Crop production in relation to cultural practices in the Chromolaena odorata fallow system in South-West Côte d'Ivoire.

Abstract

Farmers grow food crops alternated by short fallow periods of the naturally re-establishing Chromolaena odorata in response to the growing demand for land in humid Africa. It is unknown whether current cultural practices in this system are appropriate and how land use can further be intensified. Maize production was measured in experiments on three sites cleared from a two-, three- and four-year-old C. odorata fallow, respectively. On the two-year-old site, bearing the lowest quantity of vegetation biomass, maize response to removing, burning and intensively burning (i.e.: burning after adding vegetation from outside) the slashed vegetation was studied as well. Various weeding frequencies and fertilizers were applied in the experiments to better assess the effects of the length of the fallow period and the clearing methods. The experiments were conducted during three consecutive cropping seasons to provide an indication of changes in yield over time. During the first season after clearing, maize yield decreased from 3.8 on the four-year-old fallow to 2.6 t ha-1 on the two-year-old fallow. This reduction was due to a lower availability of P and N, and to a higher competition from weeds. Burning or intensively burning the vegetation raised yields of the unfertilized crop, up to 1.2 t ha-1. It increased the availability of phosphorus but did not clearly reduce competition from weeds. In the second and third season, yield fell to 1.5 t ha-1 irrespective of the fallow age. Burning slowed down the yield decline over time. Application of P- and N-fertilizers raised crop yields under all conditions and maintained them up to 5 t ha-1 during the three seasons. Weeding twice instead of once reduced the yield decline over time on the young fallow only. Results suggest that in the C. odorata crop production system a fallow period of two years is too short to be fully effective, that burning the C. odorata slash benefits crop production, even when its amount is limited, and that growing maize for more than one season is worthwhile only where fertilizers are applied.