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1. The allelopathic potential of weeds through the release of toxic substances into the environment either by root exudation or from decaying plant material has been demonstrated in about 90 species (Putnam 1986). 2. Rice germination, growth and yield were significantly reduced by leachates from C. rotundus and this inhibition might be due to the presence of allelopathins. Among the whole plant extracts of Lantana camara, Cyperus rotundus, Ageratum conizoides, Echinochloa crusgalli, Commelina benghalensis and Xanthium strumanium, except for those of E. crusgalli and C. benghalensis, all others delayed rice seed germination. 3. The allelopathic effect of methanol extract from goose weed (Sphenoclea zeylanica) was examined by Premastira and Zungsontiporn (1996) using bio assay techniques and opined that the phytotoxic effect of the extract increased with the age of S. zeylanica. Extracts from inflorescences (seed) were the most phytotoxic, followed by extracts from leaves, stems and roots. 4. Among the aquatic weeds tried viz., Ceratophyllum sps., Hydrilla sps., Nymphoides sps., alga and chara sps., to evaluate the effects of 1 – 10% solutions of the extracts, on rice seed germination and radical length of rice, showed that the extracts of Ceratophyllum sps., Hydrilla sps. increased rice seed germination. 5. The effect of the extracts on radical length was very variable (Nahar et al 1993). Ichizen et al (1997) from their pot trials concluded that, Eleocharis acicularis suppressed the growth of Scirpus juncoides, Cyperus difformis and Monochoria vaginalis and increased rice yield. Cyperus rotundus, C. iria and Fimbristylis miliacea does not influence rice (variety Jyoti) seed germination, but C. difformis, Panicum repens, Brachiaria ramosa, Monochoria vaginalis, Scirpus sps., and Ludwigia parviflora significantly reduced the germination.
Weed Management in Rice, DRR Training Manual