Effect of timing of weed control on dominance by Setaria faberi and Digitaria ciliaris in orchards

A study was conducted in 1995 to compare the structure of summer weed communities following winter weed control at two different times. The comparison was based on the measurement of vegetative and reproductive organs of constituent species in three orchards in Kagawa and Okayama. At the time the study began, these orchards were dominated by Setaria faberi Herrm. and Digitaria ciliaris (Retz.) Koeler. When winter weeds were controlled in late March, S. faberi became dominant, showing more than 60% relative dry weight (an index of dominance); D. ciliaris was subdominant. By contrast, the plots having had weed control in late April were dominated exclusively by D. ciliaris (about 80% relative dry weight), followed by S. faberi. Seed number of the dominant species, either S. faberi or D. Ciliaris, was greater than that of the subdominant. The relative dry weight was positively correlated to relative density in both species. When the relative density of either species increased, that species became dominant. When S. faberi became dominant after early weed control, its relative dry weight increased proportionately to the increase of relative mean dry weight (an index of plant biomass). S. faberi was significantly greater in weight and length than D. ciliaris at harvest, partly because S. faberi started emerging slightly earlier in April (one leaf stage difference). By contrast, when D. ciliaris was dominant after late weed control, no clear relationship was found between relative dry weight and relative mean dry weight. There was no distinct difference in plant size between S. faberi and D. ciliaris at harvest. These results suggest that S. faberi which emerged after early winter weed control may have dominated the fields by virtue of having suppressed the growth of D. ciliaris. Consequently, the change of dominant species may have been affected both by plant density of each species and by species-interaction.