed in ammonia.

Figure 2

Figure 2: Simplified schematic diagram illustrating the digestive utilisation of crude proteins by a ruminant.

* N.B. The large intestine, which has not been shown for better clarity, hosts celluloytic bacteria (but not protozoa). This is where celluloytic fermentation and bacterial synthesis (the source of PIM) takes place, allowed by the urea in the blood (passing through the wall), the source of NH3, and by the few constituants which are still degradable (source of energy). There is no absorption of amino acids at this stage in the digestive tract. A small proportion of these microbial proteins are fermented into ammonia which will join the overall ammonia pool, the major proportion which remains being excreted in the faeces

Figure 3

Figure 3: Effect of the concentration of ammonia in the rumen on intake and digestibility (in sacco) of straw fed to cattle (Leng, 1990).

Figure 4

Figure 4: Simplified schematic diagram of microbial synthesis in the rumen.

Ammonia is a precursory ingredient, essential for the microbial growth of most of the bacteria species in the rumen; these take it up and use it for the synthesis of their own constituent amino acids. It is even considered as the main source of nitrogen for many bacteria, in particular those involved in the digestion of cellulose and starch. According to MAENG et al. (1976), 82 % of the rumen bacteria can self-develop using only ammonia. However, it was observed by the same authors, that a supplement of amino acids, in association with ammonia supplied through urea, stimulated the synthesis of microbial proteins.

Because the transformation of the dietary nitrogen into microbial nitrogen is mainly done through the ammonia reserve, many authors have emphasised the importance of the need for a minimum quantity of ammonia in the rumen for better synthesis of the microbes and for optimisation of the food degradation process. According to HARRISON and McALLAN (1980) and LENG (1990), amongst others, these minimum concentrations lie between 50 and 100 mg/litre of rumen juice. The concentration of ammonia in the rumen has a positive effect on the digestion and the intake of low quality forages (Figure 3).

The utilisation of ammonia for the microbial synthesis is closely linked to the amount of energy (in the form of ATP) which is produced by fermentation of the carbohydrates, but also it depends on the presence of certain minerals, in particular sulphur and phosphorous (DURAND et al., 1987).

These phenomena may be illustrated diagrammatically as shown in Figures 2 and 4.

The collection of research results in this domain allow one to conclude that, on average,

145 g of Crude Proteins are synthesised in the rumen for each kg of Fermented Organic Matter (FOM)

The microbes are then carried along with the “digesta” into the abomasum and the small intestine where they are subjected to the classic digestive process. Their composition consists of 80% proteins, well balanced in essential amino acids and of these, between 80 and 85% are digested, supplying what are known as PDIM (“Protéines Digestibles dans l'Intestin d'origine Microbienne” i.e. microbial true protein which is truly digestible in the small intestine) (see INRA, 1988). This is the nomenclature under the French PDI system (“Protéines vraies réellement Digestibles dans l'Intestin grêle” i.e. true Protein truly Digestible in the small Intestine). These PDIM play a very important role in covering the nitrogen requirements of the ruminants, above all when these receive their basic rations from low quality forages.

Apart from these proteins, there are those which originate in the feed but which have bypassed the microbial degradation process in the rumen (the degree of degradation is very variable and depends upon the nature of the protein sources). These latter proteins are digested according to a coefficient called the Coefficient of True Digestibility which varies between 50 and 75; they supply what are known, in French, as the PDIA (“Protéines Digestibles dans l'Intestin d'origine Alimentaire” i.e. dietary protein undegraded in the rumen, but truly digestible in the small intestine).

The sum of the PDIA and the PDIM constitutes the true Protein truly Digestible in the small Intestine (PDI).

The French PDI system, together with other modern international systems, allows evaluation of the respective roles played by the feed and the microbes concerning the supply of protein to the intestine of the host animal. The true protein value of a feed may be calculated on the basis of two factors as follows:

firstly, PDIN = PDIA + PDIMN

Where PDIMN is the amount of PDIM that could be synthesised in the rumen from the degraded dietary N, when energy and other nutrients are not limiting,

and secondly, PDIE = PDIA + PDIME

where PDIME is the amount of PDIM that could be synthesised from the energy available in the rumen, when degraded N and other nutrients are not limiting.

These two values are not additive. It is the smallest value which must be taken into consideration for a given feed. This implies that, in order to balance a ration, one must ensure that PDIN = PDIE through suitable mixing of feeds, some rich in PDIN and others rich in PDIE.

Ruminants are thus less dependant upon the quality of nitrogen source than monogastrics because they are able to transform simple nitrogen compounds, such as urea, into microbial proteins of high biological value.

It is therefore unnecessary, at least in order to satisfy the ruminant's normal maintenance needs and those for modest production rates, to give them high quality proteins, as most of these will be degraded into ammonia which could, alternatively, have originated from simpler forms of nitrogen. This possibility has considerable economic significance in developing countries, given the scarcity and/or the high cost of vegetal proteins such as cakes.

1.3. Digestive utilisation of low quality forages

1.3.1. Chemical composition

Due to their particular physiology, which leads to very early lignification, tropical forages are generally of lower quality than their temperate counterparts (Figures 5a and 5b). A schematic representation of the variations in digestibility, according to the age of the plant, is shown in Figure 6. Digestibility diminishes less rapidly, over time, for tropical forage varieties than for temperate ones (be they legumes or grasses) but it starts from an initially lower value during the young vegetal stages.

The straw and stalks of cereals (such as rice, wheat, maize, sorghum, millet,…) or from annual or perennial grasses (such as Andropogon gayanus, Panicum Sp.,…) are of even lower nutritional quality.

Table 1 shows, for example, the chemical composition of several samples of these forages, harvested in a variety of tropical countries.

These low quality forages pose three major nutritional inconveniences:

Figure 5a

Figure 5a: Compared frequencies (%) of temperate and tropical grasses and legumes according to the overall crude protein content (after Minson, 1990).

  1. a high level of complex parietal carbohydrates: cellulose, hemicelluloses and lignin which compose the cell wall (Figure 7a) and which make up a substantial proportion of the total organic matter of the plant (between 60 and 80 %).

    Cellulose is the most abundant structural constituent. It represents between 32 and 47 % of the total forage dry weight (Table 1). It consists of a homosaccharide made up of long straight chains of β 1–4 glucose called cellobiose, interlinked through micro-fibres which lead to the formation of fibres with certain zones showing strong crystallinity. True cellulose (not to be confused with crude fibre) is, potentially, entirely digestible (Figure 7b).

    Figure 5b

    Figure 5b: Compared frequencies (%) of temperate and tropical grasses and legumes according to their digestibility (after Minson, 1990).

    Hemicelluloses, in contrast to cellulose, originate from amorphous hetero-polymers made up from the hexose family (glucose, mannose, galactose) and above all from the pentose family (xylose, arabinose). The molecular chains of these macromolecules are relatively short. They constitute a polysaccharidic matrix which is often associated with the phenolic constituents surrounding the cellulose fibres (THOMSON, 1983). The hemicelluloses are only partially digestible (Figure 7b).

    Lignin is a hetero-polymer of phenol, related to the hemicelluloses. The relationships between lignin and the hemicelluloses are not yet precisely understood. Organisation of the micro-fibres of cellulose takes place and results in the creation of a dense and mechanically strong lattice. Lignin is not digestible at all (Figure 7b).

    Figure 6

    Figure 6: Schematic representation of the reduction in digestibility (in points per day) of temperate and tropical grasses according to their age or stage of growth (after INRA, 1988).

    Table 1: Extreme values of the chemical composition and of the digestibility of some tropical crop residues (source: Chenost, 1993 and 1995; Chenost et al., 1993; Kayouli, 1979, 1988, 1994 a and b)


    Number of samples Origin DM % Crude Fibre Crude Protein Digestibility in sacco (72 h) or Rexen (*) cellulase digestibility
    (% of DM)
    Rice straw 35 Niger, Cambodia 91 35–40 3–5 35–41
    Rice straw 15 Madagascar, Mauritania 90   3–7 30–35(*)
    Wheat straw 30 Tunisia 89 37–43 2–5 39–35
    Sorghum stalks 18 Niger, Togo, Burkina F. 90 32–45 2–8 32–44
    Maize stalks 8 Tanzania, The Gambia 90   3–5 34–46 (*)
    Millet stalks 23 Niger, Togo 90 35–46 2–7 32–40
    Panicum spp. 15 Niger, Togo, Burkina F. 90 36–45 2–5 35–45
    Andropogon gayanus 7 Niger, Burkina F., The Gambia 90 40–47 2–3 30–38

    The proportion of the cell walls and their degree of lignification increases with the age of the plant, at the same time negatively affecting their digestibility.

    Figure 7a

    Figure 7a: Schematic representation of the cell walls of forages.

    Figure 7b

    Figure 7b: Variation in digestibility of the constituants of membranes (Van Soest and Wine, 1967).

  2. a weak nitrogen level: crop residues are weak in crude protein (N × 6.25) content (from 2 to 5 %). It is the same situation with perennial grasses whereby their crude protein content reduces sharply with age. During the dry season and after the flowering stage, which occurs early during the rainy season, this value only represents a very low figure (see Table 1). The weak nitrogen content of these forages is also highly inaccessible as it is linked to the lignified cellular walls.

  3. a weak level of mineral and vitamin content. In fact all these forages show an acute shortage of mineral content, together with a similar lack of micro-elements (Ca, P, Na) and of trace elements. They also lack vitamins, particularly A and D3.

1.3.2. Digestive utilisation Background

The degradation of parietal carbohydrates, the main components of low quality forages, is shown in Figure 2. The microbes in the rumen colonise the ingested feed particles by attaching themselves to them and the cellulolytic strains partially degrade (or hydrolyse) the cellulose and the hemicelluloses through using the enzyme, cellulase. This hydrolysis ends up with formation of oses (glucose, xylose, etc.) which are then fermented by the microbe population which thus draws upon energy supplies (in the form of ATP) and produces volatile fatty acids for the host animal (see § 22).

The degradation of the cell walls requires the microbes to attach themselves to the feed particles so that the enzymes can penetrate inside the fibrous structures, hence the need for the microflora, which secretes these enzymes, to have sufficiently wide access paths through the ligno-cellulose complex. Unfortunately, low quality forages show a high proportion of lignified walls, incrusted with lignin in a very complex manner (above all in the case of the grasses). Thus the lignin impedes the microbial colonisation of the fibres and, because of that, the action of degradation of the cellulolytic enzymes.

All this means that these forages have only weak digestibility, particularly as regards the stems (and thus the straw) which only falls in the range of 35 to 55% (Table 2).

In addition to their low digestibility, the lignified walls resist for a long time the microbial degradation and the peristatic mastication (of rumination) and they are thus only digested slowly. The particles resulting from this degradation remain longer in the rumen than is the case for high quality forages, before being reduced to a sufficiently small size as to be able to break through the omasal reticular orifice. These particles, resulting from low quality forages, are known to stay for up to five days in the rumen (INRA, 1988). The particles thus also take up a disproportionate amount of space in the rumen. This “encumbrance”, which determines the rate of ingestion, depends directly on the forage digestion rate (the physical regulation of the ruminant's appetite, BLAXTER et al., 1961; BALCH et al., 1962). It has importance for low quality forages which will thus only be able to be ingested in meagre quantities.

Table 2: Examples of the variation (extreme values) of the nutritional value of straws
STRAW Organic matter digestibility (OMD) (%) Intake (g / kg LW 0,75)
Dry matter Digestible organic matter
RICE (1) 35–55 25–65 9–25
BARLEY (2) 43–48 35–51 15–21
WHEAT (3) 35–46 23–35 8–10

(1): 20 references, Asia et Australia (Doyle et al., 1986)

(2): 7 determinations, Syria (Capper et al.,1989)

(3): 15 determinations in vivo (INRA,1988)

Wide variations due to the species (barley being superior to wheat), variety (particularly for rice) the agronomic conditions of the crop (fertilizer, climate,…), and to harvesting conditions (cutting height weeds, drying conditions, storage,…)

A recent study undertaken in Burkina Faso serves as an example (CHUBBIER-ZOUN-GRANA, 1995); this shows that the digestibility in vivo of Andropogon gayanus drops from 56% at the green stage down to 31 % at the dry stage when the plant has become very rich in lignified cell walls. The amount of dry matter (DM) which is ingested by sheep drops from 63 to 26 g DM/kg LW0.75 (in other words, from 900 to 375 g for a 35 kg sheep) as the forage advances from the stage of tillering during the rainy season to complete drying during the dry season.

It should be mentioned here that the nutritional value of these forages, in particular that of straw, shows wide variation as is seen from Table 2. This variation depends essentially upon the botanical family and the species, upon the conditions affecting the maturity process (both climatic and agronomic) and upon the harvest and storage conditions to which the crop is subjected. Conditions for good digestive utilisation

In order for the cellulolytic fermentation process to be correctly carried out, the microorganisms in the rumen must be able to find the nutritive elements which they need for self development and to enable them to degrade (through cellulolysis) the polysaccharides of the cell walls of the straw or the low quality forage, together with the physical and chemical conditions for maintenance of good cellulolysis.

  1. The nutritive elements needed by the rumen microorganisms (Figure 4)

    These are the same as for all living organisms: above all, energy, nitrogen, minerals and vitamins:

    - Energy

    Energy is mainly contained in the polysaccharides of the straw or forage. It is slowly liberated during degradation (fermentation) of the complex carbohydrates from the cell walls by the rumen's microorganisms. This energy roughly corresponds to the overall digestible energy of the forage and is only very gradually made available to the microorganisms in a manner allowing assimilation.


    The cellulolytic bacteria (or microflora) are essentially made up of proteins (p.2). It is thus essential that, apart from the energy provided for fermentation of the cellular walls, the microbes also be able to locate sufficient nitrogen for synthesis of their own proteins.

    However straw, as in the case of any old forage, is weak in nitrogen and additionally has only low digestibility.

    This means that, in order to ferment correctly these low quality forages and to allow their full potential of degradation, one must, as a first priority, provide sufficient quantities of the necessary nitrogen. The nitrogen needs of the microorganisms depend upon the amount of fermentable energy which is already present.

    The following example offers a practical illustration of the problem; it shows that it is possible, without any cause for ambiguity, to determine the amount of non-protein nitrogen (NPN) which is required for the case quoted:

    Consider a sheep which ingests 0.7 kg per day of dry matter consisting of straw with a level of digestibility at 40%, an organic matter content of 90% (which signifies a mineral content of 10%) and a crude protein content of 3%.

    The deficit nitrogen in low quality forages must be provided in a manner which may be used by the microorganisms (the PDIN, the nitrogen which is fermentable or readily “degradable” in the rumen); it may originate from a vegetal source (young forage rich in nitrogen), from a non protein source, or from an industrial source (such as urea).

    In the above example:

    1 g of urea provides 1.45 g of PDIN (1 g × 28/60 N × 0.78 (the rate the microbes capture the nitrogen) × 6.25 Crude Protein × 0.8 × 0.8 PDIN),

    One must, therefore, provide 15.7/1.45 = 10.8 g per day of urea to the sheep

    The practical aspects of how to do this will be described later in the text (Chap.6).

    This nitrogen supplement must be given in several feeding stages throughout the day to avoid any risk of food poisoning which might result through an excessive build-up of ammonia in the rumen. Figure 8 shows a simplified diagram of urea metabolism by a ruminant.

    - Minerals and vitamins

    Straw is also deficient in minerals and vitamins, these being insufficient to cover needs for synthesis and activity of the microbes.

    This deficiency concerns the major elements, particularly P, Ca and Mg, but equally the trace elements, Cu, Zn, Mn, Fe and S which are required for synthesis of sulphurous amino acids in which the cellulolytic bacteria are rich. Precise needs are still relatively unknown and form the subject for some current research.

    Whilst awaiting a better understanding of these requirements, it is preferable to forestall any risk of mineral or vitamin deficiency when the ration consists of straw. This may be arranged.

    Figure 8

    Figure 8: Metabolism of urea by a ruminant

    Vitamins are hardly present at all in straw or forage which has been harvested at an advanced stage of maturity. They should generally be added to the mineral feed supplement. The vitamins A, D3 and E are particularly lacking. The table below gives an idea of supplements given to animal races from the temperate zones.

    The majority of developing countries lack the facilities necessary for the elaboration of these supplements. This aspect forms the basis of a special section in Chapter 6 (§ 623).

    Example of a mineral and vitamin supplement (MVS) for straw:
    Composition of the MVS %
    CaPO4. 2H2O (di-calcium phosphate) 55
    NaCL (sodium chloride) 26
    MgSO4. 1OH2O (magnesium sulphate) 9
    Na2SO4. 1OH2O (sodium sulphate) 7
    Sulphur (flour of,) 1
    Trace elements (see below) 2
    Composition of the trace element mix %
    ZnSO4. 7H2O (zinc sulphate) 47.40
    MnSO4. H2O (magnesium sulphate) 23.70
    FeSO4. 7H2O (iron sulphate) 23.70
    CuSO4. 5H2O (copper sulphate) 04.70
    CoSO4. 7H2O (cobalt sulphate) 00.09
    SeO3. Na2 (sodium selenate) 00.04

    (The MVS should be given at a rate of: 100 g/day per TLU (Tropical Livestock Unit) and for European races; 80 g/day for calves of one year; 180 g day for cattle of 600 kg).

    Vitamins: A, D3, E, particularly a supplement of vitamin A:

    either in a water soluble form in the MVS the supplement should be comprised of between 20,000 and 50,000 IU/day (where IU signifies International Units)

    or in the form of an intra-muscular injection applied once a month, at a rate of between 1 and 2 million IU per injection.

  2. The physical and chemical conditions for successful cellulolysis (Figures 9a and 9b)

    The cellulolytic activity of bacteria diminishes when the pH drops below 6.5. Adding concentrates to the ration which are rich in rapidly fermentable carbohydrates (sometimes inevitable in the case of certain supplements, as described later in Chap.6) can cause a drop in pH of the rumen due to the resulting rapid and pronounced production of volatile fatty acids (VFA). In the case of molasses, which are often used to support the presence of urea, one must arrange for them to be ingested slowly and regularly (see again Chap.6) so as to avoid any abrupt variation of the pH value in the rumen.

    Figure 9: Effect of the pH value of the rumen on:

    Figure9 a

    a- Activity of the cellulolytic and amylolytic bacteria of the rumen

    Figure9 b

    b- The nature of the fermentation actions in the rumen

    The cellulolytic activity also depends upon the regularity of the supply of nutritive elements to the microbial flora and upon the renewal or the regeneration of the flora. A practical consequence of this observation is that one must adopt a feeding system for these supplements which ensures their regular supply and an intake as uniform as possible throughout the day.

1.3.3. The nutritional value, as related to intake and digestibility

By respecting those conditions which favour cellulolysis (essentially, the minimum and regular supply of the deficient nitrogen, minerals and vitamins) one ensures:

The process of freeing the digestible elements and placing them at the disposal of the microbes will also be quicker and more intense.

This will provoke the following results:

These improvements are only possible if optimum quantities of feed supplements are supplied so as to ease the process of cellulolysis. Above these levels, phenomena concerning digestive interactions between the forages and the concentrates appear and the supplement then substitutes the forage. These problems will be examined in greater detail in Chapter 6 which presents a discussion of optimum feed supplements.

1.4. Conclusions and strategies which allow maximum value to be obtained from low quality forages

Annual and perennial grasses from natural pastures (which are consumed during the dry season and often at a late stage of maturity), together with the straw and stalks from cereal crops, constitute what are known as low quality forages; they are characterised by a high content of lignified cell walls and are very weak in nitrogen, mineral and sugar contents in forms which can readily be assimilated.

Ruminants are only able to take advantage of these forages due to their special digestive physiology. The simultaneous effects of rumination and microbial fermentation, which take place in their paunch - or rumen - through the action of the hosted microorganisms, enables these forages to be degraded into fine particles and for the nutritive elements to be extracted. These are placed at the disposal of the animal through end products from this fermentation process; these end products are made up of volatile fatty acids (VFA) and microbial matter, which is itself digested by enzymes in the abomasum - the true stomach - and the large intestine.

Nevertheless, not only do these forages have low digestibility, their digestion only takes place slowly and hence they are only ingested in feeble quantities by ruminants. When they constitute the only feed offered to the animals they will, in general, not sufficiently cover even their basic maintenance needs.

There are various possibilities to improve the nutritional value of these low quality forages:

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