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The role and potential of forage legumes in alley cropping, live mulch and rotation systems in humid and subhumid tropical Africa


Abstract
Introduction
Alley cropping
Live and in situ mulch systems
Conclusions
References

K. Mulongoy and B.T. Kang
International Institute of Tropical Agriculture
PMB 5320, Ibadan, Nigeria

Abstract

Fragile, low activity clay soils, which are prone to erosion and are characterised by inherently low fertility, are common in humid and subhumid tropical Africa. On these soils, forage legumes play an important role in developing sustainable and low input crop production systems. Woody and herbaceous species such as Leucaena leucocephala, Gliricidia sepium, Flemingia congests and Sesbania rostrata have shown good potential for inclusion in alley cropping systems. Mucuna pruriens var utilis is one of the most promising sources of in situ mulch in small- and large-scale crop production. Live mulches of Psophocarpus palustris and Centrosema pubescens smother weeds effectively and have been shown to sustain high maize yields with little fertilizer N input. Despite the encouraging results obtained with forage legumes on less acid soils, further research is needed to select species that can be included in low-input crop production systems on strongly acid soils.

Introduction

Although a wide range of soil types are found in the humid and subhumid regions of tropical Africa, highly weathered and low activity clay (LAC) soils (Ultisols, Oxisols and Alfisols) are the most important groups used for upland crop production (Kang and Juo 1981). According to Dudal (1980), Ultisols make up 46.7% and Alfisols 12.5% of the land area in humid tropical Africa, while in the subhumid zone Ultisols and Oxisols comprise 25.6% and Alfisols 27.4% of the land area. Some of the major management problems with using these LAC soils for food crop production in the humid and subhumid regions of tropical Africa were reviewed by Hartmans (1981) and Kang and Juo (1981).

Ultisols and Oxisols are strongly acid and leached soils, occuring mostly in areas with perudic and udic moisture regimes. These soils are characterised by low cation exchange capacity (CEC), very low inherent fertility, multiple nutrient (N. P. K, Ca, Mg, Zn) deficiencies and nutrient imbalances. Toxic levels of Al or Mn or both are among the main chemical constraints to growing crops on these strongly acid soils. Alfisols, which have high base saturation and are less acid, are characterised by inherently low nutrient (N. P. K, S and Zn) status and low structural stability. They are therefore prone to erosion and compaction. Maintaining soil fertility and controlling soil erosion and compaction are thus major management problems in using these soils for crop production.

The traditional agriculture that is widely practiced on the LAC soils is changing due to rapid increases both in human population and in the rate of urbanisation. These changes have resulted in the shortening of fallow periods, which are needed to restore and maintain soil productivity and to control weeds. Consequently, crop yields have declined in tropical Africa despite the introduction of improved cultivars. There is therefore a need to identify soil and crop management practices that can maintain soil fertility under more intensive land use. In recent years, interest has increased in the inclusion of herbaceous and woody forage legumes in crop production systems as sources of mulch, green manure and supplementary browse, to develop sustainable and low-input production systems (Brewbaker et al. 1982: Sumberg 1984: Kang and Duquma, 1985). The role of forage legumes in alley cropping, live mulch and in situ mulch systems will be discussed in this paper.

Alley cropping

Concept

In the humid and subhumid tropics, tree crop production, whether in plantations, on smallholdings or in compound farming has generally been most successful on LAC soils, as tree crops are ecologically suited to the environment and cause little or no damage to soil. Traditional farmers retain certain trees and shrubs in their crop production systems to restore soil fertility exhausted by cropping (Moorman and Greenland, 1980; Getahun et al, 1982). In efforts to replace or improve the traditional bush-fallow cultivation, investigations have been carried out on alley cropping, an agroforestry system in which arable crops are grown between rows of woody shrub or tree fallows (Kang et al, 1981; Wilson and Kang, 1981). The shrubs or trees are pruned periodically during the cropping season to prevent shading and to provide green manure for the companion food crop. Thus, the trees and shrubs grown in the hedgerows recycle nutrients and provide green manure, firewood and staking material, as is observed in the bush-fallow system. Alley cropping has the advantage over the traditional system of combining the cropping and fallow phases.

Forage legume species for alley cropping

Kang et al (1984) reviewed the important characteristics of trees and shrubs suitable for alley cropping. Legumes that meet most of the required characteristics for the humid and subhumid tropics include species of the genera Albizia, Calliandra, Cassia, Flemingia, Gliricidia, Inga, Leucaena, and Sesbania. Information about some of these fast-growing, nitrogen-fixing trees has been assembled by the National Academy of Sciences (NAS, 1980; 1983).

Among the Albizia species, A. lebbek is the most commonly grown; it can be pruned continuously and provides green material even in the dry season (Whyte el al, 1953). Many Cassia species are unpalatable and toxic, but some, e.g. C. siamea, are used for fodder in some tropical countries. C. siamea is commonly used as hedges in Nigeria and Zaire, and has also been recommended as a shade tree and for soil improvement in Rwanda (Behmel and Neumann, 1982). Yamoah (1985) observed that C. siamea suppressed weed growth and increase maize yields more than Gliricidia sepium and Flemingia congesta in alley cropping with maize on an Alfisol.

Sesbania species are herbaceous shrubs or small trees, used mainly for green manure. Sesbania grandiflora, S. sesban (Skermann, 1977), S. aculeata (syn. S. cannabina or S. bispinosa) and, in Mali, S. rostrata (T. Traora, personal communication), are also used as fodder. They have shallow roots and may compete for nutrients and moisture when grown with food crops. Sesbania rostrata is an annual shrub that grows in hydromorphic soils in the Sahel region of West Africa (Berhaut, 1976). It is unique in its profuse stem nodulation. Its beneficial contribution as green manure in rotation with rice grown in microplots in Senegal has been reported by Rinaudo et al (1983). It is also a potential N source for alley cropping with hydromorphic rice (Mulongoy, 1985). Tests done at IITA show that two prunings of S. rostrata planted at 10 cm x 200 cm spacing yielded 3 t DM/ha, which can contribute 70 kg N/ha during the rice cropping season. Sesbania grandiflora has also been tested in alley cropping at IITA but with little success. This species is severely damaged by insects in the area and cannot stand repeated pruning.

Leucaena leucocephala and Gliricidia sepium have been widely tested in alley-cropping systems, and both species are suitable for alley cropping on the less acid Alfisols and associated soils. Their prunings can be used for mulch, green manure and supplementary high quality browse for small ruminants, particularly during the dry season (Sumberg, 1984). On acid soils (pH <5.0), however, both species grow poorly and require liming and phosphate fertilization (Duguma, 1982). There is therefore a need to breed and select woody forage legumes species that grow well with low inputs on acid soils.

Hutton (1982) reported several promising interspecies crosses of leucaena for acid soils. Recent observations in eastern Nigeria showed that Flemingia congesta performs well in alley-cropping systems on acid soils (IITA, 1983). The junior author observed that in Yurimaguas in Peru, Inga edulis and Erythrina species have good potential for alley cropping on acid Ultisols. The fodder value of these species still needs to be determined.

For rapid establishment, these woody leguminous forage species sometimes require inoculation with rhizobium. Leucaea nodulates effectively with only a narrow range of Rhizobium strains (Date and Halliday, 1980). To ensure rapid establishment and N fixation, inoculation with the appropriate rhizobia is therefore recommended. Munns and Mosse (1980) also reported a significant effect of inoculation with mycorrhiza on the use of phosphate, growth, and nodulation of leucaena in pot trials using Alfisols from southern Nigeria. The importance of mycorrhizal inoculation under field conditions still needs to be assessed.

Nutrient contribution from leguminous trees and shrubs grown in hedgerows

Some promising results have been obtained in using Leucaena leucocephala and Gliricidia sepium in alley cropping with various food crops on the less acid Alfisols (Kang et al, 1985). Observations in southern Nigeria indicate that five annual prunings of leucaena and gliricidia grown in hedgerows spaced 4 m apart yielded over 140 and 230 kg N/ha per year, respectively (Kang and Duguma 1985). Despite the high N yield, supplemental N application was still required to obtain high maize yields (Kang et al, 1981). This was due in part to low efficiency of N in the prunings, as was also reported by Guevarra (1976). It is estimated that prunings from leucaena hedgerows spaced 4 m apart contribute 40 to 60 kg N/ha to the associated maize crop. Besides N. leucaena also recycles large quantities of other nutrients. In one trial conducted on an Alfisol in southern Nigeria, nutrient yields in leucaena prunings amounted to 10.6 kg P. 153.4 kg K, 73.7 kg Ca and 15.9 kg Mg/ha.

The long-term cumulative effect illustrated in Figure 1 is very important in alley-cropping systems. Higher maize yields in the long-term alley-cropping plot as compared to the control plot cannot be attributed solely to the N contribution of the prunings but reflect a general improvement in soil conditions.

Figure 1. Grain yield of maize variety TZPB grown on Psammentic ustorthent as affected by nitrogen application and 6 years of alley cropping with Leucaena leucocephala (Kang and Duguma, 1985).

Live and in situ mulch systems

Concept

Most tropical soils are inherently low in nutrients and are prone to erosion, especially after deforestation and subsequent cultivation with conventional mechanical tillage (Hartmans et al, 1982). Also, traditional farmers in the tropics spend 40 to 60% of their labour time on weeding. Hand or mechanical weeding causes structural deterioration of surface soil and exposes the soil to erosion. Chemical weed control is usually too expensive for these small farmers.

Although mulch farming has proven to be effective in controlling soil erosion and in improving soil properties, resource-poor farmers have always been reluctant to adopt conventional mulching, because of the large amount of labour needed to gather, transport and apply the mulch. The search for inexpensive and more attractive mulching methods has led to the development of in situ and live mulch.

The term in situ mulch refers to the residues of dead or chemically killed cover crops which are used on the same land on which they were grown (Wilson, 1978). The land is usually left for at least one cropping season under the cover crop to produce the mulch (Hartmans et al, 1982).

The live-mulch system is a crop production technique in which food crops are planted directly in a low-growing cover crop with minimum soil disturbance (Akobundu, 1980). This practice is based on the concept of using cover crops in tree plantations. Its main advantage is that it smothers weeds, and it can play an important role in soil conservation and maintenance of soil fertility.

Forage legumes for in situ and live mulches

Leguminous cover crops are often used for in situ and live mulches because of their ability to fix atmospheric N when they are effectively modulated by rhizobia. The high forage value of these legumes may make them more attractive to farmers as part of their livestock and crop production system (Botton, 1958).

The choice of legumes for live and in situ mulch systems is based on several characteristics. They should be easy to establish, produce sufficient biomass in a short time with little or no fertilizer inputs and nodulate effectively with indigenous rhizobia. Species that grow rapidly and vigorously will be able to smother weeds effectively. They should not increase pest and disease incidence and should be easy to eradicate by inexpensive means. For the live-mulch system, they should be perennial and should not compete with the associated crops.

Various forage legumes possess a number of these attributes. A few species were tested for in situ and live mulch on Alfisols at the IITA main station in Ibadan (Okigbo and Lal, 1977; Lal et al, 1979; Akobundu, 1980; Wilson et al, 1982; Akobundu and Okigbo, 1984). Results of live-mulch trials in which maize, cowpea and rice were grown with Arachis prostrate, Indigofera spicata, Centrosema pubescent and Psophocarpus palustris, showed that growing maize in live mulches of the last two legumes was the most promising system. The climbing habit of P. palustris and C. pubescens can be controlled by spraying with the growth hormone, CGA 47283, at 2 kg a.i./ha (Akobundu, 1980). Indigofera spicata, Pueraria phaseoloides (Kudzu), Stylosanthes guianensis and Mucuna pruriens var utilis (syn. Stizolobium deeringianum or velvet bean) have been found to be good sources of dead mulch for minimum-tillage systems. Pueraria is adapted to low altitudes and was widely tested as a cover crop and fodder plant before the 1960s in Central Africa and Liberia. Pueraria interplanted in rice fields in Sierra Leone suppressed weeds, increased rice yields and provided fodder after the rice had been harvested (Whyte et al, 1953). Mucuna has been studied widely in Nigeria, Zaire and Zimbabwe for green manure and temporary pastures (Falkner, 1934; Whyte et al, 1953; 1979).

There are a number of other forage legumes that may have potential as live in situ mulch, such as Centrosema plumieri, Desmodium ascenders, D. canum, D. scopirus, Calopogonium mucunoides and Mimosa pudica. The characteristics of these species have been described by Whyte et al (1953), Botton (1958), Skerman (1977) and Burt et al (1983).

Adapted cover crops usually nodulate freely in tropical soils. Some of them, such as Centrosema, Stylosanthes and Desmodium species require inoculation with specific rhizobia for proper nodulation and N fixation (Skerman 1977; Date and Halliday, 1980). In acid soils, legumes generally nodulate less, due to poor survival of the rhizobia and failure of tolerant strains, even if they are abundant, to infect the root hair. The acidity can be corrected with liming if lime is available. Legumes also have a high P requirement for nodule development and optimum plant growth. Plants growing in association with mycorrhiza use native and added P more effectively than plants without mycorrhizae. Many of the tropical legumes are dependent on the presence of mycorrhizae for P uptake (Mosse, 1981). Nodulation and symbiotic nitrogen fixation of some species are even contingent on mycorrhizal infection. Crush (1974) showed that Centrosema sp. seedlings nodulated only when infected with mycorrhizae or when P was applied. Stylosanthes grown in several tropical soils failed to grow adequately or to nodulate without mycorrhizal infection even when P was applied (Mosse et al, 1976).

Soil restorative and protective value of in situ and live mulches

The soil restorative and protective value of organic mulches is well known (Lan, 1984). Scientists working in the tropics showed early interest in using leguminous cover crops for soil restoration (Faulkner, 1934). Results of a series of continuous-cropping experiments using leguminous cover crops, particularly Mucuna pruriens var utilis, conducted in Ibadan, Nigeria, from 1922 to 1951, showed that mucuna improved soil properties but that its effect was not long lasting. Studies by Lal et al (1978) and Wilson et al (1982) also showed improvement in soil physio-chemical properties and biological activities, as measured by earthworm cast production, under Stylosanthes guianensis, Centrosema pubescent, Pueraria phaseoloides and Mucuna pruriens grown for only a short period, as compared with natural fallow. The cover crop improved soil bulk density and soil moisture retention and gave better protection against erosion. We also observed higher microbial biomass under Psophocarpus palustris live mulch than in bare plots (Table 1).

Table 1. Some chemical and microbiological characteristics of the soils (0-10 cm) under P. palustris live mulch on land that had been continuously cropped to maize for five seasons.

Soil characteristics

Live mulch

Bare soil

Casts

Soil

pH

6.1

5.5

5.6

organic C, %

3.36

1.80

1.73

Total nitrogen, %

0.51

0.21

0.18

Bray-1 P. ppm

30.5

5.3

9.6

Ca2+, meq/100g

12.7

2.5

1.4

Mg2+, meq/100g

3.2

0.4

0.2

Mn2+, meq/100g

0.08

0.04

0.06

K+, meq/100g

1.0

0.2

0.1

Na+, meq/100g

0.24

0.19

0.16

CEC, meq/100g

17.3

3.54

2.2

Microbial biomass C,meq/100g

44.7

19.4

17.6

Source: K Mulongoy (unpublished data).

Nitrogen fixation and nitrogen contribution

Forage legumes can fix atmospheric N when they are effectively nodulated, and thus can contribute to the N economy of cropping systems. This potential is currently still underutilised, particularly in the tropics. Little information is available on N fixation by forage legumes in the field in tropical Africa. On low-N soils, the amount of N fixed is closely correlated with legume dry-matter production (Skerman, 1977; Vallis et al, 1983). Annual dry-matter yields of forage legumes that are useful in live and in situ mulch systems range between 1500 and 7500 kg/ha in Africa (Skerman, 1977; Mulongony and Akobundu, 1985), with N yields ranging from 30 to 300 kg/ha per year.

A number of reports have indicated that forage legumes increase the organic matter and N contents of the soil (Tables 1 and 2). Most of these studies dealt with legume-grass mixtures. Observations by Lal et al (1979) and Wilson et al (1982) showed small increases in soil N content ranging from 0.01 to 0.06% over a 2-year period.

Table 2. Some physio-chemical characteristics of the soils (0-10 cm) after 2 years of cover crops.

Cover crop

Bulk density

Organic

Total

Bray-1 P

CEC

(g/cm)

C(%)

N(%)

(ppm)

(meq/100 g

Centrosema pubescent

1. 33

1.53

0.18

33

10.0

Mucuna utilis

1.33

1.57

0.21

35

10.5

Psophocarpus palustris

1.14

1.57

0.20

37

10.9

Pueraria phaseolodids

1. 32

1.50

0.17

24

7.7

Stylosanthes gracilis

1.33

1.63

0.21

38

8.8

Control (bare soil)

1.42

1.37

0.17

19

8.4

LSD (0.05)

0.04

0.23

0.03

22

3.5

Source: Lal et al (1979).

Mulongoy and Akobundu (1985) studied nitrogen uptake by maize grown in live mulches of Psophocarpus palustris, Centrosema pubescens and Arachis repens. In newly established live-mulch plots, the cover crops and maize competed for N. Poor nodulation and N fixation, aggravated by application of a growth retardant (CGA 47283) used to reduce the climbing tendency of the live mulch, corroborated this observation. Positive N contributions (46, 48 and 2 kg N/ha for the three legumes respectively) were obtained in the fifth cropping season i.e. 2.5 years after establishment of the live mulches and continuous cropping with maize. The positive N contribution may have been due partly to the accumulation of organic matter under the mulch.

Effects on nematodes

Plant parasitic mematodes are among the major soil-borne plant pests that are suppressed effectively by the bush-fallow system (Wilson and Caveness, 1980). Forage legumes show a range of susceptibility to species of nematodes. Some of them are not only resistant but create conditions adverse to the pest and, therefore, could control populations of parasitic nematodes when they are used in live or in situ mulch systems. For instance, the population of Meloidogyne incognita, which is not considered as a serious maize pest, increased under Psophocarpus palustris but the number of Helicotylenchus pseudorobustus, the spiral nematode which is harmful to maize, decreased (Hartmans et al, 1982). Such beneficial interactions between the food crop and the legume justify more research on the influence of in situ and live mulches on certain plant pests.

Effects on weeds

Forage legumes, particularly prostrate types such as Centrosema pubescens, Pueraria phaseoloides and Mucuna pruriens, compete with and smother weeds successfully when well established. Research by INEAC in Zaire also has indicated the advantage of using leguminous cover crops such as Pueraria javanica, Stylosanthes guianensis and Calopogonium mucunoides to eradicate Imperata cylindrica in the savannah region (Jurion and Heneray, 1969). At IITA main station, Akobundu and Okigbo (1984) tested several ground-cover crops over a 2-year period to determine their effects on weed competition and maize yield. Maize was planted directly into already established cover crops. Weed infestation was heaviest in Desmodium triflorum, Indigofera spicata and in no-tillage control plots; was moderate in Arachis repens and maize-stover plots; and very low in Centrosema pubescens and Psophocarpus palustris plots. Good maize yields were obtained where weed competition was minimised by the legume cover crop. It is important to note that the thick mulch obtained with some cover crops such as P. phaseoloides can seriously reduce the maize stand, especially with mechanical planting. Martin and Touchton (1982) reported a similar effect on sorghum grown in a legume cover crop.

Effects on crop yield

The reason for using leguminous cover crops in rotation and intercropping systems is to obtain sustained and high crop yields with minimum N fertilizer input.

Earlier work with cover crops at Moor Plantation in Ibadan, Nigeria, from 1922 to 1951 (Faulkner, 1934; Vine 1953) showed that, though the cover crops have no long lasting effect, inclusion of Mucuna pruriens in the rotation system, supplemented with low fertilizer rates, could maintain adequate maize yields. Faulkner (1934) estimated that the increase in yield of a maize crop following a Mucuna crop was in the order of 700 to 900 kg grain/ha. Kannegieter (1966) also reported increases in crop yield in Ghana following a short-term fallow on which Pueraria phaseoloides was grown.

Agboola and Faysmi (1971) observed that intercropping maize with M. pruriens reduced maize yield but intercropping with Calopogonium mucunoides did not affect the maize yield. The same authors (Agboola and Fayemi, 1972) also showed that C. mucunoides intercropped with maize fixed 370 kg N/ha but did not benefit the early corn crop. However, it was of benefit as a green manure for the late cropping season.

Wilson (1978) reported increased yield and improved quality of tomatoes grown with an in situ mulch of P. phaseoloides. Lal et al (1978) also observed that, after 2 years, in situ mulches of Centrosema pubescens, P. phaseoloides and Stylosanthes guianensis increased the yields of cowpea, soya bean, maize and cassava. Among the leguminous cover crops evaluated on Alfisols and associated soils at IITA for in situ mulch, M. pruriens was the most promising for both large- and small-scale crop production. In areas with a pronounced dry season of longer than 2 months, the plant dies naturally, leaving 4 to 5 t of dry mulch/ha, suitable for no-till planting. To control volunteers, spraying with low doses of herbicides may be necessary.

As live mulch, C. mucunoides gave increases in maize yield equivalent to applying 55 kg fertilizer N/ha (Agboola, 1980). Akobundu (1980) also observed that after five seasons of continuous cropping, maize yields were higher from plots with a live mulch of Psophocarpus palustris and Centrosema pubescens than from bare plots under minimum or conventional tillage. Maize in the live-mulch plots showed little or no N response, whereas in the no-tillage system, maize responded to application of more than 60 kg N/ha. It thus appeared that leguminous live mulches contributed to the N needs of the maize crop. Although good results were obtained on Alfisols, Faulkner (1934) reported disappointing results on strongly acid Ultisols in southeastern Nigeria, mainly due to poor establishment of the cover crops.

Conclusions

Some research has been done into developing planted fallows which restore soil fertility sooner and are easier to manage than natural fallows. Paramount to this research is the development of stable, viable and environmentally sound low-input production systems on the fragile LAC soils. Research results thus far have shown that, on LAC soils in the humid and subhumid tropics, fallows have to be included in the farming systems in order to protect the soils and maintain satisfactory productivity. This can vary from one season fallow and use of a cover crop (such as Mucuna) to many years of managed fallows such as alley cropping and live-mulch systems that are economically productive (e.g. leucaena stakes, browse etc). Forage legumes can also reduce the need for N fertilizer in the production system and assist in nutrient recycling. Alley cropping and in situ mulch systems are ready for testing by farmers. More cooperative work, as is currently done between IITA and ILCA in alley farming research, also needs to be carried out on the use of some of these leguminous forage crops for animal production. One important area that needs more research is the selection leguminous species for use on planted fallows and as forage that will perform well on acid soils with low chemical inputs.

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