Abstract
The effects of four aqueous extracts (0, 1.0, 2.0, and 3.0% w/v) of itchgrass [Rottboellia cochinchinensis (Lour.) W. Clayton] and cogongrass (Imperata brasiliensis Trin.) on rice germination and the effects of residues (0, 1.0, 2.0, and 3.0% w/w) incorporated in the growth medium on early rice plant development, were tested. Allelochemicals present in weed extracts reduced germination of rice by 11-15% at the highest concentration of extracts. A different effect of residues on total dry matter of seedlings was observed. The lowest concentration of water extracts of weeds increased epicotyl length and reduced the germination index. Only itchgrass extracts had a negative effect on rice root elongation. Growth reduction of rice induced by residues of both weeds was similar and plant height and leaf number per plant were reduced by 22 and 43%, respectively. In contrast, rice radicle length was more sensitive to the itchgrass residues than to the cogongrass residues. Residues of both weeds at the highest level caused a decrease of 50% in N concentration of rice plants. Potassium content was unaffected by cogongrass while P was decreased only by the highest level of residues. The effects were dependent on levels of residues and on weed species. The results demonstrated the allelopathic potential of itchgrass and cogongrass residues on germination and the early development of rice.
