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II. Background to Intercropping Systems

II. Background to Intercropping Systems

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CEREAL-LEGUME INTERCROPPING SYSTEMS



43



as millet and sorghum dominate (Andrews, 1972; Baker, 1979). In areas with

annual rainfall greater than 600 mm, cereals and legumes of varying maturities

are used. In the tropical and subtropical regions, the cereal component is usually maize, sorghum, millet, or, to a lesser extent, rice, and the legume is usually

cowpea, groundnut, soybean, chickpea, bean, or pigeonpea. Both early- and

slow-maturing crops are combined to ensure efficient utilization of the whole

growing season (Baker, 1979). The millet or sorghum and pigeonpea combinations of the Hyderabad area of India are a typical example (Ruthenberg, 1980).

Maize seems to dominate as the cereal component and it is combined with many

different legumes.

In India, short-duration sorghum and millet (90days) are intercropped with

pigeonpea that matures 90days later than the cereal (Willey, 1979). In high rainfall areas of West Africa, a common crop combination is maize and cowpea

(Okigbo and Greenland, 1976), whereas in South and Central America, maize

and different types of beans dominate (Francis et al., 1976). Combinations of

rice and other cereals or legumes are also found in high rainfall areas with a

single extended rainy season such as Southeast Asia (Ruthenberg, 1980). In

some temperate regions with warm climates, intercrop systems consist of

wheat, oats, or barley as the cereal component and field bean, vetch, lupin, or

soybean as the legume component (Table I).

Geographical patterns for different crop combinations are discernable

(Table I), and these generally follow general climatic classificationssuch as the

one of Koppen (Trewartha, 1954).

B.



INDICES FOR EVALUATING

PRODUCTIVITY AND EFFICIENCY



Different indices have been suggested for evaluating productivity and efficiency per unit area of land of cereal-legume intercrop systems (Willey, 1979,

1985; Beets, 1982). These include comparisons of absolute yields, protein

yields, caloric equivalent, and in economic terms, gross returns from intercrops

and sole crops. Evaluation in economic terms is considered inappropriate due to

seasonal price fluctuations of inputs and the lack of cash economy in most areas

where intercropping is practiced (Beets, 1982). Yields and prevailing prices of

crops tend to fluctuate, composition and quality of crop products will vary, and

energy contents and growth durations of the component crops differ, so combined yields are of little value (de Wit et al., 1966; Beets, 1982). Van den Bergh

(1968) suggested relative yields for comparing performance of crops in intercropping mixtures. Willey (1 979) also suggested standardizing component crop

yields in relation to sole crop yields in order to evaluate intercropping efficiency.



I . Relative Yield Total

The mixture yield of a component crop expressed as a proportion of its

yield as a sole crop from the same replacement series is the relative yield of the



Table I

The Distribution of Various Intercrop Systems in Different Climatic Types



Crop combinations

Maize intercropped with

Beans



Climatic

type after

Koppen"



Irrigated

or

rain fedb



Country and location of study



Reference



Af

Am

Am

Aw

Aw

Aw



R

R

R

R

R

R



Costa Rica, Turrialba

Sri Lanka, Peradeniya

Colombia, Palmira

Uganda, Kampala

Tanzania, Morogoro

Brazil, Pernambuco



9.56"N,

7.16"N.

3.33"N,

0.19"N,

6.49"N,

8.14"S,



83.48"W

80.37"E

76.17"W

32.53"E

37.40"E

38.00"W



Hart (1975)



cowpea



Af

Af

Am

Am

Aw

Aw

BS



R

R

R

R

R

R



5.54"S,

9.56"N,

7.23"N,

7.16'N,

6.49"N,

8.14"N,

5.33"N,



76.07 "W

83.48"W

3.56"W

80.37"E

37.40"E

38.00"W

0.1S"W



Wade and Sanchez (1984)

Chang and Shibles (1985a)

Remison (1978)

Gunasena et al. (1979)

Enyi (1973)

Mafra el of. (1979)

Haizel (1974)



Groundnut



Am

Aw

Aw

Ca

Am

Am

Aw

Ca

Da

Da

Da



R

R

R

R

R

R

R



Peru, Yurimaguas

Costa Rica, Turrialba

Nigeria, Ibadan

Sri Lanka, Peradeniya

Tanzania, Morogoro

Brazil, Pernambuco

Ghana, Accra

Sri Lanka, Peradeniya

Tanzania, Morogoro

India, Hyderabad

India, Meerut

Trinidad, St Augustine

Sri Lanka, Peradeniya

Zimbabwe, Harare

Australia, Camden

U.S.A., Minnesota

U.S.A., Ames

U.S.A., Amherst



7.16 ON, 80.37 "E

6.49"N, 37.40"E

17.22"N, 78.26"E

29.00"N, 77.42"E

3.38"N, 61.31"W

7.16"N, 80.37"E

17.43"S,

31.OS"E

34.04"S, 150.40"E

40.06"N. 91.46"W

42.02"N. 93.39"W

42.23"N, 72.31"W



Gunasena et af. (1979)

Evans (1960)

Nambiar et al. (1983)

Gangwar and Karla (1982)

Dallal (1974)

Gunasena et a/. (1979)

Beets (1977)

Searle et al. (1981)

Crookston and Hill (1979)

Chui and Shibles (1984)

Putnam et a/. (1985)



%



Soybean



1, R



1, R



I

I

I



Gunasena et al. (1979)

Francis et al. (1982a)

Willey and Osiru (1972)

Enyi (1973)

Mafra et of. (1979)



Blackgram

Greengram



Ca

Am



R



India, Meerut

Nigeria, Ibadan



29.00"N,

7.23"N.



Clover

Rice



Ca

Af



I

R



U.S.A., Urbana

Phillipines, Manila



40.07'N,

88.12W

14.37"N, 120.58"E



Millet



Aw

AW



R



Sorghum



R



Nigeria, Samaru

Nigeria, Samaru



11.1l0N,

11.1l0N,



7.38"E

7.38"E



Cassava



Af



R



Costa Rica, Turrialba



9.56"N,



83.48"W



Aw

Aw

Aw



R

R

R



Uganda, Kampala

Tanzania, Morogoro

Brazil, Pernambuco



Cowpea



Aw

Aw

Ca

BS



R

R

R

I



Tanzania, Morogoro

Brazil, Pernambuco

India, New Delhi

U.S. A., Riverside



0.19"N, 32.35"E

6.49"S,

37.40"E

8.149,

38.00"W

6.49"s.

37.40"E

8.14"S,

38.00"W

28.37"N, 77.13"E

33.59"N, 117.22"W



Groundnut



Aw

Ca



R

R



Tanzania, Morogoro

India, New Delhi



6.49"S,

28.37"N,



37.40"E

77.13"E



Soybean



Aw

Ca

Da



R

R

I



Blackgram



Ca

Aw



R

R



Puerto Rico, Isabela

India, New Delhi

U.S.A., Urbana

India, New Delhi

Tanzania, Morogoro



18.30°N,

28.37"N,

40.07ON,

28.37"N,

6.49"S,



67.20"W

77.13"E

88.12OW

77.13"E

37.40"E



Ca



R



India, New Delhi



28.37"N.



77.13"E



Singh (1981)



Aw

Aw

Ca



R

R

R



Tanzania, Morogoro

Brazil, Pernambuco

India, New Delhi



6.49"N.

8.14"S,

28.37"N,



37.40"E

38.00"W

77.13"E



Enyi (1973)

Mafra et at. (1979)

Singh (1981)



R



77.42"E

3.56"E



Gangwar and Karla (1982)

Agboola and Fayemi (1972)

Kurtz et a/. (1952)

IRRI (1974)

Andrews (1972)

Baker (1979)



Hart (1975)



Sorghum intercropped with

Beans



111

P



Chickpea

Greengram

Pigeonpea



Osiru and Willey (1972)

Enyi (1973)

Mafra et 01. (1979)

Enyi (1973)

Mafra et 01. (1979)

Singh (1981)

Shackel and Hall (1984)

Evans (1960)

Wagmar and Singh (1984)

Elmore and Jacobs (1984)

Singh and Jain (1984)

Wahua and Miller (1978)

Singh (1981)

Chowhury and Misangu (1981)



(Continued)



Table I (Continued)



Crop combinations



Climatic

type after

KoppenO



Irrigated

or

rain fedb



Country and location of study



Reference



Sorghum intercropped with (cont.)

Cotton



Aw



11.1l0N,



7.38"W



Baker (1979)



Aw

Aw



R

R

R



Nigeria, Samaru



Sorghum

Millet



Nigeria, Samaru

Nigeria, Samaru



11.11"N,

11.11"N,



7.38"W

7.38"W



Norman (1974)

Andrews (1974)



Cowpea



Aw



R



Nigeria, Samaru



11.11 ON,



7.38"W



Andrews (1974)



Groundnut



Aw



R



India, Hyderabad



17.22"N,



78.26"E



Reddy and Willey (1981)



Greengram



Aw



R



Tanzania, Morogoro



6.49"S,



37.40"E



May (1982)



Cotton



Aw



R



Nigeria, Samaru



11.1l0N,



7.38"W



Cowpea



Af



R



Phillipines, Manila



14.37"N,



120.58"E



Harwood and Price (1976)



Mungbean



Af



R



Phillipines, Manila



14.37"N, 120.58"E



Harwood and Price (1976)



Peas



BS



R



Cyprus, Prastio



35.10"N.



33.45 "E



Hadjichristodoulou (1973)



Vetch

Soybean



BS

Ca



R

R



Cyprus, Prastio

U.S.A., Oklahoma



35.10°N,

35.28"N,



33.45 "E

97.33"W



Hadjichristodoulou (1973)

Crabtree and Rupp (1980)



Lupins



Da



R



Greece, Thessaloniki



40.38"N,



22.58"E



Field beans



Da



R



U.K., Reading



51.28"N,



0.59"W



Millet intercropped with



Baker (1979)



Rice intercropped with



Wheat intercropped with



Barley intercropped with



'Koppen's classification as in Trewartha (1954).

bR, rain-fed, I, irrigated.



Papadakis (1941)

Martin and Snaydon (1982)



CEREAL-LEGUME INTERCROPPING SYSTEMS



41



crop (de Wit and van den Bergh, 1965; van den Bergh, 1968). The sum of the

relative yields of component crops is called the Relative Yield Total and is

denoted by RYT (de Wit and van den Bergh, 1965; Harper, 1977). When the

RYT is equal to or less than 1, there is no advantage to intercropping.

Although the calculation of RYT was originally based on the replacement

series in competition studies, where proportions of the components in binary

mixtures are varied but the overall crop densities remain constant (de Wit,

1960; de Wit and van den Bergh, 1965), the calculation can in fact be applied

to any density situation in intercropping. However, Mead and Riley (1981)

have argued against its use because the objectives of intercropping are essentially agronomic, that is to find the best ways of growing crops together.

2. Land Equivalent Ratio

Willey and Osiru (1972) proposed the concept of the Land Equivalent

Ratio (LER) as an index of combined yield for evaluating the effectiveness of

all forms of intercropping. LER is defined as the total land area required

under sole cropping to give the yields obtained in the intercropping mixture.

It is expressed as:

LER =



(Yjj/Y,i)



+ (Y,i/qj)



where Y is the yield per unit area, Yi and Yj are sole crop yields of the component crops i and j, and yij and Tiare intercrop yields (Mead and Willey, 1980).

The partial LER values, Li and L j , represent the ratios of the yields of crops i

and j when grown as intercrops, relative to sole crops. Thus,

Li



=



(xj/Yi) and Lj = (Yi/qj)



LER is the sum of the two partial land equivalent ratios so that

LER



=



L,



+ Lj



When LER = 1, there is no advantage to intercropping in comparison with

sole cropping. When LER > 1, a larger area of land is needed to produce the

same yield of sole crop of each component than with an intercropping mixture. For example, when LER = 1.25, 25% more land is needed to produce

the same yield from the components as sole crops.



3. Area Time Equivalent Ratio

The Area Time Equivalent Ratio (ATER) was proposed by Hiebsch (1980)



and McCollum (1982) as a modification of the LER. This takes into account



48



FRANCIS OFORI AND W. R. STERN



the duration of the crop, i.e., the time it occupies from planting to

harvesting; it also permits an evaluation of crops on a yield-per-day basis

(Hiebsch and McCollum, 1987). It does not appear to have been adopted

widely. It is calculated as:

ATER = (Liti



+ Ljtj)/T



where Li and Lj are relative yields or partial LERs of component crops i and

j, ti and tj are the durations (days) for crops i and j , and T is the duration

(days) of the whole intercrop system.

4. Staple Land Equivalent Ratio



In situations where the primary objective is to produce a fixed yield of

one component (staple) crop, usually the cereal, and some yield of the

legume, Reddy and Chetty (1984) proposed the concept of the Staple Land

Equivalent Ratio (SLER) as an extension of the LER. It is based on the

assumption of a basic requirement for minimum supply of a major staple

crop such as the cereal. It is estimated as:

SLER =



(q/qi) + Pij ( Y j i / Y j j )



where &/yiis “the desired standardized yield” of staple i, Pijis the proportion of land devoted to intercropping, and Yji/yjj is the relative yield of crop

j. This index is peculiar to India and does not appear to have been used widely

there either.



5.



Comparison of RYT, LER, ATER, and SLER



Examples are given in Table I1 of some RYT, LER, ATER, and SLER

values calculated from two experiments that examined maize intercropped

with either pigeonpea or soybean in replacement series (Dallal, 1974, 1977).

For both intercrop systems, all four functions indicate higher biological efficiency in the maize-pigeonpea combination because components are of

contrasting maturities. The lower efficiency of the maize-soybean combination, however, could be due to competitive factors because with similar

maturities, the components would be exploiting the same environment.

The values of RYT and LER were similar in both intercrop systems but as

has already been mentioned, RYT is generally based on the replacement

series (de Wit, 1960; de Wit and van den Bergh, 1965). Besides the RYT,

both ATER and SLER are restricted to specific intercrop situations. ATER



Table I1

Comparison of Calculated Values of Four Indices for Evaluating Cereal-Legume Intercropping Efficiene

Maize-pigeonpea (Dallal, 1974)



Crop duration (days)b

Sole crop yield (kg/ha)

Intercrop yield (kg/ha)

Relative yield total (RYT)

Land equivalent ratio (LER)

Area time equivalent ratio (ATER)

Staple land equivalent ratio (SLER)



Maize-soybean (Dallal, 1977)



Maize



Pigeonpea



Maize



Soybean



110

3130

2979



170

1871

1481



110

5353

5118



110

1634

517



1.74

1.74

1.41

1.35



‘Derived from the data of Dallal (1974, 1977).

%ese values have been rounded off to the nearest 10.



1.28

1.28

1.28

1.28



50



FRANCIS OFORI AND W.



R. STERN



is only appropriate in systems with component crops of contrasting maturities

such as 110-daymaize and 170-daypigeonpea (Dallal, 1974);when components

are of similar growth durations, ATER values are similar to RYT and LER

(Table 11) (Hiebsch and McCollum, 1987). SLER is only applicable when it is

desired to attain a specificyield of a staple cereal crop and yield from the legume

component is a bonus (Willey, 1979). SLER therefore cannot be used to

evaluate intercropping efficiency in situations where it is desired to produce

yield from equally acceptable component crops, and this is the most common

goal of cereal-legume intercrop systems (Willey, 1979).

From this, LER is considered to be the most appropriate general function to

determine the efficiency of cereal-legume intercrop systems and could be applied to any form of intercropping. However, when the difference between

growth durations of component crops is substantial, time becomes an important element and ATER is considered to be a more appropriate index of efficiency of the system. In this chapter, except where otherwise stated, intercropping

efficiency or yield advantage due to intercropping will be estimated by the LER.



c. THELANDEQUIVALENT

RATIO:

ITSADVANTAGES

AND DISADVANTAGES

The Land Equivalent Ratio (LER) is the most frequently used index to determine the effectiveness of intercropping relative to growing crops separately

(Willey, 1985). Generally, the value of LER is determined by several factors including density and competitive abilities of the component crops in the mixture,

crop morphology and duration, and management variables that affect individual crop species (Enyi, 1973; Natarajan and Willey, 1980a; Fawusi et al.,

1982).

It has been suggested that in density studies of cereal-legume intercrop

systems, the sole crop yields used as standardization factors for estimating LER

should be at the optimum densities of the crops (IRRI, 1974; Huxley and

Maingu, 1978). This avoids the confounding of beneficial interactions between

components with a response to change in density (Trenbath, 1976). The values

of LER follow the density of the legume component rather than that of the

cereal (Ofori and Stern, 1987).

Differences in competitive ability affect the relative performance of component crops and thus the LER values of different cereal-legume intercrop

systems. In maize-bean combinations, Davis and Garcia (1983) found that

yield reductions were related to plant height. Maize yields were reduced by 17%

when intercropped with the most competitive bean cultivars (Group IVB, the

aggressive climbers), by 14% with the intermediate cultivars (Group IVA), and

by 8% with the weak climbing cultivars (Group IIIB). The LER values

calculated using the mean yield of 114 cultivars of beans as a standardization

factor were 1.72 for maize cultivar ICA intercropped with bean Group IVA,



CEREAL-LEGUME INTERCROPPING SYSTEMS



51



and 1.30 with bean Group IIIB. In another study, which compared three

contrasting maize cultivars intercropped with beans, LER values for the

short maize cultivar, La Posta, and the medium height cultivers, Suwan-1,

were 1.04, and with the taller cultivar, ICA H-210, it was 0.94. Using

sorghum and soybean, Wahua and Miller (1978a) obtained an LER value of

1.11 with tall and 1.09 with short sorghum, not a significant difference.

Differences in growth durations of component crops affect the

magnitude of the LER. The LER values in crops with similar maturities are

usually less in crop combinations with contrasting maturities (Table 11)

(Trenbath, 1976; Willey, 1979). Enyi (1973) studied maize or sorghum intercropped with either cowpea or beans of similar growth durations and found

that productivity was less when compared to intercropping of these cereals

with 240-day pigeonpea. The estimated partial LER of maize was 0.72 with

pigeonpea, 0.64 with beans, and 0.50 with cowpea. With sorghum, a partial

LER of 1.65 was obtained with pigeonpea, 1.01 with beans, and 0.93 with

cowpea. The almost similar and lower partial LER values for the associations with beans and cowpea may be due to competition for growth-limiting

factors, because peak demands on the environment by these crops might

have coincided with those of the cereals. The higher partial LERs obtained

with sorghum compared to maize indicates that it was offered less competition from the associated legumes and therefore was able to maintain higher

plant yields (Andrews, 1974).

The availability of water also appears to influence the LER. In maizebean (Fisher, 1977) and sorghum-cowpea intercrop systems (Mafra et al.,

1981; Rees, 1986), LER values increased with the availability of water and

diminished when water was limiting. However, Natarajan and Willey

(1980b) found that LER increased under limited water situations.

1. Advantages for LER



As an index of combined yield, LER provides a quantitative evaluation of

the yield advantage due to intercropping (Willey, 1979).

Although component crops may give greatly different yields, the estimate

of relative yields with sole crops at optimum or recommended densities as

references gives comparable scales for both components, permitting comparisons of various crop combinations.

LER could be used either as an index of biological efficiency to evaluate

the effects of various agronomic variables (e.g., fertility levels, density and

spacing, comparison of cultivar performance, relative time of sowing, and

crop combinations) on an intercrop system in a locality or as an index of

productivity across geographical locations to compare a variety of intercrop

systems (Chetty and Reddy, 1984).



52



FRANCIS OFORI AND W. R. STERN



LER is identical to the RYT of de Wit and van den Bergh (1965), and can

be used for any set of intercropping treatments (Willey, 1979).

The partial LER values give an indication of the relative competitive

abilities of the components of intercrop systems. In the intercropping mixture, the species with higher partial LER is considered to be more competitive for growth-limiting factors than the species with lower partial LER

(Willey, 1979). Partial LER is more applicable to intercropping experiments

than the relative crowding coefficient, k, used in measuring competitive

ability in competition studies (de Wit, 1960; Hall, 1974a,b).

2. Disadvantages of LER



LER is based on land area only and does not take the duration of component crops into consideration. However, crop production is a function of

both crop duration (time) and land area because land occupancy by a given

intercrop system is frequently of longer duration than for sole crops. In

these situations, the concept of Area Time Equivalent Ratio (ATER)

developed by Hiebsch (1980) is appropriate; a detailed analysis of its applicability is given by Hiebsch and McCollum (1987).

Several methods have been suggested in the literature for calculating LER

using different sole crop values as standardization factors. These include

averaging all the sole yields in each block or replication (Fisher, 1977); using

the average sole crop yields in the entire experiment (Mead and Stern, 1980;

Oyejola and Mead, 1982); using the average sole crop yield at each treatment level in studies that involve graded levels of a factor such as fertilizer

or herbicide (Mead and Willey, 1980); and using the yield of the best sole

crop treatment of each crop (Huxley and Maingu, 1978; Mead and Willey,

1980). The choice of sole crop yield for standardizing mixture yield in the

estimation of LER is not clear and a generalization is not possible. The

method used will depend on the aim of the experiment.

As an index of biological efficiency, LER is based on harvested products

and not on desired yield proportions of the component crops predetermined

at sowing (Mead and Stern, 1980; Mead and Willey, 1980; Riley, 1984).

This is overcome by the “effective LER” proposed by Mead and Stern

(1980), and again later by Mead and Willey (1980) as an alternative to LER

for evaluating the biological efficiency of a given required proportion of

component crops in an intercrop system.



111. COMPETITIVE RELATIONSHIPS

BETWEEN COMPONENT CROPS

In plant populations, competition is defined as the situation in which

each of two or more plants growing together in the same area seek the same



CEREAL-LEGUME INTERCROPPING SYSTEMS



53



growth factor, which is below their combined demands (Clements et al.,

1929; Donald, 1963).

Willey (1979) pointed out that the efficiency of production in

cereal-legume intercrop systems could be improved by minimizing interspecific competition between the component crops for growth-limiting

factors. Growing component crops with contrasting maturities so that they

complement rather than compete for the same resources at the same time is

one way of achieving this. It is substantiated by the yield advantages

reported from various studies: 85-day bean and 120-day sorghum gave a

55% total yield increase (Willey and Osiru, 1972; Reddy et al., 1980) obtained a 31% yield advantage with 82-day millet and 105-day groundnut; and

Natarajar) and Willey (1980a) found a 62% yield advantage with 82-day

sorghum and 173-day pigeonpea. In contrast, lesser advantages have been

reported in crop combinations in which interspecific competition is evident

due to similar or almost overlapping growth durations. In this category,

Wahua and Miller (1978a) obtained an 11Vo advantage with a sorghum-soybean combination; 11Yo was obtained with 112-day maize and 116-day soybean (Dallal, 1977); and 8% with a maize-cowpea system (Wanki et al.,

1982). No yield advantages were found in maize-cowpea (Haizel, 1974) and

sorghum-cowpea (Andrews, 1972; Rees, 1986) intercrop systems in which

components were of similar growth durations.

Competition between component crops for growth-limiting factors is

regulated by basic morpho-physiological differences and agronomic factors

such as the proportion of crops in the mixture, fertilizer applications, and

relative time of sowing (Harper, 1961; Trenbath, 1976). Where component

crops are arranged in defined rows, the degree of competition is determined

by the relative growth rates, growth durations, and proximity of roots of

the different crops. The cereal component, with relatively higher growth

rate, height advantage, and a more extensive rooting system, is favored in

the competition with the associated legume. The cereal is described as the

dominant component and the legume as the dominated component (Huxley

and Maingu, 1978). In considering the relative yields of cereals and legumes

in intercropping systems, a survey of 40 published papers shows that the

yield of the legume component declined, on average, by about 52% of the

sole crop yield, whereas the cereal yield was reduced by only 11% (Table

111). Thus the general observation is that yields of legume components are

significantly depressed by cereal components in intercropping.

A. THELIGHTFACTOR

The rate of dry matter production in crops depends on the efficiency of

the interception of photosynthetically active radiation (PAR) (Biscoe and

Gallagher, 1977; Monteith, 1977).



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II. Background to Intercropping Systems

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