agronomic modification of resource use and intercrop productivity

Field Crops Research, 34 (1993) 357-380 357 Elsevier Science Publishers B.V., Amsterdam

Agronomic modification of resource use and intercrop productivity

D a v i d J. M i d m o r e The Asian Vegetable Research and Development Center, P.O. Box 42, Shanhua, Tainan, 741,

Taiwan, ROC

ABSTRACT

Suitable land areas for food production remain fixed or are diminishing, yet farmers and agronomists are faced with the task of increasing production. Raising productivity, through a more effective use of natural (e.g. light) and added (e.g. fertilizer) resources, is possible through intercropping" provided component crop demands for resources are well understood. Management of intercrops to maximize their complementarity and synergism, and to minimize competition between them follows simple natural principles, and its practice is limited only by the imagination of farmers and agronomists.

Successful crop mixtures extend the sharing of available resources, over time and space, exploiting variation between component crops in such characteristics as rates of canopy development, final canopy width and height, photosynthetic adaptation of canopies to irradiance conditions and rooting depth. Occasionally, commensalism is effected. Loosely defined as one organism gaining benefits from another without damaging or benefitting it, it is exemplified when one crop modifies the microenvironment to suit another. Prime examples are the benefits of shading during crop, particularly trans- planted crop, establishment under hot or dry conditions, the supply of nitrogen and solubilization of phosphorus by legumes for companion crops, and the suppression of weeds through direct competition or allelopathic effects.

The onset of competition between intercrops can be delayed by judicious choice of relative planting dates. The differential influence of weather (in particular temperature) on component crop growth and development can be modified through reasoned planting dates, and relative proportions of crop component yields can be targeted. In general, to ensure its high yield the main crop should be planted first. Choice of plant population density and crop geometry, including row orientation, permits a planned sharing of natural resources and manipulation of competitiveness to suit targeted yields. Increases in rectangularity in the crop geometry of the main crop tends to enhance transmission of light to shorter crops for longer periods before canopy closure. Crops harvested for their vegetative yield appear less sensitive to supra-optimum population densities within mixtures than do seed crops.

The period over which intercrops compete for resources can be shortened by the supply of external inputs, in as much as they permit greater exploitation of the finite supply of light. Supplementary irrigation has been shown to raise total productivity in various intercrop systems, but little research effort has been turned towards mineral nutrients. Addition of N fertilizer to legume intercrops reduces the relative over-yielding, i.e. compared to mixtures without N fertilizer, but not without overall improvement in total yield. Benefits of residual N on succeeding crops following legume intercrops are also not unsubstantial, and deserve attention when evaluating the merits ofintercropping.

In order to sustain enhanced productivity from intercrops, it will become increasingly more important to substitute natural resources where feasible for purchased inputs. Since the major focus of intercrop research has been on small-scale resource-poor systems, a serious gap in our knowledge on high input intercrop systems will hinder their rapid spread.

Correspondence to: D.J. Midmore, AVRDC, P.O. Box 42, Shanhua, Tainan, 741, Taiwan, ROC.

0378-4290/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

358 D.J. MIDMORE

INTRODUCTION

Low-external-input farmers have, in various geographic regions, learnt to manage and sustain their production systems without a substantial drain on the environmental resource base. Recent demographic pressure has forced agricultural planners and development agencies to review the role of multiple cropping as a means to enhance agricultural production, since the extent of suitable agricultural land is static or diminishing.

While capitalizing on the ecologically sound concepts of multiple cropping (complementarity of crops in time and space, long-term sustainability, stabilizing of pest, weed and disease populations, synergism), intensification of crop production through multi-cropping will lead to a greater removal of resources, extra resources which either must be externally supplied or more ef- ficiently utilized within the cropping systems.

The efficiency with which inputs, either on-farm or off-farm, are incorpo- rated into agricultural products is largely influenced by the agronomic management of the cropping system. While agronomic practices and varieties recommended for sole crops are to a certain degree site specific (Simmonds, 1979), agronomic practices recommended for intercropping must contend with the confounding effect of management interventions and the environment on two or more crops within the intercrops. These confounding effects may induce alterations in competitiveness or synergism, or in the degree of commensalism.

Much research on intercrop systems over the past decade has led to a clearer understanding of the forms and functions of intercrops which mould various outcomes of such systems. This understanding, however, is of little conse- quence unless it can be associated with the actual or perceived needs and de- sires of the producer. Thus, the agronomist must attempt to devise the range of management practices likely to satisfy the producer's needs. For example, farmers might adopt widely different intercrops if their desire is to maximize economic return or to provide for dietary variety and food self-sufficiency (Ojeifo and Lucas, 1987 ). Ideally, computer models will permit an objective choice of management practices based upon a concise inventory of resource availability (Caldwell and Hansen, 1990), but until these have been developed, empirical decisions must be taken based upon principles gleaned from farmer and researcher experience. It is the latter that forms the substance of this paper.

Competition vs. complementarity The present discussion on the use of resources by intercrop species assumes

for simplicity that competition and complementarity in resource use by intercrop component species are mutually exclusive, although this may not be so. Sharing of light amongst species with contrasting adaptation to irradiance

RESOURCE USE AND 1NTERCROP PRODUCTIVITY 3 5 9

levels may go hand in hand with competition for mineral nutrients within the soil. Additionally, complementarity and sharing of resources early in the intercrop cycle may gradually evolve into competition at some later stage with an indistinct period separating the two.

Complementary use of resources by intercrops implies minimizing competition. Use of a different resource pool represents the most common example of complementarity. The temporal use of irradiance within intercrops of contrasting development and phenology (i.e. their peak demands for the same resource do not overlap in time due to differences in phenology (Willey et al., 1983) or planting date (Midmore et al., 1988a)) is a prime example illustrating the more efficient use of naturally available resources by intercrops than by each crop if grown alone. The spatial use of soil moisture by crops of contrasting demand, e.g. chili pepper and soybean (Hulugalle and Willatt, 1987 ), or contrasting root extraction zones (Lakhani, 1976), also illustrate the efficient sharing of resources between component crops. Indeed, the complementary use of resources can also be associated with effects on the growth of one component crop not related to the shared resource. For example, while chili pepper uses soil moisture resources more effectively after soybean reaches the reproductive stage, leaf water potential of intercropped chili pepper is higher than sole-cropped chili pepper, most likely due to a windbreak effect by the taller soybean (Hulugalle and Willatt, 1987). Earlier emergence of potato relayed into maize shade in warm climates represents another such effect (Midmore et al., 1988a). Complementary use by component crops of the same resource pool is less common, but exemplified by the mixing of short C3 and tall C4 type plants, which differ in efficiency in use of tropical sunlight (Willey et al., 1983; Midmore, 1990). Complementary use of resources therefore takes place over space both vertically and horizon- tally, and over time, and in any combination of these two.

The stage at which complementarity evolves into competition for resources is amenable to manipulation through choice of agronomic management. De- lay in the crossover point between complementarity and competition is often the goal of improved agronomic practices. For example, greater degree of rectangularity (or strip cropping) when planting intercrops will delay the onset of competition, but will concomitantly reduce the opportunity for complementarity compared with square (or row) planting patterns. Just as competition between component species can lead to over- or under-yielding by the same package of agronomic practices for intercrops in different sites (Ama- dor and Gliessman, 1990) or seasons (Rees, 1986), within-site variation in management practices can also lead to over- or under-yielding of crop mixtures.

Since the array of management interventions open to resource-poor farmers is somewhat narrower than for resource-endowed farmers, each group will be dealt with separately.

360 D.J. MIDMORE

RESOURCE-POOR SYSTEMS

Choice of component crops, site and timing of planting, spatial geometry and population densities are amenable to management by resource-poor farmers. Without exception, the choice of suitable crop mixes that are complementary in their use of resources, based upon farmer objectives, is inti- mately related to the efficient use (i.e. interception and conversion) of available irradiance.

Choice of component crops Combinations of crops in mixtures have been devised by researchers, and

most certainly beforehand by farmers, to optimize use of natural resources, to capitalize on generated (e.g. cereals using N fixed by legumes) or converted resources (e.g. release of bound P by legume roots), and to reduce external inputs (e.g. freedom from weed control due to weed suppression or from pest control due to stabilization of pest populations).

For natural resources. Optimal natural resource use is attained when mixtures are not comprised of highly competitive crops. Evidence suggests that intercrop stability over space and time is likely to be favoured by the choice of less-aggressive cultivars, of similar final canopy heights (Cenpukdee and Fu- kai, 1992c). Under adverse conditions, e.g. N deficiency or drought, growth is reportedly dominated by the aggressive species (Fukai et al., 1990). How- ever, low soil N and P improved the competitiveness of cowpea and decreased that of the dominant maize (Chang and Shibles, 1985a,b), resulting in greater complementarity in resource use and higher land-equivalent ratio (LER). Competitiveness of component crops therefore depends to a large degree on each crops' response to the limiting factor(s). This is discussed more fully by Fukai and Trenbath ( 1993 ).

Whereas severe competition between two component crops can result in reduced biomass and yields of one component, if the intercrops form part of a sequential relay intercrop, that is one component is removed and another planted in its place, temporal compensation in yields may occur (Fig. 1 ). For example, tall grain maize suppressed early growth of intercropped chili pepper, but after harvest of maize and replacement by snapbean, yield of the latter was favoured in association with the less-vigorous chili pepper treatment (Fig. 1 ).

Once competition for light begins, certain systems, particularly alley crops, have been developed which lend themselves to artificial manipulation, pri- marily through pruning. Reduction of shade afforded by the taller alley crop Leucaena to levels acceptable to the understorey crop resulted in enhanced per plant yield of potato (Kuruppuarachchi, 1990), and maize and cowpea (Kang et al., 1985).

RESOURCE USE AND INTERCROP PRODUCTIVITY 361

c~ .£z

qc}

@ > -

16-

14-

12-

10-

8-

6-

4-

2-

0 Short sweet

maize

Tall grain

maize

Fig. t. Influence of intcrcropping short sweet or tall grain maize with pepper on their yields, and on yield of snap beans relay-planted into pepper after removal of maize (D.J. Midmore and V. Kleinhenz, unpublished). From bottom to top: maize, pepper, snapbean.

Studies on intercropped maize with coppiced teak (Verinumbe and Okali, 1985 ), using trenching to deconfound root competition, illustrated the greater importance of competition for light than root-mediated competition in the reduction of maize yields, and highlights the need to match supply of light to demands of understorey intercrops.

However, yield reductions in the less competitive species in some systems, particularly those involving plantation crops, is less amenable to agronomic modifications. Testing of ten arable crops intercropped with three different tree species showed the existence of significant interaction effects for yields (Srinivasan et al., 1990). Choice of apparently shade-tolerant COWl)Ca and green gram, and the tree Casuarina equisetifolia would optimize the complementarity between tree and arable species over a 3-year period.

For generated and converted resources. The transfer of nitrogen (N) fixed by legumes to adjacent non-fixing species has been the subject of debate for many years. Nodulation in legumes has been shown to increase when planted with species unable to fix N. A greater stimulus for nodulation in beans was noted when interplanted with maize (Boucher and Espinosa, 1982 ), as were consis- tently better intercrop yields when maize and legume roots intermingled in the same, versus alternate, planting holes (May and Misangu, 1982). Nitro-

362 D.J. MIDMORE

gen has been shown to pass from one N-fixing crop to another during concurrent life cycles (Martin et al., 1991 ). However, a delicate balance must exist between mixtures of crops of contrasting canopy height. An excessive transfer of N to the non-fixing species favours overshading of the N-fixing species, and a reduction in N fixation. On the other hand, as indicated later, additional N favours self-shading of legumes (Nambiar et al., 1983 ) and shading by associate crops (Chui and Shibles, 1984), which inhibits nodulation and by infer- ence, reduces N fixation by legumes.

A more recent finding on the benefits of crop association has been the dem- onstration that some species mobilize more soil P than necessary for their own use (e.g. white lupin, Horst and Waschkies, 1987; pigeon pea, Ae et al., 1990). Companion crops grown in association with these species have a more- efficient P uptake rate per unit root length, and consequently greater yield and P uptake. Two-fold increases in P uptake were evident for wheat with roots intertwined with roots of white lupin (Horst and Waschkies, 1987) and total P uptake by a pigeon pea/sorghum intercrop was 60-100% greater than by the sole crops (Ae et al., 1990).

Maximization of the benefits of this commensalism, as for that of N fixation, will be dependent upon agronomic management of the competitive balance between species. Earlier planting of the N-fixing or P-availing legume should afford protection from competition by companion crops, especially if their root systems are later to intermingle. In this way the scope for N fixation or P availing is not curtailed by a companion crop competing for another resource. If the cropping system dictates a simultaneous or later planting of the legume, due for example to climatic conditions, strip-planting of component crops will reduce the potential loss of the favourable N or P release, but will also limit the extent of transfer of P and N between the associated crops.

For reduced herbicide and pesticide inputs. Maintenance of a complete crop canopy over the soil inhibits weed seed germination and reduces the need to weed. Early canopy growth likewise smothers weeds and reduces weed/crop competition, particularly for soil nutrients and soil water. Farmer benefits from intercropping for weed control are particularly evident under low-input conditions (Leihner, 1979 ), and increases in component crop yields have been attributed to improved weed control. An effective weed control of cassava intercropped with beans equalled that of preemergent herbicide application on sole cassava plots, yet the intercropped cassava, besides equalling sole cassava yields, resulted in some bean yield. Although similar effects can be achieved through the planting of high population densities of sole crops, land carrying capacity may not permit their full development to maturity, or high populations might lead to sterility such as was demonstrated for maize (Can- nell, 1983b). Higher population pressure of the component intercrop species together, rather than the sole crops alone, do often, however, provide the in-


tercrop with a greater competitive advantage over weeds than sole cropping (Mugabe et al., 1982).

Weed control in intercrops is particularly effective for slow-growing yet ul- timately tall crops when interplanted with short-season, low-habit crops. Ag- ronomic practices which maximize ground cover favour more effective weed control (Table 1 ), while adding to total land-equivalent ratio (LER). Suppression of weeds by some intercrops is not only dependent upon ground cover. Allelopathy was implicated by Amador and Gliessman (1990) who found that lower weed biomass in corn/beans/squash polycultures was only partially related to greater canopy light interception.

Besides suppressing weeds, the undersowing of low-canopy crops such as melon reduced soil temperature fluctuations and conserved soil moisture (Ikeorgu et al., 1989 ), especially when growth of companion crop canopy was sparse. Competition for water during long drought periods may disproportionately reduce the taller crop yield in mixtures, but other benefits of the understorey crop, e.g. erosion control and reduction of soil temperature, may be more valued by the farmer. For example, although squash yields are much less in mixed plantings with maize and beans than in sole plantings, farmers value the squash largely for its control of weeds (Gliessman, 1983). The use of short understorey cover crops results in a more effective use of water for the polyculture under conditions of sufficient soil moisture (i.e. more water passes through intercrop canopies than through crops with weeds). However, this may also be true under drought conditions, especially if the understorey crop has a low transpiration rate or it exploits soil water reserves from a different stratum than that of the taller crop ( s ) (Hulugalle and Willat, 1987 ).

Just as the relative advantage of mixed cropping on weed control is greatest at low-input levels, so too is that for pest control (Thung and Cock, 1979). Exploitation of diversity in spatial arrangements, physical and temporal bar- riers, microclimate modification, olfactory effects, and colour and trapping effects amongst intercrop components affect pest or disease development, or that of their natural enemies (Potts, 1990). Careful scrutiny of the effects of

TABLEI

Effect ofintereropping vegetable soybean with sugarcane on weed ground cover (% _+ SE) at harvest of soybean, and after a further 26 days (following weeding of all treatments) and sowing of a subsequent mungbean intercrop (unpublished data ofD.J. Midmore and M.H. Wu, 1991 )

Sample time Initially cane alone

No subsequent Subsequent mungbean mungbean

Cane plus vegetable soybean followed by mungbean

At soybean harvest 48 + 6 - 0 + 0 26 days later 41 + 6 11 + 5 6 + 6

364 D.J. MIDMORE

altering species mixes, population densities and spatial arrangements, canopy to canopy contact, and input management on the host/environment/patho- gen complex is imperative to optimize the stabilizing effect of intercropping on pest and disease incidence (Trenbath, 1993 ).

Timing of planting Choice of planting dates, whether for simultaneous planting of all crops

within a growing season or the relative timing of component crops, can con- tribute greatly to the yield outcome of intercrop systems.

Unless component crops respond similarly to climatic stimuli, even a slight alteration in the concurrent planting date may differentially influence relative development, especially canopy height and width, an effect that is evident when comparing intercrop performance from year to year (e.g. the induced difference in sorghum/cowpea intercrop performance by annual variation in rainfall, Rees, 1986) or from season to season (the induced difference in intercrop performance of bean with cassava due to rainfall, Leihner, 1979).

Cool-adapted species can capitalize on the unused space and light between rows of a slow-growing, spring-planted thermophile, leading to an early efficient interception of irradiance. Delay of planting date, which favours growth rate of the thermophile, hastens the transition from complementarity to competition in favour of the thermophile. Early planting does the reverse, favour- ing competition by the cool-adapted species. If competition occurs before maturity of the cool-adapted component, its yield potential declines (Midmore, unpublished ). Hastened spring development (e.g. through soil covering with clear plastic mulch) of the thermophile, if a C4 species and especially if growth is very slow under cool conditions, extends the period of complementarity when grown with cool-adapted C3 species. Thus, early smothering of the thermophile is avoided and later shading by the thermophile increases total radiation-use efficiency (Liu and Midmore, 1990).

To delay or hasten the relative planting date of one versus the other crop (s) in a mixture provides farmers more flexibility in an unpredictable environment, which might override their original calendar for planting. For example, in spring-planted soybean-cassava mixtures in Australia (Tsay et al., 1985 ), soybean sowing could be delayed until 5 weeks after cassava planting without reduction in soybean yield, but under tropical conditions in Colombia, even simultaneous planting of soybean and cassava resulted in less soybean yield relative to sole-crop yield (Thung and Cock, 1979 ). Judicious choice of relative planting date and variety can enhance over-yielding in mixtures. When mild competition from one crop limits canopy growth to the optimum level in the other component crop, yield of the latter is enhanced (Thung and Cock, 1979; Fukai et al., 1990). Although relative canopy height (Leihner, 1979 ), as affected by relative planting date, has been implicated as responsible for variation in yields of component crops, more recent evidence (Cenpukdee


and Fukai, 1992c) highlights the importance of canopy width in determining competitiveness, and consequently mixture yields, especially when canopy heights of component crops are similar. If complementarity in light use gives way to a long period of competition thereafter, the crop components ideally should not differ greatly in height from that stage onwards (Cenpukdee and Fukai, 1992b) unless crop mixtures involve shorter C3 and taller C4 plants able to exploit spatial availability of irradiance in the vertical axis. Especially where alterations of relative planting date have marked influence on relative canopy heights and widths (as in the study by Cenpukdee and Fukai, 1992c), the choice of sowing date may have an overriding effect on the range of suitable population densities, as they affect final yield outcome. For example, delaying planting of tall pigeon pea until 35 days after cassava led to greater total biological and economic yield than did a doubling of the pigeon pea population (Cenpukdee and Fukai, 1992c).

Alteration of relative planting dates, besides modifying the relative periods of complementarity and competition, also influences the extent to which plants of component crops reach their yield potential. For example, a 22% reduction in sole-crop yield was evident following a 3-week delay in soybean sowing (Nnko and Dota, 1982 ) as was a 40% reduction in pigeon pea yield following a 5-week delay in planting in Australia (Cenpukdee and Fukai, 1992c). This must be taken into account when determining the net profitability of intercrop systems.

In general, when component crops are sown at different times, the earlier- sown crop has an earlier competitive advantage (Ofori and Stem, 1987a). This is commonly due to a quicker access to and delimitation of the resource pool, particularly for light. Even when the competitive ability of one crop is markedly greater than the other in a mixture (e.g. maize versus cowpea, Ofori and Stem, 1987a) competitive ability and yield of the maize were reduced if sown after the less-competitive cowpea. Nevertheless, the relative reduction for a given delay in planting was greater in cowpea than in maize. Within sequential relay crop systems, advantage can be taken of differential competitiveness between crops to temper the yields of successive crops within the system. For example (Fig. 1 ), differential suppression of chili pepper yield by maize varieties of contrasting height and maturity led to compensatory increases in snapbean yield wherever growth and yield of chili pepper was reduced.

Farmer choice of relative planting dates may not only be dictated by the desire to reduce interspecific competition, but also to permit cropping of two crops when climatic conditions do not permit their sequential cropping. For example, highest yields of green gram were recorded by Sharma et al. ( 1991 ) when sown in standing wheat because intercropped green gram escaped ex- tremely hot days in June as a result of an earlier harvest. Similarly, relay planting of autumn potato into standing maize in China during the hot sum-

366 D.J. MIDMORE

mer is essential to provide protection for emergence under hot conditions, and to ensure harvest before the onset of frost (Midmore et al., 1988b). With- out relay cropping the season would be too short for full maturity of the potato crop.

In the 1950s and 60s in Taiwan, relay intercropping was a popular practice since it offered farmers the scope to intensify the use of land and resources for enhanced agricultural output (FFTC, 1974). Besides providing relay intercrops with more time to mature in the field and a precocious and efficient use of residual moisture, relay intercropping also led to better market prices for early produce.

From the above it is evident that complex relationships between species and their rates of growth and development and their competitive interactions modulate the outcome of intercrop yields. Temperature appears to be one of the primary factors involved, but no doubt photoperiod, particularly as it leads to determinate growth habit, and water availability, are important in their own fight. Alteration in time of planting of all components may therefore modify competitiveness of component crops and hence crop yields. Reasoned choice of relative planting time diminishes or enhances relative competitiveness between component crops, and the perceived main crop should be planted first. However, where labour supply is scarce and agriculture is largely mech- anized, synchronous planting of intercrops is more suitable, and future research emphasis will inevitably focus on simultaneous planting, unless the benefits of relay planting outweigh financial returns to mechanization.

Spatial geometry and planting density Of the agronomic options open to resource-poor farmers, perhaps the selec-

tion of spatial geometry (i.e. the positioning of plants of one component relative to that of the other component crop (s)) and planting density of component crops offer the greatest scope to maximize interspecific complementarity. Crop mixtures in which components vary in plant height are amenable to manipulation of spatial geometry, principally to provide more space (i.e. irradiance) for the shorter (understorey) crop through reduction of space for the taller (dominant) crop (Chui and Shibles, 1984). Maintain- ing sole-crop population for the dominant crop, yet altering the spatial geometry to increase space for understorey crops within rows (i.e. grouping plants two or three to a hill, Chui and Shibles, 1984; Fig. 2 ) or between paired dominant crop rows (Mohta and De, 1980), increased the understorey crop yields above those achieved when less space was available to them. Reducing the space for the dominant crop, besides limiting their access to solar radiation, will reduce the size of their soil moisture and nutrient pool, with conse- quent limitations on their competitive ability over that of the understorey crop. Increasing the resource capture by the understorey crop through alteration of its spatial geometry often leads to over-yielding. Over-yielding herein

RESOURCE USE AND INTERCROP PRODUCTIVITY

800-

o

6 0 0 -

@

-c, 4 0 0 -

0o p ONE

200

X

x x

- - x x X X X X x X X x

TWO

X X X X X X X X X X x X X X X X x X x x x X x X

T H R E E

367

Number of maize seeds per hill

Fig. 2. Influence of maize spatial arrangement and soybean population density on the yield of intercropped soybean. One, two or three maize seeds per hill were planted at 70 × 30, 70 × 60 or 70 × 90 cm respectively. Soybean was distributed uniformly between maize hills. Sole crop soybean averaged 3410 kg ha-1 (data of Chui and Shibles, 1984). Soybean density: open bars, 4.76 plants/m2; crossed bars, 9.52 plants/m E.

refers to yields of component intercrops which surpass the sum yields of mon- ocrops of the same species. For example, paired-row interplanting of groundnut with a sunflower crop, which provides more within-row plant space and delays the onset of intraspecific competition, led to greater harvest index and yields than single-row planting of groundnut intercropped at the same population density (Dayal and Reddy, 1991 ).

For easier logistic management of intercrops, the component species are often planted in strips, rather than simply planting in alternating rows or mixing within rows. With strip plantings consideration of row orientation increases in importance in relation to diurnal shade levels and periodicity and their associated effects on understorey crop growth (IITA, 1983). Predictable consequences of planting understorey crops on E or W sides of N/S-orien- tated dominant crops, and similarly on N or S sides of E/W orientations, on the radiation regime for understorey crops augur well for an efficient logistic set of management practices (Table 2). Narrower between-row (or strip) spacing of the taller crop within a mixture might be expected to reduce row- orientation effects on the understorey crop. However, although maize yields were not influenced by row orientation when planted at normal spacing, yields of single rows of the shorter intercropped mungbean were favoured when planted in the N/S direction (Dhingra et al., 1991 ). As with sole crops, the importance of row direction for radiation capture in intercrops is diminished

368 D.J. MIDMORE

TABLE 2

Shading effect of Leucaena (4 m between rows) on alley-cropped first-season maize and second- season cowpea (data source: IITA, 1983)

Radiation incident on crops (%)

West side Middle East side

First season Before pruning

Early growth phase 42 66 50 Late growth phase 94 93 98

After pruning Early growth phase 84 92 91 Late growth phase 88 92 93

Second season

Before pruning 41 82 49 After pruning 89 98 90

with proximity to the equator (Mutsaers, 1980), although even at 12°S latitude a significant influence of row orientation was observed on soil temperature and emergence of potato planted within rows of flowering maize set 1.5 m apart (Midmore et al., 1988a).

Timely pruning of alley crops (Kang et al., 1985 ) and removal of dominant relay crops (Midmore et al., 1988b) are also among the tools available to farmers to minimize interspecific competition. However, when released from competition by earlier harvest of one component crop within a mixture, the harvest index (HI) of the remaining crop may either be favoured (e.g. re- moving soybean from a mixture with cassava, Tsay et al., 1985) or diminished (e.g. pigeon pea from a mixture with cassava, Cenpukdee and Fukai, 1992a). In the former study, soybean restricted early top growth of cassava, with a resultant increase in HI. Prolonged competition between pigeon pea and cassava, including below-ground competition, in the latter study reduced canopy growth, particularly leaf growth, which resulted in a low radiation-use efficiency and low HI.

The planting of paired rows, or strips of component crops, besides lending itself to easier mechanization than alternate rows of intercrops, reduces the influence of competition to less than that apparent in alternate row planting (Cenpukdee and Fukai, 1992a) and also capitalizes on the so-called border effect (Baldev and Ramanujam, 1980). That is, compensatory yield of outer rows of the dominant crop over-compensate for the reduced, or ideally unaf- fected, yields of the edge rows of the understorey crop. Differential competition for light and soil nutrients between species at the strip interface could result in no yield reduction in the understorey crop (Lai and Wen, 1990).


The border row effect is particularly important where relay-crop systems are practised. The logistic grounds for implementing relay-cropping, rather than sequential cropping, overrides the potential disadvantage of lower yield in one of the species at the intercrop interface (Hart et al., 1985 ).

Considerations of density and relative proportion of component crops within intercrops are closely tied to farmer expectations in terms of relative crop yields and profitability. If farmers wish to maintain yield generally of the dominant component at close to sole-crop yield levels while reaping yield from an associate crop, i.e. to maximize combined yield above sole-crop yield, fail- ures will be minimized if mixtures include a crop harvested for its vegetative yield (Cannell, 1983a). Vegetative yield adopts an asymptotic relationship with plant population, hence compared with many crops cultured for reproductive yield, these crops have less-critical optimum population densities, and maximum vegetable yield is achieved at minimum density stress (Cannell, 1983a). The integrated effects of interspecific competition within mixtures involving vegetative crops (and at times reproductive crops, Rosset et al., 1984) reinforces the interference production principle of Vandermeer ( 1981 ) which suggests that over-yielding will occur when interspecific competition is weaker than intraspecific competition. The degree of shade tolerance amongst understorey crop species does however vary. Choice of leafy vegetables will permit intercropping at shade levels unsuitable for storage root crops, which in turn tolerate more shade than during the reproductive stages of legumes and cereals. Combining two or more crops known to exhibit very specific optimum population densities when grown as sole crops, seldom results in over- yielding, since supra-optimum populations (or density) often leads to sterility of reproductive organs (Bonaparte and Brown, 1975 ). For all their impe- diments as noted by Snaydon ( 1991 ) in the analysis and interpretation of competition, replacement designs most likely still have a role in the identifi- cation of productive mixtures of crops with reproductive yield, since total combined populations can only rarely exceed sole-crop populations without a loss of total yield. Additive designs are best suited to the study of mixtures combining crops with reproductive and vegetative yields, and derived data are more amenable to valid statistical analysis, particularly if bivafiate fac- torial designs are employed (Snaydon, 1991 ).

While combination of some crops grown in association at full stands is im- practical if planted together, staggering of planting, generally to delay early growth of the dominant crop in a mixture, permits planting of full crop stands of component crops (Midmore et al, 1988a), but does not inevitably lead to yield advantage over simultaneous sowing (Ofori and Stern, 1987a). Alter- natively, staggering of planting to synchronize harvest of the dominant crop with release from shading at a critical developmental stage in the understorey crop (i.e. a relay crop sequence) also permits planting of each crop at its sole- crop density. Both systems are, however, subject to the confounding effects

370 D.J. MIDMORE

of environmental and genetic factors which influence the onset or release of competition, particularly for light (Midmore, 1990).

RESOURCE-ENDOWED SYSTEMS

Relative advantages of mixed cropping are reputedly greatest at low input levels (e.g. without insecticides in beans-cassava mixtures, Thung and Cock, 1979), but greater yields per unit land area will still accrue in intercrops endowed with external inputs.

Competitive extraction of available resources by one crop within a mixture can be mitigated by the farmer if he has access to supplies of that resource (e.g. water, mineral nutrients). External inputs provided by the farmer can therefore obviate interspecific competition for these environmental resources. With the exception of use of reflectors, the farmer has no control over the supply ofirradiance, although the results of addition of water and mineral nutrients are generally manifested through improved access to the finite pool of irradiant energy. The benefits of external inputs to intercropping systems are therefore two-fold: to improve the immediate capture of irradiant energy, and to ensure temporal long-term sustainability of the system by matching of outputs with inputs.

Soil moisture As illustrated by Fukai and Trenbath (1993), competition among inter-

crops depends to a large extent on component crops' response to a limiting factor. As an example, a greater degree of tolerance to drought by one crop component, whether due to exploitation of a greater soil moisture resource pool or a greater water-use efficiency, confers a competitive advantage over less-drought-tolerant components within a mixture suffering drought. While this augurs well for maximum utilization of radiation and stability of (low) yield, i.e. given the prevailing limitation of soil moisture, the total yield will not deviate greatly from the average, but the proportion of crop products from a mixture may not satisfy the expectations of the producer. Judicious irrigation can relieve competition for soil moisture, raise total yield to a level con- strained by the next-in-line limiting factor, and ensure that the harvest vol- umes of the crop components are close to the expectations of the farmer.

Irrigation apparently relaxed competition in tomato/cucumber mixtures, since the yield advantage of intercrops was greater in irrigated plots (Schultz et al., 1987; Table 3 ). The greater yield of the irrigated cucumber intercrop over the non-irrigated cucumber suggests that yield in the latter suffered water stress, and perhaps crowding by tomato. For non-irrigated intercrops, LERs decreased dramatically as cucumber planting density was quadrupled, whereas LERs remained constant over the same density range if irrigated (Schultz et al., 1987). Irrigation therefore apparently could relieve the effect of over-


TABLE3

Land-equivalent ratio (LER) and component yields summarized with respect to manipulated hornworm level and irrigation. Significance of the difference between treatment LER and LER = 1 is indicated (data source: Schultz et al., i 987)

Irrigation Hornworm LER Yield (t h a - ~ ) level

Intercrop Intercrop Monoculture cucumbers tomatoes tomatoes

Nonirrigated

Irrigated

Low 1.09 nS 7.7 5.1 8.0 Med. 1.15* 7.9 5.0 7.2 High 1.39"* 9.3 5.4 6.5

Low 1.18** 11.3 6.1 8.4 Med. 1.47*** 15.2 5.5 6.7 High 1.38.* 12.9 5.5 6.5

crowding. Under rainfed conditions, however, the generally noted greater water-use efficiency (WUE) of intercrops than sole crops of legume-cereal mixes (e.g. sorghum/pigeon pea, Willey and Natarajan, 1980; mungbean/ pearl millet, Singh and Singh, 1980) as a result of similar consumptive use of water but greater yields in the intercrops, is not universal. For some mixtures (e.g. maize/cowpea, Hulugalle and Lal, 1986) a higher WUE in relation to sole cropping was only evident provided soil water was not limiting. Diver- sion of limited water resources from one crop by addition of another in a mixture, for example away from sorghum by the addition of cowpea (Rees, 1986), may lead to disproportionately large reductions in reproductive yield not observed under moist conditions (e.g. for maize/beans, Fisher, 1977). Stronger competition by cowpea was most likely related to its greater ability to root to depth (Rees, 1986 ). For that reason, in dry regions with unreliable rainfall, wide row spacing and low-density intercropping (i.e. below those recommended for sole crops) is slightly advantageous and is more likely to make better use of resources in favourable conditions than standard-row, me- dium-density intercrops, or sole crops (Rees, 1986 ).

When adequate soil moisture and other inputs are guaranteed, crop mixes, planting geometry, population densities etc. can be optimized so as to miti- gate competition for light; while under conditions of poor management and unpredictable environments the competition for light is a less important factor in choosing crop mixtures. Even so, when measured against the yields of sole crops grown under identical conditions, crop mixtures actually result in greater relative over-yields when grown under conditions of limited soil moisture (e.g. LERs > 2 for sorghum/groundnut and sorghum/millet, ICRISAT, 1981 ), even though crops provided with adequate moisture obviously produce greater absolute yields.

3 7 2 D.J. MIDMORE

Moderation of soil moisture use by the understorey crop through shading (e.g. of groundnut by millet, ICRISAT, 1981 ) or a windbreak effect (chili pepper by soybean, Hulugalle and Willatt, 1987), which lead to improved WUE, however, result in greater intercrop yields under limited soil moisture conditions but are perhaps less important when soil moisture is adequate.

Mineral nutrients Application of mineral nutrients to the soil may release interspecific com-

petition between component crops for the soil-based pool of nutrients, may alter the balance in competition between component crops for mineral nutrients and subsequently expressed as competition for irradiance, or it may directly alter the hierarchy of dominance between component crops.

Release of interspecific competition for a nutrient (or simply increasing the supply to component species) resulting in an increase in LER and total biomass production is most likely due to an increase in total irradiance interception over the season. This could be effected either through enhanced early growth and canopy cover by the mixture, or through improved maximum canopy cover, or a combination of the two. Data from a pigeonpea/rice mixture showed an increase in LER from 0.85 without P fertilizer to 1.53 with 26.2 kg P ha-~ (and a 60O/o gain in biomass), which suggested improvements in maximum canopy with added P (Mahapatra et al., 1991 ). Data for light interception were not presented. By use of autoradiography, other intercrop studies have however shown that root competition for the immobile macro- nutrients P and K is unlikely (Fusseder, 1986). Nevertheless, complementarity in the use of resources which brings about yield advantages in mixtures is greater when growth and yield of at least one component crop is somewhat limited and yield potential is low (Chang and Shibles, 1985b). With additional P, the resource in short supply in the abovementioned study, complementarity was less well expressed since increased maize shade caused cowpea yield depression.

More attention has been paid to the response of intercrop systems to the application of N, largely because the effect of N is dramatic, particularly in mixtures involving legumes. The addition of N to legume-based intercrops generally favours growth of the non-legume at the expense of the legume. With minimal nitrogen, growth of the legume is less restricted than that of the non- legume (Fukai et al., 1990). In contrast, additional N, besides directly antag- onizing rhizobial N fixation in the legume, also enhances lateral (Cenpukdee and Fukai, 1991 ) or vertical (Chui and Shibles, 1984) growth of the non- legume, which is expressed as greater competitiveness for radiation interception. Greater competitiveness, however, is not necessarily manifest in greater yields, especially in crops or varieties within crops for which the harvest index is very sensitive to high N (Cenpukdee and Fukai, 1991 ). However, increased shading over the legume, with increases in competitiveness effected


by N fertilizer application to the non-legume, does reduce the contribution of N fixation to the legume crop (Chang and Shibles, 1985a), thereby reducing relative over-yielding compared to mixtures without N fertilizer.

Where the legume is responsive to added N and has the opportunity to shade the non-legume crop (e.g. cowpea when mixed with tomato or okra, Olasan- tan, 1991 ), yields of the non-legume may effectively decline at higher N application rates. Consequently, only at low soil N status (0 or 30 kg N ha- 1 ) was complementarity of intercrops, as indicated by largest LERs, evident. At higher N rates the broad-leaved upright okra was superior to the trailing tomato in counterbalancing competitiveness of the cowpea (Olasantan, 1991 ).

The response of intercrops to added N is conditioned by factors such as soil moisture availability, plant population and canopy structure of component species, and differential temporal demands for N by component crops. These are generally manifest through subsequent competition for radiation interception.

Inconsistent effects of N fertilization on relative competitive abilities of maize and soybean across sites have been attributed to differences in soil moisture and N availability (Russell and Caldwell, 1989). Under limiting soil moisture, partial LERs of maize increased, while those of soybean decreased, as a response to increased fertilizer N over the range of density combinations. Under the same environmental conditions, where crops exhibited visible signs of moisture stress, the optimum density combinations were dependent upon N levels, whereas at a contrasting moister site the optimum combinations were unchanged over N levels.

Land-equivalent ratios of cereal/legume intercrops, in general, have been reported not to benefit from increased rates of N fertilizer (see Ofori and Stem, 1987b), with variations in LERs largely conditioned by the legume response to added N. Indeed, intercropping the cereal reduced its ability to respond to added N through fertilization, an effect which was accentuated at high cowpea population (Ofori and Stem, 1987b). Although it was argued that maize dominated cowpea in competition for growth-limiting factors, the data suggest that cowpea prevented maize from fully taking advantage of fertilizer N. The combination of high population density of maize and high fertilization caused shading and yield depression of cowpea when intercropped with maize (Chang and Shibles, 1985b), most likely since cowpea could not acquire all its N from fixation, and this finding, together with data from the studies of Ofori and Stern (1987b) and others, suggests that intercropping efficiency is greater under low than high fertility.

When intercrops vary greatly in their temporal demand for growth-limiting factors, or have the capacity to compensate favourably for earlier deficien- cies, there is apparently greater scope for response of intercropping efficiency to fertilizer application (and most likely to an increase in total plant population). For example, the advantage of intercropping a quick-maturing soybean

374 D.J. MIDMORE

with a long-season cassava was reduced if no N fertilizer was applied at planting (Tsay et al., 1989). Intercropped soybean severely reduced early leaf area of, and dry-matter production by, the intercropped cassava, but later led to an increased harvest index over the sole cassava, particularly if N was applied at planting. Release of competition between legume and cereal intercrops, through early harvest of the legume (as for forage cowpea, Waghmare and Singh, 1984a) leaving fixed N in the soil for subsequent use by the intercropped cereal, led to greater cereal yields than through intercropping with full-season grain legumes (Table 4). Yields of cereal and legumes increased over the range 0 to 120 kg N ha- 1. Although the positive response of legume yield to increasing N could be argued on the basis of compensation for shade- induced inhibition of N fixation, data suggest that at 65 days after sowing little difference existed between incident radiation at the soil surface across N treatments (minimum 39% to maximum 44%).

This highlights the need to quantify the relationships between the various factors governing growth, N fixation, and yield determination. In relation to light distribution, this has been addressed by Liebman (1989). Applications of fertilizer N can influence the relative apportionment of leaf strata between intercrops, and the benefit for return on investment in leaf area for energy interception. Whereas fertilizer N increased leaf area of barley low in the canopy (and hence did not improve on interception of radiation), leaf area of pea did not respond positively to added N (Liebman, 1989 ). In fact, greater seed yield of pea was achieved under low N, when a greater proportion of the pea leaf area was in the upper canopy.

Occasionally, legumes are intercropped with non-legumes solely to provide fixed atmospheric N to the non-legume (Pandey and Pendleton, 1986). Ni-

TABLE 4

Grain yields of sorghum, and succeeding wheat crop, as a response to sole cropping or intercropping of sorghum (data from Waghmare and Singh, 1984a,b)

Crop system Yield (t ha- ~ )

Sorghum Wheat

1978 1979 1978 1979

Sole sorghum 3.09 3.44 3.40 3.61 Sorghum + greengram 3.32 3.71 4.05 3.75 Sorghum+groundnut 3.29 3.25 4.33 4.01 Sorghum+grain cowpea 3.25 3.73 4.30 4.03 Sorghum + fodder cowpea 3.79 4.09 4.69 4.11 Sorghum + soybean 3.15 3.35 3.61 3.47

Standard error of the difference 0.11 0.12 0.29 0.22 between means

RESOURCE USE AND INTERCROP PRODUCTIVITY 3 7 5

trogen transfer can occur from root to root contact (Martin et al., 1991 ) or through release following ploughing-in of the legume prior to the non-legume harvest (Pandey and Pendleton, 1986). Interspecific competition for irradiance is minimized by judicious choice of crop geometry and apt harvest dates. Incorporation of two rows of 6-week-old soybean, sown as an intercrop with maize, contributed the equivalent of 28 kg N ha- ~ to the maize crop in unfer- tilized plots but, with 100 kg N ha- ~ fertilizer, adequate inorganic N obviated the contribution of N from soybean to the maize (Pandey and Pendleton, 1986).

Influence of intercropping on the efficiency of use of N (expressed as kg yield/kg N taken up, Tanaka et al., 1984) is an important consideration while evaluating the merits of intercropping. In contrast to a decrease in nitrogen- use efficiency (NUE) for sole maize over the range 0-120 kg N ha-1 (Ofori and Stern, 1987b), NUE for intercropped maize and cowpea was relatively stable over the same range of N. The same trend was evident for another cereal/legume mixture, sorghum and soybean (Baker and Blamey, 1985 ), where apparent recovery of applied N was also greatest in the sole cereal, and recovery of N by the intercrop was intermediate to that of the sole crops.

The benefit of intercrops, especially involving a legume, for improved soil fertility are not confined to the current intercrop system, but may also be imparted through release of residual nutrients for succeeding crops. While N applied to legumes intercropped with maize was apparently inhibitory to N fixation, N uptake by a subsequent wheat crop was similar following sole maize with 100 kg N ha -~ or intercropped maize with no N (Searle et al., 1981 ). Even higher levels of residual N might have been obtained if legume tops had been returned to the plots. In a like manner, rotation with legume/cereal intercrops increased grain yield of succeeding wheat by 0.2 to 1.3 t ha- ~ (Table 4 ). For a target 4 t ha- ~ wheat crop this represented potential savings of 39 to 87 kg N ha- ~ (Waghmare and Singh, 1984b). No interaction was evident between N rate (0, 40, 80, and 120 kg ha - l ) and the intercrop system on wheat grain yield, and the same treatment (i.e. sorghum+fodder cowpea) which resulted in least competition with sorghum and which led to maximum sorghum yield (Table 4), also resulted in greatest wheat grain yield.

In the above examples, it is inferred that the legumes provide residual N to succeeding crops through N fixation. Under high N fertilizer rates the legumes may also, through uptake, prevent leaching of N, as too may non-legume intercrops, with a slower release of nutrients over time to companion or successive crops through mineralization of plant residues. Although the data which substantiate this mainly involve legumes (Yadav, 1982) they do illustrate this important benefit of intercropping, particularly notable at higher N fertilizer rates.

376 D.J. MIDMORE

C O N C L U S I O N S

The greater complementarity of intercrops in utilizing natural resources at low input levels has been a recurrent result of experiments aimed at raising productivity under conditions of sustainability. While competition is to a greater degree kept in check under low inputs, there is no a priori reason why competition should not be manipulated at greater levels of input, until complete and efficient radiation interception and conversion to harvestable products is effected. The addition of one input to raise yields in a sole-crop system must be balanced or complemented by other inputs (e.g. additional N can only be utilized if sufficient soil moisture is present). The same holds true for intercrops. With intercrops, however, the influence of interspecific competition, evolving from interspecific complementarity, limits the preferential use of additional inputs and, if not compensated for in some other way (e.g. hornworm-induced reduction of tomato foliage growth resulting in additional interception of irradiance by cucumber when irrigated, Schultz et al., 1987, Ta- ble 3), results in shifts of dominance which may, or may not, fit the expectations of the farmer-producer.

Much knowledge has been assembled on the optimum complementarity by intercrops in their use of above-ground space particularly through manipulation of plant population and crop geometry, but recommendations for the proper use of inputs (e.g. fertilizer and irrigation management), which will enable the farmer to manipulate interspecific competition and yield outcome of intercrops, are however sadly lacking. Point or staggered application of inputs to one component is a potentially practical tool for the farmer who wishes to have greater control over the output of mixtures, but research-based recommendations are lacking. Furthermore, the agronomist is barely cogni- zant of the series of intimate biochemical interactions both above and below the soil surface which enhance or check productivity of crop mixtures. A sound knowledge of the basis for superior intercrop productivity will lay the basis for matching interspecific interactions with opportunities to mechanize aspects of intercropping. Significant advances in this challenge have been made in recent years (Nankar, 1990).

In conclusion, while our understanding of the consequences of differential light interception for interspeciflc competition within crop mixtures is sound, many gaps still exist in our knowledge of how these are effected, and how the alterations in the balance of subterranean competition for nutrients and soil moisture are manifest. An increasingly important role for PC-based pro- grammes which model interspecific interactions in resource use will evolve to fill the many gaps in our knowledge (Caldwell and Hansen, 1990 ).


REFERENCES

Ae, N., Arihara, J., Okada, K., Yoshihara, T. and Johansen, C., 1990. Phosphorus uptake by pigeon pea and its role in cropping systems of the Indian subcontinent. Science, 248: 477- 480.

Amador, M.F. and Gliessman, S.R., 1990. An ecological approach to reducing external inputs through the use of intercropping. In: S.R. Gliessman (Editor), Agroecology. Researching the Ecological Basis for Sustainable Agriculture. Springer-Vedag, New York, pp. 146-159.

Baker, C.M. and Blarney, F.P.C., 1985. Nitrogen fertilizer effects on yield and nitrogen uptake of sorghum and soybean, grown in sole cropping and intercropping systems. Field Crops Res., 12: 233-240.

Baldev, B. and Ramanujam, S., 1980. Pulses in scientific intercropping - a critical appraisal. Indian J. Genet., 40A: 1-10.

Bonaparte, E.E.N.A. and Brown, R.I., 1975. The effect of intra-specific competition on the phenotypic plasticity of morphological and agronomic characters of four maize hybrids. Ann. Bot., 39: 863-869.

Boucher, D. and Espinosa, J., 1982. Cropping systems and growth and nodulation responses of beans to nitrogen in Tabasco, Mexico. Trop. Agric. (Trinidad), 59: 279-282.

Caldwell, R. and Hansen, J., 1990. Simulation of intercropping with IBSNAT models. Agro- technol. Transfer, 12: 1-4.

Cannell, M.G.R., 1983a. Plant management in agroforestry: Manipulation of trees, population densities and mixtures of trees and herbaceous crops. In: P.A. Huxley (Editor), Plant Re- search and Agroforestry. ICRAF, Nairobi, pp. 456-487.

Cannell, M.G.R., 1983b. Plant population and yield of tree and herbaceous crops. In: P.A. Hux- ley (Editor), Plant Research and Agroforestry. ICRAF, Nairobi, pp. 489-502.

Cenpukdee, U. and Fukai, S., 1991. Effects of nitrogen supply on cassava/pigeonpea intercropping with three contrasting cassava cultivars. Fert. Res., 29: 275-280.

Cenpukdee, U. and Fukai, S., 1992a. Cassava/legume intercropping with contrasting cassava cultivars. I. Competition between component crops under three intercropping conditions. Field Crops Res., 29:113-133.

Cenpukdee, U. and Fukai, S., 1992b. Cassava/legume intercropping with contrasting cassava cultivars. 2. Selection criteria for cassava genotypes in intercropping with two contrasting legume crops. Field Crops Res., 29: 135-149.

Cenpukdee, U. and Fukai, S., 1992c. Agronomic modification of competition between cassava and pigeonpea in intercropping. Field Crops Res., 30:131-146.

Chang, J.F. and Shibles, R.M., 1985a. An analysis of competition between intercropped cowpea and maize. I. Soil N and P levels and their relationships with dry matter and seed productivity. Field Crops Res., 12: 133-143.

Chang, J.F. and Shibles, R.M., 1985b. An analysis of competition between intercropped cowpea and maize. II. The effect of fertilization and population density. Field Crops Res., 12: 145- 152.

Chui, J.A.N. and Shibles, R., 1984. Influence of spatial arrangements of maize on performance of an associated soybean intercrop. Field Crops Res., 8:187-198.

Dayal, D. and Reddy, S., 1991. Intercropping of rainfed groundnut (Arachis hypogaea) with annual oilseed crops under different planting patterns. Indian J. Agric. Sci., 61: 299-302.

Dhingra, K.K., Dhillon, M.S., Grewal, D.S. and Sharma, K., 1991. Performance of maize and mungbean intercropping in different planting patterns and row orientations. Indian J. Agron., 36" 207-212.

FFTC, 1974. Multiple Cropping Systems in Taiwan. Food and Fertilizer Technolgy Center. Taipei, Taiwan, 77 pp.

378 D.J. MIDMORE

Fisher, N.M., 1977. Studies in mixed cropping. I. Seasonal differences in relative productivity of crop mixtures and pure stands in the Kenya Highlands. Exp. Agric., 13:177-184.

Fukai, S. and Trenbath, B.R., 1993. Processes determining intercrop productivity and yields of component crops. Field Crops. Res., 34:247-27 I.

Fukai, S., Tsay, J.S., Wilson, G.L., Cenpukdee, U. and Grady, D.P.S., 1990. Cassava-legume intercropping in the subtropics. In: R.H. Howeler (Editor), Proceedings 8th Symposium of the International Society for Tropical Root Crops. ISTRC, Bangkok, Thailand, pp. 293-300.

Fusseder, A., 1986. Distribution of the root systems of Zea mays L. and Lupinus luteus L. in mixed cropping with respect to competition for phosphate and potassium. Z. Pflanzern~ihr. Bodenkd., 149: 541-547.

Gliessman, S.R., 1983. Allelopathic interactions in crop-weed mixtures: applications for weed management. J. Chem. Ecol., 9: 991-999.

Han, X., Liu, X. and Kong, Y., 1985. Studies on climate-ecological adaptation of relay cropping in China. In: International Multiple Cropping Systems Conference. Chinese Association of Agricultural Science Societies, Nanjing, Jiangsu, China, pp. 231-238.

Horst, W.J. and Waschkies, C., 1987. Phosphorus nutrition of spring wheat ( Triticum aestivurn L. ) in mixed culture with white lupin (Lupinus albus L. ). Z. Pflanzerniihr. Bodenkd., 150: 1-8.

Hulugalle, N.R. and Lal, R., 1986. Soil water balance ofintercropped maize and cowpea grown in a tropical hydromorphic soil in Western Nigeria. Agron. J., 77: 86-90.

Hulugalle, N.R. and Willatt, S.T., 1987. Seasonal variation in the water uptake and leaf water potential ofintercropped and monocropped chillies. Exp. Agric., 23: 273-282.

ICRISAT, 1981. ICRISAT (International Crops Research Institute for the Semi-Arid Tropics) Annual Report 1979/80. ICRISAT, Patancheru, A.P. India, pp.203-210.

Ikeorgu, J.E.G., Wahua, T.A.T. and Ezumah, H.C., 1989. Effects of melon (Citrullus lanatus Thunb. ) and okra (Abelmoschus esculentus (L.) Moench) on the soil moisture and leaf water status of intercropped cassava/maize in Nigeria. Trop. Agric. (Trinidad), 66: 78-83.

IITA, 1983. International Institute for Tropical Agriculture Annual Report 1982. IITA, Ibadan, Nigeria, pp. 154-155.

Kang, B.T., Grimme, H. and Lawson, T.L., 1985. Alley cropping sequentially cropped maize and cowpea with Leucaena on a sandy soil in Southern Nigeria. Plant Soil, 85: 267-277.

Kuruppuarachchi, D.S.P., 1990. Intercropped potato (Solanum spp. ): Effect of shade on growth and tuber yield in the northwestern regosol belt of Sri Lanka. Field Crops Res., 25: 61-72.

Lai, Z. and Wen, G., 1990. Row-ratios and plant density in potato/maize strip-cropping. Field Crops Res., 25:51-59.

Lakhani, D.A., 1976. A crop physiological study of mixtures of sunflower and fodder radish. Ph.D. Thesis, Reading University.

Leihner, D.E., 1979. Agronomic implications of cassava-legume intercropping systems. In: E. Weber, B. Nestel and M. Campbell (Editors), Intercropping with Cassava. International Development Research Centre, Trivandrum, pp. 103-112.

Liebman, M., 1989. Effects of nitrogen fertilizer, irrigation, and crop genotype on canopy rela- tions and yields of an intercrop/weed mixture. Field Crops Res., 22: 83-100.

Liu, J. and Midmore, D.J., 1990. A review of potato intercropping practices in Western Hubei, China. Field Crops Res., 25: 41-50.

Mahapatra, P.K., Dikshit, U.N., Pradhan, N.K. and Uttaray, S.K., 1991. Effect of phosphorus on pigeonpea (Cajanus cajan) and rice (Oryza sativa) intercropping. Indian J. Agric. Sci., 61: 308-310.

Martin, R.C., Voldeng, H.D. and Smith, D.L., 1991. Nitrogen transfer from nodulating soybean (Glycine max (U) Merr.), to corn (Zea mays L.) and non-nodulating soybean in intercrops: direct ~SN labelling methods. New Phytol., 117: 233-241.

May, K.W. and Misangu, R., 1982. Soybean-cereal intercropping and its implications in soy-


bean breeding. In: C.L. Keswani and B.J. Ndunguru (Editors), Intercropping in Semi-Arid Areas. International Development Research Centre, Morogoro, Tanzania, pp. 79-83.

Midmore, D.J., 1990. Scientific basis and scope for improvement of intercropping with potato in the tropics. Field Crops Res., 25: 3-24.

Midmore, D.J., Roca, J. and Berrios, D., 1988a. Potato (Solanum spp. ) in the hot tropics. IV. Intercropping with maize and the influence of shade on potato microenvironment and crop growth. Field Crops Res., 18:145-157.

Midmore, D.J., Berrios, D. and Roca, J., 1988b. Potato (Solanum spp.) in the hot tropics. V. Intercropping with maize and the influence of shade on tuber yields. Field Crops Res., 18: 158-176.

Mohta, N.IC and De, E., 1980. Intercropping maize and sorghum with soyabeans. J. Agric. Sci. Camb., 95: 117-122.

Mugabe, N.R., Sinje, M.E. and Sibuga, K.P., 1982. A study of crop/weed competition in intercropping. In: C.L. Keswani and B.J. Ndunguru (Editors), Intercropping in Semi-Arid Areas. International Development Research Centre, Morogoro, Tanzania, pp. 96-101.

Mutsaers, H.J.W., 1980. The effect of row orientation, date and latitude on light absorption by row crops. J. Agric. Sci. Camb., 95: 381-386.

Nambiar, P.T.C., Rap, M.R., Reddy, M.S., Floyd, C.N., Dart, P.J. and Willey, R.W., 1983. Effect of intercropping on nodulation and N2-fixation by groundnut. Exp. Agric., 19: 79-86.

Nankar, J.T., 1990. Scope and prospects for intercropping of potato with sugarcane in Mahar- ashtra State, India. Field Crops Res., 25:123-132.

Nnko, E.N. and Doto, A.L., 1982. Intercropping maize or millet with soybean, with particular reference to planting schedule. In: C.L. Keswani and B.J. Ndunguru (Editors), Intercrop- ping in Semi-Arid Areas. International Development Research Centre, Morogoro, Tanzania, pp. 33-36.

Ofori, F. and Stem, W.R., 1987a. Relative sowing time and density of component crops in a maize/cowpea intercrop system. Exp. Agric., 23:41-52.

Ofori, F. and Stem, W.R., 1987b. The combined effects of nitrogen fertilizer and density of the legume component on production efficiency in a maize/cowpea intercrop system. Field Crops Res., 16: 43-52.

Ojeifo, I.M. and Lucas, E.O., 1987. The growth and development of Corchorus olitorius (L.) grown alone and intercropped with tomato (Lycopersicon esculentum (Mill.) ). J. Agric. Sci. Camb., 109: 39-45.

Olasantan, F.O., 1991. Response of tomato and okra to nitrogen fertilizer in sole cropping and intercropping with cowpea. J. Hortic. Sci., 66:191-199.

Pandey, R.K. and Pendleton, J.W., 1986. Soyabeans as green manure in a maize intercropping system. Exp. Agric., 22: 179-185.

Ports, M.J., 1990. Influence of intercropping in warm climates on pests and diseases of potato, with special reference to their control. Field Crops Res., 25:133-144.

Rees, D.J., 1986. Crop growth, development and yield in semi-arid conditions in Botswana. II. The effects of intercropping Sorghum bicolor with Vigna unguiculata. Exp. Agric., 22:169- 177.

Rosset, P.M., Ambrose, R.J., Power, A.J. and Hruska, A.J., 1984. Overyielding in polycultures of tomato and bean in Costa Rica. Trop. Agric. (Trinidad), 61: 208-212.

Russell, J.T. and Caldwell, R.M., 1989. Effects of component densities and nitrogen fertilization on efficiency and yield ofa maize/soyabean intercrop. Exp. Agric., 25: 529-540.

Schuiz, B., McGuinness, H., Horwith, B.J.V., Phillips, C., Perfecto, I., Rosset, P., Ambrose, R. and Hansen, M., 1987. Effects of planting densities, irrigation, and homworm larvae on yields in experimental intercrops of tomatoes and cucumbers. J. Am. Soc. Hortic. Sci., 112: 747-755.

Searle, P.G.E., Comudom, Y., Shedden, D.C. and Nance, R.A., 1981. Effect of maize+legume

380 D.J. MIDMORE

intercropping systems and fertilizer nitrogen on crop yields and residual nitrogen. Field Crops Res., 4: 133-145.

Sharma, S.N., Prasad, M., Prasad, R. and Singh, S., 1991. Productivity and nitrogen require- ment of potato (Solarium tuberosum)-based sequential and relay cropping systems. Indian J. Agric. Sci., 61:311-315.

Simmonds, N.W., 1979. Principles of Crop Improvement. Longman, London, 408 pp. Singh, R.P. and Singh, K.C., 1980. Intercropping systems with grain legume component for

drylands. Indian J. Genet., 40A: 136-146. Snaydon, R.W., 1991. Replacement or additive designs for competition studies? J. Appl. Ecol.,

28: 930-946. Srinivasan, V.M., Subramaniam, S. and Rai, R.S.V., 1990. A note on allelopoly and tree age up

to which intercropping is feasible. J. Trop. Forest Res., 3: 89-92. Tanaka, A., Yamaguchi, J., Miura, S. and Tamaru, H., 1984. Comparison of fertilizer nitrogen

efficiency among field crops. Soil Sci. Plant Nutr., 30: 199-208. Thung, M. and Cock, J.H., 1979. Multiple cropping cassava and field beans: status of present

work at the International Centre of Tropical Agriculture (CIAT). In: E. Weber, B. Nestel and M. Campbell (Eds.), Intercropping with Cassava. International Development Research Centre, Trivandrum, pp. 7-16.

Trenbath, B.R., 1993. Intercropping for the management of pests and diseases. Field Crops Res., 34:381-405.

Tsay, J.S., Fukai, S. and Wilson, G.L., 1985. Soybean response to intercropping with cassava. In: S. Shanmugasundaram and E.W. Sulzberger (Editors), Soybean in Tropical and Sub- tropical Cropping Systems. AVRDC, Tainan, Taiwan, pp. 13-24.

Tsay, J.S., Fukai, S. and Wilson, G.L., 1989. Growth and yield of cassava as influenced by intercropped soybean and by nitrogen application. Field Crops Res., 2 l: 83-94.

Vandermeer, J., 1981. The interference production principle: an ecological theory for agriculture. BioScience, 31: 361-364.

Verinumbe, I. and Okali, D.U.U., 1985. The effect of coppiced teak regrowth on soil in a teak- maize agroforest. For. Ecol. Manage., 12: 37-41.

Waghmare, A.B. and Singh, S.P., 1984a. Sorghum-legume intercropping and the effects of nitrogen fertilization. I. Yield and nitrogen uptake by crops. Exp. Agric., 20:251-259.

Waghmare, A.B. and Singh, S.P., 1984b. Sorghum-legume intercropping and the effects of nitrogen fertilization. II. Residual effect on wheat. Exp. Agric., 20:261-265.

Willey, R.W. and Natarajan, M., 1980. Some aspects of resource use in sorghum/pigeonpea intercropping. Indian J. Genet., 40A: 16-26.

Willey, R.W., Natarajan, M., Reddy, M.S., Rao, M.R., Nambiar, P.T.C., Kannaiyan, J. and Bhatnagar, V.S., 1983. Intercropping studies with annual crops. In: Better Crops for Food. Ciba Foundation Symposium 97. Pitman Books, London, pp. 83-97.

Yadav, R.L., 1982. Minimizing nitrate-nitrogen leaching by parallel multiple cropping in long- duration row crops. Exp. Agric., 18: 37-842.

agronomic modification of resource use and intercrop productivity

Documents