Resumen
- The critical period of rice-weed competition was studied for the rice varieties Oryzica 1 and CICA 8, in 1989 and 1990. Weed species were Eleusine Indica, Echinochloa crus-galli, Echinochloa colonum, Cyperus dlftormis, Cyperus esculenlus, Cyperus ina, Leplochloa filiformis and Eclipla alba. Competition effects were similar for both varieties. Weeds emerging with the crop were the most damaging ones. Rice yields increased wijh longer weed-free periods up to 70 days atter emergence (d.a.e.). Weed emergence and growth atter 70 d.a.e. was suppressed by the crop. Both rice cu~ivars required a critical height of 44-50 cm to suppress weed growth at 70 d.a.e. Though both varieties were equally weed suppressive by 70 d.a.e., leaf area index of CICA 8 was much larger than that of Oryzica 1 when rice grew weed-free after emergence. Economic analysis showed that under heavy weed infestations, In a system where rice cannot be early and continuously flooded, three post-emergent herbicide applications at 9, 18, and 44 d.a.e were justified.