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

Responses of upland rice (Oryza sativa L.) and the weeds Amaranthus spinosus L., Cyperus rotundus L., Eleusine indica (L.) Gaertn., and Rottboellia cochinchinensis (Lour.) W. D. Clayton to nitrogen (N) and time of N application were studied in the glasshouse both in monoculture and in mixture. The N response of two rice cultivars and R. cochinchinensis in monoculture levelled off between 69 and 103 mg N kg-1 air-dried 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 weeds 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 the competitive ability of rice when in mixture with E. indica, R. cochinchinensis, and A. spinosus, but not with C rotundus. Although N response of a plant species generally declined with age, interspecific differences were observed. Timing of N application can be exploited in rice-weed management.