Tropical Forage Tree legumes:
Key Development Issues

by

Max Shelton
The University of Queensland, Australia

 

Photo - Feeding gliricidia to smallholder buffalo in Bali
Popular saying in Rajasthan

"Death will not visit a man, even at the time of famine, if he has a Prosopis cineraria, a goat and a camel since the three together can sustain him during the most trying conditions".

Source: Kishan Kumar 1999.

 

A short version of this paper has been included in Unasylva Vol. 51, No. 200, pages 25-32.


Table of Contents

Summary of key issues and priorities

1. Introduction

2. Traditional use was often not forage

3. The important issues in R & D

3.1 Qualities sought in forage tree legumes

3.2 Is forage supply the most important use?

3.3 Taxonomic confusion

3.4 The debate over exotic versus native species

3.5 Diversity, quality and availability of planting material

3.6 Agro-ecological range of species

3.7 Nutritive value, palatability, and preference

3.8 Diseases and insects

3.9 Management and conservation

3.10 Weediness

3.11 Integrating forage tree legumes into farming systems and farmer uptake

4. The main species

5. References


Summary of key issues and priorities

Introduction

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 (see Reference list). Tree legumes offer many benefits. 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 purpose of this monograph was to:- (a) Review the benefits derived from planting tree legumes; (b) consider which species are important for forage; (c) analyse the important issues in improvement, management and use, (d) review the level of uptake and commercial potential of forage tree legume technology, and (e) suggest some on-going R & D priorities.

The full text follows. This summary is linked to the main text for those wishing more detail.

Benefits and species

Many benefits are claimed for forage tree legumes (see section 3.1). 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 (see section 3.6). However, no single species delivers all stated benefits, and there is no single species suited to the entire range of conditions (Box 1.1). 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 (see section 3.2). 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.

Only 20 species are listed as significant providers of fodder, and several new opportunities are mentioned (see section 4). These are largely in the Leucaena genus where recent work has demonstrated the outstanding potential of the KX2 hybrid, as well as some other species for specific niches.

Exotic versus indigenous use

Traditional use of tree legumes was often not for forage (some Acacia spp. excepted) (see section 2). Whilst recent movements (over the last 50 years) of tree legume germplasm have largely been for agroforestry purposes, forage use has been one of the primary objectives. Significantly, most commercial use of tree legumes for forage has been with exotic species. Indigenous species appear to have been confined to subsistence feeding systems perhaps due to their lower productivity and lower quality, and to communal management regimes which place few limits on use. It is likely that the most appropriate path will be judicious use of both native and exotic species.

There is currently lively debate concerning the relative merits of using exotic and indigenous species, fuelled by concern about exotic species becoming weeds, and deterioration of the native range of many important genera (see section 3.4). For instance, the Acacia communities in the Sahel, North Africa and the Near East have deteriorated almost beyond recovery due to excessive demand for fuelwood, grazing, and agricultural land. Amelioration and conservation measures for indigenous species, over-exploited in their native range is a priority. This will require low cost participatory approaches which emphasize preventative rather than remedial measures.

Using and conserving diversity

The development and improvement of tree legumes for farm use is contingent on the availability of germplasm from the Centres of Origin of species to underpin improvement programs. It is therefore imperative that the diversity of genetic resources is collected and described taxonomically, and that the native ranges are protected from exploitation and over use. The in situ conservation status is listed as threatened for some species but not others. A combination of several conservation approaches may be necessary including in situ (maintenance of natural population), ex situ (e.g. germlasm banks and botanic gardens) and circa situm (maintenance while in agricultural use e.g. as hedge row) conservation (see section 3.9).

While there is a range of material available in germplasm banks, there is much unstudied germplasm requiring investigation. The further development and release of new cultivars, of many species in current use, suffers from both lack of diversity and inferior germplasm availability. This is due to the narrow genetic origins of most naturalised populations of tree legumes. This phenomenon applies to the most important forage species in use (Leucaena leucocephala, Gliricidia sepium, Prosopis, Calliandra calothyrsus), and their narrow genetic base ensures that there is little resistance to pests and diseases.

Some genera, such as Prosopis and Albizia, still require fundamental taxonomic review, including further exploration, collection and mapping of germplasm in its natural habitat. For these species, there is very limited information on climatic and edaphic adaptation as available for well- studied genera such as Gliricidia and Leucaena (see section 3.3).

New techniques can now be adopted to expedite plant improvement programs. They include new molecular techniques for describing and mapping genetic diversity, and vegetative propagation approaches to conservation and release of elite clonal provenances.

However, it is now exceedingly difficult to obtain support for taxonomic studies and conservation of undomesticated genetic resources and communication of the importance of this work needs to be promoted (see section 3.4).

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 (see section 3.5).

Forage quality

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 (see section on palatability). 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 (see section on livestock preferences).

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 (see section on tannins).

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 (see section 3.7).

Pests

Diseases and insects of forage tree legumes limit productivity worldwide. As the use of tree legumes is expanding rapidly, pest problems are increasing in occurrence and severity. Whilst there are lists of pathogens and insects of tree legumes, more detailed information is needed on the extent and severity of damage incurred, insect and pathogenic variability, and host plant relationships. Country and region surveys would better describe the location and extent of problems. The existing networks are an appropriate way to gain information on disease and insect occurrence. Unfortunately, there is often a lack of specialist expertise to address these problems (see section 3.8).

Weed risk

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 (see section 3.10).

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 dispersial 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.

Farmer uptake

Adoption remains unsatisfactory despite years of promotion and development. Whilst there are large areas where forage trees are regularly used for forage purposes, the potential for expansion into new areas and new tropical farming systems is great. Active promotion of the use of new varieties, and of agroforestry packages (e.g. alley cropping and fodder banks), is occurring in Australia, Africa, the Pacific, Southeast Asia, Latin America, and the United States but with generally disappointing results. Scientists need to improve their effectiveness at promoting acceptance and adoption of new species and technologies (see section 3.11).

There are many reasons put forward for the poor levels of uptake including lack of understanding of specific needs of farmers and of their socio-economic environment. There may be a mismatch between the environmental goals of development organisations and the personal goals of farmers. There is often inappropriate planting material and support infrastructure provided. It is suggested that uptake can be advanced firstly, by raising awareness through publications / training / networks; and secondly, by working more closely with farmers using participatory on-farm approaches, where social constraints (risks, relevance, labour, environment) and economic constraints (incentives, markets and returns) are fully addressed.


Tropical Forage Tree legumes:
Key Development Issues

 by 

Max Shelton
The University of Queensland, Australia

Sesbania grandiflora on paddy walls, Lombok, Indonesia

 

"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)

 


1. Introduction

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 (see Box 1.1). 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.

It is therefore timely to critique our findings as we move towards the new millenium.

The purpose of this monograph is not to review all that is known about forage tree legumes. Detailed information can be found in the literature. A list of many important publications is given in the bibliography. Rather, the aim is to review the progress of R & D on forage tree legumes in tropical and subtropical farming systems; and in particular, to consider the following questions:

(a) Are the claimed benefits from planting tree legumes exaggerated?

(b) Which species are important and why?

(c) What are the important issues that have emerged regarding their improvement, management and use?

(d) What is the level of uptake of forage tree legumes world-wide?

(e) What are the on-going research priorities and potential for further expansion of use of tree legumes for forage?


2. Traditional use was often not forage

We tend to think of the world-wide interest in forage tree legumes as being 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).

A recent study of Erythrina in rural populations of Costa Rica showed that food, fuelwood, medicine, construction, living fences, and shade, but not forage, were the principal traditional uses (Nygren 1993).

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.


3. The important issues in R & D

3.1 Qualities sought in forage tree legumes

A review of the qualities sought in a forage tree legume quickly shows that we are indeed looking for miracle trees (see Box 3.1). An analysis of these qualities reveals that none of our current species meet these requirements in their entirety:-

Agronomic. Apart from Sesbania spp., tree legume seedlings rarely show rapid early growth and routinely require ongoing protection against weed competition and predation until the juvenile phase has passed (Shelton 1994b).

Many species require inoculation with specific Rhizobium strains for best results (Leucaena - Lesueur et al. (1998); Sesbania sesban - Dart (1994); Prosopis spp. - Dommergues (1992); Erythrina spp. - Westley and Powell (1993); Calliandra calothyrsus - Evans (1996). Poor agronomic performance, caused by lack of an effective Rhizobium symbiosis, may be erroneously attributed to unsuitable environmental adaptation. Inoculum is often not available commercially and therefore not used in field plantings in the developing world.

While all species tolerate lenient defoliation, many species are weakened by repeated cutting, and are often damaged by direct grazing e.g. C. calothyrsus and S. sesban, especially when grazed by goats (Shelton et al. 1996). Leucaena leucocephala is a notable exception with its exceptional ability to tolerate severe defoliation, by either cutting or grazing, over extended periods of time (10-30 years).

There is a great range of adaptation to environmental stresses. Species are available for cold, acid and waterlogged environments, but we cannot expect this combination of tolerances in all species. For instance, whilst cold tolerance has been identified in Leucaena (L. trichandra, L. diversifolia, L. pallida), adaptation to acid and waterlogged soils has not (Mullen et al. 1998b). Many species are ravaged by diseases and insects (Lenné and Boa 1994, Walter and Parry 1994) and suitable methods of control are not known. Many African Acacia and American Prosopis species are thorny. The abundant seed production of some aggressive species, required for easy propagation, often leads to unwanted invasion of disturbed ecosystems.

Nutritional qualities. Clearly, high nutritional quality is desired for tree legumes managed for forage purposes. Whilst total protein levels in herbage of all species are usually adequate, the availability of this protein is often compromised by low digestibility, due to the formation of indigestible protein-tannin complexes. These are caused by high levels of naturally occurring poly-phenolic compounds (McNeill et al. 1998, Dalzell et al. 1998). The palatability of some species is low for ruminant livestock, especially for animals without prior experience of the fodder. Anti-nutritive compounds commonly occur in many species (see section 3.7).

We must be realistic in our goals when selecting forage trees for farming systems. It is important to match environment and species with need (see Box 3.2). When multiple objectives are required, in multiple habitats, it is likely that an integrated approach using several species will be appropriate.

3.2 Is forage their most important use?

This paper concerns the utilisation of tree legumes for forage, although this may be just one of several uses.

There can be no doubt that there is increasing demand for high quality legume forage for use in livestock feeding systems where the main objectives are commercial gain and sustainability. This can be argued for both smallholders and large-scale ranchers.

The International Livestock Research Institute expects that demand for milk and meat will double in developing countries between 1993 to 2010 (Dr Hank Fitzhugh, DG of ILRI, personal communication). Inadequate feed resources is the key constraint, especially for smallholders in peri-urban locations. ILRI suggests that the use of forage trees, in conjunction with crop residues, will be one of their key strategies to meet this increased demand.

Workers in other regions see similar opportunities. In the development of agroforestry in grazing areas of Zimbabwe, systems designed to improve forage production will make a significant contribution to farm productivity because of the importance of cattle in the farming systems and the present forage shortage (Cambell et al. 1991). In Australia, large scale leucaena plantings are occurring because farmers appreciate that leucaena-grass systems are both sustainable and highly productive. This production system allows them to produce cattle for high value domestic and export markets in East and Southeast Asia (Larsen et al. 1998) (see Box 3.6).

In southeast Asia, an increasingly affluent urban population is consuming more meat, and demanding higher quality meat. Smallholders are responding to this new and profitable opportunity with feeding strategies designed to fatten animals for slaughter at a younger age. This requires either high quality locally grown feeds or use of expensive concentrates. Farmers are finding that forage tree legumes meet this need enabling them to achieve increasing levels of profitability. Box 3.4 describes the system of "forced feeding" of leucaena in the Philippines. Box 3.3 describes the use of gliricidia for fattening goats in Aceh Province of Indonesia.

Where there is no premium available for the sale of livestock products, the value of forage trees is less appreciated. Indigenous forage tree species have generally been used for subsistence feeding rather than in commercial systems. Wickens et al. (1995) describe how fuelwood and grazing were the principal uses of the former Acacia communities in the Sahel, North Africa and the Near East (Box 3.5). However, due to overuse, some have now deteriorated, almost beyond recovery.

In eastern Sudan, where commercial livestock production is not practised, small-scale industries are almost 100% dependent on mesquite for fuelwood (Elnur A. Elsiddig, unpublished report to FAO) (Box 3.8).

To conclude, whilst forage is just one of the uses of tree legumes, it may be argued that it offers the best opportunity for commercial enterprise, where livestock markets exist. It is applicable and commercially beneficial to both small and large-scale operators. Significantly, most commercial use of forage from tree legumes has been with exotic species. Indigenous species appear to have been confined to subsistence feeding systems perhaps due to their lower productivity and lower quality. The communal management regimes employed in traditional systems place few limits on use, and this has led to over exploitation.

3.3 Taxonomic confusion

Accurate classification and documentation of genus and species relationships within important plant groups, is a prerequisite for effective plant improvement and utilisation of tree legumes (Hughes 1998b). Plant classifications provide the names that we use in our agronomic, nutritional and farming system programs. Accurate naming, and knowledge of the diversity available, influences the direction of plant evaluation. Without this, key taxa may be omitted and uncertainty is often the main result. This has been the case, at various points, with most forage tree legume plant improvement programs.

In some cases, important genera have yet to be described and evaluated. Only a small proportion of the total diversity in Albizia has been surveyed, and there is considerable unmapped potential in this genus (Hughes and Pottinger 1997). Further critical investigation is required to test the forage potential of Albizia.

There is incomplete understanding and confusion regarding the taxonomy, genetics and ecology of Prosopis (Dutton 1992). Hybridisation, intra-specific polymorphism and heterogeneity make it very difficult to identify some Prosopis spp. (Ibrahim 1992).

After a long period of disagreement, Leucaena has finally been described in detail and 22 species have been named (Hughes 1998a and b), compared with only 17 species in an earlier report (Hughes and Harris 1995).

Another source of taxonomic confusion is the hybridisation which often occurs when species, normally well separated in their native range, are brought together in evaluation programs. Hybrid seed is unknowingly collected and spread. Examples of this phenomenon have been documented in Leucaena (Hughes 1998a), Erythrina (Neill 1993), and Gliricidia (Lavin 1996). In Papua New Guinea, where Leucaena leucocephala and L. diversifolia were introduced separately, a vigorous inter-specific hybrid has appeared spontaneously. It is highly favoured and known locally by the incorrect name L. mexicana.

Unfortunately, in the present economic environment, it is difficult to find support for taxonomic studies, yet such activity underpins all plant improvement programs, and ultimately influences the quality of new varieties made available for farmers.

3.4 The debate over 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 (Box 3.7).

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)(see Box 3.6).

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).

Exotic species can have significant effects on associated ecosystem species. In Puerto Rico, regeneration of understorey native and naturalised trees and shrubs under exotic tree species (Casuarina equisetifolia, Eucalyptus robusta, and Leucaena leucocephala), were most abundant beneath Leucaena leucocephala and least abundant under Casuarina equisetifolia (Parrotta 1995). In Hawaii, 4 species of native birds rarely feed on the fruits of the exotic nitrogen-fixing tree Myrica faya which is invading Hawai'i Volcanoes National Park. However, five species of exotic birds were seen ingesting the fruit (Woodward et al. 1990). In South Africa, invasive exotic plants such as Acacia longifolia and A. mearnsii, were detrimental to native, ground-living, invertebrate fauna. There was no significant effect on species richness and diversity, but there was a different assemblage of species associated with exotic compared to indigenous vegetation. Management should therefore be sensitive to the needs of the ecosystem to ensure conservation of desirable species when native vegetation is replaced by exotics (Samways et al.1996).

Over the last three decades, there has been movement of plant material around the world on an unprecedented scale, but there are few restrictions covering movement. Hughes (1994) advocated a more cautious approach to species introduction and a more thorough assessment of the advantages and limitations of native and exotic species to lessen the risks of introduction of a weed.

Many native plants are incompletely studied. Some species are only now undergoing preliminary domestication and are still harvested by the traditional gathering activities associated with wild species. A case in point is Acacia albida (Faidherbia albida) which is now the focus of international collaborative efforts to extend its versatility of utilization (Nouaille 1992). There is large variability in performance of individual trees as little plant improvement has occurred, and little is known of the silviculture of the species (Cromwell et al. 1996).

It is likely that the most appropriate path through this minefield is judicious use of both native and exotic species (see Box 3.7). For instance, in tropical humid Africa, research may continue on species such as Leucaena, Gliricidia and Sesbania, but emphasis may gradually shift to local species as adoption would be more rapid and widespread. Schrempp et al. (1989) noted from their work in the eastern highlands of Ethiopia that preferred species in fields were indigenous species such as A. albida, while preferred species off-field were fast growing exotics such as A. mearnsii, A. saligna, Eucalyptus spp. and P. procera.

3.5 Diversity, quality and availability of planting material

(a) Lack of diversity in original introductions

In most cases, the movement of germplasm around the world began a long time ago. Usually the original seed that was introduced into an area was harvested from a few readily accessible trees. This meant that the early developments were based on unimproved, inferior varieties that contained little genetic diversity.

The movement of small amounts of seed of Leucaena leucocephala subspecies leucocephala from Mexico to Southeast Asia in the 1600s is the most celebrated example of this phenomenon. This species is a highly self-fertile polyploid, so that further movement of the introduced variety, was of almost identical genetic material. The spread of readily harvestable seed from country to country, region to region, organisation to organisation, farmer to farmer, has resulted in over-reliance on an extremely narrow genetic base (Hughes 1998a and b). This original leucaena, now known as the "weedy type", has invaded disturbed sites, and become a weed in many countries. We now appreciate a much greater diversity in Leucaena, some of which has significant potential value for agriculture, with much lower weed risk.

Similarly, Dutton (1992) reported that most seed of Prosopis planted around the world was of unknown origin and from a narrow genetic base. Pasiecznik (1999) confirmed that the thorny Prosopis shrubs, widespread in Africa and India, came from introductions of inferior germplasm, and this has lead to a poor appreciation of the genus. Research trials have shown that there is superior germplasm for different rainfall zones and soil types, and information on this new material needs to be disseminated.

The introduction of Gliricidia sepium from Trinidad to Sri Lanka was made with seed from one tree (Stewart et al. 1996). They suggest that the genetic diversity in many introduced populations will not be sufficient to ensure long-term stability.

There are other examples. Only two seed samples of Calliandra calothyrsus were first introduced into Indonesia (Java) from Guatemala in 1936 to provide shade for coffee. After 1974, seed was further spread by forest rangers for fuelwood use and now covers more than 30,000 ha in Java alone (Kartasubrata 1996). The first introductions of Erythrina species into Europe were for botanical and ornamental purposes and began in the early 1700s. Other introductions into Europe, Australia and the United States occurred in the late 1700s and early 1800s (McClintock 1993).

There is an added problem as farmer choices have narrowed diversity. In Flores Indonesia, a wide diversity of tree legumes was grown in farming systems in the 1960s. Species such as Acacia, Albizia, Calliandra, Cassia, Gliricidia, Pterocarpus, Sterculia and Tamarindus were all grown in diverse mixed farming systems. With intensification and commercialisation, there was greater reliance on few species notably leucaena (Djogo et al. 1995). The arrival of the psyllid in this region was particularly devastating.

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.

(b) Understanding diversity

Although the genus Leucaena has been well described both taxonomically (Hughes 1998a), agronomically and nutritionally (Shelton et al. 1998), this is not the case with all important genera. Much more investigation is required to test the forage potential of the untapped species and provenances in the Albizia genus (Hughes and Pottinger 1997).

Further, our ability to describe variation in plant material has greatly improved, and this may warrant a re-investigation of some genera. Traditionally, morphological and agronomic traits have been used to characterise patterns of diversity in plants. It is now known that these represent only a small proportion of the genome. Such traits are influenced by environmental factors, thus limiting their use for description of genetic relationships and variability. Molecular approaches such as the use of isozymes, and other genetic markers, which may be independent of environment and production responses, are likely to provide a more powerful method to gauge species relationships and origins (Dawson and Chamberlain 1996).

Macqueen (1996) confirmed that studies of molecular data, polyploidy and hybridisation research, rather than morphological work, were needed to understand the complex patterns of variation in Calliandra.

(c) Accessing high quality germplasm

It is clear that many farmers are using inferior planting material and that overcoming this limitation will not be a simple matter. Cromwell et al. (1996), in surveys of farmers using multi-purpose trees (MPTs) in Honduras, Sri Lanka and Malawi, found that the quality and reliability of MPT germplasm supply was limited. Purchase from formal sources was often expensive, and projects often obtained seed locally, as it was cheaper and more accessible. For this reason, germplasm was often:

  • Of unknown genetic quality,
  • Collected and distributed with weak protocols,
  • Selected on timber criteria,
  • Distributed with no knowledge or understanding of provenance quality, provenance origins, or the importance of genetic diversity,
  • Not reliably available.
Other workers have found that the market is not discerning. Much of the demand for Gliricidia has been met with seed of inferior quality with no premium paid for quality (Simons 1996). A major and serious limitation for Prosopis spp., which is restricting adoption, is lack of availability of seed of well documented provenances or improved varieties. There is also no source of clones of best vegetatively propagated material (Dutton 1992).

This has not been the case with leucaena, as high quality seed marketed under species, cultivar or provenance name, is available.

(d) Problems when introducing new material to farmers

We now also appreciate that it is difficult to introduce new varieties to areas where inferior varieties are already well established. There are two reasons for this.

(a) Farmers may not be prepared to purchase seed of improved varieties e.g. of Gliricida, as they consider it a low value crop (Simons 1996). They can use existing material at no cost. New germplasm would need to be markedly superior. Although seed of the best provenances of Gliricidia is now becoming available from seed orchards around the world, much of the current demand for gliricidia is being satisified by inferior unlabelled material, and it may be difficult to promote superior provenances such as Retalhuleu (Simons 1996). Simons (1996) suggested that new varieties will need to be at least 30% better in terms of woody and leaf biomass to interest farmers.

(b) When new material is introduced into existing areas, there may be hybridisation, loss of purity and therefore loss of advantage. This will be less of a problem for vegetative propagation and selfed seed. There may also be inbreeding depression if farmers collect seed from just a few trees for propagation eg. those that seed prolifically.

However, other experience is that farmers will plant new varieties if they recognise key benefits. For instance, farmers in Batangas Province in the Philippines were immediately enthusiastic about the new F1 hybrid KX2 leucaena (A. Castillo, personal communiucation). They appreciated its improved growth potential and its resistance to psyllids. Whereas K636 (cv. Tarramba) may not be accepted as quickly as it is more similar to the giant leucaenas already present in the Philippines.

In order to introduce new varieties it may therefore be important to:-

(a) Ensure that the variety has clear benefits e.g. insect / disease resistance, or greatly improved productivity,

(b) Create an appropriate local name for the new variety,

(c) Provide seed or planting material of high quality, and

(d) Ensure that seed is readily available through traditional channels at a reasonable price.

There are advantages in promoting farmer level (smallholder) seed production schemes to provide income for farmers, and local availability of seed. Their enterprises can also be used to create incentive to conserve the native range of threatened species. Disadvantages are lack of quality control, and difficulty in marketing away from their immediate regions.

3.6 Agroecological range of species

Agroforesters require forage tree legumes adapted to a wide range of environments. Species are needed for climates ranging from the humid tropics of some Pacific Island countries, the seasonally dry tropics of Southeast Asia, the cooler high altitude tropics of East Africa, and the arid zones of the Sahel in North Africa. Similarly, adaptation to a great range of edaphic conditions is sought from the alkaline vertisiols of the brigalow soils in Central Queensland to the very acid high aluminium grasslands of South America.

A summary of the general ecological adaptation of key species is given in Table 1.

Table 1. Adaptation of some fodder tree legumes to various environments
Species
Acid soils (pH<5.5)
Cool temp. (15-25oC)
Low 
rainfall (<500mm)
Medium rainfall
(5-1000 mm)
High 
rainfall (>1000mm)
Poor drainage
High salinity
Acacia aneura
T
T
T
NT
NT
NT
NT
Acacia angustissima 
T
NT
NT
NT
T
NT
NT
Acacia nilotica
NT
NT
NT
NT
NT
T
T
Acacia tortilis
NT
NT
T
NT
NT
NT
NT
Albizia chinensis
T
T
NT
T
T
NT
NT
Albizia lebbeck
T
T
T
T
T
NT
T
Albizia saman
T
NT
NT
NT
T
T
NT
Calliandra calothrysus
T
NT
NT
NT
T
NT
NT
Chamaecytisus palmensis
NT
T
NT
T
NT
NT
NT
Cratylia agentea
T
NT
NT
NT
T
NT
NT
Desmodium rensonii
T
NT
NT
NT
T
NT
NT
Desmodium virgatus
NT
NT
NT
T
T
NT
NT
Erythrina spp.
T
NT
NT
NT
T
NT
NT
Faidherbia albida
NT
NT
 
NT
NT
T
NT
Flemingia macrophylla
T
NT
NT
NT
T
T
NT
Gliricidia sepium
T
NT
NT
NT
T
NT
NT
Leucaena diversifolia
NT
T
NT
T
T
NT
NT
Leucaena KX2 hybrid
NT
T
NT
T
T
NT
NT
Leucaena leucocephala
NT
NT
NT
T
T
NT
NT
Leucaena pallida
NT
T
NT
T
T
NT
NT
Leucaena trichandra
NT
T
NT
T
T
NT
NT
Prosopis juliflora
NT
NT
T
NT
NT
NT
NT
Sesbania grandiflora
NT
NT
NT
NT
T
T
T
Sesbania sesban
NT
T
NT
NT
T
T
T
T = tolerant; NT = not tolerant

Source: Roshetko et al. (1996), Shelton (1994)

There is a wide range of ecological adaptation among tree legumes, although there are no single species suited to the entire range of conditions. Managers must select carefully to ensure successful growth of tree species in their environment. The topic of environmental adaptation is treated in detail elsewhere (Shelton 1994a). The vexed question of suitable leguminous trees for acid soils was covered in a workshop and reported in Evans and Szott (1995) and continues to be an important research objective for many workers. Several species such as Cratylia argentia, Desmodium velutinum and Flemingia macrophylla, are valued in South America because of their acid soil tolerance, but need to be more thoroughly tested for nutritional quality for ruminant feeding (Kexian et al. 1998). No tolerance of severely acid soils (pH < 5.0, with high aluminium saturation) was found in Leucaena (Mullen et al. 1998a).

3.7 Nutritive value, palatability, toxicity and preference

High nutritive value for livestock is an essential pre-requisite for successful adoption of forage species. Without high quality, farmers may not achieve the economic animal responses they require to justify their investment. On the other hand, in areas where feed resources are grossly inadequate, or other uses are equally important, farmers may accept a more modest contribution from browse, especially if the species is indigenous and does not require specific introduction and management.

Having established the agronomic advantages and ecological adaptation of promising new varieties, they must then be assessed for nutritive value. Many researchers view this as a matter of high priority (Dicko and Sikena 1992). Plants which grow well but contribute little to livestock production are of little value as forage species. For instance, the species Cratylia argentia, Desmodium velutinum and Flemingia macrophylla, are valued in South America for their acid soil tolerance, but Flemingia macrophylla has low intake, high condensed tannin content, low digestibility and protein quality (Kexian et al. 1998).

Firstly, what is nutritional quality? Many agroforesters have only a partial understanding of this concept. Judgements are often made based on readily available chemical composition data which may be misleading. Forage quality is complex and is assessed in many ways (Table 2).

Table 2. Methods of assessment of forage quality of tree legumes
 
In vitro measures of forage quality
In vivo measures of forage quality
Actual measures of forage quality
(a) In vitro dry matter digestibility (a) Rate of fermentation and passage of feed materials (a) Liveweight gain
(b) In vitro measurement of degradation rates (b) Volatile fatty acid production and composition, relative to rate of microbial protein synthesis (b) Milk production
(c) Proximate analysis (crude protein, fibre, lignin, ash contents) (c) Rumen ammonia (c) Fibre production
(d) Mineral and amino acid composition in edible tissue (d) Faecal nitrogen, plasma urea, and N balance
(e) Protein/energy ratios (e) Delivery of by-pass nutrients including non-microbial protein, to the hind gut
(f) Presence of anti-nutritive compounds e.g. tannins (f) Palatability measures
(g) In vivo dry matter digestibility
(h) Voluntary intake

 

The most important measure of forage quality is intake of digestible dry matter (nutritive value), and ultimately the production of animal product. Whilst this is known for well researched species such as Leucaena leucocephala (Middleton et al. 1995), Sesbania sesban (Gutteridge 1994b), Calliandra calothrysus (Shelton et al. 1996), and Gliricidia sepium (Stewart 1996), there is much less information on other species. Much of the data available are chemical composition only, and therefore of limited value. The concepts of nutritive value of tree legumes are described in detail in Norton (1994a,b,c).

Tannins

One aspect of forage quality that deserves special mention is the secondary plant compounds which are common in tree legumes. They appear to have no functional role, although they may impart ecological advantage by limiting or preventing damage from insects, fungi, bacteria, protozoa or grazing animals.

In particular, many tree legumes species contain condensed tannins (CT). These compounds are highly polymerised proanthocyanidins composed of flavanoid units with molecular weight from 1000-20,000. Tannins may have positive and negative effects on feed quality for ruminants. They bind with protein reducing digestibility of dietary protein in the rumen, but the effect may be positive if protein is released post-ruminally. The relative binding capacity of CTs varies among species eg. L. leucocephala CT appears to be "better" than L. pallida CT (McNeill et al. 1998).

However, it is clear that high levels of CT are detrimental to forage quality. Dalzell et al. (1998) showed that there was a strong relationship between in vitro digestibility and the ratio of crude protein to CT in tissues. Levels of CT above approx. 5-6% reduced digestibility.

Many genera contain high levels of tannins (>10%) e.g. Acacia (Woodward and Reed (1997), Calliandra (Shelton et al. 1996), Prosopis (Ibrahim 1992), Leucaena (Dalzell et al. 1998), and Flemingia (Kexian et al. 1998). However, there is great variation in CT levels both between and within species. This was shown in Leucaena where L. collinsii, L. lanceolata, L. macrophylla, L. magnifica, L. shannonii, L. trichodes, and L. lempirana had low CT content while L. pallida, L. trichandra, and L. diversifolia had high CT contents.

Some genera, such as Acacia, are therefore unlikely to contain high quality species. They are clearly valuable for supplemental forage but most could not supply adequate minerals when used as sole feeds (Karachi et al. 1997). Volatile fatty acid (VFA) analysis showed that mixed rumen microbes, after 12 hours, produced only 15 æmol/ml from A. angustissima fermentation compared to 63.9 æmol/ml from Sesbania sesban (Osuji et al. 1997).

Similarly, Prosopis, Flemingia, Calliandra, Erythrina, whilst important, can be regarded as species of lower quality. In contrast, key species from Leucaena, Gliricidia, Sesbania and Chamaecytisus (Osuji et al. 1997) are generally of higher quality, but there can still be significant inter- and intra-specific variation as was found in Leucaena (Dalzell et al. 1998).

There is also evidence that diets containing forage trees can influence rumen microbe composition. Extracts of A. angustissima inhibited the growth of pure cultures of rumen bacteria, while those from Sesbania sesban increased growth. Aacia cyanophylla decreased the numbers of protozoa in Ethiopian highland sheep (Osuji et al. 1997).

Palatability

Palatabilty is another complex issue with tree legumes. There are reports of low palatability in Gliricidia, Sesbania, and Leucaena whilst similar material at other locations was relished by livestock. It is now clear, that `palatability' is not constant and is influenced by prior learning, time to accustom to new feeds, smell, method of presentation and breed of animal (M. Faint, personal communication).

Length of time of exposure to feeds is an especially crucial parameter in `palatability'. In 5-day trials at ILRI in Ethiopia, MPTs such as Leucaena leucocephala and Sesbania sesban and less well known species such as Acacia venosa, Acacia persiciflora, Acacia melanoxylon, Acacia hockii, Acacia polyacantha, Tamarindus indica, Chamaecytisus palmensis, Tipuana tipu, Indigofera arrecta and Atriplex nummularia, had high palatability. Flemingia macrophylla, Erythrina abyssinica, Acacia salicina, Acacia coriacea, Albizia schimperana, Ceratonia siliqua, Casuarina glauca and Erythrina burana, had poor palatability. Gliricidia sepium and Calliandra calothyrsus, had only a medium palatability ranking (Kaitho et al. 1996).

Educational programs are required to inform researchers, extension workers and farmers of the value of "apparently unpalatable" plants, including methods to overcome the initial reluctance of inexperienced animals to consume new materials.

Livestock species preferences

In semi-arid and arid Africa, cattle, sheep, equines, wildebeast, most antelopes and gazelles graze forage tree legumes in the dry season to balance their diets. During the wet season, they prefer grass. Species, such as goats, camels, eland, impala, kudu, elephant, giraffe, black rhino and a number of antelope, are primarilty browsers of forage tree legumes (Wickens et al. 1995). The Orma people in the Tana and Lamu Districts of Kenya, who keep cattle, goats, sheep, camels and donkeys (in descending order of importance) found that browse preferences varied with species (Anttila et al. 1994). The ability of herbivores to graze browse trees often depends on their ability to handle thorns, woody materials, or high tannin foliage. Goats have greater preference for high tannin species than sheep or cattle, because of their ability to secrete proline rich saliva to reduce the astringency of the tannins (Kaitho et al. 1997, Kexian et al. 1998).

There is opportunity for mixing both livestock and plant species to take advantage of the varying preferences of livestock species.

Monograstric animals consume very little forage from tree legumes although there are many examples of MPTs being used for supplementing diets of monogastric livestock. However, the general consensus is that they have a limited role to play in monograstric feeding, and only as leaf protein concentrate. In general, they are not a suitable feed due to (a) their high content of anti-nutritive compounds, which non-ruminants have greater difficulty handling, (b) their high fibre content (Dutton 1992) and (c) their low energy content. Seeds are sometimes fed to monogastrics but may need detoxification procedures before being fed (D'Mello 1992).

3.8 Diseases and insects

Diseases and insects of forage tree legumes limit productivity worldwide. As the use of tree legumes is expanding rapidly, pest problems are likely to increase in occurrence and severity, yet the extent of knowledge of diseases and insects of tree legumes is generally poor.

There are data providing lists of pathogens but little information on their significance or on pathogenic variability (Lenné 1992). Disease and insect pests are reviewed for specific species and genera including Gliridicia (Boa and Lenné 1996), Leucaena (Boa and Lenné 1995), Erythrina (Westley and Powell 1993) and Sesbania (Murphy 1990). There are some important tree legume species with little information available (Lenné and Boa 1994).

There are also summary tables of insects pests on tree legumes (Walter and Parry 1994) but little is known about insect and host plant relationships and other aspects of their ecology. For this reason, there is often ignorance concerning acceptable control measures. Chemical control may be the easiest approach as a range of broad spectrum insecticides are available. However, in many cases, chemical control may not be an acceptable approach as (a) chemicals are sometimes not accessible to farmers, (b) they are expensive, (c) broad sprectrum chemicals have other harmful effects, and (d) animals may consume the sprayed leaf material with detrimental results. The leucaena psyllid (Heteropsylla cubana) is the most studied insect pest. While chemical control measures are effective and biological control using predatory insects is partially effective, the most practical, the most cost-effective, and the most ecologically sound approach is the use of resistant varieties. Yet we still do not understand the mechanisms of resistance in Leucaena necessary to develop effective screening programs (Mullen et al. 1998a).

The narrow genetic base of many species is frequently reported. The condition arose due to the movement of small quantities of seed from their centres of origin and subsequent multiplication for extended use. This has lead to widespread use of similar genetic material (see section 3.5) which will expedite speedy dissemination of pest species. The most notorious example is the leucaena psyllid.

It is vital that more detailed information is assembled on the diseases and pests affecting the cultivation and productivity of tree legumes. Country and region surveys are needed to describe the location and extent of problems. The existing networks are an appropriate way to gain information on disease and insect problems currently experienced (Lenné and Boa 1994). Catalogues and manuals illustrating the key insect and disease species are required to assist field workers, not only with identification but also with formulation of control measures. Preparation of quarantine guidelines to ensure the safe movement of seed to limit the spread of pests is another priority.

Unfortunately, there is often a lack of specialist expertise to address these problems.

3.9 Management and Conservation issues

The development and improvement of tree legumes for farm use is contingent on the availability of germplasm from the Centres of Origin of species to underpin improvement programs. It is therefore imperative that the native ranges of tree legumes are protected from exploitation and over use. Some examples of the current status for selected species are given below.

According to Wickens et al. (1995), the former Acacia communities in the Sahel in North Africa and the Near East have deteriorated almost beyond recovery. This has been due principally to excessive demand for fuelwood, but also to overgrazing, and demand for more agricultural land; all driven by increasing population pressures. The result is almost irreversible. Rehabilitation of these areas will be very slow where desertification and soil movement has occurred as there is little soil seed reserve. They stress the need for low cost participatory approaches, emphasising preventative rather than remedial measures.

Stands of Faidherbia albida in Wadi Aribo in western Sudan are endangered due to indiscriminate lopping for browse by camel nomads (Wickens et al. 1995).

Due to its colonising nature, G. sepium is not under threat at the species level. But certain important provenances such as Retalhuleu in Guatemala are under serious threat from human encroachment and river erosion (Stewart et al. 1996).

The majority of Albizia species is severely depleted in their native range in Mexico and Central America. Most species remain abundant in only a few areas. Promotion of greater use of the species would assist with their in situ conservation (Hughes and Pottinger 1997).

In Leucaena, the majority of species are of no conservation concern. However, three species, L. matudae, L. magnifica and L. involucrata are rare and of strong conservation concern. There are less than 400 known individual plants of L. magnifica (Hughes 1998a).

Prosopis africana is seriously threatened in the semi-arid lowlands of West Africa in Burkina Faso, Niger, Mali and Senegal. ICRAF has organized seed collections of this species to capture the genetic diversity before invaluable genetic resources are lost (Tchoundjeu et al. 1998). Patterns of genetic diversity in populations of Calliandra calothyrsus, distributed throughout Central America and southern Mexico, were examined using isozyme analysis. Four distinct population groups were identified and their conservation status was reported by Chamberlain (1998).

There are various methods used to conserve genetic resources. Hughes (1998) discusses the merits of in situ (maintenance of natural population), ex situ (e.g. germlasm banks and botanic gardens) and circa situm (maintenance while in agricultural use e.g. as hedge row) conservation. In vitro techniques for conservation and multiplication of germplasm, and elimination of disease, have been applied to the conservation of Leucaena leucocephala, Erythrina brucei and Sesbania sesban by Ruredzo and Hanson (1988). Perhaps a combination of all approaches may be necessary. As with taxonomic studies, it is now exceedingly difficult to obtain support for conservation of undomesticated genetic resources in their native range.

3.10 Weediness

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 (Box 3.8),
  • 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).

Species may also become a weed in their own environment. Albizia tomentosa is a weed in disturbed areas in Mexico (Hughes and Pottinger 1997) and Acacia aneura is often weedy in southwest Queensland when poorly managed (Beale 1994).

Free movement of seed in international R & D activities may result in related taxa being planted in close proximity leading to natural hybridisation perhaps increasing weed risk (Hughes 1998a and b). This appears to be supported by molecular studies of genome structure, which indicate that gene exchange between cultivated plants and wild and weedy relatives is often considerable (Nouaille 1992).

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. But this is an unrealistic constraint on farming systems and indeed the environment (see Box 1.1). Forage tree legumes are already major contributors to our farming systems. They have the potential to become even more important in our livestock industries thus enhancing the quality of life of rural communities. However, it will be imperative to pursue our objectives actively, but with environmental responsibility. The key is to carefully evaluate the level of risk, reject high risk introductions, and carefully manage introductions to minimise the chances of weed outbreak.

When introducing new species to an environment it may be necessary to first:-

  • Assess seed production and longevity,
  • Assess seed dispersial mechanisms,
  • Review susceptibility of seedlings and trees to grazing (thorns, toxins, anti-palatability will reduce animal access)
  • Study climatic and soil characteristics to predict potential areas susceptible to invasion,
  • Study insect predators and pathogens in native range,
  • Evaluate methods of control e.g. pests, fire, chemical and mechanical methods,
  • After introduction, install long-term monitoring and rapid action systems
  • Ensure that farmers have support to make full use of MPT introductions
Other approaches should also be used. For example, to minimise the weed risk status of Leucaena, it will be necessary to (a) educate farmers to achieve good grazing management to minimise seed production and to prevent seedling growth; (b) spray or slash isolated seedlings; (c) introduce less seedy varieties, including the KX2 hybrid; (d) develop sterile hybrids which will eliminate risk; and (e) utilise biological control methods such as the leucaena bruchid beetles (Acanthosceloides macrophthalmus) which greatly reduces the amount of viable seed produced. A number of these strategies can be combined to reduce the weed risk of this species.

Perhaps the most important step is to ensure that the rural community adopting the new species have the tools to make full use of the MPT. There are many examples of apparent weediness occurring because villagers may be unaware of the many uses of new plants.

Nevertheless, tree legumes should not be introduced where risk is high, or where nearby disturbed vegetation might be ecologically threatened.

3.11 Integrating forage tree legumes into farming systems and farmer uptake

Examples of successful adoption of exotic and indigenous tree legumes, for multi-purpose uses including forage, are too numerous to list. Outstanding examples are Leucaena leucocephala in Australia (Middleton et al. 1995) and Asia (Moog et al. 1998), Gliricidia sepium in southeast Asia (Stewart 1996), Sesbania grandiflora in Indonesia (Gutteridge 1994b), Calliandra calothyrsus in Indonesia (Palmer et al. 1994), and Acacia spp. in Africa (Wickens et al. 1995), to mention a selected few (see Boxes 3.2 to 3.5).

Nevertheless, despite high levels of promotion, farmer uptake has been lower than anticipated . Recent attempts to achieve adoption of new varieties and agroforestry packages, and more complex agroforestry packages such as alley cropping, have been only partially successful, and in some cases unsuccessful (Gutteridge 1998). Difficulties in achieving high levels of adoption for Leucaena are reported for Africa (Dzowela et al. 1998), South America (Argel et al. 1998) and Asia (Moog et al. 1998).

Although the forage bank concept has been shown to be feasible in tropical Africa, rate of adoption has been low due to socio-economic constraints such as land tenure insecurity and lack of infrastructure support (Cromwell et al. 1996).

The value of alley cropping especially, is hotly debated in Africa (Cromwell et al. 1996). Research groups have become cautious about the sustainable benefits of the system and in particular the value of the mulch in terms of increased crop yields on farm. Part of the problem has been identified by Grist et al. (1999) who found, using an alley cropping model, that while the use of Gliricidia in Imperata grasslands can increase soil fertility, farmers were likely to incur a loss in the first year of development, and that it would take approx. 4 years to begin making a profit.

There are many reasons put forward for the lower than anticipated levels of adoption (Smith 1992, Cromwell et al. 1996) including :-.

(a) Projects usually assume which species and agroforestry system are required by farmers. In reality, reasons for farmer choices are often complex and determined by their specific needs and resource constraints. There needs to be a range of options available to meet their various needs.

(b) Projects may achieve short-term success due to farmer trust in outside interventions, but not long-term success, as farmers do not have the opportunity to observe such benefits before the end of the life of a project.

(c) Farmers do not conceptualise the multi-functions of MPTs the way researchers do. The attributes that farmers appreciate need to be better understood.

(d) It may be difficult to achieve environmental and conservation benefits at the macro community level, whilst at the same time provide tangible benefits to individual farmers. For instance, for Prosopis, Dutton (1992) suggests that the benefits must involve:- Ecological sustainability (improved soil fertility, control of soil erosion, reduced contamination of water resources, improved self-sufficiency for on-farm energy, reduced emmissions of greenhouse gases); sociological sustainability (changed attitudes to management of species, employment opportunites, improved self-reliance); and, in addition, target groups need to benefit from their labour through value added product and cash income.

(e) MPT planting material is often not distributed to farmers in a form that they are familiar with e.g. seeds distributed where farmers normally use seedlings. Farmers are often not clear as to how to manage planting material once they received it.

(f) Planting material is initially distributed by a project, but post-project, farmers are unable to obtain further supplies. There is very limited evidence of active seed suppliers, especially community based supply mechanisms. Seed multiplication orchards producing planting material of the elite varieties are required.

On the other side of the ledger, many suggestions have been made to increase levels of adoption:-
(a) The priority is for participatory on-farm testing of new varieties and systems. It is vital that farmers are brought more directly into the decision making process to ensure that social constraints (risks, relevance, labour, environment) and economic constraints (incentives, markets and returns) are adequately addressed. It is important that exotic species fit into their existing year round feeding systems and are sustainable and persistent under regular use.

(b) Simons (1996) suggested that substantial advantage in woody and leaf biomass yields is needed to interest farmers in new varieties. He suggested that farmers may not be prepared to buy improved germplasm of low value crops as they can use existing material for free. New germplasm would have to be markedly superior, as is the case with the KX2 Leucaena hybrid being introduced into the Philippines.

(c) The use of farmer groups to expedite training visits to demonstration sites and other successful farmers, has been a useful approach (Larsen et al. 1998).

(d) A collaborative approach of scientists, agroforesters, foresters, extension workers and farmers (animal and plant persons) is necessary.

(e) Cook et al. (1989) stressed the importance of understanding the economics of agroforestry systems from the farmer's point of view as well as from the broader perspective of the benefits to society. Project implementation should therefore take into account local markets and opportunities for off-farm employment offered by tree products, as well as the opportunity costs perceived by farmers in making adoption decisions. A full cost-benefit analysis of new agroforestry systems is essential. There is often a lack of information on the economics and long-term benefits of new systems.

(f) The importance of communication / training / extension and research networks needs to be stressed. Accessing existing networks and databases will help to achieve this objective. Adequate training of specialists and technicians in all aspects of the management and use of tree legumes is important (Dutton 1992).

Without improved levels of adoption, and more explicit demonstration of the relevance and benefits of forage tree legumes, the good will and support of funding and donor agencies will be limited.

4. 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 3. Most used tree legume species for forage purposes
(key references in parenthesis)
Higher quality species
Lower quality species
  • Albizia lebbeck (Lowry 1989)
  • Chamaecytisus palmensis (Snook 1982)
  • Cratylia argentea (Argel and Lascano 1998)
  • Desmodium rensonii (Djojo et al. 1995)
  • Desmanthus virgatus (Gutteridge 1994a)
  • Gliricidia sepium (Stewart et al. 1996)
  • Leucaena leucocephala (Shelton et al. 1998)
  • Leucaena diversifolia (Shelton et al. 1998)
  • Sesbania grandiflora (Gutteridge and Rekib 1995)
  • Sesbania sesban (Gutteridge and Rekib 1995)
  • Acacia aneura (Beale 1994) *
  • Acacia nilotica (Carter 1994)
  • Acacia tortilis (Wickens et al. 1995) *
  • Albizia chinensis (Zabala 1997)
  • Albizia saman (Roshetko 1995)
  • Calliandra calothyrsus (Evans 1996)
  • Erythrina spp. (Westley and Powell 1993)
  • Faidherbia albida (Wickens et al. 1995) *
  • Flemingia macrophylla (Gutteridge 1994a)
  • Prosopis juliflora (Dutton 1992)
* 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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