INTRODUCTION
In southern Ethiopia, 46.5% of farmers have an average land holding of
0.1 to 0.5 ha with a further 25.4% having 0.51 to 1 ha (CACC, 2003). This
acute land scarcity necessitates farmers to use other alternatives to
improve their productivity. Intercropping systems play an important role
in subsistence and food production in developing countries (Tsubo and
Walker, 2002). Tefera and Tana (2002), Agegnehu et al. (2006),
Gebeyehu et al. (2006) and Ghosh et al. (2006) have shown
the advantage of intercropping over sole cropping in the tropics.
Multiple cropping is common in Southern Ethiopia with diverse associations
comprised of maize (Zea mays L.), tef (Eragrostis tef
(Zuc.) Trotter), rapeseed (Brassica carinata A. Braun), common
bean (Phaseolus vulgaris L.), sweet potato (Ipomoea batatas
(L.) Lam.), enset (Ensete ventricosum (Welw.) Cheeseman),
coffee (Coffea sp.) and banana (Musa acuminate Colla) (Worku,
2004). The dominant association, among these, is maize-common bean intercropping.
Suitable cultivars should be identified in order to maximize the benefit
from multiple cropping. For instance, Yadav and Yadav (2001) reported
that higher advantage from intercropping was obtained when clusterbean
was intercropped with the shorter and early maturing HHB 67 pearl millet
rather than with the tall and late maturing cultivar MH 179. The choice
of compatible cultivars would be very important in a crop like common
bean where there is great variation in the growth habit and morphology
of cultivars.
Differences in growth habit and vegetative traits among genotypes may
lead to differential performance under sole and intercropping systems.
For instance, Tefera and Tana (2002) suggested that the significant variation
among groundnut (Arachis hypogaea L.) cultivars in yield and yield
components under intercropping with sorghum (Sorghum bicolour L.
Moench) cultivars revealed that sole cropping may not provide the appropriate
environment for selecting varieties intended for use in intercropping.
Also, Gebeyehu et al. (2006) reported a similar finding from maize-bean
intercropping involving climbing types. On the other hand, Santalla et
al. (2001) working on bush bean cultivars, suggested that the evaluation
of bean genotypes for agronomic and quality traits under sole cropping
provide sufficient information to select varieties efficiently for intercropping
systems with maize.
The number of released bean cultivars has been increasing in Ethiopia.
However, their performance under intercropping system has not been tested
rigorously. This is also true for maize where the more robust hybrids
are gaining acceptance by farmers and are increasingly replacing open
pollinated varieties. Aims of this research were to (1) examine the relative
performance of common bean genotypes in sole stands and in association
with hybrid maize, (2) observe if there is genotype by cropping systems
interaction, (3) investigate differences within and between the three
growth habit groups under the two cropping systems and (4) identify the
most suitable bean genotypes for intercropping with hybrid maize.
MATERIALS AND METHODS
The experiment was conducted in Southern Ethiopia during the 2005 and
2006 cropping seasons at Awassa, which is located 7 °05`N and 38 °30`E
and asl 1660 m.
Design and treatments: Treatments were made from a combination
of two factors: cropping systems and common bean genotypes. The cropping
systems consisted of sole cropping and intercropping with maize. The maize
cultivar, Pioneer 3253 was planted in 2005 and BH 540 was planted in 2006.
Both are recommended for the experimental area. The second factor involved
seven released, one local and two potential genotypes of common bean representing
three different growth habit groups. There were four determinate and six
indeterminate types out of which eight were bush and two were semi-climbing
types. One of the cultivars, Red Wolaita, is a popular local cultivar
widely used for sole cropping and intercropping among farmers.
The factorial combinations of the ten bean genotypes and the two cropping
systems (Sole and intercropping) comprising twenty treatments were arranged
in a split plot design with three replications. Cropping systems was the
main plot factor while common bean genotypes were assigned as a sub-plot
factor. Also, sole maize plots were planted in three replications.
Agronomic management: Plantings of intercropped maize, sole maize
and sole common bean were carried out on 15 and 14 April in 2005 and 2006,
respectively. A pre-planting dose of phosphorus was applied on the intercrop
and maize sole plots at a rate of 46 kg P2O5 ha-1
as a one time application while nitrogen was applied on the same plots
at the rate of 54 kg ha-1 as split application. Half of the
rate of nitrogen dose applied with phosphorus and the remaining half was
given four and two weeks after emergence in 2005 and 2006, respectively.
Sole common bean plots received phosphorus at the rate of 23 kg P2O5
ha-1 and nitrogen at the rate of 9 kg ha-1
and were applied as a single dose just before planting. Intercropped common
bean plots were planted four weeks after maize emergence on 22 May in
2005 while they were planted two weeks after emergence, on 9 May in 2006.
There was no additional fertilizer applied for intercropped common bean.
The planting date for intercropped common bean was shifted in 2006 based
on the fact that the advanced maize growth, which was near canopy closure,
affected the common bean stand in 2005.
The intercropping was an additive type where the two components were
combined with their full sole crop populations. The arrangement of intercropping
was a row type where two common bean rows were planted between successive
maize rows. The row arrangement was made east-west to allow better light
penetration. Intercropped and maize plots were 4.8 m wide and 3 m long.
Sole plots of common bean were planted on 2 m wide and 3 m long plots.
Maize seeds were hand planted with two seeds per hill with 80 cm inter-row
and 30 cm intra-row spacing. The stand was thinned to a population of
41,666 plants ha-1 a week after emergence. Both the sole and
intercrop common bean plots were hand sown with two seeds hill-1
with an inter-row and intra-row spacing of 40 and 10 cm, respectively.
The stand was thinned to a density of 250,000 plants ha-1 a
week after emergence. Both sole and intercrop maize took 135 and 140 days
from emergence to physiological maturity in 2005 and 2006, respectively.
Data collection and analysis: Grain yield of intercropped and
sole maize was determined from plants harvested of the two central rows
(4.8 m2). Number of rows per ear and number of seeds per row
were determined from six randomly taken cobs. Similarly, grain yield of
intercropped and sole common bean was determined from the two central
rows (2.4 m2). Number of pods per plant was determined from
eight randomly selected plants while number of seeds per pod was determined
from 15 randomly picked pods. Grain yield of both maize and common bean
was adjusted to 13% moisture content.
A combined analysis of variance was done using the General Linear Models
of the Statistical Analysis System (SAS, 1999). Year was considered as
a random effect while cropping system and genotype were taken as fixed
effects. The F-test was used to check for homogeneity of error variances
between the two years (Gomez and Gomez, 1984). Responses of bean genotypes
were also contrasted to examine the distribution of variation among genotypes
between and within the different growth habit groups. The efficiency of
the intercropping system as compared to sole cropping was analysed using
the Land Equivalent Ratio (LER) method (Mead and Willey, 1980).
where, Yij and Yii are intercrop and sole crop
yields of component i, respectively while Yji and Yjj
are intercrop and sole crop yields of component j, respectively.
Land equivalent ratio evaluates the productivity by considering relative
performance under the sole and intercropping rather than the yield per
se of a given cultivar. This implies that larger LER values may arise
not only from larger intercrop yields but also from smaller sole crop
yields. This may lead to a choice of cultivars with medium or poor performance
as long as they show better relative performance. Thus, the mean of the
sole crop yields of all the genotypes was used as standardization factor
for estimating partial LER of the common bean genotypes instead of individual
sole crop yields (Mead and Stern, 1980; Oyejola and Mead, 1982; Santalla
et al., 2001).
RESULTS
There was a difference in rainfall amount and distribution between the
growing seasons of 2005 and 2006 while there was no marked variation in
temperature. Amount of rainfall received during the 2006 growing season
(656 mm) was larger by 17% compared to the amount in 2005 (561 mm) and
by 18% compared to the ten years average (554 mm) for the same period.
There was also a better distribution in 2006 whereby only 31% of the days
during the growing period received no rainfall while it was 57% of the
days in 2005. This variation could be one of the contributing factors
for the differences observed between the two years in the response of
parameters to the study factors, especially in common bean.
The common bean component: Grain yield was significantly affected
by year, cropping system and genotype (Table 1). The
difference in grain yield among genotypes is distributed all over the
computed contrasts except within semi climbing (Table 1).
Yield increased by 22% in 2006 compared to 2005 (Table 2).
On the other hand, intercropping with maize reduced bean grain yield by
80% compared to its sole counter part. On a group basis, the bush and
determinate types produced 67 and 16% more yield compared to semi climbing
and indeterminate types, respectively (Table 2). Within
the determinates, Melke and AFR-772 produced the highest yields which
were also the overall top yielders (Table 3). Within
indeterminates, DOR-554 and Roba are the highest yielders.
Year, cropping system and genotype have shown a significant effect on
the first yield component, pod number per plant (Table 1).
The difference among genotypes for number of pods per plant is attributed
to the same contrasts that showed variation for grain yield except determinate
vs. indeterminate. Pod number per plant increased by 35% in 2006 compared
to 2005. On the other hand, it dropped by 71% under intercropping compared
to sole cropping, following the grain yield trend (Table
2). Bush genotypes produced more pods per plant than semi climbing
types (Table 2). The within group comparison showed
that genotypes with superior performance were not necessarily the highest
ranking in pod number per plant though most are in the top category (Table
3). This could be explained by the fact that trends for pod number
per plant and seed weight were variable among genotypes. For instance,
the determinate types produced relatively smaller number of pods per plant
and seeds per pod but remarkably heavier seeds while the reverse were
true for indeterminates (Table 3).
| Table 1: |
Combined analyses of variance on yield, yield components
and maturity of common bean genotypes under sole and intercropping
with maize at Awassa, in 2005 and 2006 |
 |
| *, **, *** indicate significance at 0.05, 0.01 and 0.001
probability levels, respectively |
Year and genotype influenced the second yield component, seed number
per pod (Table 1). The difference in seed number per
pod among the genotypes was distributed all over the computed contrasts:
between and within group contrasts. Relatively larger number of seeds
per pod was produced in 2006 compared to 2005 (Table 2).
There were a yearxcropping system, a yearxgenotype and a cropping systemxgenotype
interactions. However, the interactions were not large and consistent
enough to be remarkably different from the main effect.
There was a significant variation between years and among genotypes for
seed weight (Table 1). The variation among genotypes
was located in all computed group contrasts, except for the within indeterminate
and the within semi climbing contrasts. Seeds were heavier in 2005 compared
to 2006, which may be attributed to the restricted number of pods in 2005
due to water stress leaving fewer pods to compete for assimilates (Table
2). A remarkable variation was observed for seed weight among the
group comparisons in that determinates produced seeds larger by 96% compared
to indeterminates while bush types produced seeds larger by 41% compared
to semi climbing types (Table 2). There was also a significant
year by genotype interaction for grain weight. This interaction did not
alter the main effect considerably in that genotypes fall in a similar
seed weight category in either year except minor change in ranking within
their class.
Days to maturity were influenced by genotype and this variation was located
under within group contrasts (Table 1). Maturity period
was similar among the different growth habit groups. Maturity period did
not vary significantly between cropping systems, either (Table
2).
Simple correlation coefficients of yield with various parameters were
made separately for each cropping system or year and for the data pooled
over cropping systems or years. Grain yield of common bean genotypes under
sole cropping has shown a positive correlation with their yield under
intercropping for 2006 (r = 0.62*) and for the pooled data (r = 48*) but
not for 2005 (r = 0.55, p = 0.10). Correlations between ranks for bean
grain yield under the two cropping systems were also significant for 2006
(r = 0.63*) and for the pooled data (r = 0.58**) but not for 2005 (r =
0.52, p = 0.11). Number of pods per plant made a positive correlation
with grain yield under sole cropping (r = 0.61*), intercropping (r = 0.62*)
and for the pooled data (r = 0.95***). Number of seeds per pod made positive
relationship for the pooled data only (r = 0.60**) while seed weight did
not make significant association either for each cropping system or for
the pooled data.
| Table 2: |
Means of year, cropping systems and contrasts for grain
yield, yield components and maturity of common bean genotypes from
a maize-common bean intercropping at Awassa |
 |
| NS: Not Significant, LSD0.05: Least Significant
Difference at 0.05 probability level |
| Table 3: |
Means of yield, yield components and maturity of common
bean genotypes intercropped with maize, at Awassa |
 |
| BU: Bush, SC: Semi Climbing |
The maize component: Grain yield was not significantly varied
between the two years and when intercropped with the 10 common bean genotypes
(Table 4). The overall mean grain yield of intercropped
maize averaged across common bean genotypes was no different compared
to the mean of the sole counter part (Table 5).
Unlike grain yield, harvest index and yield components including number
of rows per cob, number of seeds per row and seed weight were significantly
different between years (Table 4). Intercropping with
the various common bean genotypes did not influence any of the yield components.
The plants carried no more than one cob per plant under all treatments,
on average (Table 5). However, number of rows per cob,
number of seeds per row and harvest index were higher in 2005 compared
to 2006 while the reverse was true for seed weight.
Intercropping efficiency: The partial Land Equivalent Ratio (LER)
for the maize component was not varied much between the two years (1.01
for 2005 and 1.02 for 2006) and among the four growth habit groups (between
1.01 and 1.04). Partial LER of maize when intercropped with the 10 genotypes
was nearly one or greater (Table 6) indicating that
there was no yield loss for the maize component when associated with the
beans.
| Table 4: |
Combined analyses of variance on yield, yield components
and harvest index of maize intercropped with different genotypes of
common bean at Awassa, in 2005 and 2006 |
 |
| *, **, ***, indicate significance at 0.05, 0.01 and
0.001 probability levels, respectively. |
| Table 5: |
Means of year, contrasts and cropping systems for yield,
yield components and harvest index of maize from a maize-common bean
intercropping at Awassa |
 |
| NS: Not Significant; LSD0.05: Least Significant
Difference at 0.05 probability level; a: Means of sole
versus intercropping for comparison |
| Table 6: |
Means of partial LER and total LER from intercropping
maize with different genotypes of common bean at Awassa |
 |
| BU: Bush, SC: Semi Climbing |
The partial LER for the bean component varied between the two years which
were 0.11 for 2005 and 0.27 for 2006 showing a 145% increase in 2006 compared
to 2005. As a group, determinate types gave a higher partial LER (0.21)
than the indeterminates (0.17) while bush genotypes showed a greater partial
LER (0.21) than semi climbing types (0.10). Within the determinate category
partial LER values for Melke (0.31) and AFR-772 (0.29) were the highest
while within the indeterminate group, DOR-554 (0.27) and Tabor (0.24)
produced the highest values (Table 6). The two determinate
bush types, Melke and AFR-772 gave the highest overall bean partial LER
values. A significant yearxgenotype for bean partial LER showed that the
magnitude of partial LER differences between the two years for each genotype
was not similar (Fig. 1). However, the three top genotypes
for partial LER remained the same in both years except a change in ranking.
|
| Fig. 1: |
Genotypexyear for bean partial land equivalent ratio
(LER) from a maize-common bean intercropping at Awassa |
Total LER reflected the trend of the bean partial LER in that it was
influenced by genotype (Table 6). The bush types produced
the highest mean total LER (1.23) as compared to semi climbing types (1.14)
while there was no much difference between the determinates and indeterminates,
as a group. Total LER for specific combinations were best for AFR-772
(1.32) and Melke (1.29) within determinates and for DOR-554 (1.34) and
Tabor (1.28) within indeterminates (Table 6). Over all,
DOR-554, AFR-772, Melke and Tabor showed the best intercropping advantage
in that order.
DISCUSSION
The common bean component: Grain yield varied among the common
bean genotypes both between and within groups. Determinate genotypes were
superior to indeterminates while bush types were better than semi climbing
types. The better performance of the determinate and the bush types, in
the absence of growth duration differences and staking, could be attributed
to better light distribution throughout their canopy as a result of their
upright growth. Such distribution improves light utilization. For instance,
Davis et al. (1984) reported that staking increased yield in climbing
types while no response was observed for bush types. Furthermore, in rice,
Setter et al. (1997) observed that lodged plants showed sub optimal
stratified light interception of the canopy and reduced assimilation rate
compared to erect plants, due to self-shading. On the other hand, Clark
and Francis (1985) observed greater yield potential under monoculture
for climbing compared to bush cultivars. However, their climbing types
have longer maturity duration than the bush group. Occurrence of significant
variation within each group requires examination of performance of each
genotype separately and this indicated that there were productive genotypes
within each category, except within semi climbing.
There was no significant genotype by cropping systems interaction for
grain yield indicating that performance of genotypes did not vary considerably
under the two cropping systems. Also, grain yields and ranks of common
bean genotypes under sole cropping have shown a significant positive correlation
with their yield and ranks under intercropping. These showed that selection
of common bean cultivars for sole cropping could sufficiently identify
suitable genotypes for intercropping with hybrid maize. Similarly, from
maize-bean intercropping involving determinate and indeterminate bush
genotypes, Santalla et al. (2001) and Francis et al. (1978)
observed significant correlations between sole and intercrop yields and
suggested that the evaluation of agronomic traits for sole cropping provide
sufficient information for maize-bean intercropping systems. On the other
hand, Gebeyehu et al. (2006) working on maize-bean intercropping
involving climbing genotypes and Hauggaard-Nielsen and Jensen (2001) working
on barley (Hordeum vulgare L.) intercropped with determinate and
indeterminate pea (Pisum sativum L.) genotypes, observed significant
genotype by cropping systems interactions and advocated for a separate
selection scheme to develop appropriate cultivars for specific adaptation
to intercropping. Differences in growth habit and morphology of the component
cultivars involved may have contributed to reported differences in the
response of genotypes to cropping systems. For instance, significant genotype
by cropping system interactions are more consistently observed with climbing
beans compared to bush beans (Francis, 1985).
Grain yield of common bean decreased remarkably under intercropping compared
to sole cropping and this was associated with very low pod number per
plant. Considerable yield reductions of the legume component were reported
in various studies. For instance, Fininsa (1997) and Gebeyehu et al.
(2006) reported 67 and 75-91% reduction in common bean yield when intercropped
with maize, respectively while Hauggaard-Nielsen and Jensen (2001) reported
a 35 to 64% yield reduction for determinate pea when intercropped with
barley. Yield of the shorter legume component like bean could be reduced
from shading by the tall cereal component like maize, depending on the
density of the cereal, among other things. According to Gardiner and Craker
(1981), at 55000 maize plants per hectare, the associated bean intercepted
20% of light and yield was decreased by 70% compared to the sole bean.
The maize component: Intercropping of maize with common bean did
not reduce maize yield during both years. This shows that the bean component
did not exert much competition on the maize component either because of
the competitiveness of the maize hybrids and/or the less aggressive nature
of the bean genotypes. Similarly, Gebeyehu et al. (2006) reported
comparable yields between sole and intercropped hybrid maize cultivar
in association with climbing bean genotypes and ascribed it to the competitive
ability of the maize.
Intercropping efficiency: All associations involving the various
genotypes showed a LER value of greater than one indicating the superior
productivity of the combinations rather than growing the two crops separately.
However, specific combinations which showed LER values of 1.3 and above
would be recommended which is considered practically acceptable for intercropping
production (Onwueme and Sinha, 1991). Maize, with the higher partial LER
and greater yield contribution is the more competitive component in the
system. Ofori and Stern (1987) indicated that the cereal component, with
relatively higher growth rate, height advantage and a more excessive root
system is favoured in the competition with the associated legume. Lima
Filho (2000), in maize-cowpea replacement intercropping, indicated that
intercropped maize maintained higher values of leaf water potential, stomatal
conductance, transpiration and photosynthesis than as sole crop.
As a group, bush genotypes produced a higher bean partial LER and total
LER compared to semi climbing types. This may be because, the erect bush
types could be in a better position to intercept light that is filtered
through the tall maize component more uniformly throughout their canopy,
as indicated in a previous section. Whenever the semi climbing types use
the maize stalk for support, most of their leaves will be directly underneath
the maize canopy where available light is at its lowest. Also, the two
semi climbing genotypes used in this experiment are comparatively low
yielders under sole cropping and since the mean of all the genotypes were
used for standardization, they lost the advantage which otherwise could
have gained. Due to significant differences within the bush group, it
is important to look for specific genotype combinations, which offer the
best LER.
Most of the productivity in the intercrop mixture, 79%, was contributed
by the maize component while 21% is contributed by the bean component.
However, the magnitude of the intercropping advantage was influenced by
the legume component. The contributing factors for the intercropping advantage
could be related to resource acquisition and efficiency of its utilization.
From a maize-cowpea replacement intercropping, Lima Filho (2000) reported
that intercropped cowpea maintained higher leaf water potential than the
sole crop because of reduced evapotranspiration. This was caused by decreased
radiation load on the legume component due to shading by the taller maize
component. Regarding the resource use efficiency, the shaded legume component
usually uses intercepted light more efficiently. For instance, Marshall
and Willey (1983) reported that the intercropped groundnut intercepted
27% less radiation, but used it with 48% greater efficiency under intercropping
with sorghum.
CONCLUSION
The experiments indicated that determinate genotypes were superior in
grain yield compared to indeterminate ones while bush types were better
than the semi climbing types. This was also reflected in the intercropping
advantage of these growth habit groups. There was a positive correlation
for grain yield and ranks of bean genotypes under sole cropping and intercropping.
Unlike the bean component, hybrid maize did not suffer yield reduction
when grown in association with bean genotypes. Absence of yield reduction
in hybrid maize when associated with the bean component should encourage
more farmers to practice intercropping, as maize is the principal crop
of the area. Farmers could get more out of maize-bean intercropping by
using improved bean genotypes such as DOR-554, AFR-772 and Melke than
Red Wolaita. For similar growth period categories, bush types provide
a better intercropping advantage. Due to absence of significant genotype
by cropping systems interactions, it is possible to use determinate and
indeterminate bush genotypes that are isolated as superior for sole cropping
as components for intercropping with hybrid maize. Further research aimed
at investigating the physiological basis of differences in performance
among genotypes of the various growth habit groups under sole and intercropping
would be worthwhile.
ACKNOWLEDGMENT
The financial support by the Ministry of Finance of Ethiopia is gratefully
acknowledged.
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