MECHANISMS OF COMPETITION FOR LIGHT BETWEEN RICE (ORYZA-SATIVA) AND REDSTEM (AMMANNIA SPP)

Redstem is an important weed in California water-seeded rice fields be cause of its aquatic habit, wide distribution, interference with harve st, and resistance to the herbicide bensulfuron. Our objective was to understand the mechanisms of competition for light between rice and re dstem, with the goal of improving redstem control. A replicated greenh ouse experiment was done in 1993 and 1994. Rice was water-seeded at a rate of 400 seeds m(-2), and redstem was seeded simultaneously at appr oximate densities of 0, 50, and 100 seeds m(-2) in continuously floode d 0.77 m(2) basins. Plants were harvested once at final harvest in 199 3 and twice in 1994, with an additional nondestructive sampling 34 day s after seeding (DAS). Despite slower early growth, redstem height exc eeded rice height about 45 DAS. At the midseason harvest in 1994 (56 D AS), no effects of redstem competition on any rice response variables were detected. However, at final harvest (110 and 118 DAS, 1993 and 19 94, respectively) redstem competition at both treatment densities redu ced rice tiller density, panicle density, shoot drymass, and grain dry mass. Redstem competition reduced rice growth only after penetrating t he canopy. Shade cast by redstem through rice maturity decreased shoot and grain production and increased tiller mortality Lodging caused by redstem further affected rice growth. Season-long competition from re dstem at mean densities of 67 and 110 plants m(-2) reduced rough rice yields 31 and 39%, respectively, making redstem the most competitive b roadleaved rice weed yet studied. Improved understanding of rice-redst em interactions indicates that using alternative herbicides to bensulf uron is unlikely to increase yield losses to redstem, and that control may be improved by increasing rice plant densities or slightly delayi ng early season chemical control. Because these strategies are mechani stic, they may also be useful for controlling other rice weeds with gr owth patterns similar to redstem.