Abstract:
The natural fallow through which the fertility of tropical soils is restored and the weed pressure is alle
viated can no longer be sustained. However, improved bush fallow using fast-growing, deep-rooting and
good biomass-accumulating shrubby legumes provides a good alternative. The suitability of Senna obtu
sifolia in improved fallow management was investigated by studying its competitive ability relative to
Chromolaena odorata (siam weed) and Euphorbia heterophylla (poinsettia weed) in replacement series
studies at four total densities (29, 58, 115 and 230 plants m-2) in pure stands and at three total densities
(58, 115 and 230 plants m-2) in equiproportion mixture. Replacement series diagrams were drawn and
relative competitive ability indices (Relative Yield, RY; Relative Yield Total, RYT; Relative Replace
ment Rate, RRR) were determined from the plant dry weight (PDW). In the replacement series dia
grams of Senna/Chromolaena combination, the curves of the PDW/pot and those of the RY were convex
for the two species. The RY values were 0.64±0.12 and 0.59±0.05 for S. obtusifolia and C. odorata re
spectively. The RYT and RRR, averaged over all densities, were 1.23±0.08 and 1.27±0.17 respectively.
However, in the Senna/Euphorbia combination, the curves were convex for S. obtusifolia and concave
for E. heterophylla. The RY values were 0.81±0.06 for S. obtusifolia and 0.23±0.05 for E. heterophylla re
spectively. The RYT and RRR values for the combination were 1.05±0.07 and 11.07±2.40 respectively. The
results indicated that though S. obtusifolia and C. odorata attained equilibrium growth with RYT sig
nificantly deviated from unity (RYT>1) when grown in mixture, S. obtusifolia exerted greater replace
ment pressure than C. odorata. Conversely, in the Senna/Euphorbia combination, there was a competi
tive relationship (RYT»1) in which S. obtusifolia was the aggressor and E. heterophylla the subordinate
species. The study showed that while S. obtusifolia exhibited obvious competitive superiority in mixture
with E. heterophylla, the superiority was less obvious in mixture with C. odorata.
