The Use of Chromolaena odorata as Fallow in a Semi-permanent Cropping System in Southwest Côte d'Ivoire

J. J. P. Slaats
Department of Agronomy, Wageningen Agricultural University,
Wageningen, The Netherlands



Chromolaena odorata invaded the Taï region in Southwest Côte d'Ivoire after the area opened up for timber production, leading to a subsequent breakdown of the traditional shifting cultivation system. This species presently dominates vegetation during early stages of the fallow period. Farmers in the Taï area have started to use C. odorata fallow land for maize cropping. Wageningen Agricultural University is doing a comprehensive evaluation on using natural stands of C. odorata as fallow in a semi-permanent cropping system. This study comprises the growth and development of C. odorata fallow vegetation, its management for subsequent crop production, and its development during and after the cropping period. C. odorata appeared to dominate fallow vegetation during the first five years. Grasses and herbs in the understory disappeared from the second year onwards.
Consequently, crop yields were higher and competition of non-crop plants was reduced after a three year fallow period. Burning and weeding slowed the development of C.odorata during cropping, while regeneration in the subsequent fallow period was mainly reduced by frequent weeding and long cropping periods. The use of C. odorata as fallow in a semi-permanent cropping system has good prospects since it is more intensive use of land than shifting cultivation. It also requires low external input to agriculture. Prolonged cropping and intensive weeding, however, threaten the continuity of this system.


The use of Chromolaena odorata as fallow in a semi-permanent cropping system was studied in the Taï region, Southwest Côte d'Ivoire.This region of the lowlands is located at 5°N, and has a humid tropical climate of the Köppen Aw-type. Rainfall is distributed bimodally with an annual mean of 1800 mm; mean temperature is around 26°C with very little diurnal and yearly variation. Cropping seasons run from April to June, and from July to November. In the past, this region was covered by evergreen rain forests and was sparsely populated. The inhabitants practiced a shifting cultivation system for food crop production. About 30 years ago the region was opened for timber extraction. This was followed by forest clearing for perennial cash crops. After a massive influx of immigrants, large parts of the forest were transformed into tree crop plantations. The remaining areas of forested land diminished rapidly. Consequently, fallow periods on these lands were shortened and cropping periods were prolonged.
Since its introduction to the eastern part of Côte d'Ivoire in the early fifties, C. odorata has spread in northern and western directions (Delabarre 1977, Gautier 1992). Due to geographical isolation and high forest cover, this species was not seen in the Taï region until the early eighties. Its invasion has been successful due to disturbance of the natural vegetation as a result of the timber industry and breakdown of the shifting cultivation system (de Rouw 1991). At present, C. odorata is found in areas where forests have been cleared, particularly in frequently cropped fields around the villages. In these areas, C. odorata dominates the early stages of succession.
Original inhabitants of the Taï region who still own forested land consider C. odorata a nuisance because it delays regeneration of the forest. On the other hand, immigrants who possess little forest land have started to use C. odorata fallow land for maize cropping. A system is now developing where a two-four year fallow is alternated with one year of maize growing (Slaats 1992).
As part of the research program "Analysis and Design of Land-use systems in Taï, Southwest Côte d'Ivoire", Wageningen Agricultural University is doing a comprehensive evaluation using natural stands of C. odorata as fallow in a semi-permanent cropping system. This study comprises the growth and development of C. odorata fallow vegetation, its management for subsequent crop production, methods to improve crop yields, and its development during and after cropping periods.
Arguments in favor of using C. odorata as fallow include its wide distribution and adaptation to ecological conditions of the humid tropics, as well as its role in the successional process of forest regrowth. Eradication of C. odorata, an alternative management option, is not economically feasible. If C.odorata is eradicated, grasses are likely to dominate early vegetation stages of the fallow period. Generally, grasses are considered an unfavorable fallow crop. Moreover, C. odorata is able to suppress undergrowth of herbs and grasses within a short time, and has a rather high biomass production which can store large amounts of nutrients. This paper presents the major findings of research on management aspects of C. odorata use as fallow in semi-permanent food production.


Growth and development of C. odorata fallow was studied on farmers' fields that had zero to five year old fallow vegetation. Observations were made on biomass, plant density and composition of fallow vegetation during the growing period.
Results show that C. odorata is the major weed species during the cropping season. It establishes from both stump sprouts and seedlings. Four months after harvesting, fallow vegetation covered 90% of the soil, with a biomass of 4.3 tons/ha, of which 70% was C. odorata. In subsequent years, the fallow vegetation developed into a 3 meter high thicket, made up of a dense mass of tangled, half-woody C. odorata branches. Beginning the second fallow year, the undergrowth of grasses and herbs had already disappeared. During development of the fallow vegetation, density of C. odorata plants fell from about 150 at 4 months after crop harvest, to 1 plant/m2 in the 5 year old fallow vegetation.
The dominant position of C. odorata in development of fallow vegetation is reflected in biomass changes over time (Fig. 1). From the beginning to the fifth year of fallow, C. odorata was the main component of vegetation biomass. By the third year of fallow, total vegetation reached a maximum biomass production of 22.3 t/ha, and C. odorata reached a maximum of 20.8 t/ha. In older vegetation, biomass production decreased as did the proportion of C. odorata in the total biomass. However, the amount of litter increased rapidly. These changes indicate the vegetation is dying back. On 5 year old fallow fields, C. odorata canopies started to disintegrate, allowing shoots of tree stumps to pierce through.
It is assumed C. odorata will gradually disappear in subsequent years as a result of this tree shading.

Figure 1. Development of Chromolaena odorata fallow over a 80-month period

Figure 1. Development of Chromolaena odorata fallow over a 80-month period


Crop production on C. odorata fallows was studied in several field experiments. This paper highlights effects of the length of fallow on subsequent crop production. For the first three seasons, this effect was studied in three cleared fields, differing in ages of fallow vegetation from 2 to 4 years. The experimental design combined two frequencies of weeding (one versus two weedings per cropping season), and three levels of fertilizer application (no fertilizer, P, P and N) in each field. The test crop was a hybrid maize.
Effects of the length of fallow and weeding frequency on crop yields during the first and third season are presented in Figure 2. Yield figures are averaged for all fertilizer treatments. Results of the second season are not given since they are very similar to the third season. In the first season, crop yields were higher when the preceding fallow period was longer. In the third season, yield levels were almost the same in all three fields, having fallen to lower levels than the first season's yield levels. On the 2 year old field, a second weeding resulted in significantly higher yields during both seasons. On the 3 and 4 year old fields, weeding twice did not significantly raise crop yield. Responses to fertilizer application were highly significant on all fields during the three seasons.
The research findings indicate that a longer fallow period results in long term reduction of weed competition and short term yield increase. The benefits were highest when the fallow period was prolonged from 2 to 3 years. By giving N and P fertilizers at a rate of 100 kg N/ha and 25 kg P/ha, the yield level was almost doubled.


After the C. odorata fallow vegetation has been cleared, the plant re-establishes rapidly by sprouting stumps and emerging seedlings. Hence, it may become a weed during cropping. This dual role of C. odorata, the main component of the fallow vegetation and weed, was the reason for studying the effects of clearing methods and weeding on the re-establishment of the plant during cropping. Clearing methods used were removing of slashed fallow vegetation or burning at two intensities. For low burning intensity the standing vegetation was burnt in situ after slashing. At the high burning intensity, the amount of slashed vegetation was doubled by adding vegetation from the outside and then burning. Weeding was done by scraping the top soil and clipping sprouts of stumps with a short hoe at 4 weeks after clearing. Observations were made on the number, spread and soil cover of C. odorata sprouts and seedlings.
Sprout development was reduced more when slashed vegetation was burnt as opposed to when it was removed. By burning, parts of the stumps were killed while sprouting was delayed on living stumps; this resulted in a lower number of sprouts per surface unit and in poor soil covers. After weeding, sprout development was no longer affected by the method of clearing. Soil cover and sprout numbers were similar using either clearing method, primarily because weeding resulted in fewer stumps due to unintentional uprooting. Seedling development was rather slow at the start of the cropping season for all clearing methods. Burning had reduced the number and cover of seedlings, before as well as after weeding.
Both clearing methods led to a slow seedling development at the start of the cropping season. These results indicate that C. odorata sprouts have a higher potential than seedlings to compete with crop plants, and that sprout development was restricted by burning and hoe weeding.


Research on the re-establishment of C. odorata after cropping has focused on the effects of management practices. These effects were studied by following the regrowth of fallow vegetation subsequent to cropping on former experimental fields. The fields differed in length of cropping and fallow periods, and in clearing methods used prior to cropping. Within each field, there were two levels of weeding frequency: 1 or 2 weedings per season, and 3 levels of fertilizer application (no fertilizer, P, P and N). One to two months after harvest, the cover of several components of the soil cover of fallow vegetation were recorded. After four months, their biomass was quantified.
The re-establishment of C. odorata is reduced by frequent weeding during cropping, and by a prolonged cropping period. In Figure 3, the biomass of both sprouts and seedlings for two weeding frequencies is given. The data were gathered from a field with a 2 year old fallow, where three maize crops had been grown prior to these observations. Figure 3 indicates that weeding twice reduced the biomass of sprouts, but hardly affected seedlings. In Figure 4, the biomass of both sprouts and seedlings from 3 different fields is presented. The data refer to plots which have been weeded once per season during the cropping period. The figure shows that both sprout and seedling biomass were reduced by prolonged cropping. Both figures clearly illustrate that intensive management of cropping systems on C. odorata fallows slow down the re-establishment of this fallow type.


Results showed that C. odorata fallow vegetation reached a rather high biomass production within a short period, while the undergrowth of grasses and herbs had disappeared. The number of C. odorata plants also had been reduced. These characteristics contribute to relatively high yields and a limited weed problem during cropping. The development of C. odorata as a weed during cropping can be effectively reduced by burning and weeding. Its re-establishment after the cropping period, however, is hampered by intensive management practices. A fallow period of 3 years, alternated with one season of cropping under an extensive management regime, is considered to be the optimal use of C. odorata as fallow in terms of crop growth conditions.
Due to its abundant presence and specific properties, use of C. odorata enables more intensive land-use to be achieved without forcing farmers to greatly intensify production methods. Hence, the use of C. odorata as fallow in a semi-permanent food cropping system may form an attractive alternative for longer bush fallows, thus contributing to conserving the last remnants of tropical rainforest.


The author is grateful to the Ministry of Higher Education and Scientific Research in Côte d'Ivoire for granting permission to carry out this research, and to farmers in the Taï region for their hospitality and cooperation. Financial support from the Tropenbos Foundation for the project, and for my participation in this workshop, is gratefully acknowledged. The author thanks Professor Dr. Ir. M. Wessel for a critical review of this paper.


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