Rice-Weed Dynamics-94


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Project Leader and Principal UC Investigators

Theodore C. Foin, professor, Division of Environmental Studies, UC Davis

James E. Hill, extension agronomist, Dept. of Agronomy and Range Science, UC Davis

Kevin Gibson, Division of Environmental Studies, UC Davis

Barney Caton, Division of Environmental Studies, UC Davis

As chemical options diminish, future weed management strategies will depend more heavily upon the knowledge of the biological interaction between rice and weeds. Scientists working on this project are focusing on five main objectives:

• Greenhouse studies of competition for light between rice and weeds, particularly redstem, with an emphasis on properties responsible for yield reduction.

• Growth analysis experiments at four nitrogen levels to identify the interactions of fertilizer on rice and three weed species.

• Analysis of how rice densities affect rice-weed dynamics at different fertility levels.

• Field experiments to confirm the potential control of redstem by coordinating herbicide application to maximize weed suppression by the rice canopy.

• Development of a model of rice-weed competition.

Rice-Weed Competition

RWDynamic1.jpg (338356 bytes)The evidence that competition for light is the most important determinant in riceweed competition is mounting.

Researchers repeated competition experiments with simultaneous seeding at different ratios that documented how redstem's aggressive growth reduces rice tillering and thus yield. The results were the same as in 1993. Redstem's competitive strength stems from its ability to redirect growth from leaves, secondary branches and roots into elongation of the main stem. It uses this adaptive behavior to stay within or to reach above the developing rice canopy. Therefore, early control of redstem is essential to keep it out of the rice canopy.

In 1994 experiments showed that redstem that germinated 13 days after planting or later was unable to penetrate the canopy. This suggests that herbicide control for redstem be targeted for the period two weeks to one month after rice planting. After this time, given a uniform rice stand with no gaps, the rice canopy should effectively suppress most or all of the later-germinating redstem plants.

Fertilizer Dynamics

Researchers also conducted a growth analysis of smallflower umbrella sedge, watergrass and redstem. A statistical analysis showed that while varying nitrogen levels can account for variations in rice growth, nitrogen levels alone cannot explain weed growth adequately.

Another experiment compared the highest panicle weight for rice and lowest redstem plant weight at varying nitrogen levels. The lowest redstem weight was found at a combination of high rice density and high nitrogen treatment. This suggests that redstem can be suppressed by combining higher seeding rates with good fertility management.

Herbicide Timing

Researchers carried out a field experiment to examine the efficiency of herbicide applications targeted at redstem. At 57 days after planting (DAP), an application of Londax cut weed density in half and increased grain yield by 8 percent. A treatment of MCPA at 55 DAP completely eliminated the redstem problem but resulted in an 11 percent reduction in rice yield. This was "almost certainly" due to rice injury caused by the late timing of MCPA application; extensive damage to tillers was noted at harvest.

Computer Modeling

As new knowledge about the interaction of redstem with rice is developed, scientists are incorporating that information into computer programs designed to estimate weed impacts on rice production. The first is called CANWER, a multiple-layer canopy development model, which has been completed and is being tested extensively.

The second is called RICECOMP, the direct reformulation of the original CARICE model, and is still in the design and programming stage. Current work is incorporating the new knowledge about nitrogen effects into the program.


This research re-emphasizes the primary role that light plays in determining how weeds compete with rice. Cultural control efforts should be directed at closing the rice canopy as quickly as possible. Chemical control should maximize weed suppression from the rice canopy.

The other major conclusion the scientists working on this project want to stress is that seed density and fertility levels can be used to improve weed control. When rice is more efficient than its competitors in sequestering nitrogen, higher planting densities and higher nitrogen availability serve to close the rice canopy more quickly and thereby achieve better competitive advantage to rice.

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