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5.2 Forage Tree Legumes in Alley Cropping Systems

B.T. Kang and R.C. Gutteridge


Introduction
Alley Cropping
Benefits of Alley Cropping
Crop Performance
Effect of Alley Cropping on Soil Properties
Effect of Alley Cropping on Soil Erosion
Effect of Alley Cropping on Weed Suppression
Management of Alley Cropping Systems
Conclusions
References


Introduction

One of the greatest challenges facing agriculture in the tropics is the need to develop viable farming systems for the rainfed uplands that are capable of ensuring increased and sustained crop production with minimum degradation of the non-renewable soil resource base.

Much of the agricultural land in the humid tropics is currently Used for traditional farming based on the bush fallow. This is a low productivity but biologically stable system with long fallow periods that can sustain agricultural production for many generations (Kang and Wilson 1987). However, in many regions, shortening or abolition of the fallow period has resulted in increased land degradation, invasion by weeds and substantial crop yield decline. The use of fertiliser inputs alone has largely been ineffective in overcoming these problems (Lal and Greenland 1986), and there is a need to develop an integrated soil fertility management approach to address these issues.

The incorporation of woody species into crop production systems is one option that has received significant attention in recent years (Kang et al. 1990).

Alley Cropping

Alley cropping or hedgerow intercropping is an agroforestry practice in which perennial, preferably leguminous trees or shrubs are grown simultaneously with an arable crop. The trees, managed as hedgerows, are grown in wide rows and the crop is planted in the interspace or 'alley' between the tree rows (Figure 5.2.1). During the cropping phase the trees are pruned and the prunings used as green manure or mulch on the crop to improve the organic matter status of the soil and to provide nutrients, particularly nitrogen, to the crop. The hedgerows are allowed to grow freely to shade the inter-rows when there are no crops. Alley cropping retains the basic restorative attributes of the bush fallow through nutrient recycling, fertility regeneration and weed suppression and combines these with arable cropping so that all processes occur concurrently on the same land, allowing the farmer to crop the land for an extended period.

Fig. 5.2.1. Alley cropping of maize and Leucaena leucocephala in experimental plots.

Benefits of Alley Cropping

Some of the beneficial effects that have been claimed for alley cropping include:

· improved crop performance due to the addition of nutrients and organic matter to the soil/plant system,

· a reduction of the use of chemical fertilisers,

· an improvement in the physical nature of the soil environment. The addition of mulch can lower soil temperatures, reduce evaporation, and improve soil fauna activity and soil structure resulting in better infiltration, reduced runoff and improved water use efficiency,

· on sloping land, the tree rows act as a physical barrier to soil and water movement, resulting in significant reductions in erosion losses (Paningbatan et al. 1989),

· the provision of additional products such as forage, firewood or stakes when a multipurpose tree legume is used as the hedgerow, and

· an improvement in weed control. During the fallow period shading of the interspaces may reduce weed growth, while in the cropping phase, the mulch may inhibit germination and establishment of weeds (Ssekabembe 1985).

Crop Performance

Experimental evidence supporting claims of beneficial effects of alley cropping is provided by a number of studies conducted largely in humid or subhumid regions on high base status soils. Kang et al. (1981) increased maize grain yields from 1.9 t/ha in unfertilised control plots to 3.5 t/ha in plots mulched with Leucaena leucocephala (leucaena) from 4 m wide rows. A similar magnitude of response was obtained by Dofeliz and Nesbitt (1984) in the Philippines again with leucaena at 4 m row spacings.

In the third year of cropping, a Gliricidia sepium (gliricidia) alley system on a degraded alfisol soil in Nigeria gave 2.42 t/ha of maize while control plots yielded 1.74 t/ha (Atta-Krah and Sumberg 1988).

B.T. Kang (unpublished data) has demonstrated the long-term yield sustainability of alley cropping in trials conducted over 10 years in southern Nigeria (Figure 5.2.2). With additional inputs of N. P. K fertiliser, maize yields in the leucaena alley cropping plots were maintained at an average of 3.5 t/ha while in fertilised control plots yields fell to 2 t/ha.

Although the results from alley cropping in humid regions on high base status soils have been quite positive, the performance of the system in other agroecological zones has been less encouraging. In the semiarid lowland tropics, Singh et al. (1989) reported that the yields of castor, cowpea and sorghum alley cropped with leucaena hedgerows spaced at 10 m for a period of 4 years were lower than in the control treatment. Yield declined from 30 to 150% of the sole crop yield as the distance from the hedgerows declined from 5 m to 0.3 m. These authors attributed much of the yield decline to severe moisture competition. In Peru, Szott (1987) conducted alley cropping trials using Inga edulis and Cajanus cajan on a Typic paleudult. Yields of alley cropped cowpea, maize and rice were extremely low and were equal to or less than that of the control treatment. Evensen and Yost (1990) initially reported positive results from the alley cropping of upland rice and cowpea with rows of Paraserianthes falcataria on a Tropeptic haplorthox in west Sumatra, Indonesia, particularly with addition of a low rate of lime. However, yields declined after 4 years and were restored only after fertiliser input was increased.

Fig. 5.2.2. Effect of alley cropping with L. leucocephala on grain yield of maize grown on degraded Oxic paleustalf compared with control (no tree) treatment in Nigeria (fertiliser rate, years 1-3, 90N, 40P, 40K; years 4-10, 45N, 12P, 25K kg/ha) (B.T. Kang, unpublished).

Effect of Alley Cropping on Soil Properties

An important benefit of alley cropping is the addition of large amounts of organic materials from the prunings as mulch or green manure (Table 5.2.1) which can have favourable effects on soil physical and chemical properties, on microbiological activity and hence on soil productivity. Several studies (Kang et al. 1985, Lal 1989b, Kang and Ghuman 1991) have demonstrated significant positive effects of alley cropping on soil fertility parameters such as organic C levels, total N and extractable P levels over a range of climatic and soil conditions. The magnitude of these effects, however, varied with hedgerow species and management as this influenced the quantity and quality of prunings. Factors such as C:N ratio, lignin and polyphenol contents influence the decomposition rate of the mulch, the subsequent release of nutrients and their uptake by the crop. Gutteridge (1990) showed that mulches from Sesbania sesban, gliricidia and leucaena were effective sources of N for maize growth while those from Calliandra calothyrsus, Acacia cunninghamii and A. fimbriata were ineffective in the short term. This may have been due to the high polyphenol and/or lignin content of the latter species.

Table 5.2.1. Biomass nutrient content of leaves and twigs of two tree legumes grown on an Alfisol at Ibadan, Nigeria (duo and Kang 1989).

Species


Biomass of leaves & twigs

Nutrient content of leaves and twigs
(kg/ha/year)

(t/ha/year)

N

P

K

Ca

Mg

Leucaena leucocephala

7.4

247

19

185

98

16

Gliricidia sepium

5.5

169

11

149

66

17

Guevara (1976) found that only about 38% of N in leucaena prunings was recovered by a maize intercrop while Evensen (1984) noted, that compared with urea, mulching with leucaena leaf was only 41% as efficient in supplying N to maize.

The efficiency of utilisation of N from the prunings can often be improved by incorporation. Read (1982) found that incorporation of leucaena leaf produced a higher N uptake than when applied as a surface mulch. Evensen (1984) increased the efficiency of mulched leucaena leaf to 63% that of urea by incorporation. Kang et al. (1981) also found that incorporation improved maize yields both in the presence and without additional nitrogen fertiliser (Table 5.2.2). Use of fresh rather than dry prunings also improved the rate of N release and uptake by maize (Read 1982).

Hedgerows have the ability to recycle nutrients and although this aspect has not been widely studied, Hauser (1990) demonstrated this phenomenon in an alley cropping system with leucaena. He found higher concentrations of N. K, Ca and Mg in the surface soil than in the subsoil under the hedgerows. This was attributed to leaf litter fall and nutrient uptake by the trees from the subsoil. In the centre of the alley plots, the reverse situation occurred with lower nutrient levels in the surface soil due to crop uptake and higher levels in the subsoil due to leaching. This result shows that alley cropping can reduce the downward displacement of nutrients.

Effect of Alley Cropping on Soil Erosion

A large number of experimental results have confirmed the significant role of alley cropping in reducing runoff and soil erosion (Young 1989, Hawkins et al. 1990, Kang and Ghuman 1991). Lal (1989a) showed that erosion in plots tilled and alley cropped with gliricidia and leucaena was reduced by 73 and 83% respectively compared with a tilled control treatment. R.C. Gutteridge (unpublished data) found that rows of leucaena planted at 5 or 10 m intervals across the slope were as effective as conventional contour banks in reducing erosion on a 10% slope in southeast Queensland. In a trial lasting 3 months on a Typic tropudalf, erosion was greatly reduced by the presence of Desmanthus virgatus hedgerows spaced at 6 m intervals (Paningbatan 1990). A total of 1,424 mm of rain fell during the experimental period. Total soil loss was 127 t/ha in the control treatment, 41 t/ha with Desmanthus hedgerows and contour cultivation, and 0.2 t/ha with hedgerows, application of prunings as a mulch and zero tillage. Young (1989) attributed the beneficial effects of alley cropping in controlling soil erosion partially to the barrier effect of the hedgerows, but mainly to the presence of prunings applied as mulch.

Table 5.2.2. Grain yield of maize fertilised with nitrogen and/or leucaena prunings. Prunings were incorporated or applied as a surface mulch (after Kang et al. 1981).

Leucaena prunings (t/ha)



N rate (kg N/ha)



Leucaena prunings

Incorporated

Surface mulch

Grain yield (kg/ha)

5

0

2,313

2,013


50

3,035

2,300


100

3,435

3,020

10

0

3,213

1,855


50

2,578

2,338


100

3,068

3,023

LSD = 688 (P < 0.05)

Effect of Alley Cropping on Weed Suppression

The germination and growth of most weed species are usually stimulated by exposure to light. Thus some control of weeds may be effected if a closed canopy can be maintained during the fallow period in an alley cropping system. Anoka et al. (1991) found that the shoot biomass of Imperata cylindrica decreased by about 80% under uncut hedgerows of gliricidia and leucaena in Nigeria. Yamoah et al. (1986) also reported lower weed yields under hedgerows of Flemingia macrophylla, gliricidia and Cassia siamea when they remained uncut for 2 years.

There also appears to be a shift in weed composition following alley cropping. Siaw et al. (1991) showed a significant change towards more broadleaf weeds after alley cropping with leucaena and Dactyladenia barter) compared with the control treatment.

In most alley cropping systems, the weed suppression effect of the hedgerows is not fully exploited and further studies of the effect of different hedgerow species, fallowing and manipulation of cutting regimes may improve the effectiveness of the system in reducing weed infestation.

Management of Alley Cropping Systems

Biologically, the effectiveness of alley cropping systems depends to some extent on the soil type and agroecological zone in which the system is used but it is also very dependent on management strategies adopted. Factors such as choice of tree species, orientation, layout and manipulation of the hedgerows and crop husbandry practices are all important in determining the outcome of the alley cropping system.

Choice of tree species

The choice of tree species for alley cropping is extremely important and to a large extent determines the success or failure of the system. Rachie (1983) detailed a number of attributes which should be considered when selecting a tree species for alley cropping. These include:

· a rapid growth rate,
· ability to withstand frequent cutting,
· good coppicing ability (regrowth after cutting),
· ease of establishment from seeds or cuttings,
· nitrogen fixing capacity,
· deep-rooted with a different root distribution to the crop,
· multiple uses such as forage and firewood,
· ability to withstand environmental stresses such as drought, waterlogging, and extremes of pH,
· high leaf to stem ratio,
· small leaves or leaflets,
· dry season leaf retention and
· freedom from pests and diseases.

The first three of these attributes also mean that the tree will be competitive with the associated crop.

A wide range of tree species has been used in alley cropping experiments or demonstrations (Table 5.2.3) but leucaena has been by far the most favoured species (Kang et al. 1990). A number of comparative trials in humid/subhumid zones on high base status soils have shown leucaena to be superior to other species and this may partly explain its widespread use (Kang and Reynolds 1986). However, on acidic low base status, soils leucaena has not been as successful as species such as Flemingia macrophylla (Kang et al. 1991) and Erythrina peoppigiana (Kass et al. 1992). There is a need for a wider range of tree species suited to low activity, acid, infertile soils. Ethnobotanical surveys in regions dominated by these soils may help identify appropriate species.

Table 5.2.3. Tree species used in alley cropping.

Species

Countries

Legumes


Cajanus cajan

Nigeria


Calliandra calothyrsus

Indonesia, Western Samoa


Cassia siamea

Kenya, Nigeria


Erythrina peoppigiana

Costa Rica


Flemingia macrophylla

Nigeria, Rwanda


Gliricidia sepium

Nigeria, Costa Rica, Sri Lanka, Philippines


Inga edulis

Costa Rica


Leucaena leucocephala

Nigeria, Kenya, Philippines, Sri Lanka, Indonesia, India, Thailand, Australia


Paraserianthes falcataria

Indonesia


Sesbania sesban

Rwanda, Kenya, Australia, Ethiopia


Sesbania grandiflora

Nigeria, Western Samoa

Non-legumes


Alchornea cordifolia

Nigeria


Dactyladenia barteri

Nigeria


Gmelina arborea

Nigeria


Grevilea robusta

Kenya

Hedgerow manipulation

One of the major benefits of alley cropping is the mulch provided by the hedgerow species, in the form of prunings, to the associated crop. Factors such as cutting height and frequency, hedgerow spacing and intra-row density will all influence the quantity of prunings produced.

In humid and subhumid areas, tree row spacings range from 2 to 7 m with 4-6 m most commonly used (Lawson and Kang 1990). Tree spacings within the rows should be as close as possible and experience with species such as leucaena, gliricidia and Sesbania sesban indicates that trees should be spaced at 10-15 cm or as near as possible to a solid hedge along the row. This helps to favour leaf production over stem, provides a more effective barrier to soil movement on sloping lands and creates a better microenvironment for crop growth. Closer spacing both within the row and between the rows also allows for improved distribution of nutrients to a greater proportion of the intercrop. On the other hand, close spacing between the hedgerows reduces the amount of land available for the crop and can result in increased competition for the growth factors of light, moisture and nutrients between hedgerow and crop.

Competition for light

In Nigeria, Kang et al. (1985) demonstrated competition for light in a maize/leucaena alley cropping study. The maize rows adjacent to leucaena received 51-69% of the available light compared with 75-81% received by mid-alley rows. In a subsequent crop, leucaena was pruned to 75 cm during crop growth and no significant yield reductions were observed. In southeast Queensland, Australia, Mekonnen (1992) found that maize grain yields in rows adjacent to leniently cut hedgerows of L. diversifolia were reduced by 88% largely due to competition for light.

Further work in Nigeria found that decreasing alley width from 4 to 2 m reduced both maize and cowpea yield associated with higher partial shading at the narrow spacing. Four tree species were tested in this study and their ability to depress yields of maize was in the order Leucaena > Gliricidia > Alchornea > Dactyladenia (Figure 5.2.3) and was well correlated with the size and leafiness of the tree (Lawson and Kang 1990). Other studies on this aspect have indicated considerable variation in light transmission among species. The more erect branching habits of gliricidia and Flemingia macrophylla cast less shade than the spreading lateral branches of Cassia siamea which shaded the centre of a 4 m wide alley (Anon. 1983). In another study, Getahun (1980) showed that Sesbania grandiflora transmitted more light than leucaena which was in turn better than Alchornea cordifolia. The fast growing pulpwood species Paraserianthes falcataria and Gmelina arborea showed considerably greater capacity for shading making them unsuitable for alley cropping unless cut back very frequently.

Although more frequent pruning at a lower pruning height can minimise the shading effect of the hedgerow, the effectiveness of the hedgerows for biomass production and nutrient recycling is also reduced (Duguma et al. 1988).

These studies have demonstrated that hedgerow species have the ability to shade adjacent crops in alley cropping systems. Thus timely pruning must be incorporated into the management calendar particularly to assist shorter statured crops such as cowpea which endure greater levels of shading and consequent greater yield reduction than taller statured crops.

Competition for moisture and nutrients

Recent reviews of alley cropping research (Ssekabembe 1985, Kang et al. 1990) have indicated few detailed studies of the effects of competition for moisture and nutrients. This may be partly explained by the reduced importance of moisture stress in lowland humid sites where most work has been conducted. On the other hand, it may be due to the assumption that trees place their roots deeper in the soil profile than most crops and that competition is therefore avoided (Berendse 1979). A study by Kang et al. (1985) on a degraded Entisol supported this latter assumption. They found that maize grown in association with leucaena drew moisture mainly from 0 to 30 cm soil depth while leucaena tapped the 60-90 cm zone. Verinumbe and Okali (1985) grew maize between coppiced teak trees (Tectona grandis) and separated the effects of shading and root competition by the use of barriers and judicious pruning. They found that shading alone depressed maize yield by 40%, root competition alone had no effect but shading and root competition combined depressed yield by more than 60%. Other studies by Ewel et al. (1982) with larger fruit and timber trees indicated that competition between tree and crop for moisture does occur and that it increases with the vigour of the above-ground biomass production.

Fig. 5.2.3. Effect of partial shading of hedgerow species on grain yield of associated maize crop (DB = Dactyladenia barter), ALC = Alchornea cordifolia, GL = Gliricidia sepium, L = Leucaena leucocephala) (Lawson and Kang 1990).

In drier areas, and even in humid zones on acid soils, competition between the hedgerow and the crop for nutrients and moisture can be very severe as the woody species and the crop have a tendency to concentrate their roots in the surface soil because of subsoil acidity. Studies by Fernandes et al. (1990), Basri et al. (1990) and Evensen and Yost (1990) showed significant reductions in the performance and yield of crops particularly when grown in the first few rows adjacent to the hedgerows. Root pruning of hedgerows can partially reduce competition but it is not always effective in the long term. The question of competition for moisture and nutrients requires further research. There is limited evidence from subhumid areas where interactions between competition, conservation and enhancement effects of trees, make interpretation of results more complex (Nair 1987).

Conclusions

The biological merits of alley cropping make it an important conservation farming practice for smallholders and resource-poor farmers. However, with minor modifications, it could also be adapted to the broadacre farming systems of the world.

The system exploits moisture and nutrients deep in the soil profile. It permits nutrient recycling, improves soil structure, provides good soil erosion control and reduces the need for chemical fertilisers.

Following a decade of intensive research on alley cropping in various parts of the tropics, a better understanding has now emerged of the potential and limitations of the system and the areas requiring further research.

Competition for light can largely be eliminated by judicious pruning of the tree species during the cropping phase. Below-ground competition for moisture has not been well defined and further research is required especially in subhumid regions. Identification of additional tree species suited to low base status, acid soils is also an important research goal as well as defining better management practices to enhance the beneficial effects of the system.

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