2. GUIDELINES FOR ESTABLISHING FEEDING SYSTEMS FOR RUMINANTS

2.1 GENERAL CONSIDERATIONS

2.1.1 Introduction

In order to develop feeding systems it is necessary to relate information on the nutritional characteristics of feed resources with the requirements for nutrients according to the purpose and rate of productivity of the animals in question. In the industrialized countries, this information has been incorporated in tables of “Feeding standards” which interpret chemical analyses of feed resources in terms of their capacity to supply the energy, amino acids, vitamins and minerals required for the particular productive purpose. These standards are steadily becoming more sophisticated with the aim of improving their effectiveness in predicting rates of performance of intensively-fed livestock and to derive least cost formulations.

2.1.2 Limitations to “conventional” feeding standards

The relevance of feeding standards for developing countries, particularly those in the tropics, has been questioned from the socio-economic (Jackson 1980) and technical (Graham 1983; Preston 1983) viewpoints. It has been apparent for many years that feeding standards based on assigned nutritive values (eg: net energy) are misleading when unconventional feed resources are used (eg: Preston 1972; Leng and Preston 1976; Gaya et al 1981), since the levels of production achieved may be considerably less than the level predicted. More importantly, this often led to the rejection of many available feed resources which apparently were too low in digestible energy to supply the energy needed for production. It also encouraged researchers to copy feeding systems used in temperate countries which are relatively “predictable”, but which require feed resources that are unavailable and/or inappropriate on socio-economic grounds, in most developing countries.

2.1.3 An alternative approach

The justification for a new approach to the development of feeding systems for ruminants, not based on conventional “feeding standards”, is that:

• The efficiency of the rumen ecosystem cannot be characterized by any form of feed analysis

• Feed intake on some diets bears no relationship to digestibility and is much more influenced by supplementation

• Availability of amino acids cannot be inferred from the crude protein content of the diet

• The energy value of a diet, and the efficiency of its utilization, are largely determined by the relative balances of glucogenic energy, long chain fatty acids and essential amino acids absorbed by the animal

In the early 1960s, Professor Max Kleiber had expressed a similar concern for these issues and stated (as quoted by Kronfeld 1982) “…metabolizable energy is not a homogeneous entity; instead it represents an assembly of nutrients or metabolites each of which is used with a specific efficiency for a particular purpose”. To this could be added that the availability of these nutrients, and their interactions, affect the efficiency of energy utilization.

The misconceptions inherent in any system based primarily on feed analysis are that it is almost impossible to predict:

• Whether the feed can support efficient rumen function

• The nature, amounts and the proportions of the end products of fermentation digestion

• The potential for rumen escape of nutrients and their digestibility in the small intestine.

For these technical reasons, and also because of differing socio-economic circumstances, it has been proposed that a more appropriate objective, especially for developing countries, is to “match livestock production systems with the resources available” (Preston and Leng 1986).

This chapter sets out the guidelines for applying these concepts to the development of feeding systems which aim to optimize the utilization of locally available feed resources and to build on traditional practices. Farming systems in developing countries are notoriously difficult to change and innovation must be introduced gradually without inducing excessive risk which may, in the poor conditions of small farmers, directly affect the well-being of the family.

2.1.4 Animal response to non-conventional feed resources

It is relevant to point out that the doubts concerning the usefulness of feeding standards for ruminants in tropical countries surfaced during development work in Cuba (Preston and Willis 1974) in the 1970's when livestock production systems were being established on non-conventional feed resources (ie: molasses-based diets). In these cases, although nutrient requirements were satisfied, according to traditional feeding standards, the responses of the animals did not correspond to the predicted levels of performance.

This research demonstrated that small inputs of bypass protein (Peruvian fishmeal) increased dramatically growth rate and feed efficiency of cattle (Figure 2.1). In contrast, this feeding system was not able to support high levels of milk production (Figure 2.2), because of the greater demands of lactation for glucogenic compounds and the relative deficiencies of these in the digestion end-products on molasses-based diets because of the low-propionate, high-butyrate fermentation (Marty and Preston 1970).

The high potential yield of animal products from a hectare of sugar cane stimulated the subsequent research in Mexico, Mauritius and the Dominican Republic that attempted to establish cattle production systems applying the principles developed for feeding molasses (both feed resources had similar concentrations of soluble sugars) (see Preston and Leng 1978a,b).

Research on the feeding value of derinded (Donefer, E. and James, L. cited by Pigden 1972) and chopped (Preston et al 1976) sugar cane demonstrated that:

• Feed intake was low even though digestibility was high (60–70%)

• The animals on this feed apparently needed glucose or glucose precursors because all the sugars are fermented, rumen propionate levels are no higher than observed on high-fibre diets, and the presence of a dense population of ciliate protozoa (Valdez et al 1977) reduces the availability of microbial protein to the animal (Bird and Leng 1985).

The implication of these two findings is that rumen function did not provide the required balance of nutrients for productive purposes (see Leng and Preston 1976).

Figure 2.1

Addition of fishmeal to a basal diet of ad libitum molasses/urea and restricted forage dramatically improves growth rate and feed conversion of cattle in Cuba (from Preston and Willis 1974)

Figure 2.2

Effect of replacing maize with molasses on the pattern of rumen fermentation and milk yield in Holstein cows. As dietary molasses increased propionic acid decreased and there was a concomitant fall in milk yield (Clark et al 1972)

Recognition of the role of fermentable N and bypass protein in low-N diets led to research aimed at increasing productivity of cattle and sheep on a range of fibre- and sugar-rich low-N feeds (Leng et al 1977; Preston and Leng 1984). Prior to this work, these feed resources were considered to have little value other than to support maintenance and were universally referred to as ‘low quality’ fibrous feeds. This led to attempts to improve the digestibility of fibrous feeds by, in particular, alkali treatment (Jackson 1977, 1978). However, the value of alkali treatment was partially obscured by the failure to recognize that the first limitation was not digestibility but the imbalance of nutrients at the level of both the rumen and the whole animal (Leng 1982; Preston and Leng 1984).

Combining alkali treatment and appropriate supplementation has led to practical rice straw-based feeding systems being applied on farms in Bangladesh (Dolberg et al 1981a; Davis et al 1983) and Sri Lanka (Perdok et al 1982; Jayasuriya 1984).

The significance of these developments is not so much the use of molasses or of straw in animal feeding, since both these feeds have been incorporated into diets of ruminants in industrialized countries for many years. The issue is the magnitude of the contribution of molasses and straw to the total dietary dry matter. In industrialized countries their contribution rarely exceeds 10–15% of the diet in the case of molasses, and 20 to 40% for straw, the rest of the ratio being cereal grains, highly fertilized grasses and legumes and oil seed cakes. In contrast, in developing countries the feeding regimes aim to incorporate these feeds as the principal component of the diet because these are the locally available resources; and there are restrictions on the use of grain for livestock feeding for financial, political and socio-economic reasons.

One characteristic of diets based on crop residues, sugar-rich agroindustrial byproducts and/or mature grasses is the magnitude of the animal response when these feed resources are supplemented with small quantities of bypass protein. These responses to bypass protein are much greater than are observed when a diet is based on cereal grain (Table 2.1).

A second feature of these diets for ruminant production is that even when they are supplemented with bypass protein, the efficiency of utilization of digestible energy is generally poorer than on diets with similar digestibility but with a large part of that energy contributed by cereal grain (see Pigden 1972; Creek et al 1976). The inefficiency of energy utilization appears to be most pronounced when the host animal has a high demand for amino acids and glucose precursors (eg: the lactating cow) (Clark et al 1972; Chopping et al 1976; Perdok et al 1982).

^* Contained formaldehyde-treated cottonseed meal, meat meal and fishmeal

(1) Ørskov et al (2) Bird et al (1979) (3) Abidin and Kemption (1981) (4) Preston and willis (1974) (5) Durate et al (1983) (6) Smith et al (1979) (7) Saadullah et al (1982) (8) Lindsay and Loxton (1981)

There are many reasons why molasses and straw, when fed as the main component of a diet, perform differently than when fed as relatively minor components. Some of these differences can be explained on the basis of interactions and associated effects among nutrients, and between nutrients and the site of digestion.

In order that responses in animal productivity to supplements can be predicted accurately on a particular diet, it is necessary to take account of the constraints to metabolism. These relate specifically to the relative amounts of amino acids, glucogenic energy, VFA energy and long chain fatty acid energy in the end products of fermentative and intestinal digestion, since this is what determines the animal's productivity.

Productivity of ruminants is influenced primarily by feed intake which in turn is determined by feed digestibility and the capacity of the diet to supply the correct balance of nutrients required by animals in different productive states. Therefore the two major variables that need to be considered are:

• The amounts and balance of nutrients required

• The quantitative availability of nutrients from the diet

The balance of nutrients required depends upon:

• The amounts of dietary components unchanged by rumen fermentation that are absorbed (amino acids, glucose and long chain fatty acids)

• The rates of production of the end products of fermentative digestion (which can be highly variable)

• Productive functions (pregnancy, lactation, growth, work, maintenance, depletion or repletion of bodyweight)

• Environmental factors (disease, parasitism, temperature and humidity, and other sources of stress)

The availability of nutrients from a diet is highly dependent on:

• The microbial ecosystem in the rumen which influences the availability of microbial protein, VFA energy and glucogenic energy

• The chemical composition and physical form of the diet which influence the amounts of protein, starch and long chain fatty acids which escape the rumen fermentation.

At the present time, it is not possible to predict the nutrients required by ruminant livestock and to match these with nutrients available from digestion, because of the many interactions among the animal, its rumen microbial ecosystem and the diet.

The most widely available low-cost feeds for ruminants in the majority of developing countries are usually native pastures, crop residues and to a lesser extent agroindustrial by-products. The expensive, and often unavailable (or exported), feeds are the protein meals, derived from oilseed residues and the processing of animals, fish and cereal grains.

Generally, energy (the basic feed resource) and fermentable nitrogen (urea) are relatively inexpensive ingredients, while the sources of amino acids and glucogenic compounds (the protein meals, cereal grains and cereal by-products) are very expensive. Since it is fermentation of carbohydrate which provides the energy for microbial growth, abd as the feed is often low in digestibility, it is generally desirable to supply fermentable energy on and ad libitum basis. It is therefore contra-indicated to restrict the basal diet.

As a rule of thumb, 3 g of fermentable N per 100 g of fermentable organic matter are required to meet the needs for efficient microbial growth. It is not always necessary to provide this amount since some feed protein will be fermented to ammonia and some urea-N may enter the rumen in saliva. These processes reduce the amount of non-protein nitrogen needed. In addition there is evidence that the rumen microbes need small amounts of amino acids and other nutrients for efficient microbial growth.

The potential of the diet to satisfy the requirements of the animal for amino acids, glucogenic precursors and long chain fatty acids, depends on the pattern of fermentation and on the dietary protein, lipids (or their constituent fatty acids) and starch that escape fermentation and are digested in the intestines.

The extent to which the protein in a supplement escapes the rumen is partly a function of its rate of degradation (solubility) in the rumen. It is likely to be influenced greatly by the rate of flow of fluid and small particles out of the rumen. This latter characteristic will be influenced by processing of the feed (by physical or chemical means), the presence of some green forage, the amount of protein reaching the duodenum and external factors such as temperature and exercise/work.

The same factors will influence the supply of glucose and glucogenic precursors in terms of the likely bypass of starch to the duodenum. However, the nature of rumen fermentation will have a major influence in terms of the supply of propionic acid for glucose synthesis.

2.2 RELATING NUTRIENT SUPPLY TO PRODUCTIVE STATE

2.2.1 Introduction

There is insufficient information available to permit the precise quantification of the proportions of the different nutrients required for different productive states. Nevertheless, an approximation of the needs of animals can be attempted. The suggested scheme attaches relative priorities to the groups of nutrients according to the physiological and biochemical processes underlying the expression of the particular productive state (see Table 2.2).

The groups of nutrients to be varied for different productive states are:

• VFA energy

• Glucogenic energy

• Amino acids

• Long chain fatty acids (LCFA)

VFA energy arises from the rumen fermentation of all types of organic matter principally carbohydrates. The principal way of increasing VFA energy in a particular feed is to increase intake and/or the rumen degradability by supplementing with bypass protein and/or alkali treatment (mainly ammoniation).

Manipulation of the rumen to provide extra protein and glucogenic precursors is still at the experimental stage. Dietary supplementation is the most obivious way of manipulating the supply of absorbed amino acids, glucose and glucose precursors.

Most supplements are expensive and their use in ruminant nutrition competes with monogastric animal and human nutrition. If the primary feed resource is a product of low nutritive value, which would have been wasted if it were not fed to ruminants, it can be argued that the ruminant uses these concentrate supplements more efficiently than monogastric animals. For this reason the term “catalytic” supplement has been used to describe these effects (Preston and Leng 1980). Suckled milk, given in small amounts (<2 litres daily) as a supplement for calves given a straw or molasses based diet, is a good example of a “catalytic” supplement.

It is mandatory that research should produce response relationships to distinguish economic from biological optima. As a rule of thumb, the role of the supplement ceases to be “catalytic” when it exceeds about 30% of the diet dry matter. Beyond this point it assumes a major role and substitution occurs.

The productive functions and the need for supplementary nutrients are discussed in order of the least to the most demanding.

2.2.2 Work

Work requires ATP (adenosine triphosphate) generated from the oxidation of long chain fatty acids, with obligatory requirements for glucogenic compounds and for amino acids (to repair the wear and tear of tissues and replace protein secretions) (see Leng 1985).

The working animal can often obtain sufficient nutrients from a nitrogen-deficient diet so long as it balances the protein to energy ratio needed for tissue turnover by “burning” off acetate which is in excess of requirements. However, bodyweight loss may restrict the period of work. If the work period is to be prolonged and weight loss is to be minimized, then the nutrients available must be balanced so as to satisfy the needs of the working animal. The digestibility and the intake of the basal diet may also have to be increased by supplementing with urea to correct a deficiency of fermentable nitrogen in the rumen. This may be the only manipulation necessary but supplements rich in fat and bypass protein could be beneficial particularly where the animal is in a productive state (eg: pregnant or lactating). If weight loss continues because work is prolonged, it may be necessary to increase the degradability of the basal diet, for instance by ammoniation (urea ensiling).

The mature, unproductive ruminant does not appear to require nutrients over and above those provided by an efficient fermentative digestion.

Since the heavily working animal uses largely long chain fatty acids and glucose (Pethick and Lindsay 1982a; Leng 1985), the supplements used should contain or provide these substrates. This is particularly important in the case of long chain fatty acids, since their absorption and use for fat deposition, mobilization and use for work, will be much more efficient and will require less glucose oxidation than fat synthesis from acetate and subsequent utilization in muscle metabolism.

2.2.3 Maintenance

Maintenance alone obviously requires less energy expenditure than work so there is a proportionately higher demand for amino acids (relative to energy) than in the working animal. This will always be provided by a rumen system which is adequate in fermentable nitrogen. Animals in negative energy balance for an extended period on low-nitrogen roughage-based diets extract more digestible energy from the basal diet when this is supplemented with fermentable nitrogen (see Table 2.3).

2.2.4 Growth

Growing animals have a very high requirement for amino acids for tissue synthesis and glucose for oxidation in specific tissues (eg: brain). In addition considerable amounts of glucose must be oxidized to provide the NADPH (reduced nicotinamide-adenine dinucleotide phosphate) required to synthesize fat from acetate.

It is imperative to recognize that high growth rates cannot be supported on the products of fermentative digestion and that bypass protein supplements are essential to take advantage of the VFA energy absorbed.

^* US supplied 55 g/d N; 

^** PP supplied 1 kg/d of a protein pellet containing 80%cottonseed meal, 10% fishmeal and 10% meat meal (protected with formaldehyde)

Many factors influence the level of protein supplementation to be used. Response relationships must be established which relate protein supply to animal productivity for each basal (carbohydrate) resource and for the available protein meals. The response pattern will vary according to the nature of the basal diet and the particular protein supplement.

Data taken from Bangladesh and Cuba demonstrate this rationale. Cattle on ammoniated (urea-ensiled) rice straw, when supplemented with only 50 g/d fishmeal, increased their liveweight gain threefold (Figure 2.3). On a molasses-based diet of higher energetic potential, 450 g/d of fishmeal were needed to raise liveweight gain from 300 to 900 g/day (Figure 2.1).

2.2.5 Reproduction

Improvements in fertility brought about through nutrition are usually attributed to increased energy intake. There is, however, information to show that the supply of glucogenic precursors relative to total energy is an important feature of the improved energy status which results in increased fertility.

2.2.5.1 Conception and puberty

Recent studies have demonstrated that even when the protein supply is adequate, the “quality” of the energy can also be a limiting factor. At the same metabolizable energy intake (the basal diet was low-N Coastal Bermuda grass pasture), puberty was reached at lower liveweights when glucose availability in the animal was enhanced (Table 2.4).

The effects of bypass protein on conception rates of cows grazing sub-tropical pasture during the dry season are shown in Table 2.5. A supplement providing fermentable energy (molasses) was much less effective confirming the report of Moseley et al (1982) that it is the “quality” of the energy (ie: energy in the form of glucogenic compounds) which is the critical issue.

2.2.5.2 Growth of the foetus

The growth of the conceptus has little effect on the protein and energy demand of ruminants until the last third of gestation when most of the foetal tissues are deposited. Because of the time course of growth of the conceptus which increases the daily need for nutrient to only a small extent, it appears that rumen function even on diets of low digestibility can support the birth of a viable offspring of normal weight. This was shown in studies in which urea was included in the drinking water of ewes on nitrogen deficient pasture (Table 2.6).

Figure 2.3

A small supplement of fishmeal dramatically increases growth in live and carcass weight in young cattle fed a basal diet of ammoniated (urea-ensiling) rice straw in Bangladesh (from Saadullah, M. personal communication)

Increases in calf birth weight were recorded when pregnant cattle, given a basal diet of hay of low digestibility (45%), were supplemented with urea. However, to prevent bodyweight loss and/or promote weight gain of the dam through pregnancy, it was necessary to provide additional bypass protein (Table 2.3).

It appears that urea supplementation enhances milk production to a level that ensures survival of the offspring by the strategy of giving urea. But to allow the young animal to grow, milk yield must be further stimulated by feeding a bypass protein meal.

2.2.5.3 Male reproduction

Male reproduction has been enhanced under grazing conditions by supplementary feeding. Lindsay et al (1982) showed that bulls could be maintained in good condition on poorly digestible, low-nitrogen spear grass pasture by providing 1 kg daily of a protein supplement (Table 2.7) More importantly the circumference of the scrotum decreased considerably when no supplement was fed; and it is known that a bull with a lower scrotal circumference is less fertile and has a lower libido (Blockey 1980). This shows quite clearly that protein nutrition influences male fertility.

As with female fertility there appears to be evidence for beneficial responses to manipulating propionate production in the rumen. At the same feed intake, bulls reached puberty earlier and at puberty had a greater scrotal circumference and larger testicles (Table 2.8).

2.2.6 Milk production

The major constraint to milk production on diets based on crop residues and agroindustrial byproducts appears to be the availability of glucogenic compounds to provide the glucose for lactose synthesis and for oxidation to provide the NADPH for synthesis of fatty acids.

There is good evidence that in large ruminants about 50% of the fatty acids of milk arise from dietary fat. A dietary source of lipid can thus reduce considerably any imbalance caused by relative deficiencies of glucogenic energy and amino acids in the end products of rumen digestion. For many feeding systems in the tropics the level of fat in the diet could be a primary constraint to milk production. This could be particularly important in diets based on molasses or sugar cane.

^* Mean yields measured on days 1, 11 and 21

^** Not determined

^* from 29 to 175 days

Supplementation of lactating animals, particularly on diets based on tropical pastures, crop residues and sugar-rich agro-industrial byproducts, should aim to correct the imbalances of nutrients for milk production. Bypass protein usually increases feed intake and as a consequence promotes milk production. But to balance energy quality, fat must be mobilized and glucose diverted from oxidation and tissue synthesis to lactose production. In these circumstances animals tend to lose body weight (Ørskov et al 1977). Dietary fat may reduce this effect. Adding a source of bypass starch in such a diet balances the ratio of glucogenic precursors to protein and energy and will tend to prevent body fat mobilization.

The points to be stressed are that:

• Bypass protein because of its effects on feed intake almost always stimulates milk production and depending on the imbalance in nutrients (fermentation pattern) may cause animals to mobilize body reserves. This may be prevented by the use of high-fat, high-protein meals that supply both protein and long chain fatty acids for digestion post ruminally.

• Bypass starch or manipulation of the rumen to give higher propionate production (eg: by supplementation with monensin), because it balances nutrients for milk production, may prevent mobilization of body reserves without large effects on feed intake and therefore on milk production. But because it balances the nutrients for milk production, efficiency of energy utilization is increased and bodyweight is often increased.

2.2.7 Wool or hair production

The effect of nutrition on wool production appears to be dependent almost entirely on the quantity, and quality, of the balance of amino acids absorbed. Therefore, feed intake is the primary limitation to wool or fibre growth although at any one feed intake, wool growth can be stimulated by altering the balance of protein relative to energy in the products of fermentative digestion (eg: removing protozoa from the rumen).

Thus on diets that require fermentative digestion, including those based on sugars or fibre, a bypass protein supplement will increase wool growth (Table 2.9).

2.2.8 Carry-over effects of balancing nutrients in early life

Under pastoral conditions with wet/dry seasons, young stock post-weaning are subjected almost invariably to a deficiency of protein relative to energy in the absorbed nutrients from the digestive tract. This results in reduced feed intake and energy deficiency.

In societies that depend on milk as a dietary staple, the young calf and people compete for the available milk supply. The reduced amount of milk available for the calves can be highly detrimental, particularly when the herd is grazing dry pastures or is being fed on crop residues. The cows will yield less at this time due to the imbalanced feed available. Thus the calf suffers on both counts (an imbalanced basal diet together with a reduction in the supply of bypass nutrients from milk).

The male stock, because they are less valuable and not usually given supplements, may die from inanition resulting from protein deficiency. This is manifested as a low intake of the available feed, usually straw or grass, which is low in fermentable nitrogen and lacks bypass nutrients. The calf normally obtains the latter from suckled milk, but this is highly restricted because it is used preferentially by the family.

In countries such as India, female offspring are more prized and are often given supplements of young grass and sometimes byproducts such as cottonseed meal. Their survival rate is much higher than the males.

Male stock that survive are often reared as replacement oxen, and their ultimate body size is important since in countries where feed resources are scarce, using a single ox for work (instead of the traditional pair) is an obvious advantage in conserving valuable feed resources. However, as body size is related to work capacity, a large animal is needed if the move to a single ox is to be successful.

There are indications that permanent stunting of cattle occurs if they are underfed in the pre- and post-weaning period. The stunting is probably due to depressed feed intake particularly during dry seasons when the level of both fermentable nitrogen and bypass protein limits feed intake of the available feeds (pasture and straw). Two sets of information support this thesis.

^* Wool or hair clipped from a 10 cm square mid-side batch

• Studies by Hennessy (1984) with Hereford cattle grazing native pastures which were low in N in the dry season showed that, if a protein supplement was given during early life, the adult body size was increased by 60 kg compared with animals that were unsupplemented (Table 2.10).

• Surveys of cattle bodyweight in traditional grazing systems as compared to ranching in Africa indicated that mature bodyweight was greater in cattle on the ranches. Calf birth weights, weaning weight and live weight at two years of age were also higher in the animals under ranching conditions (Leng and Brumby 1985). The competition between calves and humans for milk is apparent throughout Africa. The low birth weight of cattle is also indicative of nitrogen or protein deficiency and/or low feed availability to the dam.

The two sets of data strongly suggest that inadequate protein nutrition and/or low feed availability at critical periods may lead to permanent stunting of cattle. The small size of cattle in traditional systems may be partly the result of inadequate nutrition in early life.

2.3 PRINCIPLES OF SUPPLEMENTATION

The proposed scheme (Figure 2.4) is empirical but is considered to be appropriate for the conditions of most developing countries.

2.3.1 Select the basal carbohydrate-rich resource

The first step is to select the basal carbohydrate resource according to availability, potential fermentability and price. Supplementary nutrients should then be provided in accordance with their relative priorities and costs.

2.3.2 Fermentable N

The first supplement to be considered should be a source of fermentable nitrogen (usually urea or ammonia) to ensure the level of rumen ammonia is above 150 mg/litre of rumen fluid. The generally recommended minimum level of rumen ammonia to support efficient use of fermentable carbohydrate for microbial growth is 50 mg/litre. However, this appears to be too low to optimize the rate of degradation of fibrous substrate, since the disappearance rate of cellulose and fibre from nylon bags in the rumen was increased when the concentration of ammonia was raised to 200 mg/litre (see Figure 2.5).

^* This group was not mated in 1978

Figure 2.4

Principles of supplementation

• Select the basal carbohydrate-rich resource

• Provide fermentable nitrogen (urea or ammonia)

• Provide a highly digestible forage (young grass, a legume or beet pulp)

• Provide a source of bypass protein (oilseed meal, fishmeal, cereal bran, tannin-containing legume)

• Provide a source of long chain fatty acids (more research is needed before strong recommendations can be made about this)

When the substrate was alkali-treated maize cobs (Alvarez et al 1983), rate of dry matter loss from nylon bags in the rumen increased linearly as rumen ammonia concentration was raised from 30 to 120 mg/litre of rumen fluid. Similarly, the optimum level of rumen ammonia for maximum rate of fermentation on starch-based diets, was above 200 mg/litre (Mehrez et al 1977). However, it must be stressed that the rate of breakdown of starch in the rumen is probably never a constraint for the utilization of grain-based diets. On the contrary, it may well be an advantage on such diets to have a lower than optimal rumen ammonia level to slow down the fermentation rate.

In contrast, rate of degradation is of paramount importance when the diet is based on crop residues; because it is the rate of degradation of fibre which eventually limits feed intake and therefore animal productivity.

When rumen ammonia levels are lower than 150 mg/litre it is recommended that the effects of adding urea should be monitored under the prevailing field/farm situation. As a general rule, if a deficiency is suspected, urea should be added at the rate of about 1–2% of the organic matter in the diet. It is desirable th; supplementation ensures an almost continuous supply of ammonia-nitrogen in the rumen and the use of molasses/urea blocks or high-urea (10%) liquid mixtures with molasses is a convenient way of ensuring this. Ammoniation using ammonia gas, or through ensiling with urea, are other ways of providing a continuous supply of rumen ammonia with the associated advantage of upgrading the carbohydrate component. A recent development is the generation of ammonia from mixtures of dry chemicals (eg: ammoniate sulphate and quicklime) when these are mixed with water (Mason et al 1985).

2.3.3 Highly digestible forage

The second supplement should be a source of highly digestible forage, preferably legume (or beet pulp) given at about 10–20% of the diet (Juul-Nielsen 1981; Gutierrez and Elliott 1984; Silva and Ørskov 1985). The exact action of this type of supplement on rumen function is not fully understood. In some way it helps to ensure a more efficient rumen environment for the digestion of cell wall carbohydrate perhaps by providing micro-nutrients (eg: peptides, amino acids, minerals, vitamins) which increase fungal biomass and/or the rate of bacterial colonization of the fibre.

Figure 2.5

The effects of rumen ammonia levels on the rate of degradability of the insoluble components of cotton wool and oaten chaff (fibre) as measured by the nylon bag technique. The sheep were given a diet of oaten chaff and had access to molasses/urea blocks containing 10, 15 or 20% urea. The data show that the rumen ammonia level should exceed 150 mg/litre of rumen fluid in order to maximize the rate of fermentation of cellulose or fibre (Krebs and Leng 1984)

2.3.4 Bypass protein

The third supplement should be an oilseed meal, cereal bran or an animal by-product meal (supplying protein and fat) and should be given in amounts not to exceed 30% of the total diet dry matter. The 30% limit is to prevent depression/substitution of the digestible energy of the basal diet. Lesser amounts may be more economical, and it is imperative that feeding trials be carried out to define response relationships. In this way the amount of supplement can be related to the rate of animal productivity. The optimum level (in economic rather than biological terms) and the degree of response to the supplement, will depend upon the fermentability of the basal diet.

2.3.5 Long-chain fatty acids (LCFA)?

Supplementation with a source of long-chain fatty acids (LCFA) is a strategy that promises to be of considerable benefit, especially on diets with a low content of lipids (eg: crop residues and molasses). However, more research is needed before making recommendations.

2.4 CATEGORIZATION OF FEED RESOURCES

The principles underlying the development of feeding systems are based on:

• Identifying the locally available dietary resources which deserve better attention

• Understanding the nutritional constraints associated with their efficient utilization by ruminants

• Formulating supplements with the objective of optimizing the nutrient supply to the animal given the basal carbohydrate resource

In some instances it may be justified to use imported materials especiallywhere small amounts have dramatic (catalytic) effects. Urea and fishmeal (or cottonseed meal) are good examples when they are used as supplements in diets based on molasses or cereal straws.

2.4.1 Fermentable carbohydrate

Many naturally occuring materials can be fed to ruminants but relatively few of these are available in sufficient quantity to permit them to be selected as the “principal source of fermentable carbohydrate”. The main feed resources that fall into this category include:

• Pastures (a distinction should be made between pastures during the dry and wet seasons)

• Crop residues (eg: straws from rice, millet, sorghum, maize and wheat)

• Cut forages and high-biomass crops (eg: sugar cane, elephant grass and other grasses)

• Agroindustrial byproducts of which molasses is the most important. Other byproducts include pulps from the citrus, pineapple and the sisal industries, and reject fruit and waste from bananas

2.4.2 Fermentable nitrogen

A source of fermentable nitrogen must be added when the basal diet does not give rise to sufficiently high levels of rumen ammonia. The most important source is urea; animal excreta also fall in this category. The protein of some high protein forages (eg: sweet potato foliage) is rapidly degraded in the rumen to ammonia. However, this process implies a destruction of protein which should be avoided wherever possible (eg: it may be better to use this forage in the feeding of monogastric animals).

It is emphasized that fermentation (deamination) of protein is not only wasteful of protein but it is energetically inefficient. A kilogramme of protein yields only about 30–60g of microbial protein compared with about 200g microbial protein from the same amount of carbohydrate. Because the protein is converted to VFA and ammonia, feeding a highly soluble protein as a basal fermentable organic matter source can actually imbalance the protein to energy ratio in the end products absorbed, if none of the protein escapes degradation in the rumen.

2.4.3 Supplements which contribute to an efficient rumen ecosystem

The characteristics of a feed that contribute to an efficient rumen ecosystem are:

• Physical factors which influence the motility of the rumen wall, the amount of digesta held in the rumen and its flow to the lower digestive tract. The data in Table 2.11 show that intake decreases (and rumen liquid volume decreases) as the physical nature of the forage supplement in a molasses-based diet becomes less stimulatory (due to being ground too finely) or is removed completely.

• Nutrients, other than ammonia, which stimulate microbial growth in the rumen (eg: peptides, amino acids, branched-chain acids). On fibrous diets, these critical nutrients may help to stimulate the size of the pool of colonizing microbes, free floating in rumen fluid. These nutrients may also promote fungal growth.

Many of the above factors appear to be present in green forage (see Figure 2.6). Leguminous plants are probably better than grasses in this respect since they provide in addition a source of bypass protein. Normally it is sufficient to have 20% dry matter of a diet in the form of green forage to avoid any deficiency of the type mentioned. However, even smaller amounts have given beneficial effects on animal performance.

2.4.4 Bypass protein

Once the supply of fermentable nitrogen is assured and a small supplement of green forage has been included in a diet, then the next limitation to rate of productivity will be the availability of amino acids at the level of the intestine. For many of the feed resources that will be used in tropical countries, the value of bypass protein lies mostly in its effect in stimulating voluntary intake. It is thought this comes about through the improvement in the balance of nutrients in the digestion end products, and hence their more rapid metabolism. This is in addition to the role of bypass protein in complementing the amino acids supplied by microbial protein. Slowly degradable protein, in addition to providing bypass protein, may also supply amino acids and peptides for microbial growth.

Figure 2.6

Supplementing a diet of sisal pulp (included urea and minerals) for sheep with freshly harvested African Star grass improved the rumen ecosystem as evidenced by the 50% increase in the rate of cellulose digestion (in nylon bags in the rumen). This in turn led to an 80% increase in feed intake (Gutierrez and Elliott 1984)

2.4.5 Bypass starch and glucogenic precursors

Absorption of glucose usually leads to an increase in the glucogenic energy ratio. In addition, energy losses associated with glucose synthesis in the animal and also fermentative losses in the rumen are avoided. Supplements which increase propionic acid relative to the other VFA increase glucogenic energy and also have lower fermentative losses (less heat and methane are produced). The important role of these nutrients is to improve the efficiency with which metabolizable energy is utilized for productive purposes.

Starch which bypasses the rumen contributes glucose directly through gastric digestion in the intestine (see Elliott et al 1978; Ferreiro et al 1979). Although all sources of starch are fermented completely with time in the rumen, there are marked differences among them in their rates of degradation. Starches from maize, rice, banana and to a lesser extent sorghum, appear to have characteristics which permit them to escape partially the rumen fermentation; in contrast the starch present in cassava (and probably also in sweet potato roots) is fermented in the rumen rapidly (see Figure 2.7).

2.4.6 Long chain fatty acids

Supplementation with long chain fatty acids appears to have two opposing effects. Increasing the long chain fatty acid component of a diet, low in fat, will increase the efficiency of feed utilization especially for milk production. But on high-fibre diets (such as crop residues) lipid added above 5% of the diet will depress fibre digestion. However, recent work with protected fats and soaps has shown that addition of long chain fatty acids in these forms will increase substantially feed utilization for milk production (see Palmquist 1984).

Available sources of fatty acids include the oilseed residues (particularly pressure extracted cakes) and milling offals or brans and in some countries animal byproducts such as tallow. The effectiveness of any lipid source will be enhanced by protection (formaldehyde/protein complexes - Ferguson 1975) or by saponification with calcium salts (Palmquist and Jenkins 1982).

2.4.7 Feeds and other materials with a capacity to manipulate the rumen microbial biomass

Manipulation of rumen fermentation with natural feeds is becoming more feasible as knowledge of the processes of rumen digestion develops. In general manipulation aims at enhancing the proportions of propionate and of amino acids in absorbed nutrients; and in increasing digestibility.

Figure 2.7

The rate of degradation of the dry matter in nylon bags in the rumen of maize and rice grains is slower than of cassava root meal. The host cattle were fed on chopped sugar cane supplemented with urea and minerals (from Santana and Hovell, 1979)

Propionate enhancement has been associated primarily with the use of the chemical additive monensin in grain-rich fattening diets in cattle in Europe and North America. On molasses-based diets, poultry litter appears to perform a similar role (Fernandez and Hughes-Jones 1981; Marrufo 1984).

2.5 ALTERNATIVE SOURCES OF SUPPLEMENTS

The primary limiting nutrients for production on most tropical feed resources are fermentable nitrogen, glucogenic precursors and bypass protein and dietary long chain fatty acids. Urea, oilseed cakes, cereal milling byproducts and animal byproduct meals are the logical supplements when available. However, there are many situations where farmers do not have access to these supplements either because they are not locally available or are too expensive. In addition, there is often a reluctance to use urea because of the fear of toxicity.

2.5.1 Livestock excreta

Excreta from all types of livestock have been used in livestock rations. It is obvious that excreta in general must be a poor source of fermentable carbohydrate and protein. However long chain fatty acids may build up in litter as small amounts will be present in faeces and they are only slowly degraded by micro-organisms in the litter. Microbial growth in the litter will therefore tend to concentrate these fatty acids. Excreta from ruminant animals are high in refractory cell wall carbohydrate with smaller amounts of microbial cells (from the caecum) and some urea if the urine is incorporated with the faeces. The monogastric species produce the most valuable excreta; and especially in the case of poultry, there may be considerable contamination with wasted feed grains. Excreta from poultry are rich in nitrogen mostly as uric acid which is hydrolyzed to ammonia by rumen microorganisms.

Excreta (often depathogenized with formalin or by ensiling) have been used widely in the developed countries as a component of cereal grain-based diets in which their main contribution is as a source of non-protein nitrogen and minerals.

In developing countries, only poultry litter has found ready acceptance as a component of livestock feeds. It appears to play a particularly appropriate role in high-molasses diets, where it complements the readily fermentable sugars and the low levels of fermentable N and of phosphorus. The apparent beneficial effect of poultry litter on rumen propionate production in cattle fed a molasses-based diet has already been mentioned; and it is well documented that this is reflected in higher levels of animal performance (Meyreles and Preston 1982; Meyreles et al 1982).

The data in Table 2.12 show that poultry litter is less effective than fishmeal or an oil cake meal for supplementing cattle given a molasses- or a pasture-based diet, from which it can be inferred that it provides little or no bypass protein. This is to be expected in view of its chemical characteristics.

2.5.2 Legume forages and foliages from food crops

An alternative resource which can serve as a source of fermentable N and of bypass protein is a forage crop grown on the farm, or produced as a byproduct or residue from a food crop. A legume crop has a further advantage because of its capacity to fix atmospheric nitrogen and thus spare the need for fertilizer N. Based on the premise of using protein economically, then the intake of these forages should be restricted and therefore it is preferable to grow the legume as a pure sward.

The amount of bypass protein in a legume (green or dry and used as a supplement) has not been estimated. It may be beneficial to grow tannin-rich legumes where these are grown as supplements to crop residues. Whereas if they are to provide a high proportion of the diet, the low tannin legumes may be more appropriate (eg: feeding leucaena with molasses/urea) (Hulman et al 1978).

The tree legumes such as Gliricidia, Erythrina and Leucaena have very great potential because they are high yielding and perennial. They are also deep-rooted and may have access to water and nutrients (eg: phosphorus) unavailable to smaller plants. In the tropics, tree legumes have a special role, since they can also be used for shade (eg: in coffee plantations), as “live” fences and as sources of fuel. Some of them (eg: Gliricidia and Erythrina) can be established easily from cuttings hence their use as “live” posts.

On low-nitrogen diets supplemented with legumes, there is still a need to ensure that ammonia concentrations in the rumen are adequate by supplying fermentable nitrogen usually as urea. It is also important that the protein source, which is usually in short supply, should be partitioned between as many animals as possible.

Pasture was pangola grass and the cattle were also fed 1.5 kg/day of molasses containing 2.5% urea

The other valuable forages are from cassava and sweet potatoes and, to a lesser extent, bananas. Some results from using these materials as supplements in molasses-based diets are described by Ffoulkes and Preston (1978) and Rowe and Preston (1978).

In temperate countries legumes have long been used as an alternative to nitrogenous fertilizers to increase pasture biomass production per hectare. It has also been recognized that they are superior in nutritive value compared with grasses, apparently because of their higher protein content. The metabolizable energy in legumes is also used more efficiently for productive purposes as compared with grasses of the same digestibility (Waldo et al 1982).

Tropical grasses support lower levels of animal production compared with temperate grasses, mainly because they are lower in nitrogen and are less digestible (see Minson 1982). Low productivity from tropical pastures has stimulated considerable research aimed at developing grass-legume associations for tropical conditions. Presence of legumes in the sward has led to increase in animal production, but mainly in terms of productivity per unit area rather than per animal (see Mannateje 1982). But maintaining grass-legume associations in a pasture requires very careful managment.

The present discussion is restricted to the role of legumes as supplements in feeding systems based on low-nitrogen crop residues and byproducts.

2.5.3 Attributes of legumes as supplements

In developing countries where competition for land to grow crops or grazing is high, the area likely to be sown to fodder legumes will be almost always a small proportion of the total. It follows therefore that the role of a legume must be to increase the efficiency of utilization of the basal diet (ie: a low-N pasture or a crop residue) at low levels of supplementation (usually less than 20%) and used “catalytically”.

As a priority, the legume should have a high protein content to supply both fermentable and bypass protein; there will be additional benefits if the legume contains other critical nutrients (eg: lipids, minerals, vitamins and other plant compounds) which enhance the rumen ecosystem so as to increase microbial growth, rate of fibre digestion, propionate production and escape of dietary protein (eg: contains tannins).

There are two sets of data which indicate the suitability of legumes as sources of fermentable nitrogen and bypass protein: comparative studies with grasses (almost entirely for temperate species) and animal response trials.

The data in Table 2.13 show that, compared with ryegrass, white clover contains more nitrogen and provides more protein that is available for intestinal digestion in sheep. This almost certainly indicates that a proportion of the legume protein escapes rumen fermentation. The fact that the efficiency of utilization of metabolizable energy is higher for a legume as compared with a grass, is further evidence that the digestion of legumes provides a better balance of nutrients for productive purposes than is the case with grasses. However, protein from white clover only appears to escape rumen fermentation at high intakes of the clover. Therefore when it is used as a supplement to a fibrous feed it may only provide fermentable N. Clover also provides highly digestible carbohydrate which will stimulate digestibility of the basal diet. It also provides lipids which may help to spare glucose oxidation for adipose tissue or milk fat synthesis.

It is likely that legume forages rich in tannins will be superior as sources of bypass protein since tannins link with proteins during mastication, and appear to reduce microbial degradation of plant proteins (Reid et al 1974). The high levels of tannins in Lotus pedunculatus, whilst protecting protein from degradation, reduce digestibility of fibre by inhibiting the activity of bacteria (Chesson et al 1982) and fungi (Akin and Rigsby 1985). Barry (1985) considered that the ideal concentration of condensed-tannins was 20–40 g/kg diet dry matter; and that higher levels (76–90 g/kg) were detrimental. He also found that sheep could adapt to high tannin levels. Provided that tannin-rich plants are only used as supplements (eg: less than 25% of the diet dry matters), there is unlikely to be a serious problem and their presence in the diet may well be beneficial (Barry and Manley 1984). Examples of tropical legumes which are known to contain tannins are: Leucaena, Gliricidia and Sesbania.

Recent research in New Zealand (Figure 2.8), comparing the utilization by sheep of mature and immature ryegrass and white clover, demonstrates three points:

• At low total feed intake virtually no protein bypassed the rumen on any of the herbages. As intake increased protein bypass also increased.

• There appeared to be no difference in the bypass characteristics of young ryegrass and white clover (at close to ad libitum intake about 25% of the protein appeared to escape the rumen) whereas on the mature ryegrass virtually none of the protein escaped.

• The efficiency of microbial growth was similar on all herbages and was apparently not affected by the level of intake.

Figure 2.8

Relationship between organic matter intake and flow of microbial protein (NAN= non-ammonia-N) and ruminally undergraded (bypass) N to abomasum in sheep fed early- (o) or late-cut (o) ryegrass or clover (o) (from D.W. Dellow and J.V. Nolan, cited by Preston and Leng 1986)

Thus, if temperate clovers and immature grasses are to be used as bypass protein supplements, their effectiveness will depend on the level of intake of the basal diet.

While a proportion of the protein in some legumes appears to be able to escape rumen fermentation, the greater part - at least in the fresh plant - is rapidly degraded by rumen microorganisms. This is well illustrated by the data in Table 2.14 which show that the quantity of amino acids flowing to the small intestine of sheep was highest when clover was dehydrated, was intermediate in frozen material and was lowest on the fresh herbage. Assuming a constant microbial growth rate in the rumen on all diets, it can be estimated that less than one quarter of the protein consumed escaped to the small intestine when the fresh material was fed (see MacRae 1976).

In these trials, the legume was the only component of the diet. If fresh legume forages are given as supplements (less than 20% of dietary DM intake) to a diet based ondry forages, its bypass protein contribution may be very small. It must be considered therefore as mainly providing fermentable nitrogen. Theoretically, including a high-protein legume at 20% of a straw-based diet should provide most of the ammonia needed by the rumen mirobes. However, if the legume is given as a single feed, say early in the morning, the ammonia may be used wastefully and a fermentable N deficiency might occur later in the day. There is therefore an urgent need to examine what level of supplementation with fresh legume forage is necessary to raise rumen ammonia levels consistently above the critical value (eg: 150 mg/litre); and how it should be distributed (ie: how many feeds per day).

In situations where the fermentable N requirement can be met from other sources (eg: urea or animal excreta) the need is to reduce the degradability of the legume protein so as to increase its bypass characteristics. This has been shown to occur when a forage is artificially dried and more so when pelleted (see Table 2.14).

Rarely will it be economic to dehydrate or pellet legume forages and sun-drying is the only feasible alternative. Nolan and Leng (1972) showed that some 60% of the protein in sun-dried lucerne apparently escaped rumen fermentation when the legume was fed as the sole diet.

As discussed earlier, secondary plant compounds such as tannins are known to protect dietary proteins against rumen microbial attack. Thus if a freshly harvested or grazed legume is to be used as a bypass protein supplement then it should be selected for a relatively high content of tannins.

This point is illustrated by the data in Table 2.15 which show that although the tannin-containing legumes (trefoil and sanfoin) were less palatable than lucerne, nevertheless they supported faster growth rates in heifers. The authors concluded that this was because more of the protein in the legumes containing tannins escaped degradation in the rumen.

Tropical legumes generally are richer in tannins than are temperate legumes and therefore should function better as sources of bypass protein. Evidence for this is provided by the results of feeding trials with tropical tree legumes discussed earlier. It must be emphasized that when legumes contain a high proportion of protected protein, then some other source of rumen fermentable nitrogen will be required, usually urea.