EFFECTS OF NITROGEN APPLICATION ON GROWTH, NITROGEN USE EFFICIENCY AND RICE WEED INTERACTION

Responses of upland rice (Oryza sativa L.) and the weeds Amaranthus sp inosus L., Cyperus rotundus L., Eleusine indica (L.) Gaertn., and Rott boellia cochinchinensis (Lour.) W. D. Clayton to nitrogen (N) and time of N application were studied in the glasshouse both in monoculture a nd in mixture. The N response of two rice cultivars and R. cochinchine nsis in monoculture levelled off between 69 and 103 mg N kg-1 air-drie d soil, whereas the other weed species continued to respond beyond 103 mg N kg-1 air-dried soil. At high N application rates, N uptake by we eds was higher than that by rice; weeds used N more efficiently. Weeds , which had high dry matter production at high N application rates in monoculture, were most competitive at low N and least sensitive to low applied N. Nitrogen application levels sub-optimal to rice reduced th e competitive ability of rice when in mixture with E. indica, R. cochi nchinensis, and A. spinosus, but not with C rotundus. Although N respo nse of a plant species generally declined with age, interspecific diff erences were observed. Timing of N application can be exploited in ric e-weed management.