Agro-Ecological Study Of Weeds In Kenyan Rice Fields.

Abstract

Weeds present during and after the 1980/81 growing season were identified, temporal distribution monitored and spatial distribution determined. The status of weed propagules, a ,source of recurrent establishment of weeds, was assessed. Then the effect of some selected weeds on the rice variety IR 2071 and finally the biotic agents which fed on or infected some weeds were examined. A total of twenty eigth weed species is reported of which 43% were major weeds. About 80% of the weeds were emergent thus exhibiting the life form of the rice crop. The most favourable habitat for the weeds was inside the rice fields though the levees of major canals, feeder canals and the shallow drains were colonised to a small extent. The temporal distribution was markedly influenced by the human activities of flooding, weeding, drainage, harvesting and re-flooding. These activities greatly overshadowed the effect of the climatic factors. Rainfall, through wetting of the substrate, in areas not covered by flood water, allowed for better growth of weeds whereas high temperature hastened the drying of the substrate resulting in the disappearance of obligately aquatic weeds. Spatial distribution, in terms of sections, is nearly complete in that only a few weed species are absent from some sections. The soil acted as a potential reservoir of weed seeds and their placement, dormancy and uneven germination within the weed seed bank are responsible for constant recurrence of weeds in the rice fields. Feeder canals which are strategically placed conveys the weed plant material capable of regeneration from major canals to rice fields. All the weeds selected reduced the yield of rice (grain weight) and the number of tillers, though not significant in some cases probably because of the competitive ability of transplanted rice. The biotic agents found feeding on or infecting some of the weeds were not widespread enough. Further work is needed to ascertain their potientiality as biocontrol agents.