Overstory #107 - Tropical Forage Tree Legumes
"It is a humbling fact for grass pasture experts to realize that probably more animals feed on shrubs and trees, or on associations in which trees and shrubs play an important part, than on true grass-legume pastures."
CAB Publication No. 10 (1947)
Much has been written on the role of forage tree legumes. The literature abounds with reports, scholarly papers, conference proceedings, and books which describe traditional uses of indigenous species and new opportunities with exotic species. Tree legumes offer many benefits. Apart from their value as feed for livestock, tree legumes are recognised for their multi-purpose contributions to the productivity of farming systems, to the welfare of people and to the protection of the environment. But it is the flexibility of their uses that makes them especially significant; they can be found on farms ranging from small-holder subsistence to large-scale commercial.
The most well known species, Leucaena leucocephala (leucaena), was once referred to as the miracle tree. This label did great damage to perceptions of the value of leucaena, especially after the movement of the psyllid insect around the world. Following this event, great hardship was suffered by those who depended on this species for their livelihood. Its limitations are now more clearly understood (Shelton and Jones 1995) and have led to a worldwide study of alternative species; both those currently in use, as well as new, but not yet domesticated species.
The world-wide interest in forage tree legumes may seem relatively recent probably because publications and promotion of tree legumes have greatly increased over the past 15-20 years. In reality, the use of tree legumes in tropical farming systems dates back to the beginning of domestic agriculture, although early use was not for forage. Indigenous peoples had excellent knowledge of the multipurpose value of the various species available.
In Mexico and Central America, where many of our most useful forage tree species originate, there was no tradition of tree forage use. For example, Mesquite (Prosopis spp.) pods were a component of diets of inhabitants of the United States and Mexican border lands for several thousand years, and later on were consumed by the white pioneers of the 1800s (Ibrahim 1992). Its use as a browse has been more recent.
There is evidence of indigenous use of unripe Leucaena pods and seeds for human consumption in the Tehuacan Valley in Mexico since the first domestication of agriculture. Archaeological studies have located Leucaena fragments in prehistoric cave settlement sites dating back to 6800 BC and it seems that Leucaena cultivation may have begun about 2000 years ago (Hughes 1998). It continues to be cultivated for human consumption in Mexico today, but rarely for forage.
In another contrast with present day fodder use, the genus Calliandra has its centre of origin in Central America, where it has little significance for any agroforestry purpose (Arias and Macqueen 1996).
The Spanish conquerors of Mexico observed local people using and cultivating Gliricidia sepium (gliricidia) for a number of non-forage purposes (Stewart et al. 1996). From this time, gliricidia was transported around the world in several waves of introductions, beginning with Spanish colonization in the 1600s, to provide shade for plantation crops (coffee, tea, cocoa). The Spanish are thought to have introduced it to the Philippines and to the Caribbean. Later in the 1800s, it was introduced to Sri Lanka and other Southeast Asian countries and finally to West Africa in the 1900s (Stewart et al. 1996).
There are some examples where the principal indigenous use of tree legumes was and is for forage. These tend to be in the drier regions of the world, e.g. the Sahel and North Africa. Even today, in these arid and semi-arid zones, tree legumes, principally Acacia spp., continue to provide a proportion of total herbage intake, and most of the protein intake, for livestock. This increases during dry periods (Baumer 1992).
More recent movements of tree legume germplasm (over the last 50 years) have largely been for agroforestry purposes, of which forage use was one of the primary proposed objectives.
Benefits and species
Many benefits are claimed for forage tree legumes. Apart from their value for livestock, they are recognised for their contributions to farming systems, the welfare of rural populations, and protection of the environment. There are now many species and varieties available for farm use with a wide range of ecological adaptation. However, no single species delivers all stated benefits, and there is no single species suited to the entire range of conditions. Therefore, we must be realistic in our goals when selecting forage trees for farming systems. Choice of species will depend on the specific requirements of the farming system in which they are to be grown. It is important to reconcile need, environment, and sustainability with choice of species. Multiple objectives or multiple habitats will necessitate an integrated approach using several species.
Whilst forage is just one of the many uses of tree legumes, it is concluded that forage use offers the best opportunity for commercial enterprise provided livestock markets exist. Most other uses are of semi-subsistence value or have an environment focus, thus limiting economic opportunity. It is significant that both small and large-scale operators are finding relevant applications for tree legumes.
Exotic versus native species
"Too often in extension work, a few exotic species have been strongly promoted without any attention being given to the rich indigenous flora and local knowledge of it." (Bekele-Tesemma et al. 1993).
Over recent years there has been increasing interest in indigenous species as an alternative to introducing exotic species, and debate concerning the appropriateness of introducing exotic species into indigenous ecosystems.
There are many reasons for this trend:
(a) Farming communities have very detailed knowledge of the use and value of indigenous species, and often this has not been documented, assessed or verified (B. Calub, personal communication, Schrempp et al. 1992).
(b) There are clear ecological advantages in using a diversity of indigenous species, compared to a monoculture of exotics.
(c) Concern, sometimes for nationalistic and patriotic reasons, about preserving and conserving indigenous germplasm.
(d) A reduced emphasis on promotion of exotic species and greater in situ use of local tree diversity, may reduce risk of unwanted weed invasion and genetic pollution through hybridisation (Hughes 1994).
There is no simple answer to this debate and decisions have to be made on individual merit. There are arguments on both sides. Combined use of native and exotic species may have merit.
Often exotic species are more vigorous and produce higher yields than indigenous species. This was the case in Malawi where L. leucocephala, Cassia spectabilis and Gliricidia sepium have been promoted over the indigenous Faidherbia albida which is slow growing (Cromwell et al. 1996). In fact, there are many regions where exotic species have made invaluable contributions. It has been estimated that 150 to 200 M people use gliricidia world-wide, the majority of whom live outside its native range (Simons 1996). Leucaena is now naturalised in the Philippines where it is the principal source of tree fodder and of fuelwood. This species underpins a sustainable, highly productive beef cattle production system in northern Australia (Middleton et al. 1995).
In India, fast growing, multipurpose exotic tree species introduced with the relatively slow growing Acacia nilotica (an indigenous tree) enhance biomass production. However, competition reduces growth of the indigenous tree. Careful planning and thoughtful species selection was recommended before implementation of exotic large-scale afforestation programmes (Neelam-Bhatnagar et al. 1993).
Sometimes indigenous species are better adapted to difficult soils. In Costa Rica, native leguminous species had more potential for reforestation and agroforestry on acid soils high in aluminum and manganese than exotic species (Tilki and Fisher 1998). In contrast, in the mountainous area of Minas Gerais, Brazil, where acid infertile soils predominate, the exotic species Acacia mangium and A. auriculiformis achieved faster growth than indigenous species when introduced into an existing B. decumbens pasture (Carvalho 1997).
Accessing high quality seed
Many farmers are unable to access high quality seed of the best varieties e.g. new releases with insect and disease resistance, or greatly improved productivity. Greater attention is required to educate the distributors of seed (private and institutional) on the importance of using the best germplasm of known genetic quality. More formalised distribution protocols may be needed to protect farmers against receipt of poor quality or unnecessarily expensive planting material. There needs to be greater emphasis placed on both institutional and private investment in the establishment of seed and clonal orchards to ensure that sufficient quantity of the best materials are available for distribution to farmers.
The current recommendation for selecting seed from a native range, is to obtain seed from at least 25, and preferably 50 trees, with sufficient distance between them (50 m) to minimise the likelihood of co-ancestry (Allison and Simons 1996). This simple approach was not appreciated when the first introductions were made.
A number of introduced tree legumes have become serious weed pests. Given the large number of introductions to many new environments, this is not surprising. Weediness of introduced exotic trees has generally occurred when:
-- The purpose for the introduction has failed, or results in only partial use of trees,
-- Seedlings and trees are protected from grazing by thorns, or low palatability,
-- Trees have abundant, precocious seed production,
-- Seeds are only partially digested by ruminant grazers, and viable seeds are spread in faeces,
-- Seed is spread on the hoofs of animals, or transported by flood waters,
-- Seeds are long-lived in the soil,
-- Young plants grow and colonise rapidly, and tolerate drought, grazing and fire,
-- Trees are long-lived,
-- There are disturbed areas nearby suitable for invasion,
-- There is unpredictable growth as trees perform beyond expectations away from natural predators, or in new climatic, edaphic or management environments.
These conditions have been partially met by a number of introduction events e.g. Acacia nilotica was introduced to provide shade and fodder for sheep in western Queensland but now infests 6 M ha of Astrebla grasslands (Carter 1994).
Over the past 80-100 years, mesquite (Prosopis spp.) has become an aggressive invader of desert grasslands in the southwest United States (Ibrahim 1992) due to interference in the natural ecological balance by man and his activities. Strategies for control and management of this problem are still not available. Grazing livestock and reduced occurrence of fire were key factors in the increase in density of mesquite. The original movement of Leucaena leucocephala subspecies leucocaphala around the world commencing in the 1600s has lead to this inferior but seedy variety becoming a weed in many tropical environments (Hughes 1994).
The question of weed risk raises many difficult questions with few easy answers. Some suggest that only indigenous species should be considered in agroforestry programs as a way to avoid possible invasion of natural ecosystems by exotic introductions. But this is an unrealistic constraint on farming systems and indeed the environment. Tree legumes are becoming increasingly more important in our livestock industries and our communities. It is imperative that we actively pursue environmentally responsible objectives. Whilst biological control measures have been partially successful (e.g. the bruchid beetle in Leucaena), as always, the key is to use preventative rather than remedial measures. It is important to carefully evaluate the level of risk, rejecting high risk introductions, and then to carefully manage introductions to minimise the chances of weed outbreak.
When introducing new species to an environment it may be necessary to first:
(a) Review risk of spread by assessing seed production, seed longevity, seed dispersal mechanisms,
(b) Review potential methods of control such as susceptibility of seedlings and trees to grazing (thorns, toxins, anti-palatability will reduce animal access); susceptibility to fire, chemical and mechanical methods; and occurrence of insect predators and pathogens in the native range,
(c) Study climatic and soil characteristics, in relation to habitat preference of introductions, to predict potential areas susceptible to invasion,
(d) Ensure that farmers have been informed as to how to manage and make full use of the introductions. There are many examples of apparent weediness occurring because villagers may be unaware of the many uses of new plants,
(e) After introduction, install long-term monitoring and rapid action systems,
(f) In improvement programs, investigate opportunity to breed sterile varieties e.g. the sterile triploid in Leucaena breeding programs.
A number of these strategies can be combined to reduce weed risk. Nevertheless, tree legumes should not be introduced where risk is high, or where nearby disturbed vegetation might be ecologically threatened.
Forage quality is essential in tree species used for commercial livestock production. Whilst there is sufficient chemical composition data on tree legumes, this can be misleading. Detailed information on the most important nutritional characteristics (intake of digestible dry matter, production of animal product) is not available for most species.
Low palatability (animal preference) is an issue for many species, yet our understanding of palatabilty is only partial. Educational programs are required to inform researchers, extension workers and farmers of methods to overcome the reluctance of inexperienced animals to consume new materials. There is opportunity to mix both livestock and plant species and match plant palatability characteristics with livestock preference, to achieve both acceptance of the feed and nutritional advantage.
Many genera contain high levels of tannins which will reduce forage quality. Levels above 5-6% appear to reduce digestibility and the release of protein for ruminants use. Some species in the genera Acacia, Calliandra , Prosopis, Leucaena and Flemingia have particularly high levels (10%) although, there is great variation in tannin levels both between and within species, and therefore opportunity for selection of lower tannin varieties.
It is concluded that species in genera, such as Acacia, Prosopis, Flemingia, Calliandra, Erythrina, whilst important, can be regarded as lower in forage quality. In contrast, key species from Leucaena, Gliricidia, Sesbania and Chamaecytisus (Osuji et al. 1997) are generally of higher quality. Nevertheless, there can still be significant inter- and intra-specific variation, as was found in Leucaena, and this offers scope to seek higher quality varieties in some genera.
The main species
There are several hundred species of leguminous trees with potential for forage listed in the literature (Houérou 1980, Atta-Krah 1989). Most have not been investigated and few are in current use in any significant way. Of the 5000 known nitrogen fixing woody species, Brewbaker (1986) suggested that only about 80 leguminous tree and shrub species may have potential multipurpose agroforestry roles, including fodder, in tropical farming systems. Roshetko et al. (1996) listed 46 species suitable for fodder, but many fewer have found significance in world animal production systems as key sources of forage supply.
The species and key references are given in Table 3. There may be additional species which have forage potential, and within each species there is genetic variation which can be exploited. However, in this brief review only those species in significant use for forage are listed. Selection for membership of this list was a subjective process although fodder value was the pre-eminent selection criteria.
Table: Most used tree legume species for forage purposes
Higher quality species
-- Albizia lebbeck
-- Chamaecytisus palmensi
-- Cratylia argentea
-- Desmodium rensoni
-- Desmanthus virgatus
-- Gliricidia sepium
-- Leucaena leucocephala
-- Leucaena diversifolia
-- Sesbania grandiflora
-- Sesbania sesban
Lower quality species
-- Acacia aneura *
-- Acacia nilotica
-- Acacia tortili *
-- Albizia chinensis
-- Albizia saman
-- Calliandra calothyrsus
-- Erythrina spp.
-- Faidherbia albida *
-- Flemingia macrophylla
-- Prosopis juliflora
* Principal application is in indigenous semi-subsistence systems
Other species have potential but are not yet in significant use. Examples include the Leucaena pallida x L. leucocephala KX2 hybrid, L. collinsii and L. trichandra, the latter species for the high altitude tropics (Shelton et al. 1998).
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This article was adapted with permission of the author and publisher from:
Shelton, M. Tropical Forage Tree legumes: Key Development Issues. FAO, Rome. Web: fao.org/ag/AGP/AGPC/doc/Present/Shelton/default.htm
The original source is a publication of the the Grassland web site of the Food and Agriculture Organization of the United Nations located at: fao.org/ag/AGP/AGPC/doc/pasture/pasture.htm
About the author
Max Shelton obtained his Ph.D. degree from the University of Queensland and is now an Associate Professor responsible for teaching and research in pasture science and agroforestry. His current research interest is forage tree legumes in tropical agriculture; he was previously involved in research and development programs on pastures for tropical plantation crops. From 1995, he has been research leader of a project "New Leucaenas for Southeast Asian, Pacific and Australian Agriculture" supported by the Australian Centre for International Agricultural Research (ACIAR). This program has involved collaboration with scientists and extension workers in 7 countries and has comprised work on the germplasm evaluation, environmental adaptation, forage quality and animal productivity of a world germplasm collection representing the entire Leucaena genus. Dr Shelton has worked extensively on forage and livestock systems in the Southeast Asian and Pacific regions over 25 years. He has published widely including editorship of a CABI book on tree legumes and four ACIAR Proceedings on Leucaena and Forages for Plantation Crops. Dr Shelton was the key instigator and inaugural coordinator of the International Leucaena R & D Network (LEUCNET).
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