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African Crop Science Journal, Vol. 9. No. 3, pp. 487-497

EFFECT OF MAIZE DENSITY, BEAN CULTIVAR AND BEAN SPATIAL ARRANGEMENT ON INTERCROP PERFORMANCE

A. MUTUNGAMIRI, I.K. MARIGA and O.A. CHIVINGE

Department of Crop Science, University of Zimbabwe, P.O.Box MP 167, Mount Pleasant, Harare, Zimbabwe

Received 17 August, 2000
Accepted 9 March, 2001

Code Number: cs01068

ABSTRACT

On-farm trials to determine the optimum combination of maize (Zea mays L.) density, bean (Phaseolus vulgaris L.) cultivar and bean spatial arrangement to produce high yields of the intercrop combination were conducted in Chinyika Resettlement Area (CRA) and at Domboshava Training Centre (DTC) during the 1996/97 and 1997/98 rainy seasons. The effects of maize at 37, 000 and 24, 000 plants ha^-1, bean cultivars 'Natal Sugar' and 'Carioca', and bean arranged in one or two rows between rows of maize or bean planted in the same row as maize were evaluated in a completed factorial arrangement. Maize density, bean cultivar, bean spatial arrangement and their interactions significantly (P<0.01) affected maize and bean grain yields at both locations for the two seasons. Changing the maize density from 90 x 45 cm (24000 plants ha^-1) to 90 x 30 cm (37000 plants ha^-1) increased maize yield by 28 and 39% and reduced bean yields by 11 and 18% in the respective seasons. Maize yield was 19% less when intercropped with Natal Sugar than with Carioca. Same row and one row bean cultivar arrangements produced similar maize yields. Land equivalent ratios (LER) were greater than one for all but one intercrop arrangements at CRA but less frequently at DTC. The maize density of 90 x 30 cm with Carioca in the same row as maize produced the highest LER value at all sites, 1.73 at Chinyudze, and 1.53 at Domboshava. The same treatment gave the highest gross return ofZ$12 649 ha^-1 at CRA. Carioca planted in the same row as maize at the maize density of 37,000 plants ha^-1is the most ideal approach to dryland maize/bean intercropping since it achieved high yields and allows easy weeding.

Key Words: Bean/maize, intercropping, plant arrangement, Zimbabwe

Résumé

Des études en champs pour déterminer les combinaisons optimales de densité de variétés de maïs (Zea mays L.), et l'arrangement spatial de haricots (Phaseolus vulgaris L.) pour des rendements élevés de la culture intercalaire étaient conduites à Chinyika Resettlement Area (CRA) et a Domboshava Training Centre (DTC) durant les saisons pluvieuses de 1996/97 et 1997/98. L'impact de plantes de maïs à 37000 et 24000 plantes ha^-1 et des variétés de " Natal sugar" et "carioca" de haricots plantées dans et entre les lignes de maïs était évalué dans une disposition du type factoriel. La densité de maïs et de haricots, leurs arrangements spatiaux et leurs intéractions ont affecté de manière significative (P<0.01) leurs rendements aux deux endroits pour les deux saisons. La substitution de la densité de maïs de 90 x 45 cm (24000 plantes ha^-1) par 90 x 30 cm (37000 plantes ha^-1) a augmenté le rendement de maïs de 28 et 39% et a réduit le rendement de haricots de 11 et 18% pour les saisons pluvieuses de 1996/97 et 1997/98, respectivement. Le rendement de maïs était réduit de 19% quand il était cultivé ensemble avec le "Natal sugar" qu'avec "carioca". La même ligne et une ligne de haricots ont produit des rendements en maïs identiques. Le taux de terres équivalentes était supérieur à 1 partout à l'exception de la combinaison (maïs - haricots) au CRA. La densité de 90 x 30 cm avec carioca sur la même ligne que le maïs a produit la valeur la plus élevée de taux de terre equivalent aux deux endroits. La valeur du taux de terres équivalentes était de 1,73 à Chinyudze et 1,53 à Domboshava. Ce même traitement a produit un prix de revient de Z$ 12649 ha^-1 à CRA. Le haricot "Cariora" planté sur la même ligne que le maïs avec une densité de 37000 plantes ha^-1 représenté l'approche idéale pour les maïs / haricots intercalés de régions arides parce qu'elle permet de réaliser un rendement élevé et un facile sarclage.

Mots Clés: Haricots/maïs, culture intercalée, arrangement des plantes, Zimbabwe

INTRODUCTION

Maize/bean is a predominant cropping system in most parts of Africa (Francis et al., 1982). Productivity of a cropping system comprising intercrops of two or more species depends upon the degree of complementarity between them. Enhancing productivity of maize and bean intercrops requires improving the interspecies complementarity or reducing competition effects (Rezende and Ramalho, 1994). This might be achieved through manipulation of plant arrangements, plant densities and planting compatible cultivars (Rao and Mittra, 1990).

Ofori and Stem (1987) proposed that the growth and yield of the legume component is reduced markedly when intercropped with high densities of the cereal component. In a maize/bean intercrop system, increasing maize density three-fold, from 18, 000 to 55, 000 plants ha^-1, reduced bean leaf area by 24% and seed yield by 70% (Gardiner and Craker, 1981).

Spatial arrangement of component crops is one of the most important agronomic factors that determine whether an intercrop system will be advantageous or not with regard to yield gains (Natarajan and Shumba, 1990). Row arrangements, in contrast to arrangements of component crops within rows, improves the amount of light transmitted to the lower legume. Such arrangements can enhance legume yields and efficiency in cereal/legume intercrop systems (Mohta and De, 1980).

Most intercropping research in Zimbabwe has focused on spatial arrangement of inter-row intercrops with little attention to intra-row intercropping (Mutungamiri, 1999). The adoption of inter-row intercropping practices by small-scale farmers is rather limited perhaps because the practice is relatively labour-intensive. In contrast, intra-row intercropping may reduce total labour demand since planting of both species can be done simultaneously, and other cultural practices, such as weeding by ox-drawn cultivators, are possible (Nyakanda et al., 1995). Smallholder farmers in Zimbabwe practise intra-row intercropping to facilitate ox-drawn cultivation and to protect beans from rain drop impact, thereby minimising bean flower dropping (Munguri, 1996).

Cultivars which minimise intercrop competition and maximise complementary effects are suitable for intercropping (Rao and Mittra, 1990). Productivity of the intercrop can be enhanced through selection of bean cultivars suitable for intercropping as they have different growth habits and growth durations, which may result in different interactions with maize in the intercrop. In Zimbabwe, farmers tend to mostly grow one specific bean cultivar, Natal Sugar, despite considerable evidence indicating that other bean varieties exhibit superior adaptation to dryland maize/bean intercropping (Mariga and Munetsi, 1990). Maize cultivars with short internodes and broad leaves shade beans relatively more than cultivars of a similar height with long internodes and narrow leaves. Tall cultivars generally give more shadding to understorey crops (Davis and Garcia, 1983).
This study was undertaken to determine the optimum combination of maize density, bean cultivar and bean spatial arrangement suitable for high yields of the maize/bean intercrop combination.

MATERIALS AND METHODS

The experiment was conducted during the 1996/97 and 1997/98 rainy seasons at Chinyudze in Chinyika Resettlement Area (CRA) and at Domboshava Training Centre (DTC). The Resettlement lies between 32° 05' and 32° 44' East and between 18° 00' and 18° 20' South in the eastern part of Zimbabwe. Its altitude ranges from 1,200 m above sea level in the west to 1,700 m in the east. The Training Centre is situated at 31° 10' East and 18° 25' South in central Zimbabwe at an altitude of 1,500 m above sea level. Both CRA and DTC are high potential areas with mean annual rainfall in the range of 700 - 1000 mm. The trials were planted in the first week of December in both seasons.

The effects of maize population, bean cultivars, and bean spatial arrangement were evaluated in a complete 2 x 2 x 3 factorial arrangement in a randomised complete block design. An early maturing (140 days) 3- way maize hybrid ^"SC5Ol" was planted at 90 x 30 cm and 90 x 45 cm for plant densities of 37,000 and 24,000 plants ha^-1. The indeterminate, bushy bean cultivars were Natal Sugar and Carioca. Bean spatial arrangements included: one (SAl) or two (SA2) rows between rows of maize and bean planted in the same row as maize (SA3). Within row spacing of bean was 10 cm giving population densities of 111,111 for the one row and within row arrangements and 222,222 for the two row arrangement. Sole crop treatments included maize at the two planting arrangements and the two bean cultivars planted at 45 x 10 cm spacing. Thus, 16 treatments were tested in three replicates.

Intercrop plots had four maize rows, 6 m long (21.6 m²) and three, four or six rows of beans for spatial arrangements one row, within row and two rows, respectively. Pure stand plots had either four rows of maize or eight rows of beans. The harvested area for maize comprised the two central rows excluding 1 m at each end (7.2 m²). The harvested area for beans in sole stands comprised the four central rows excluding 1 m at each end (7.2 m²). The intercrop harvested area for beans was either (3.6 m²) for one and two or (7.2 m²) for within row.

Compound D fertiliser (8% N, 14% P₂0₅, 7% K₂0 and 6.5% S) was station placed (dolloped) on maize at planting at 300 kg ha^-1. Ammonium nitrate (34.5% N) was topdressed at 250 kg ha^-1 at five weeks after crop emergence (WACE). The trials were hand weeded using a hoe, twice at two and five WACE. Dipterex (2.5% trichlorfon a.i.) was applied to maize at five WACE to control maize stalk borer (Busseola fusca Fuller).

Grain yields of beans and maize were determined at 12.5 % moisture content. Analysis of variance was performed on grain yield and monetary value for all treatments using the MSTATC Statistical computer package. Farm-gate prices for maize (Z$l ,200 for 1996/97 and Z$2,400 t ^-1 for 1997/98) and bean (Z$7,000 for 1996/97 and Z$12,000 t^-1 for 1997/98) were used in the analysis of monetary benefits. Biological performance of the intercrop treatments was evaluated using the land equivalent ratio (LER) method (Mead and Willey, 1980).

RESULTS

The three main effects, maize density, bean cultivar and bean spatial arrangement, had significant (P<0.01) effects on maize and bean grain yields during both seasons. There were also significant (P<0.01) interaction effects of maize density x bean cultivar, maize density x bean arrangement, and bean cultivar x bean arrangement on maize and bean grain yields in both seasons.

Bean yield. Bean yields were generally more with lower maize density x bean cultivar interaction effects at DTC in 1996/97 and Chinyudze in 1997/98 (Table 1). At DTC, Carioca performed poorly and gave the least yields with the lower maize density. At Chinyudze, the interaction effects were significant because the magnitude of cultivar effects differed for the two maize densities. Interaction effects of maize spacing x bean spatial arrangement were significant (P<0.01) for bean grain yields during both seasons at DTC and Chinyudze. At both maize densities, two rows of beans between maize rows out-yielded the other bean spatial arrangements at both sites in 1996/97 season (Table 2). However, with maize a spacing of 90 x 30 cm, one row and same row arrangements produced similar yields whilst the one row achieved the lowest bean yield at 90 x 45 cm maize spacing at DTC in the 1996/97 season. At Chinyudze the same row arrangement achieved similar bean yields at both maize densities, but higher and lower yield levels than the one and two row arrangements, respectively.

The two row arrangement of Natal Sugar out-yielded all the other combinations at both sites in the 1996/97 season (Table 3). One row of Carioca produced the lowest yield at DTC. Two rows of Carioca and Carioca planted in the same row with maize gave similar yields. At Chinyudze, one row and same row arrangements of Carioca gave similar yields at this site. Carioca generally achieved higher bean yields than Natal Sugar at the three spatial arrangements in the 1997/98 season (Table 4). At Chinyudze, Natal Sugar and Carioca produced similar yields at the one row arrangement, but, at the two row and same row arrangements, Carioca out-yielded Natal Sugar. The two row arrangement achieved the highest bean yields for each variety at both sites (Table 4). A serious outbreak of Ascochyta phaseolorum in Carioca at DTC resulted in low bean yield in the 1996/97 season.

Maize yield. Maize density x bean cultivar interaction effects were significant (P<0.01) for maize yields at DTC and Chinyudze in the 1996/97 season (Table 5). At both sites and at both maize densities, maize intercropped with Carioca resulted in higher maize yields than with Natal Sugar. Maize yields were significantly (P<0.05) lower at 90 x 45 cm spacing than at 90 x 30 cm at both sites for both bean cultivars. At each site, maize at 90 x 30 cm intercropped with Carioca gave the highest yields.
Interaction effects of maize density x bean spatial arrangement on maize yield were significant (P<0.0l) at DTC and Chinyudze in the 1996/97 season (Table 6). Maize at 90 x 30 cm spacing with one row of beans produced the highest maize yields at both sites. Maize planted in the same row with beans and maize with one row of beans in between at the 90 x 45 cm maize density gave similar yields at DTC. The three spatial arrangements at 90 x 30 cm maize spacing resulted in higher maize yields than at 90 x 45 cm maize spacing. The lowest maize yields were achieved for treatments of maize spaced at 90 x 45 cm and intercropped with two rows of beans at DTC, and with beans in the same row at Chinyudze.

Significant (P<0.01) bean cultivar x bean spatial arrangement interaction effects at DTC and Chinyudze in the 1996/97 season were observed in the different maize grain yields responses in the presence of the two factors (Table 7). At DTC, the presence of two rows of Natal Sugar and maize in the same row resulted in similar maize yields which were significantly (P<0.05) lower than that of the one row arrangement. The intercrops with Carioca resulted in higher maize yields than with Natal sugar at Chinyudze. The one row arrangement with Carioca gave the highest maize yields at each site.

At both locations in the 1997/98 season, one row arrangement at 90 x 30 cm maize density resulted in the highest maize yield (Table 8). At DTC, one row and two row arrangements at 90 x 45 cm resulted in similar maize yields. Two rows and same row arrangements at 90 x 30 cm at Chinyudze resulted in similar yields. The 90 x 45 cm spacing achieved lower maize yields than the 90 x 30 cm spacing at all bean spatial arrangements.

For either bean variety, intercropping with maize at 90 x 30 cm resulted in higher maize yields than at 90 x 45 cm at Chinyudze in 1997/98 season (Table 9). Carioca with maize at 90 x 30 cm resulted in the highest maize yields; with maize at 90 x 45 cm Carioca also resulted in higher maize yields. Carioca planted in the same row with maize gave the highest maize yield compared to other arrangements at DTC in 1997/9 8 season (Table 10). Significant bean cultivar x bean arrangement interaction effects on maize were apparent.

In the 1996/97 season, at Chinyudze, the partial LER values for maize show that sole crop maize at 90 x 30 cm out-yielded maize in all the maize/bean intercropping combinations, but, at DTC, sole crop maize exhibited a lower yield than intercrop maize with one row of Carioca in between maize rows (Table 11). The partial LER values for beans generally indicate that two rows of beans in between maize rows gave higher yields than the other spatial arrangements. In the 1996/97 season intercropping had an advantage over monocropping at Chinyudze whilst most of the treatments had LER values below 1 at DTC. The partial LER values for beans were notably lower at DTC compared to those for Chinyudze. In the 1997/98 season the sole crop maize at the standard maize spacing of 90 x 30 cm out-yielded maize in all the maize bean intercropping combinations. SC5O1 at 90 x 45 cm with two rows of Natal Sugar gave the highest LER value of 1.56 and SC5O1 at 90 x 45 cm with one row of Natal Sugar gave the lowest LER value of 0.97.

At DTC, treatment 4 (SC5O1 at 90 x 45 cm + 2 rows Natal Sugar) gave the highest return of Z$6,746 but was significantly different (P>0.05) from the return produced by treatment 5 (SC5O1 at 90 x 30 cm + 1 row Carioca (Table 12). The sole crop treatments generally gave lower returns than the intercrop treatments except for sole crop maize at 90 x 30 cm which gave similar yields as the intercrop treatments at Chinyudze. Treatment 10 (SC5O1 at 90 x 30 cm + Carioca in the same row as maize) produced the highest return of Z$12,649 at Chinyudze. This treatment was not significantly different from treatment 1 (SC5O1 at 90 x 30 cm + I row Natal Sugar).

DISCUSSION

Results from this research clearly show that farmers must strive to achieve the currently recommended 37,000 plants ha^-1 in maize, sole or intercropped, in order to optimise total productivity.

The three parameters tested in this trial did not exhibit simple relationships. The bean spatial arrangements of two bean rows in between maize rows and beans planted in the same row as maize gave lower maize yields than where beans were planted in alternate rows with maize. The results therefore suggest that a bean spatial arrangement of one bean row in between maize rows was less competitive to maize in the intercrop. In addition, the first two bean spatial arrangements place the bean plants closer to the maize plants, thus increasing inter-component plant competition. The closer intimacy increased the effects of competition on maize. However, the one row arrangement hinders the use of ox-drawn cultivation of the maize crop and is thus unlikely to be adopted by farmers except on very small farms were hand-weeding is most appropriate or at least by non-cattle owners.

Natal Sugar depressed maize yields more than Carioca. Similar results were reported by Mariga and Munetsi (1990). An ideal legume would be one with a reduced competitive effect on the companion cereal, but at the same time showing high productivity. Farmers are more likely to consider using Carioca since it is less competitive to maize which they consider to be their main crop. However Carioca's high susceptibility to disease during wet weather may necessitate the development of alternative bean cultivars for dryland maize/bean intercropping.

Plots comprising of 90 x 45 cm maize density with two rows of beans in between gave the highest bean yields compared to other intercrops. Here the bean population had the advantage of high density and reduced competition from maize. Mwaipaya (1990) suggested that beans are more likely to benefit from the fertiliser applied to the maize in this arrangement. However, this planting arrangement too may not be attractive to farmers since it hinders ox-drawn cultivation and besides, maize yields were significantly reduced in this treatment. Improved technologies that increase productivity (yields) of the secondary crop but reduce yields of the main crop are unlikely to be adopted by farmers. However, in the case of small-holder farmers in Zimbabwe where farmers only intercrop part of their maize crop, the monetary value of minor intercrops may sometimes more than compensate for the reduction in maize yields. For example, the market prices for maize and beans in the year 2000 were Z$5, 500 and Z$35, 000 to Z$40, 000 ton^-1 for the respective crops. This particular planting arrangement should also be attractive to non-cattle owners who cannot use ox-drawn cultivators. In the 1997/98, season the 90 x 30 cm maize density with two rows of Carioca in between maize rows and Carioca in the same row with maize gave the highest gross outputs. An argument commonly advanced against intercropping in Zimbabwe is that it makes weeding difficult but with beans planted in the same row as maize, weeding should be easy as it permits use of oxen drawn cultivators which are commonly used in the small-holder sector. Labour demand is also reduced as planting of both component crops can be done at the same time.

Most LER values at DTC were below 1. This was not due to treatment effects but was caused by high rainfall which resulted in rotting of some bean pods and maize kernels. This observation shows the importance of the contribution of the minor legume to the overall intercrop LER value. The results at Chinyudze, however, indicate that maize/bean intercropping is more efficient than growing sole crops of maize and beans. This probably suggests that intercropping facilitated more efficient exploitation of available resources by the crops, resulting in greater total yields and gross return.

CONCLUSION

The bean cultivar Carioca planted in the same row as maize and at the recommended maize spacing of 90 x 30 cm is the suitable crop density for dryland maize/bean intercropping since it achieved high yields and also facilitates weeding. Reducing maize density to 24,700 plants ha^-1 for maize/bean intercropping or sole cropping is not recommended. For non cattle owners and those who would want to carry out maize/bean intercrop on a small area the spatial arrangement of two bean rows in between maize rows at the 90 x 30 cm is recommended. Similar studies in future should involve detailed economic analysis and farmer evaluations. More work should be focused on the spatial arrangement of the legume to generate data for a more definitive recommendation.

ACKNOWLEDGEMENT

This study was part of an MPhil study programme of the first author. The authors are grateful to the Rockefeller Foundation'Forum on Agricultural Resource Husbandry for funding the study.

REFERENCES

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