A plant growth model for integrated weed management in direct-seeded rice. III. Interspecific competition for light

Abstract

The model DSRICE1 was developed for analyzing integrated weed management strategies for direct-seeded rice. We have shown that DSRICE1 predicts monoculture rice growth well and accounts for water-depth effects on growth. Here, the model is used to simulate competition for light between rice and two weeds, Echinochloa oryzoides (early watergrass) and Ammannia spp. (redstem). Except for minor differences in phenology, weed growth was simulated as described for rice. Direct competition for light depended on the species' vertical distributions of leaf and stem areas (live and dead) and their extinction coefficients. Water also attenuates light, so species' early height growth rates were important because they determined when plants emerged into full light. Structural sensitivity analyses of rice in competition with the two weeds revealed that water-depth effects and leaf area distributions strongly affected competition, and shading by dead leaf and stem dry mass reduced total production. Validation was based on independent data sets for redstem and watergrass competition using several statistical tests and indices. For rice–redstem competition, DSRICE1 simulated rice growth well because redstem competitive effects were small, but predictions of redstem growth were good only when observed heights were matched in simulations. Redstem competitiveness depended on height growth rate, perhaps due to its small seed size. For rice–watergrass competition, the growth of both species was predicted well, except that watergrass growth in plots with early-season drainage was underpredicted. Watergrass parameters were similar to those for rice except for faster height growth and higher photosynthesis rates. In a model application, simulations in which rice seeding was delayed for a time after flooding led to greater yield losses from redstem than from watergrass because delays reduced the advantage of rice over redstem. The usefulness of DSRICE1 for drained fields will be improved by better simulation of plant growth responses to drainage, but rice competition with redstem and watergrass in continuously-flooded fields was simulated well.