Tree foliages represent an important source of cellulosic biomass for feeding ruminants throughout the world. Tree and shrub foliages are often selected by grazing ruminants where possible and traditional people recognize those tree foliages which are sought and consumed by ruminants. However, other than traditional knowledge of tree/shrub foliages, there is a dearth of information on their benefits, even though some progress has been made recently to collate information, for example, Gutteridge & Shelton, 1994 and Shelton et al. 1995. It seems that their rôle in ruminant nutrition has not been truly defined and is likely to be different depending on whether they are used as strategic supplements or total feeds. In the next part of this publication, an attempt will be made to clarify these rôles.
Fodder from trees and shrubs (top feed) have been an important source of protein for grazing animals. However, in some cases, not only has their crude protein digestibility been observed to be low, but also several cases of livestock death have been associated with high tannin content of some foliages. The most important property of tannins is their strong affinity for enzyme and feed protein, but even this varies depending on species; for instance, Prosopis cineraria has been observed to have tannins that have a very high protein precipitating capacity-higher than tannins from other tree foliages.
Tannin content, therefore, potentially alter the use and value of tree foliages and may at times be responsible for the poor utilization of such forages by ruminant livestock. On the other hand, lack of tannins in Gliricidia is believed to leave the protein so unprotected as to be completely degraded in the rumen. Tannins from Leucaena leucocephala afford a good level of protection of the protein (Wheeler et al. 1995), but tannins from Lotus pedunculatus appear to overprotect protein from ryegrass fed to sheep (Waghorn & Shelton, 1995) with subsequent increased faecal loss of protein.
A selective review of the effects of tannins on ruminant nutrition is given below to provide the rationale for some of the recommendations discussed in the final chapter.
There are two chemically distinct types of tannins:—
hydrolyzable tannins (gallotannins and ellagitannins). These are polyesters of gallic acid and other phenolic acids derived from it, with a sugar (normally glucose). They are readily hydrolyzed by acid.
condensed tannins (flavolans). These are polymers (M.W.≈ 1,000 to > 20,000) of catechins, which are flavonoid phenols. The linkage between the monomers, typically a carbon condensation, is relatively stable under the conditions which cleave ester linkages in hydrolyzable tannins.
The generalized structures of tannins are shown in Figure 4.1 on the next page.
Figure 4.1: Examples of hydrolyzable (gallotannin) and condensed (proanthocyanin) tannins and their constituent units. In the case of gallotannin the dotted line indicates the repeating gallic acid unit (from van Soest (1982).
It has been believed for some considerable period that tannin above 5% can become a serious anti-nutritional factor in plant materials fed to ruminants (McLeod, 1974). Barry (1983) and his colleagues have demonstrated with Lotus pedunculatus that the ideal concentration of condensed tannins in this forage legume is between 2–4% of the diet dry matter, at which level they bind with the dietary proteins during mastication and appear to protect the protein from microbial attack in the rumen. On the other hand, other forages with tannins have anti-nutritional effects in the rumen and reduce N retention (Waghorn & Shelton 1995). If the protein-tannin complex dissociates under acid conditions then the protein can be digested in the lower gut. At higher levels (5– 9%) tannins become highly detrimental (Barry, 1983) as they reduce digestibility of fibre in the rumen (Reed et al. 1985) by inhibiting the activity of bacteria (Chesson et al. 1982) and anaerobic fungi (Akin & Rigsby, 1985) high levels also lead to reduced intake (Merton & Ehle, 1984); above 9% tannins may become lethal to an animal that has no other feed (Kumar, 1983).
Sheep have been shown to adapt slowly to tannins in a diet of Acacia leaves, suggesting that there are rumen organisms that in some way detoxify their effects (Reed et al. 1985). Recent studies have demonstrated a bacterium in the rumen of goats and sheep capable of growth at high tannin levels in the rumen (Brooker, J., personal communication1).
Thus a little tannin has been usually accepted as being able to protect protein of forages and allow a higher efficiency of feed utilization by the animal. However, recent results throw some doubt on this. Tannins may indirectly effect rumen function by reducing rumen ammonia levels through decreased protein degradation in the rumen. If rumen ammonia levels decrease below 80 mg N/l (see Figure 2.4 on page 21) then fibre digestibility may be depressed and digestibility is reduced well below 10 mg N/l (Leng et al. 1993). Whenever tannins are present in forages there may be a need to supplement ruminants with a non-protein nitrogen source such as urea or chicken manure. Conversely, tannins in feed may increase detrimental effects on rumen function when the basal diet is low in protein.
1 Dr J.D.Brooker, Department of Animal Science, Waite Campus, University of Adelaide, South Australia.
Condensed tannins are present in only some plant species. In general, shrub and tree foliages are likely to be higher in tannins than pasture plants, and leguminous forages from the tropics are generally higher in tannin than those from the temperate countries.
The level of tannins within a species has been found to vary considerably depending on a number of factors. For example in New Zealand, Lotus pedunculatus grown on fertile, high moisture soils has about one third the condensed tannin content of Lotus grown on hill country under water stress (Barry, T., personal communication2).
The literature in this area is often confusing as reported levels of tannins often seem to be higher than can be explained and these values often are rejected by reviewers as possibly due to errors of analysis. For example, Prosopis leaves have been reported to have tannin levels of 2.2% of the dry matter (Sehgal, 1984) but in 15 individual trees Joshi et al. (1985) reported levels ranging from 10.6 to 25.3% which suggested to some authors that there may have been major analytical difficulties. Certainly the level of tannins in the foliage of trees is highly variable and depends on environmental stress (fertility, soil-water relationships, insect attack etc.). Newer information also points to analytical flaws in preparation of leaf material for analysis and drying reduces the measured tannin levels over fresh material. New leaves often have higher tannin content than older leaves (Vaithiyonathan & Singh, 1989) and in South Africa the tannin content of Acacia grazed by Kudu increased with grazing pressure (van Hoven, 1991).
Tannins at 14–16% are present in the bark of many trees (Dalziel, 1948) indicating a very large pool in the tree that can possibly be mobilized. In general it could be expected that the green bark of new growth would contain less tannin than the brown bark, and the leaves and petals less than bark.
There is a small amount of literature indicating that the tannin content of some tree foliages may be controlled in some way and can be elevated at times of high risk of defoliation (i.e., by insect attack, cutting and harvesting, or grazing).
An investigation in South Africa of the death of a number of Kudu on a wild life farm has led to knowledge that may have major implication for management of some fodder trees (van Hoven, 1985).
2 Professor T. Barry, Faculty of Agriculture, Massey University, Palmerston North, New Zealand.
A number of Kudu (member of the deer family) died after grazing on a small area of Acacia trees (van Hoven, 1991). Subsequent studies have shown that in the wild these Kudu would approach such woodlands and after grazing on the trees on the periphery move quickly to trees well separated from those recently grazed. However, because of the enclosure these animals were forced to consume more of the foliage from one woodland. Subsequent studies implicated tannin in the death of the animals and this led to a study of tannin in the tree foliage. Tannin levels rose sharply over a 15 minute to 1 hour period, not only in the trees grazed by deer, but in the trees adjacent to those that were being damaged.
Nomads in India, grazing camels on Prosopis juliflora on the roadside carefully explained the need to move their camels to fresh trees after a short time of grazing in the one area (Leng, R.A. personal observation). Although not yet tested, this fits the idea of some trees responding to grazing by increasing the content of anti-palatability secondary plant compounds.
The implication is that there is a pool of tannin which can be readily mobilized by activation of specific enzymes sensitive to air borne materials released from damaged foliage. This obviously has been an important survival mechanism for trees on the savannahs. Also it has major implications for the use of browse trees in pasture/tree associations for cattle production. However, responses of some tree foliages to such treatment is far from clear and a number of trees do not respond to damage in any way. This area requires experimental work in a number of situations.
The effect of simulated grazing on tannin content of a number of tree leaves is shown in Table 4.1 on page 51. The information on such responses in a number of trees needs to be compiled.
Tannin buildup in foliage has enormous implications for the use of fodder trees. It may partially explain some of the contradictory nature of much of the data on the efficiency of use of fodder trees, and the common rejection of trees for fodder purposes in some areas and not in others.
The quality of harvested foliage will always depend on where it is produced, how it is harvested, the stage of plant growth, the climate, the effects of insect damage and the stocking rate. Where trees respond to damage, it is possible that occasional grazing by animals in a plantation may effect tannin levels in the leaves for up to 4 days.
As tannins appear to be both anti-nutritional and perhaps nutritionally beneficial it is important to determine what factors influence tannin levels in foliage; whether these factors can be controlled (e.g., strategic fertilizer or water application) and whether harvesting techniques and stocking rates can be developed to optimize or minimize the tannin levels.
If it is too complex to develop such techniques then it is obvious that high tannin levels may be acceptable so long as they are diluted by other feed resources that have sufficient protein to bind free tannins beneficially but this necessitates a cut and carry system for tree fodders.
The fate of tannins following ingestion by ruminants depends on the type of tannin. Most tannins form complexes with protein in the plant material, in saliva or the rumen contents. Hydrolyzable tannins hydrolyze in gastric acidity beyond the rumen, releasing protein, amino acids and small units of phenolics that probably pass to the urine. At high levels of tannin intake, both mucoproteins and the epithelial cell lining of the digestive tract are affected. This alters the integrity of the gut wall causing problems of gastritis, slowed propulsion of feeds and constipation (Kumar & Singh, 1984). Only under exceptional circumstances will herbivores consume large quantities of forages containing tannins, although recent studies from Zimbabwe suggests that the intake of browse can be increased by feeding very small quantities of polyethylene glycol (Duncan, 1994). However, from the point of view of this publication, fodder trees are considered only as potential supplements to poor quality forage and not a basal feed reserve.
Drying promotes the combination of tannin and plant protein before ingestion which may result in different responses by ruminants on forage diets to supplements of dry or fresh tree foliage. Proanthocyanidin tannins are held in special organs in the leaves to prevent their interference with the plant's own metabolic apparatus, and this factor favours tannin-tolerant animal browsers that have tannin-binding proteins in their saliva (Austin et al. 1989). In these instances the salivary factor binds the tannins and spares valuable forage protein with a higher concentration of essential amino acids.
Attempts to use tannins to promote rumen escape (“bypass”) of feed protein have had limited success. Fresh forage may be ingested before tannin-protein complexes can oxidatively cross-link, as in the case of dry feeds, although drying has been shown to have favourable effects on productivity (see later). Recently, Waghorn & Shelton, (1995) showed that, in sheep fed freshly cut rye grass, the addition of a third of the diet as Lotus pedunculatus (which supplied 1.8% condensed tannin in the total diet) resulted in the digestibility of the protein being significantly decreased from 78% to 65%. The inclusion of Lotus forage also lowered dry matter digestibility by 3–7%, which was almost all accounted for by the lowered protein digestibility. Apparently the level of protein availability in the animal from plant and other sources was unaffected, as wool growth and liveweight gain were not different in groups given Lotus forage compared with those on pure rye grass. These experiments suggest that condensed tannins overprotect protein and that the precipitation of protein-tannin complexes protect the micro-organisms in the rumen from the detrimental effects of condensed tannins.
The presence of tannins in forages stimulate salivary flow in animals. A number of studies have shown an apparent increase in microbial protein leaving the rumen after feeding moderate levels of tannin (Beever & Siddons, 1986), but the level of increase is not convincing, considering the technology used to make the measurements. Nitrogen balance is apparently improved in animals that are fed low levels of tannins, although digestibility of forage fibre may be lowered (see Norton 1994a). The effect of condensed tannins overall appears to be to make more amino acids available in the intestine. Whether this is the result of increased microbial growth efficiency or increased dietary protein availability is unclear.
Table 4.1: Tannin increases in three tree species in response to the effects of simulated grazing damage. The tannin levels were elevated for 100 hours before they began to decline.
|Species||% Rise in 15 mins||% Rise after 60 mins|
Source: van Hoven, W., personal communication3
3. Professor W. van Hoven, Centre for Wildlife Management, University of Pretoria, Pretoria, South Africa.
Polyethylene glycol forms complexes with tannins and has been fed to animals to reduce the inhibitory effects of tannins (see Pritchard et al. 1988).
Long-term ingestion of tannins by ruminants may induce enlargement of the salivary glands. Deer and goats and some monogastrics posses salivary proline-rich proteins that specifically bind tannins (Mehansho et al. 1987), but these are absent or low in the saliva of sheep and cattle (Austin et al. 1989).
Many foliages have chemicals that appear to be produced for the purpose of deterring invasion or consumption of their leaves by microbes, insects and herbivorous animals. Whilst tannins are the best known of these, there is a long list of secondary plant compounds. Cyanide, nitrate, fluoroacetate, cyanogenic glycosides, saponins, oxalates, mimosine and various sterols are but a few (see Norton, 1994c for a more extensive list). Quite recently, saponin concentrations have been implicated in low productivity in young cattle on young growth of signal grass (Brachiaria) (Lowe, S., personal communication5). These may or may not be modified in the rumen by microbial action. The primary compound or its breakdown products in the rumen may be toxic or of no nutritional consequence. Some secondary plant compounds are actively detoxified in the liver.
One of the best known secondary plant compound in tree leaves is mimosine in Leucaena, which is degraded to the toxic compound 3-hydroxy-4-pyridone by normal rumen organisms and which is usually further degraded by another microbe where animals have evolved in grazing situations containing Leucaena. This microbe is not present in ruminants that have been isolated from Leucaena and inoculation is necessary to prevent toxicity when Leucaena becomes a high proportion of the total diet (Jones & Megarrity, 1986).
The main reason for the foregoing discussion is to emphasize that these secondary plant compounds should be taken into consideration. However, the toxic compounds often only become of significance nutritionally when the plant assumes a high proportion of the diet. Although some effects of the toxic compounds may persist when these are used to supplement the diets, the beneficial effects of a high protein forage often override their effects. There are, however, important exceptions particularly in the case of fluoroacetate which is an lethal compound that claims the lives of large numbers of cattle on pasture lands with Acacia georginae (see Cunningham et al. 1981), even though it is of relatively low digestibility. Recently a rumen microbe has been genetically modified to carry and express a gene that encodes for an enzyme that hydrolyzes fluoroacetate (Gregg et al. 1994).
5 Ms S.Lowe, Department of Agriculture, The Papua New Guinea University of Technology, Lae, Papua New Guinea
Whilst cautionary notes are necessary it is important to stress the advantage of fodder trees. The toxic principles must be kept in mind they should not be given prominence, especially where small quantities of foliage are supplementing forage based diets for ruminants. Even trees not considered as sources of forage may become useful in the future, for instance, Acacia mangium which grows vigorously on acid soils is being used to supplement cattle in Vietnam (An et al. 1994), even though it is of relatively low digestibility.
Tree foliages have been given high prominence as protein supplements for ruminants fed low protein forages. However, seldom is it known whether escape protein per se is the valuable component of the foliage or whether the protein of the tree foliage is largely providing ammonia (from protein degradation), minerals, or all three. The rôle that tree foliages play in ruminant nutrition determines the required rate of supplementation, so it is extremely important to know what this rôle is.
Tree foliages low in tannins or other secondary plant compounds that might bind protein, are probably degraded rapidly and, at times, completely. In this way they provide ammonia and volatile fatty acids in the rumen. The rate of microbial growth on protein, however, is approximately half that on carbohydrate, so P/E ratios are lower when protein is degraded in the rumen in comparison with carbohydrates. On the other hand, if foliage protein is bound by condensed tannins, microbial degradation of leaf protein in the rumen will be prevented or slowed. This will allow particles high in protein to move to the lower digestive tract where some of the condensed tannin complexed with protein may be hydrolyzed, which then allows the protein to be digested. Condensed tannin under acid or alkaline conditions of the intestines may be split to sugars and organic acids, mostly gallic acids, releasing protein and amino acids that are digestible in the lower gut. However, lowered N retention in animals fed some tanniniferous forages suggest that much bound protein, is unavailable for digestion in the gut (Mangan, 1988; Kumar & Singh, 1990; Waghorn & Shelton, 1995). Provided some protein remains soluble and can provide ammonia in the rumen and perhaps escape protein, some of the detrimental effects would be reduced. In this case the protein level relative to tannins would be the most critical factor.
Few research studies have defined the rôle required of tree foliages where their feeding objective is as supplements to low digestibility forage. The extent to which tree foliage protein is degraded in or escapes the rumen is extremely important. If the tree foliage protein is totally degraded then it provides only ammonia and minerals for microbial growth (but both these may be more easily and economically provided from other sources such as MUMB, chicken manure or litter from broiler production).
Tree foliages, on the other hand, often have higher digestibilities than the pasture forage available and thus provide a more energy dense feed, allowing a higher total feed intake. In a situation where the fermentable N and minerals can be provided more economically, say, as MUMB then the use of such tree foliages is counter-indicated. The potential value of the foliage may only be realized by harvesting and processing so that it contains a higher percentage of bypass protein.
The relationships between the generation of microbial protein from various sources are shown in Figure 4.2 on the following page, where the protein available from 1 kg carbohydrate fermented in a rumen sufficient in ammonia is compared to the protein and energy available from 1 kg of a soluble and bypass protein source.
The reasons for using tree foliage as supplements to low protein forages such as tropical pasture or straw are similar to those for forage legumes under the same conditions and therefore some comments on forage legumes are pertinent to the discussion here.
The presence of legume forages and tree forages in pastures have been generally accepted to improve ruminant productivity in both temperate (Ulyatt, 1980) and tropical pastures (Milford, 1967). The problems of maintaining legume forages in pastures centre around the competent management of such pastures. The inability of highly palatable legumes to survive in pastures resides in the inability in most farming systems to manage grazing, but where legumes are established, animal productivity is always greater for legume based pastures than from pure grass pastures (Mannetje, 1984; Thompson, 1977; Walker, 1987) (Figure 4.3 on page 56). A most important attribute of legumes is that their digestibility declines more slowly with maturity and environmental temperature than does that of grasses (Minson, 1980).
Figure 4.2: A theoretical balance of nutrients arising from feeding soluble protein, insoluble protein and carbohydrate to ruminants (after Leng, 1981).
Introduction of legumes into pastures has generally improves animal production even without fertilizer inputs. Growth rates are often increased by 50–100% (see Clatworthy & Hollen, 1979; Stobbs, 1966, 1969). Fertilizer application to legume pastures has increased animal production to even greater degrees and importantly, this has often led to improved cattle reproduction in the tropical areas (Holroyd et al. 1977) possibly through improved phosphorus nutrition. Productivity is improved through individual liveweight increases plus the increased stocking rate that such practices then allow. The influence of various treatments of pasture on individual animal productivity with stocking rate is shown in Figure 4.3 on the following page (Walker, 1987).
Legumes also improve animal productivity from grass lands by increasing total edible biomass. The production increases can also be related to the high mineral concentrations in legumes and to higher protein levels in the associated grasses. In addition their seeds contribute significant concentrated nutrients in the form of protein and carbohydrate and are highly digestible if crushed in chewing. Many of the responses to legumes are similar to the responses seen from feeding MUMB and are undoubtedly attributable to supplementation of the rumen microbial ecosystem, ensuring an efficient fermentative digestion.
Figure 4.3: Cattle growth on pasture is a function of pasture type, fertilizer application and legume concentration. Productivity per unit area is maximized for the different pastures at different stocking rates: 89 kg/ha for native pasture, 223 kg/ha for tropical grass with legume, 682 kg/ha for tropical grass with fertilizer and on temperate pasture (clover) 1051 kg/ha (Source: Walker, 1987).
The ability of MUMB to lift animal productivity and production of animal products per hectare is associated with an increased efficiency of utilization of the available forage through improved rumen conditions. Whilst legumes may have a similar rôle, there is also the additional benefit on the total forage availability. The proportion of the benefits from improved efficiency and improved biomass is not clear. Operative questions arising are:—
is it more economical to feed MUMB or equivalent to increase productivity than to sow legumes into pasture and manage them?
is it more appropriate to find high yielding grasses such as Eragrostis spp. (love grasses) that grow rapidly in response to showers of rain and use MUMB supplementation to achieve higher levels of animal production from that biomass?
The same questions must similarly be asked of the use of tree foliages. Whether some forage legumes contain secondary plant compounds that protect their protein is uncertain--many of the tanniniferous legumes are sustained in pasture and are unpalatable but are eaten to some extent in the dry season when the pasture protein is low.
The potential for using bypass protein from forages to increase productivity of ruminants is evident where concentrated bypass protein supplements such as cottonseed meal (see Figure 4.1 on page 46) have been fed to ruminants on basal poor quality forages. There has been no major research attempt to develop tree or legume forages high in bypass protein, perhaps because most effort has gone into the agronomic aspects of the forages/trees in association with pastures. Some efforts have been made to grow legume forages as protein banks which can then be used to supplement low quality forages for cattle.
Tree foliages have essentially the same rôle in the nutrition of ruminants as legume forage. Trees, however, have a number of attributes which are advantageous or disadvantageous to either the pasture or the animal. These include:—
their deep rooted nature, which allows them to continue to grow often well into a dry season, whereas forage legumes although often adapted to pasture, may dry off earlier as they are mostly more shallow rooted than trees. Deep rooted trees, following their establishment, often produce more biomass than pasture in the semi-arid or arid regions.
their regeneration after harvesting or grazing. They must be managed and each tree species requires a different management system. Trees in the wet tropics do not necessarily compete for soil nutrient with pasture.
their growth in hedge rows or alleys between crops as protein banks where they are easily harvested. They can be grown as plantations where they are the sole objective of the production system. Often, strategic cutting of trees, particularly Leucaena at the end of the dry season, produces considerable extra foliage into the dry season.
their importance for control of soil erosion-their extensive root systems bind soil when pasture has dried off and the earth may be exposed. Trees also reduce the direct effects of wind erosion.
their provision of shade that is invaluable in the tropics when animals may be heat stressed.
their leaves, which are often richer in secondary plant compounds than forages. These compounds can make them unpalatable to grazing herbivores or allow the protein to escape fermentative digestion when consumed by ruminants. These secondary compounds may be advantageous in that they may be toxic for a single group of rumen organisms and may favourably manipulate the microbial mix within the rumen ecosystem. For example, some secondary plant compounds (some saponins) are toxic to rumen protozoa (Leng et al. 1992)
The plates of Figure 4.4 on the following page and Figure 4.6 on page 62 show some protein banks of Gliricidia sepium. Figure 4.4 has also the shrub (Pachecoa venezuelensis) growing for supplementary feeding of both large and small ruminants in Venezuela (Combellas, 1994). Figure 4.5 on page 61 and Figure 4.7 on page 65 show solitary fodder trees growing in Queensland, Australia.
Fodder trees may be used to feed ruminants where high protein feed resources are scarce or unavailable. They should be planted where they have advantages over more conventional forage crops. In general, it is not economic to grow trees as a high biomass crop to provide a basal diet for ruminants. An exception to this is in parts of Central Queensland and particularly The Kimberley area of Australia where Leucaena leucocephala is being grown under irrigation for grazing by cattle. Growth rates even at 20% C.P. in the foliage are about 800 g/day (Rowe, J., personal communication6). However, Leucaena and grasses under irrigation can support high stocking rates (e.g., 6–8 cattle/ha).
There are circumstances, particularly on large ranches, where the establishment of legume trees as a total diet could be condoned, but, in general, the costs of establishment of plantations will preclude such strategies. Exceptions to this might be where timber or fuel are the major salable product or where the fruit or the pods are harvested as a concentrate source for ruminants and the tree require little maintenance throughout the year. An example of the latter is with Prosopis juliflora plantations in semi-arid areas of the tropics where the trees in conjunction with Buffel grass pastures have transformed parts of the Sertao of Brazil from cactus and scrubby grazing country to easily managed tree plantations with undergrowth of grasses. In Australia, systems of biomass production for grazing cattle using Leucaena leucocephala have evolved, but these enterprises are in the dry areas where the high cost of establishment of Leucaena are offset against market opportunities arising when unfinished cattle from extensive areas can be fattened by grazing this resource (Quirk et al. 1990).
6 Professor J. Rowe, Department of Animal Science, University of New England, Armidale, New South Wales 2351, Australia.
The overall conclusion is that the rôle of tree foliages is seen in the same light as forage legumes. Tree foliage allows efficient utilization of the basal feed resource and they have environmental and other advantages (see Chapter 1) but the nutritional rôle of the forage tree is often unknown or at least not considered by farmers and scientists alike.
Figure 4.4: Tree fodders (protein banks) for cattle production in the tropics: Gliricidia sepium plus Pachecoa venezuelensis growing in Venezuela.
Undoubtedly the inclusion of tree foliages in diets for cattle and sheep given low quality forages or grazing tropical pastures has improved both production per head and production per hectare. Stocking rates are generally increased where fodder trees are included in pastures.
Two operative questions arise:—
at what rate of inclusion of tree foliages in a diet is the production response maximized
are the responses brought about through an improved rumen or an improved efficiency of utilization of the diet through the provision of bypass protein, or both?
Unfortunately no research studies to the knowledge of the writer have set out to compare responses to supplements of tree foliages with those from:—
an equivalent amount of bypass proteins or
a combination of MUMB and a known bypass protein.
In terms of bypass protein requirements, a cottonseed meal at 44% CP with 75% of the protein potentially escaping the rumen (i.e., 33 g bypass protein/100 g cottonseed meal DM) requires to be fed at between 1 and 2 kg per day to have optimal economic effect on growth rate of cattle fed a poor quality roughage (see Figure 3.4 on page 43).
Figure 4.5: Tree fodder for cattle production in the tropics: Leucaena pallidum growing at Gayndah, Queensland
In making such comparisons, it is important to recognize that urea is a concentrated source of ammonia for the rumen: 100 g urea provides about 45 g of ammonia N. Tree foliage or other forages with say, 24% crude protein (CP) in the dry matter can supply only 6 gN/100 g forage dry matter (DM). To provide the same NPN as 100 g urea requires 750 g DM of a tree foliage—approximately 3.65 kg of fresh plant material would be required to replace 100 g urea as a rumen ammonia source. In a steer of 250 kg liveweight consuming forage at a rate of 2.5% of its body weight, this represents 12% of its total dry matter intake. The amount of NPN required from such a source, however, would need to be adjusted for the crude protein content of the basal diet.
To obtain the same amount of bypass protein from a protected leaf meal at 20% CP in its dry matter, 1.65 kg DM must be fed or 8.25 kg wet leaf material. So if all the NPN and bypass protein has to be provided by the leaf meal, a 250 kg steer fed straw would need 2.4 kg DM or 12.0 kg of wet leaf material if there was a 50:50 contribution of dietary protein to NPN and bypass protein. Thus it is necessary to feed at a minimum 38% of the total feed intake as foliage to potentially balance a poor quality forage based diet.
Figure 4.6: Tree fodders (protein banks) for cattle production in the tropics: Gliricidia sepium growing at Maracay, Venezuela.
There are obvious opportunities for combining tree foliage with MUMB and tree foliage with bypass protein or both supplements. These are the areas for a major nutritional research thrust.
|Treatment||Rumen Ammonia (mgN/l)||Initial Wt (kg)||Final Wt (kg)||LWt gain (g/d)|
In the ensuing discussion only a small number of examples are given. A full review of the literature is not warranted because the majority of experiments have only set out to test whether a tree foliage will improve productivity of ruminants and not to examine the mechanism by which these benefits are delivered.
Supplementation of cattle on green Brachiaria decumbens pasture with either urea/molasses or foliage of the fodder tree Gliricidia resulted in similar increases in production (see Table 4.2); research from Cuba has shown that Leucaena leucocephala in pastures produced the same live weight gain as cattle on pasture supplemented with MUMB (Table 4.4 on the following page). Seijas et al. (1994), however, obtained some evidence for a better result on production of cattle grazing pasture with Gliricidia compared to pasture with MUMB, but there was a trend for both supplements together to produce the best response (Table 4.3 on the next page) These few experiments indicate that the value of a high protein tree forage is the same or only slightly higher than supplements aimed at improving the rumen fermentative capacity.
|Control||Gliricidia||MUMB||MUMB + Gliricidia|
|LWt gain (kg/d)||0.20||0.36||0.28||0.40|
|Gliricidia intake (kg)||0||0.46||0||0.51|
Undoubtedly, for animal production purposes, the mineral composition of tree foliages is superior to that of tropical grasses. Norton (1994a) has reviewed the mineral content of a range of tree forages as shown in Table 4.5 on page 66. There is little information on the range of trace elements but if the tree is healthy then an array of trace elements in the foliage can be expected.
|Leucaena + pasture||Pasture + MUMB|
|No of animals||12||8|
|Stocking rate (hd/ha)||3||2|
|Liveweight gain (g/d)||0.45||0.45|
Tree foliage is likely to be a significant source of minerals when fed in high amounts but animals are likely to require supplementation where dry feeds deficient in minerals are the basal diet and tree foliage makes up 20–30% of the total dry matter intake. Goodchild & McMeniman (1994) attributed increases in ruminant production to leaf foliage included in the diet as being due mainly to its mineral content and therefore the supply of minerals to the animal and the rumen microbiota.
Figure 4.7: Tree fodder for cattle production in the tropics: Albizia chinensis growing at Gayndah, Queensland
It appears from the literature that tree foliages in general provide a mixture of soluble N, minerals and vitamins. An exception to this has been reported recently. Palmer & Schlink (1992) found that fresh Calliandra calothyrsus was readily eaten by sheep. However, dried material was less edible and this fitted with a higher in sacco digestibility of the fresh material when compared to the same material dry, wilted or freeze dried.
When Calliandra forage was fed in increasing levels to groups of sheep given a basal diet of hay, both liveweight gain and wool growth were stimulated, indicating that considerable amounts of the protein was apparently bypassing the rumen (see Table 4.6 on page 67).
|Range of mineral components (g/kg DM)|
Calliandra leaf contains condensed tannins which are implicated somehow in reducing digestibility of Calliandra foliage when wilted. Palmer & Schlink (1992) demonstrated that 6 hours' wilting may reduce Calliandra foliage digestibility for ruminants by 80% and there appears to be an increase in condensed tannins by as much as 80% (Tangendjaja et al. 1992, cited in Palmer et al. 1995)) possibly arising from the polymerized tannins in the leaf.
A possible explanation is that the binding of tannins to proteins under anaerobic conditions in the rumen are weaker than protein and tannins together under the aerobic conditions of drying. However, in drying it may be possible that other reactions, such as mild Browning reactions protect the protein from ruminal microbial degeneration and also from irreversible binding with tannins. To add confusion to the area Norton (1994b) (see Table 5.3 on page 77) found that Calliandra leaf material that was dry increased the growth rate of goats as compared to similar animals fed the fresh materials. This points to differences between goats and sheep in the use of browse in the rumen.
There is a need for major research here to explain the differences, but in practice such foliages are of little benefit unless grazed by animals so that they are consumed fresh. These discussions do not exclude a range of other possibilities, such as binding of critical nutrients in wilting, thereby making them unavailable in the rumen (e.g., S, P or Mg) or the production of toxic compounds to specific groups of rumen microbes.
Supplements of tree foliage can increase growth rates of cattle above that of a supplement of urea. This has been shown in studies where tree foliage has been fed with ammoniated straw which could be anticipated to oversupply rumen ammonia on these diets. However, it does not rule out the correction of other deficiencies by minerals in the tree foliage, such as sulphur.
|Supplement (% DM intake)||LWt gain (g/d)||Wool growth (mg/100 cm2/d)|
A study in Indonesia (Table 4.7 on the next page) has shown good increases with replacement of natural tropical grasses in the diet with dried Leucaena forage. These responses appear to be in line with Leucaena's being a source of bypass protein. Both total dry matter intake and digestibility was increased when Leucaena forage became an increasing proportion of the total forage fed. Also, the production and efficiency of production was increased when Leucaena supplied 40–60% of the total dry matter intake. However, in other results from Indonesia, cattle on tropical grasses (green) supplemented with MUMB have improved productivity by over 100% (Chapter 3) and the cattle have higher rates of growth than those recorded by Wahyuni et al. (1982). This then leaves unresolved the rôle of the protein in Leucaena when fed in a grass based diet.
The two overall effects of using Leucaena as a supplement to ruminants seem to be that:—
there is little substitution of the basal crop residue by the tree foliage at low intakes
total digestible feed intake is increased.
Some of the increase in productivity of ruminants on crop residues to supplements of Leucaena have been attributed to a mineral response (Goodchild & McMeniman, 1994). Rumen ammonia levels are increased, and improved digestibility and P/E ratio in the nutrients from the rumen are probably responsible for the extra production without invoking a bypass protein effect (Table 4.8 on page 69). Goodchild & McMeniman, (1994) concluded that the responses of sheep to supplements of browse are to extra N and minerals in the rumen and the organic matter in the foliage which enhances the overall fibre digestibility of the diet. However, supplementation with any dry browse seems to have an additional effect similar to adding a bypass protein.
|% grass||% Leucaena||DM intake g/kg LWt/d||DM dietary digestibility (%)||LWt gain g/d|
The results of feeding Leucaena (not dried) to sheep (mature) fed sorghum roughage (± urea) are shown in Table 4.8 on the next page.
The nutritional and metabolic strategies that small ruminants have evolved in the tropics to utilize herbs, shrubs or tree foliage high in secondary plant compounds appear to be associated with high fecundity, ability to breed twice in one year and high capacity for milk yield.
Although scientists have, so far, not attempted to relate these strategies with nutrient availability, it appears logical that strategies for utilization of foliage high in secondary plant compounds also enable the animal to receive higher levels of dietary protein post-ruminally for digestion and absorption. Nutritional ecology of ruminants is a new area for study, involving considerable rationalization of the places of different species according to their metabolic strategies.
|Level of Leucaena inclusion in diet (%)||Intake (g/kg LWt/d)|
|Urea inclusion||Sorghum straw OMI||Total OM||Digestibility OMI||OM Dig %||Rumen NH4 (mg N/l)|
In small ruminants of tropical breeds there is an obvious positive relationship between the amount of fodder tree foliage in a diet and the level of protection, whereas the reverse is true where foliages are replaced by graminaceous species in the diets (Sánchez, M., personal communication7). It becomes imperative to recognize that research on small tropical ruminants may be quite different to that of temperate climate species (commonly known as British breeds) and sheep that have developed on open grasslands in the arid subtropics or temperate areas (e.g., the Merino) and vice versa.
The choice of diets for ruminant production should involve looking at the dietary conditions under which the species evolved—in the case of small ruminants in tropical areas these are largely forests.
7 Mr M.D. Sánchez, Animal Production Officer, Animal Production and Health Division, FAO Via della Terme di Caracalla, Rome 00100, Italy.
There is little point in going further in attempting to illustrate the supplemental rôle(s) of tree foliages here, since the research necessary has yet to be done. Norton (1994b) has reviewed the effects of various foliages of trees on the digestibility of forage diets, the voluntary intake of tree foliages and their influence on production of cattle/sheep and goats.
The above discussion defines the potential strategies to provide two types of supplements from tree foliage that are required for optimal efficiency of utilization of low quality forage by ruminants in areas with scarce resources of nitrogen or protein. The strategies must be to find forages, seeds or pods that are high in protein and minerals. These materials can then be used as catalytic supplements to provide for either the rumen soluble protein and minerals, or (after treatment to protect the protein), as a bypass protein source. They can also be used in combination with a MUMB when the leaf protein is protected and/or as a source of locally available soluble protein with a bypass protein meal when the leaf protein is unprotected. Legume tree forages have a major rôle, as their extensive root system fixes N2 and the trees grow throughout the dry season.
The enormous biological diversity within tropical trees provides a bank of materials that can be used for animal feeding, yet only some 20–30 species have been used and out of those Leucaena, Gliricidia, Erythrina and Acacia species have predominated. In fact the use of Leucaena may have been accepted more widely than others despite establishment difficulties and problems with insect pests, because of the wide publicity it received when scientists found the cure to its toxic components (see Jones & Lowry, 1990). The toxicity of Leucaena was only a problem where ruminants were isolated from the micro-organisms that degraded the toxic breakdown products of the alkaloid, mimosine. These organisms are present in the rumen of animals where Leucaena has been a component of their diets. This has resulted in many agricultural advisers knowing of the value of Leucaena because of the publications surrounding this very important breakthrough in understanding. However, this has led to neglect of other adapted trees found locally. Unfortunately infestation of Leucaena with thrips has at times killed Leucaena in large areas where it has been introduced. However, Leucaena grows apparently unaffected by these thrips in Cuba where the thrips originated, indicating some form of biological control. Considerable work is proceeding in this area.
The wealth of potential fodder trees is enormous and the agronomic work has begun on a variety of species. Future developments must use an integrated approach which not only involves biomass and crude protein production of trees in relation to biomass from other plants (e.g., grasses) but also studies the optimum use of the tree foliages. To be able to take advantage of this enormous genetic resource, animal nutrition research should be focussed on:—
studying the mechanisms of binding of proteins and tannins
identification of secondary plant compounds and how they are influenced by environment and grazing or harvesting
developing ways of neutralizing anti-nutritional effects of tannins
conversely, finding ways to utilize the potentially beneficial effects of condensed tannins which bind protein in the rumen and protect it from rumen degradation
increasing the availability of the N in the rumen and/or amino acids for absorption in the intestine
protecting the dietary proteins by processing for feeding to ruminants.
The nutrition research must, of course, be undertaken in the light of agronomic research that has solved the problems of how and where to grow the tree.