As the utilisation of forages by ruminants depends on microbial fermentative digestion, the principles of digestion in the rumen are discussed as a framework to view the requirements for biotechnology innovations in nutrition.
The rumen is the dominant feature of the digestive tract of cattle. This maintains a medium that supports a dense and varied population of microorganisms. These organisms ferment feed materials to produce mainly shortchain organic acids or volatile fatty acids (VFAs), methane and carbon dioxide and the process provides substrate (the feed) and ATP (energy) for the growth of micro-organisms.
The microbial mix in the rumen is complex and highly dependent on diet. The main agents that break down fibre, sugars, starches and proteins in the rumen are all anaerobic and include bacteria, protozoa and fungi.
The bacteria are the principal organisms that ferment plant cell-wall carbohydrates (Hungate, 1966) but the anaerobic phycomycetous fungi may at times be extremely important (see Bauchop, 1981).
Protozoa are now recognised as having an overall negative effect in the rumen, particularly where ruminants are fed forage diets low in true-protein (Bird et al. 1990). Protozoa ingest and digest bacteria and reduce the bacterial biomass in the rumen (Coleman, 1975) and consequently the protein supply to the animal. Thus, they decrease the protein to energy ratio in the nutrients absorbed (see later) and increase the requirement of animals for true protein. The net result of the presence of protozoa is an increased requirement for dietary bypass protein and on low protein diets a decreased efficiency of utilisation of feed for growth and milk production (see later) (Bird et al. 1990).
The presence of protozoa in the rumen may also reduce the rate at which bacteria colonise and degrade the ingested feed particles. In studies with sheep fed straw based diets, it has been found that the apparent digestibility of dry matter was increased by 18% after protozoa had been removed from the rumen (i.e. defaunated) (Bird & Leng, 1984; Soetanto, 1986). This research indicates that large increases in productivity may be achieved with ruminants fed fibrous diets, particularly those low in true protein by controlling or removing protozoa from the rumen. Other workers have not seen the differences in digestibility and in some instances removal of protozoa from the rumen has led to decreased digestibility of mixed, starch containing diets (Jouany & Ushida, 1990).
A deficiency of a nutrient needed by rumen micro-organisms reduces microbial growth efficiency which reduces microbial biomass and eventually reduces digestibility and feed intake, particularly of fibrous feeds.
The first priority in feeding ruminants is to ensure no deficiencies in the diet of nutrients for microbial growth in the rumen. Of major importance is that the efficiency of microbial growth (that is, the amount of microbial biomass available for digestion in the intestines per unit of digestible carbohydrate entering the rumen) also determines the proportion of digested feed that is converted to methane and VFA. Methane production accompanies the formation of acetate or butyrate, whereas methane and VFA production are inversely related to microbial cell production.
On most diets based on crop residues and low-digestibility forages, the primary limitation to the growth of rumen micro-organisms is probably the concentration of ammonia in rumen fluid The second consideration is deficiencies of minerals, particularly sulphur, phosphorus, magnesium and certain trace minerals.
Ammonia in the rumen must be above a critical level for a considerable period of the day to ensure a high rate of microbial growth and digestion and therefore feed intake. The level of ammonia that supports the optimal population of micro-organisms in the rumen the highest protein to energy ration in the nutrients absorbed, and therefore maximum digestion, will vary among diets. In general on forage based diets the ammonia level should be above 200 mg nitrogen/litre (see Leng, 1991).
It must be stressed, however, that any nutrient, (including many minerals required in the growth of micro-organisms), that is deficient in a diet will result in low microbial cell yield relative to VFA and lead to a low protein (from microbes) to energy (from VFA) in the nutrients absorbed (this is discussed under quantitative aspects of fermentation digestion below).
The ratio of protein digested and absorbed from the intestines to the VFA produced in and absorbed from the rumen is termed the P/E ratio.
One of the consequences of the ruminant mode of digestion is that fermentation results in up to 20% of the digestible energy intake being lost as heat and methane. A second major disadvantage is that proteins that are fermented in the rumen are not then sources of amino acids for the animal because they are hydrolysed and their constituent amino acids deaminated by microbes.
In general, where ruminants are fed forage based diets typical of that available in tropical developing countries, small amounts of extra nutrients are needed to balance nutrient availability to requirements. Proteins which are directly available to the animals and are protected from degradation increase the efficiency of anabolism of the absorbed nutrients in growth, pregnancy, lactation or work. (see Leng, 1991).
The end products of rumen fermentative digestion are governed by the feed, the rate of consumption of feed, the balance of nutrients in the feed for microbial growth and the balance of micro-organisms that develop in the rumen (bacteria, protozoa and fungi).
In general, a proportion of the digestible feed dry matter is converted to VFA, methane and carbon dioxide and the balance is assimilated into microbial cells. The pathways of these reactions are well known and a schematic outline is shown in Figure 3.1.
Microbial cells, that are synthesised from the feed resource use the ATP that is generated in the formation of VFA from the feed to provide the energy for synthesis. The microbes are lost from the rumen pool either by passage out of the rumen to be partially digested in the intestine or by death and breakdown within the rumen (with formation of VFA, CO2 and methane). Lysis and degradation in the rumen is inefficient as it makes the protein of microbes unavailable as such to the animal.
Because microbial cells are more reduced than the substrate fermented, the quantity of microbial cells leaving the rumen per unit of carbohydrate consumed is related to methane production. The efficiency of microbial growth is then a primary determinate of the quantity of methane produced.
Figure 3.1: Energetics of rumen fermentation (Leng, 1982)
For the purposes of the present discussion and to demonstrate the underlying principles of the concepts developed, a model for a 200 kg steer will be used to illustrate the quantitative availability of nutrients from rumen fermentation. The steer consumes 4 kg which represents 25 Mole anhydroglucose or organic matter which is completely fermented in the rumen.
It is assumed:—
that the fermentation of 1 mole of carbohydrate gives rise to either 2 mole acetate, 2 mole of propionate or 1 mole of butyrate, according to the following stoichiometry:—
|Hexose||→||Pyruvate + 2ATP + H2|
|2Pyruvate + 2H2O||→||2HAc + 2CO2 + 2H2 + 2ATP|
|2Pyruvate + 8H2O||→||2HPro + 2H2O + 2ATP|
|2Pyruvate + 4H2O||→||H Bu + 2H2 + 2CO2 + 2ATP|
|CO2 + H2||→||CH4 + 2H2O + 2ATP|
In the stoichiometry, H2 indicates reduced co-enzymes, HAc is acetic acid, HPro is propionic acid and HBu is butyric acid
that the animal's rumen is functioning at “normal” level of fermentative efficiency in which one-third of the organic matter fermented is converted to microbial cells and the rest to VFA, CO2 and CH4.
that the moles ATP generated per mole of end-product are for acetate 2, butyrate 3, propionate 3, and methane 1 (Isaacson et al. 1975).
On chemical principles, the equation of substrate use and end products from fermentation of 4 kg of carbohydrate is:—
Carbohydrate to VFA
16.7 CHO → 21 HAc + 6HPro + 3HBu + 7.5CH4 + 78ATP
Carbohydrate to microbial cell precursors
8.3CHO → 1.4 polysaccharide + 13.8 pyruvate + 2CH4 + 17ATP
25CHO → 21HAc + 6HPro + 3HBu+9.5CH4 + 1300 g dry cells.
In the example, one-third of the carbohydrate provides the substrate for microbial cell synthesis 1300 g dry microbial cells are produced at a Y ATP of about 14.5 (YATP is a measure of the efficiency of utilisation of ATP generated in fermentation of carbohydrates to VFA; it is defined as the g dry cells produced per mole ATP available.)
The upper level of efficiency (or the theoretical highest level of cell production) has a YATP of 26. On the other hand the lowest efficiency of a microbial growth in the rumen that is deficient in, say, ammonia, is possibly below a YATP of 4.
The relationship between the efficiency of cell synthesis and fermentative end products produced are shown in Figure 3.2. These values were arrived at by similar calculations as that given above.
Figure 3.2: Relationship between the production of microbial cells and volatile fatty acids and methane in fermentative digestion in ruminants
The relative efficiency of the system (indicated as YATP) is governed largely by the availability of essential nutrients for microorganisms (after Leng, 1982). The ranges of YATP are shown for:
Table 3.1: The effect of different efficiencies of microbial growth on the ratio of protein to VFA energy (P/E ratio) available from the rumen of a steer consuming 4 kg of organic matter which is totally fermentable.
|Microbial protein* synthesised (g/d)||500||800||1010||1212|
|VFA produced (MJ/d)||41||34||30||26|
|Methane produced (MJ/d)||9.4||8.5||8.0||7.6|
|P/E ratio (g protein/MJ)||12||25||34||47|
* Microbial protein may be only 75–85% digestible and this will change the P/E ratio markedly in the animal.
Based on this model, but assuming a varying efficiency, the microbial cells produced relative to VFA and methane production change are shown in Table 3.1. The main point to emphasise is that, depending on the efficiency of utilization of ATP, the amount of carbohydrate converted to microbial cells can be highly variable. It is the efficiency of microbial growth that largely controls the amount of methane produced by an animal (see Figure 3.2).
Ensuring a high ratio of microbial cells (protein) produced relative to VFA (energy) or a high P/E ratio is critical for efficient feed utilisation (see Section 3.10) and mechanisms for manipulating this ratio are discussed in the next section.
Protein that is fermented in the rumen is largely wasted as a source of amino acids to the animal because:—
dietary protein is degraded and essential amino acids are deaminated to form ammonia and VFA
fermentation of 1 g of protein generates only half the ATP that would be produced from 1 g of carbohydrate and therefore anaerobic microbial growth on protein is approximately half that on carbohydrate.
In combination these effects result in only 30 to 60 g of microbial protein becoming available to the animal for digestion for every kilogram of dietary protein that is fermented in the rumen. The fermentation of protein is, however, associated with relatively small amounts of methane production. On the other hand methane is not generated when protein bypasses the rumen.
Protein that is insoluble, or has a high component of disulphide bonds or is associated with tannins tends to bypass rumen fermentation but is digested in the intestines and in this way it augments the microbial protein and alters the ratio of protein to energy (P/E) in the nutrients absorbed. The better the balance of nutrients for microbial growth the higher the ratio of P/E in the nutrients produced in fermentation. The higher the content of bypass protein in the diet the higher the P/E ratio in the absorbed nutrients.
From the above discussion the first priority for improving the utilisation of a low digestibility forage by ruminants is to optimise the availability of nutrients from fermentative digestion by:—
ensuring that there are no deficiencies of microbial nutrients in the rumen and therefore the microbes in the rumen grow efficiently and, through fermentative activity, extract the maximum possible amounts of nutrients from the forage (i.e. the ratio of microbial cells to VFA produced is high as are production rates of these end products)
ensuring that the microbial cells (which provide most of the protein to the animal) synthesised in the rumen are not lysed and fermented in the rumen but are available for digestion and absorption as amino acids from the intestines.
The second objective of a feeding strategy should be to optimise the efficiency of partitioning of absorbed nutrients into product by:—
supplementation with critical nutrients that escape or bypass rumen fermentation to augment and balance the nutrients absorbed to those required for maintenance of homeostasis, maintenance of body temperature, exercise (or work), and the particular physiological or productive function.
As the nutrients needed for different functions differ in priority, supplementation strategies will need to vary according to climate, environment, management and production targets in any one location.
The rumen microbes have specific requirements for both macro and micro minerals to meet the needs of structural components of cells and for components of enzymes and co-factors. Little is known about the requirements of the microbial milieu for trace elements and as a ‘rule of thumb’ it is accepted that if the animal is not deficient then it is unlikely that the rumen microbes will be deficient.
As Suttle (1987) has so aptly put the situation, “it will rarely be possible to approach a suspected mineral deficiency situation with a table of minor nutrient requirements or biochemical criteria in the hand, and define a scale of the animal health (microbial health) problems”. In practice, either no mineral supplements are used or a “shot gun” mixture is given as a salt licks (McDowell et al. 1984) or as molasses (which is concentrated plant juice rich in minerals) suitably fortified with minerals (Kunju, 1986).
As with any deficiency of a nutrient the likely scenario of a mineral deficiency for rumen organisms is first a reduced growth efficiency of microbes (lowered ratio of cells to VFA produced) with or without a decrease in digestibility. As the deficiencies become more extreme the digestibility of forage must decrease along with the decrease in microbial pool size and it is only then that feed intake will decrease. Feed intake however, will be decreased as P/E ratio decreases if the animal is heat stressed (see later). Correction of deficiency will obviously reverse these effects.
Ensuring adequate ammonia N in the rumen to supply the majority of nitrogen for microbial growth is the first priority in optimising fermentative digestion of forage. Satter & Slyter (1974) suggested that 50–80 mg NH3-N/1 of rumen fluid was the optimum for maximising microbial growth yield and this has been widely accepted. However, recent studies from two laboratories in Australia have clearly indicated that the minimum level of ruminal fluid ammonia for optimum voluntary intake of low N, low digestibility forage by cattle is about 200 mg N/1, even though the digestibility of the forage (in nylon bags) was optimised below 100 mg NH3-N/1 (Krebs & Leng, 1984; Boniface et al. 1986; Perdok et al. 1988). All these studies were carried out in hot environments and the effects on feed intake are possibly explained by an improving P/E ratio in the nutrients absorbed, which reduces the metabolic heat load.
The effects of increasing ruminal fluid ammonia by infusion of urea into the rumen of steers on the intake of rice straw and its digestibility in nylon bags in the rumen are shown in Figure 3.3.
Figure 3.3: The effects of the level of ammonia in the rumen on the intake and in sacco digestibility of straw by cattle. The ammonia levels were adjusted by infusing urea in the rumen (Perdok et al., 1988)
Supplements must provide adequate levels of ammonia in the rumen for continuous growth of both fibrolytic and saccharolytic organisms. The only satisfactory approach to meeting these changing requirements for ammonia is to provide ammonia continuously. One way of doing this is to provide salt/urea or molasses/urea licks and allow the animal to take these as needed. There are indications that cattle and buffaloes given continuous access to multi-nutrient blocks based on molasses/urea are able to control fairly closely their intake of urea. Once buffaloes were accustomed to molasses/urea blocks they adjusted their intake according to the N content of the basal diet (NDDB—unpublished data). Lambs given wheat straw and molasses/urea blocks also had similar abilities and consistently maintained their rumen ammonia levels above 200 mg N/1 (Sudana & Leng, 1986).
There has been considerable controversy concerning the requirements for peptides/amino acids by rumen microbes for efficient growth. A number of studies, however, have reported results of in vivo studies which appear to have indicated no apparent requirement for amino acids for efficient growth of rumen organisms (see Leng & Nolan, 1984). The results of studies by Maeng et al. (1989) may explain some of the contradictory results. The studies of these researchers indicated that rumen microbes growing on different carbohydrate substrates have requirements for different N-substrates; celluloytic organisms may not require amino acids to the same extent as organisms growing on starches or sugar as the major substrate. For microbes utilising sugars growing on starches there was an apparently high requirement for preformed amino acids/peptides but this was not so for cellulolytic organisms.
Maeng et al. (1989) also showed an increase in efficiency of microbial growth on fibrous carbohydrates with decreasing dilution rate of rumen contents. If true this may be advantageous to ruminants given low quality forages that must be retained in the rumen for a considerable period to allow digestibility to be optimised. At the same time the improved ratio of cells: VFA yielded (i.e. P/E ratio) along with the increased availability of the total nutrients are both advantageous. Such a mechanism would advantage an animal with (1) a comparatively slow turnover rate of rumen contents (i.e. buffalo vs. cow or goat vs. sheep; see Devendra, 1989) or (2) fauna-free vs. faunated animal (see Bird & Leng, 1985) or animals at high environmental conditions versus cold stressed animals (see Young, 1983).
The organisms in the rumen that are largely responsible for the fermentation of cellulose (Ruminococcus albus, Ruminococcus flavefaciens and Fibrobacter succinogenes (previously called Bacteroides succinogenes) appear to have minimal requirements for amino acids and grow on ammonia (see Leng, 1991 for discussion). Conversely, organisms important in starch hydrolysis (Butyrivibrio fibrisolvens, Bacteroides ruminicola, Selenomonas ruminantium, Streptococcus bovis and Ruminobacter (Bacteroides) amylophilus (Hobson et al. 1988) readily incorporate amino acid N and in many cases peptides (see Leng, 1991).
Supplementation of sheep fed a poor quality forage with branched chain VFA has been reported to increase the apparent flow of microbial-N to the duodenum. The apparent stimulation of microbial growth with branched chain VFA has also been shown to increase feed intake on occasions (Hemsley & Moir, 1963). This together with the suggested requirements for peptides/amino acids by rumen organisms (which on the basis of the results of Maeng et al. (1989) must now be questioned) has tempted many scientists to explain the increased feed intake of ruminants, on poor quality forages that are supplemented with bypass protein, to the slow release of amino acids, peptides and branched chain fatty acids to the rumen milieu from the protected protein (see Hunter, 1988; Silva & Ørskov, 1988a), even though in most studies there was no evidence of increased digestibility with such supplements in predominately fibre based diets.
The above discussion indicates that the cellulolytic organisms in the rumen even of cattle on straw based diets, are rarely if ever deficient in amino acids, peptides or branched chain VFA in the rumen (see also Maeng et al. 1989). This is not to deny that these organisms may still need these nutrients in “catalytic” amounts but that they are rarely if ever at such low concentrations in rumen fluid to bring about a deficiency.
Farmers in developing countries have generally recognised the benefits to cattle of adding a small amount of fresh green herbage to straw based diets. These practices, which have evolved through trial and error, may have a number of beneficial effects which include the supply of vitamin A, essential minerals, ammonia, peptides/amino acids in an otherwise unsupplemented diet.
Recently it has been shown that where the supplemental forage in a straw based diet given to sheep is of high digestibility a boost to digestibility of the basal diet occurs even at relatively small levels of supplementation (Juul-Nielsen, 1981; Silva & Ørskov, 1988a; Ndlovu & Buchanan-Smith, 1985). The rate of digestibility of straw depends on the rate and extent of colonisation of fibre and the biomass of adherent organisms (Cheng et al. 1989) and the high digestibility forage supplement may act to seed microbes onto the less digestible straw.
On the other hand, other influences cannot be ruled out. For example, in the studies of Silva & Ørskov (1988a) in the absence of an effect of supplemental forage on digestibility, the rumen ammonia levels were often not significantly below 200 mg NH3-N/1. Where increases in digestibility of the basal forage occurred to supplemental forage the ammonia levels in the rumen were significantly below 200 mg N/1 and the supplement apparently improved the concentration to above the critical level (see Leng 1991).
The physiological research which has shown that there is an increased availability of microbial protein for digestion in fauna-free as compared to faunated ruminant (see Jouany & Ushida, 1990) has supported the feeding trials with large numbers of animals which has demonstrated significantly high feed conversion efficiencies and wool growth rate in sheep without rumen protozoa as compared to control animals.
Fauna free cattle on the same intake of a low protein, molasses/urea based diet grew at a 43% greater rate than faunated cattle on the same feed intake. The improved production was therefore an effect of a higher efficiency of feed utilisation (Bird & Leng, 1978).
The discussion to follow, on the implications of environmental temperature/humidity for the nutrition of ruminants, will indicate why a major change in P/E ratio in the nutrients absorbed (i.e. the major difference between faunated and fauna-free ruminants) will be more effective in improving ruminant production in the tropics as compared to temperate/cold countries. In the tropical areas the available forages used to feed to ruminants are generally lower in digestibility, lower in true protein and animals are rarely cold, but heat stress at times may be severe. It should also be noted that animals brought into animals houses from fairly could environments may at times suffer severe heat stress through a combination of a well insulated fleece or coat and an imbalanced diet.
The efficiency with which absorbed nutrients are converted to ruminant products (liveweight, milk etc.) is dependent on precisely meeting the animal's requirements above maintenance for individual nutrients required for the particular function (see Preston & Leng, 1987). These, at times, are influenced by body condition as affected by previous health and nutritional history (see Leng, 1989b), the demands for body temperature control (Blaxter, 1962) and the requirements for substrate oxidation in exercise (or work).
Graham et al. (1959) (see also Blaxter, 1962) showed that the quantitative oxidation of individual nutrients (largely fat) depended on the degree of heat/cold stress of the animal. A cetogenic substrate was largely used to keep an animal warm when it was required to increase its metabolic rate in cold stress. In this report it is assumed that a cold stressed animal will oxidise acetogenic substrate for heat production until ‘surplus’ acetogenic substrate is totally utilised, after which fat mobilisation provides an extra and often the major source of metabolic fuel. The apparently preferential oxidation of circulating acetate leaves a higher ratio of amino acids (and glucose) in the nutrients available for production than would be available to an animal in its zone of thermoneutrality. Conversely, an animal that is not cold has more acetogenic substrates available for anabolic purposes.
The environment can, thus, alter the partitioning of nutrients into productive functions and therefore affect the efficiencies of feed utilisation. The design of supplements, to balance diets for ruminants, needs to account for the varying demands for nutrients brought about by the thermal environment of the animal.
It is recognised that cold stress in animals often increases voluntary feed intake and rumen turnover rate. And in this way on some diets it increases microbial cells moving to the lower tract and this has been suggested to increase the P/E ratio in the nutrients available for maintenance or production (see Kennedy et al. 1986).
As an example of how environmental factors can change the nutrient balance available to ruminants for anabolism and maintenance, a model used previously to predict the relative availability of specific nutrients to a “standard steer” (see Leng, 1982) has been modified to use with sheep and includes the effects of cold stress. The model is based on the sheep (closely shorn) used in the studies of Graham et al. (1959) which were fed on a daily basis 600, 1200, or 1800 g of a dried grass pellets and subjected to short periods at environmental temperatures ranging from 8 to 40°C at a relative humidity of 50%.
The data in Table 3.2 indicate that the need to maintain body temperature may require a considerable proportion of the available acetogenic nutrients to be oxidised. In the absence of a cold stress considerably more of digestible nutrients (and in particular more acetogenic substrate) is available for maintenance and synthesis. If in thermoneutral conditions the acetogenic substrate cannot be utilised for synthesis of tissue component because of a low availability of essential amino acids and/or glucose (i.e. imbalance in P/E or G/E) (see Preston & Leng, 1987) then acetate must be dissipated as heat. If the animal is able to oxidise the excess substrate but cannot dissipate the heat-generated because environmental temperature and humidity are high then it could allow its body temperature to increase to some extent but eventually it must reduce its feed intake. If the animal's body temperature rises, metabolic rate increases through the oxidation of protein (Blaxter, 1962) which may have implications for protein requirements of ruminants in the tropics and differential responses to supplementation in the tropics as compared to temperate areas.
it means that in hot
Table 3.2: A theoretical assessment of the effects of environmental temperature on the balance of nutrients available for anabolism (The example used is from Graham et al. (1959), in which closely shorn sheep were subjected to temperatures from 8 to 40°C)
|Ration (g dried grass/d)|
|Minimal heat production (MHP)||5.8||8.3||10.5|
|Temperature at MHP (°C)||40||33||24|
|Metabolizable energy intake (MJ) at MHP||5.1||9.8||13.7|
|Heat production required to combat 5°C below critical temperature (MJ)*||2.2||2.2||2.2|
|Nutrients available (MJ)** from:|
Propionic acid (G)
Total Volatile Fatty Acids (E)
|Microbial protein available (g/d)||72||148||198|
|P:E ratio (G/MJ)++||26:1||27:1||28:1|
|Available P:E ratio (g/MJ)§||118:1||45:1||40:1|
|G:E ratio (MJ/MJ)II||0.25||0.24||0.27|
|Available G:E ratio§||7.71||0.48||0.43|
* The heat production for each degree lowering of environmental temperature below the critical temperature was assumed to increase by 0.44 MJ/24 h (Graham et al. 1959).
** The available nutrients are calculated assuming that all the digestible dry matter is digested in the rumen, that the rumen microbes have a YATP 14 and that microbial cell synthesis and VFA production are stoichiometrically related as described by Leng (1982). No allowance was made for a possible increase in dilution rate with increasing feed intake.
++ Calulated microbial protein available (g) for digestion relative to VFA (MJ).
II Propionate (MJ): acetate plus butyrate (MJ) available; the glucogenic energy ratio
§ The available P:E and G:E ratios are defined as the nutrient ratios after the acetogenic nutrients have been used for body temperature control at 5°C below the critical temperature. They are calculated assuming that the energy for heat production arises from the oxidation of acetate and/or butyrate. Graham et al. (1959) showed fat was the major source of heat and that metabolism of glucogenic or aminogenic substrate is unaffected by cold stress whereas fat (acetogenic substrate) oxidation accounted for the heat produced.
The temperature/humidity at which ruminants are cold stressed depends greatly on the level of feed intake, the insulation provided by the hair or wool coat and the environmental conditions prevailing, e.g. wind, rain and availability of shelter. Thus, the environmental temperatures at which minimum extra heat production to combat the cold stress occurs will probably move through a range of from around 10°C to 40°C.
There has been vigorous debate on whether supplementation of sheep and cattle on low quality forage based diets with urea and/or bypass protein increases intake of the basal feed resource (see Leng, 1989b). The differences in results may be hypothesised to be a result of an interaction between climate and the balance of nutrients available from a diet. When research results (Australian) on the effects of supplementation to balance nutrition of cattle on low quality forages are grouped according to climatic zones a pattern emerges (Figure 3.4).
It appears to be in the tropics and subtropics where poor quality forage intake by cattle is low without supplementation and where significant responses in feed intake occurs when a non-protein nitrogen deficiency is corrected and extra protein that escapes rumen fermentation is provided in the diet. It is strongly stressed that supplementation with urea and proteinmeals increases voluntary intake of poor quality forages by cattle under tropical conditions to approximately the same level of intake as unsupplemented cattle under temperate conditions (Leng, 1989b). In this situation the supplement is only correcting a depressed intake back to normal intake.
Figure 3.4: Intake of low digestibility forages by cattle either unsupplemented or supplemented with bypass protein or bypass protein and urea (Lindsay & Loxton, 1981; Lindsay et al., 1982a,b; Lee et al., 1984; Hennessy, 1984; Perdok, 1987; Kellaway & Leibholz, 1981)
The conclusion that can be drawn from this is that supplements which improve the P/E ratio in nutrients absorbed by cattle fed low quality forage reduces metabolic heat production. Where metabolic heat production in unsupplemented cattle fed low quality forages would increase body temperature then the animal reduces its feed intake. This reduction in voluntary feed intake is ameliorated by the supplement which allows the acetogenic substrate which would otherwise have to be oxidised to be partitioned into synthetic reactions with a resultant decrease in heat production.
The concept of small increases in P/E ratio being able to reduce metabolic heat and at times therefore allow an animal to consume more food might explain the effects of increasing levels of urea in a diet on forage intake (when digestibility is no longer increased) and also the occasional effects on feed intake of branched chain VFA supplements. The concept is that it is a supplement that improves microbial growth efficiency which has an effect on feed intake and this is only seen in the hot conditions when feed intake is depressed.
The interaction of nutrition and climate may explain why there is a stubborn disbelief by some researchers from developed countries (largely in the temperate areas) of research carried out in developing countries in the tropics. Many of the results of supplementation indicate that a protein that escapes rumen fermentation stimulates both the level and efficiency of production of milk (or live-weight gain) in ruminants fed on crop residues (see Figure 3.5).
The discussion above indicates that ruminants in hot countries have an advantage of not having to oxidise much acetogenic substrate (or body fat) to keep warm. By balancing the diet with supplements, this acetogenic substrate may be captured in products or oxidised to provide ATP for assimilation of the additional nutrients into products. In cold/cool countries supplementation with protein is less necessary, as the utilisation of surplus acetate for heat, decreases the need to balance nutrients. As long as feed intake is high (i.e. the diet is highly digestible and perhaps cold stress stimulates intake) production remains relatively high as the nutrients for heat production are extracted and the balance used in synthetic reactions. Nevertheless, increases in the efficiency of utilisation are obtained when low protein diets are supplemented with a bypass protein even in temperate countries (see Ørskov, 1970; Silva et al. 1989; Leng et al. 1977) and at times feed intake is also stimulated but it is unknown whether the animals in such studies were actually subjected to hot conditions.
Figure 3.5: Schematic relationship between diet quality (metabolisable energy MJ/kg dry matter) and food conversion efficiency (g liveweight gain/MJ ME) (- - -) (from Webster, 1989). The relationships found in practice with cattle fed on straw or ammoniated straw with increasing level of supplementation. Australia (◊, o, •) (Perdok et al., 1988), Thailand (∆) (Wanapat et al., 1986) and Bangladesh (□) (Saadullah, 1984). Recent relationships developed for cattle fed silages supplemented with fish proteins (Olafsson & Gudmundsson, 1990) (o) and tropical pastures supplemented with cottonseed meal (Godoy & Chicco, 1990) (*) are also shown. This illustrates the marked differences that result when supplements high in protein are given to cattle on diets of low ME/kg DM
It can be concluded that ruminants in the tropics that are adequately supplemented with small quantities of essential nutrients may produce at the same rate on a lower digestibility feed as an animal on higher digestibility feed in a cold environment.
To emphasise the differences in potential thermal stress of animals under different conditions the average temperature humidity index (THI) (which is an index of potential heat stress conditions for ruminants (see Johnson, 1987) on a monthly basis for Cambridge (England), Chittagong (Bangladesh), Bangkok (Thailand) and Armidale (Australia) are shown in Figure 3.6. The critical THI (72) for high milk producing cows as determined by Johnson (1987) is included in the figure. However, it must be emphasised that in addition to temperature/humidities, the critical THI will depend on the insulation provided by the animal's coat and its behaviour in seeking shelter, as well as the incidence of wind and rain in addition to level and quality of feed intake.
Many studies have shown that at the same forage intake by ruminants with an already efficient digestion that a supplement of protein that reaches the small intestine increases the efficiency of feed or metabolisable energy utilisation for growth. This is positive proof that wasteful oxidation of nutrients can occurs (See Figure 3.5). It seems reasonable that, because Blaxter (1962) and his colleagues showed that acetogenic substrate are largely “burned off” that the inefficiency of ruminants on forage based diets is a result of acetate being oxidised wastefully. This points to a major difference in considering the nutrition of ruminants in the tropics as compared with temperate countries.
do e nutrition guideline at the end,
Most forages consumed by livestock in developing countries have a low digestibility which rarely exceeds 55% and is mostly in the range of 40–45%. The calculated metabolisable energy in the dry matter (ME/DM), thus, ranges from 7.5 MJ down to 4.8 MJ. Feeding standards indicate that feeds with a metabolisable energy content, of 7.5 MJ will support growth rates of cattle of approximately 2 g/MJ of ME intake. On a forage at the lowest level of ME, cattle would be in negative energy balance (see ARC, 1980) (also for reference see Webster, 1989).
Figure 3.6: Temperature humidity index (THI) of climates in temperate countries as indicated by Cambridge (U.K.) and Armidale (Australia) as compared to tropical countries as indicated by Bangkok (Thailand) and Mymensingh (Bangladesh). The THI is calculated on the mean of the maximum-minimum temperatures and humidities
THI(°C) = Temp. (dry bulb) + 0.36 Temp. (dew point) + 41.2°C
Contrast this with results of supplementary feeding trials based on balancing the nutrition of animals with urea/minerals and bypass protein, where cattle growth rates equivalent to 18 g/MJ of ME intake have been achieved in cattle fed straw (see Figure 3.6). Obviously the presently accepted feeding standards (see ARC, 1980) have been very misleading and can not be used as a means of predicting animal performance. Of vital importance however, is that the application of the concept of balanced nutrition can improve animal growth by 2–3 fold and the efficiency of animal growth by as much as six fold over previous estimates (a range of 2–10 fold).
In addition it also shows that although growth rates of cattle are below those on grain based diets cattle on forage based diets can highly efficiently convert feed to product.
Low productivity of ruminant livestock has been accepted in developing countries as an inevitable result of the poor feed base and a low feed conversion efficiency. The concept being that there is a large heat production (energy requirement) associated with the ingestion, movement of digesta along the tract in animals on forages as compared to concentrates (see Ørskov & Macleod, 1990). This conclusion is contrary to the conclusions of Leng (1990) and the concept of balanced nutrition presented here.
The original calorimetric studies of Graham and his colleagues (see Blaxter, 1962) indicated that infused acetate or butyrate were utilised by sheep with low efficiencies, i.e. there was a high heat increment when acetate was given compared with propionate. Considerable effort has since been expended on testing the hypothesis (or disproving it) that acetogenic substrate is used wastefully. Blaxter and his colleagues (Graham et al. 1959) used diets based on dried grass which was chopped and cubed. It had a metabolisable energy content (M/D) of about 8.5 MJ/kg dry matter. As most of the protein in the diet could have been highly soluble, the P/E ratio in absorbed nutrients would have been relatively low.
The controversy concerning the efficiency with which acetogenic substrate is utilised may be rationalised at least to some extent by considering the balances of nutrients available to the ruminants in the various experiments and the ability or otherwise of the animals to synthesise fat, dissipate heat or to oxidise substrate to keep warm. For example the presence of small amounts of fish meal, that has a considerable amount of protein that escapes the rumen, in a concentrate diet (see Ørskov & Allen, 1966) provides an explanation for the differences between these authors' results and those summarised by Blaxter (1962) where sheep were fed dried grass which may have contained a highly soluble source of protein.
The need to manipulate or supplement diets for ruminants in order to ensure an adequate supply of glucose and of glucogenic compounds was discussed fully by Preston & Leng (1987), who made a comprehensive literature survey.
In outline, the rationale that is used to justify the concept of glucose being a limiting nutrient is as follows:—
Little glucose is absorbed by ruminants but they synthesise considerable glucose from precursors such as propionate and certain amino acids, largely in the liver. Glucose is certainly required, by ruminants, as a major substrate for cell synthesis, as an important oxidative energy supply in the brain and red cells and as an important nutrient for the growing foetus and for milk lactose and fat synthesis in lactating animals. Glucose needs to be oxidised in the adipose tissue, and to a lesser extent the mammary gland, to supply the reduced cofactors for fat synthesis from acetate (in this case of NADPH, generated in the pentose phosphate pathway). It is possible therefore that fat synthesis in an animal may be limited by glucose availability for oxidation. If there is a block in acetic acid utilisation, it cannot be allowed to accumulate in the blood as this would lead to acidosis, the animal therefore, needs to oxidise the excess acetate in a futile cycle in which acetyl CoA is produced with a requirement for ATP and then hydrolysed. Alternatively an animal could increase its muscular activity by standing instead of sitting or walking long distances which would ensure increased acetate oxidation.
The animal that is most at risk to a deficiency of glucose is the animal with a big demand for glucose (late pregnant or early lactating) fed forage based diets (high acetogenic fermentation in the rumen) with little bypass nutrients in the diet (receives only amino acids from microbial sources) and is tethered (no requirements to oxidise acetogenic substrate to walk) living in a tropical country (no requirement to oxidise acetate to keep warm).
As discussed above, reduction in the need for acetate oxidation and a high glucose demand can establish a situation where a deficiency of glucose could lead to an imbalanced nutrition with a need to burn off excess acetate with a high thermogenic effect. Where such a thermogenic effect cannot be tolerated (high environmental temperatures, high humidity and perhaps heavy insulative coat) the animal needs to respond by reducing its feed intake. Conversely the thermogenic effect may be prevented by balancing the diet with nutrients that supply glucogenic substrates for absorption from the intestine (see Leng, 1991).
Considerable research has indicated that the level of protein nutrition considerably influences reproduction of both male and female animals and subsequent pregnancy and lactation. Leng et al. (1987) indicated that on forage based diets each physiological function is affected adversely by a low P/E ratio in the nutrients absorbed by ruminants; this could be overcome, to some extent, by feeding protein meals which bypass the rumen.
In summary, feeding a supplement which improves the P/E ratio in the nutrients absorbed by ruminants on a low true protein, forage diet has the following potential effects, particularly in a hot climate, on reproductive efficiency of female ruminants:—
stimulates liveweight gain of dam or reduces liveweight loss (see Lindsay et al. 1982a)
improves ovulation rates (Waghorn et al. 1990)
improves placental size (Hinch, 1989)
improves birth weight (Stephenson et al. 1981) and so increases survival (Lynch et al. 1990) and because of the increased birth weight possibly lowers the incidence of retained placenta
increases milk yield and efficiencies of milk production (Saadullah, 1984).
Prevention of protein deficiency in early life also prevents stunting of final body size (see Preston & Leng, 1987). Differences in size of animals of the same breed, in the same country, is almost always a result of differences in nutrition and not inherent differences. This has recently been emphasised with N'Dama breed which has always been considered to be a small breed weighing up to 250 kg liveweight. With good nutrition and adequate management the bulls have now been shown to grow to 500 kg liveweight (Murray, 1989). Work from Nigeria and Australia has also shown that young and old bulls are also very susceptible to low P/E ratios in the nutrients absorbed (Rekwot et al. 1988). Young bulls that grow on diets that would have given a high P/E ratio in the nutrients absorbed, compared with animals fed a diet giving a low P/E ratio, had better testicular development and produced larger ejaculates with double the sperm content (Rekwot et al. 1988). Older bulls that go through a period of protein undernutrition have decreased testicular size and probably are less fertile (Lindsay et al. 1982b).
Undoubtedly any parasitic or disease condition that drains protein from the animal will increase the animals requirements for protein relative to energy (Leng, 1982). Similarly, infective agents that utilise glucose may also increase the demand for this critical nutrient. For example, trypanosomes and epyrythrozoan parasites which invade red cells, increase protein requirements by increasing red cell turnover rate and also increase the animals requirements for glucose as this is the major substrate used in the parasite's metabolism. It is suggested that improving the protein nutrition of ruminants through providing bypass protein directly to the animal (i.e. avoiding rumen fermentation) may considerably ameliorate the detrimental effects of intestinal and blood parasites (Leng, 1982) and may assist in development of early immunity. (J. Steele personal communication)
Light work requires acetogenic substrate probably acetate for muscle contraction but heavy work is probably dependent on long chain fatty acids mobilised from adipose tissue. All working muscles use some glucose and there is always a concomitant increase in glucose utilisation when free fatty acid metabolism is increased by the work load (see Pethick et al. 1983).
Imbalanced diets fed to draught animals which result in a low P/E ratio in the nutrients absorbed are not so disadvantageous as they are for a lactating or growing animal. The excess acetogenic substrate is used for work removing the necessity to otherwise enzymatically dispose of the substrate. However, the increment in requirements for glucose oxidation in muscles although small could be a limitation. In addition the need in heavy work to supply long chain fatty acids which can be oxidised to produce ATP at a faster rate than from acetate, indicates that there is an increased need for glucose not only in muscle metabolism but also to aid fat synthesis from acetate in the period of rest.
An imbalanced feed (low P/E ratio or low G/E ratio), which is associated with a high metabolic heat production, could also reduce the recovery of body temperature of draught animals when the animal is resting and, thus, reduce feed intake. A decreased fat deposition because of such an imbalanced diet during the non working season may also be a major constraint to draught capacity because of lack of body reserves for mobilisation in the working season.
The P/E ratio required to support fat deposition (in periods of low work load) or to reduce heat stress by reducing metabolic heat is likely to be much lower than for example for growth and milk production. A draught animal fed poor quality forage probably requires little more nutrients than can be supplied from an efficiently functioning rumen. This is attained for example by supplying the animal with a molasses/urea multi-nutrient block. Indian farmers soon found the benefits of molasses/urea block for draught animals when these became available (personal observation) and recently the beneficial effects of feeding molasses/urea blocks to draught buffalo have been demonstrated in Vietnam (T.R. Preston personal communication). The results of Preston's studies in Vietnam are given in Table 3.3.
Table 3.3: Liveweight change and work capacity of buffaloes given rice straw supplemented with a urea/molasses block (Preston, 1990) There were 22 animals in the trial
|Live weight (kg)|
|Dry Season (no work)|
|After 1 month||346||395|
|Wet Season (ploughing)|
|After 1 month||334||370|
|Area ploughed (m2/day)|
|After 1 month||1508||2141|
|(2 buffaloes) (m/min)|
|After 1 month||32||41|
|Recovery time (min)*|
|After 1 month||16||13|
* Time needed to recover normal heart rate.
This discussion highlights the manipulable aspects of ruminant nutrition, where substantial improvements in productive efficiency of animals fed largely on fibrous feeds could result. The differences between temperate and tropical conditions also indicate opportunities which are often not apparent in temperate countries and mitigates against direct transfer of results from the former to the latter.
In tropical conditions, protein nutrition of ruminants is more crucial than in temperate areas. The general conclusions are that:—
the primary constraint to ruminant production from fibrous feeds (in the tropics) is the low efficiency of feed utilisation often coupled with an environmental/physiological effect which results in a reduction in potential feed intake.
in practice the most effective mechanisms for improving productivity will be to improve the P/E ratio in the nutrients absorbed because of the large effect on efficiency of feed utilisation.
draught animals will also benefit from technologies that increase microbial growth efficiency in the rumen and at times improve digestibility, intake and P/E ratio in the nutrient absorbed.
contrary to many statements in authoritative reviews, only when P/E ratios have been optimised does the energy density of the feed become a primary constraint.
as the primary constraint is an inefficiency due to a low P/E ratio in the nutrients absorbed it is unlikely that other inputs (e.g. agents to repartition nutrients or to stimulate growth (B-agonists, BSt)) could be successful without first applying strategic supplementation.