Management-Oriented Simula-
tion of Rice-Weed Dynamics-93



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Project Leader and Principal UC Investigators

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

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

Barney Caton, Division of Environmental Studies

John Breen, Agronomy and Range Science

Scientists working on this project are studying the biology of specific weeds and their interactions with rice plants. These researchers are confirming the significant role that competition for light has on the growth of weeds and, consequently, the effect weeds can have on rice production and yield. Two primary objectives are steering their research:
  • Continue comparing the performance of different weeds in rice stands. Redstem was the focus of 1993 studies.
  • Develop and use models of weed-rice competition to further an understanding of their field dynamics.

Research thus far has shown fairly convincingly that competition for light is the principal reason why weeds reduce rice productivity. Scientists performed the same type of growth analysis and competition experiments that they had done previously for annual arrowhead and smallflower umbrella plant.

Redstem demonstrated its ability to significantly reduce rice tiller densities - with corresponding yield reductions. Damage was about the same as what had been documented previously with smallflower umbrellaplant. Because it significantly reduced rice tillers and yield, redstem is "clearly" a more important Weed than annual arrowhead. Light competition is almost certainly the primary reason for the reduction.

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 or "plastic" behavior to stay within or to reach above the developing rice canopy. The key to controlling redstem will be to keep it from penetrating the rice canopy. Control efforts should be directed to early suppression (up to 28 days after seeding), because the rice canopy itself can supply adequate control from that point forward.

Computer Modeling

A thorough analysis of the CARICE computer model, which simulates canopy development and assumes that weed impacts are principally a function of their ability to get into the canopy, was completed during 1993. In 1994 scientists will develop other areas dealing with stand development, photosynthetic conversion, canopy development and use of weather and light data.

Another computer module under development is called CANWER, which stands for Canopy-Oriented Weed-Rice Growth Simulator. Its purpose is to estimate weed impacts on rice production. This "multilayer" model is more complex than CARICE. By developing the two different models for comparison, researchers hope to determine how much canopy detail is needed to accurately simulate rice- weed growth and competition. Completion of the program is expected during 1994.

Preliminary results from both models and experimental research indicates that weeds do their principal first damage by reducing rice tillering. In some cases, total tiller density is the key variable, while for others, initial rice tiller densities are more important. Planting a dense stand may help offset some of this tiller reduction. Researchers called this a "critically important" consideration. In all cases studied to date, tiller reduction is more important than panicle filling in the reduction of rice productivity.

The upshot of this research for rice growers is that early control of weeds - even without full control - can be effective. Early weed control means rice gains a critical competitive advantage, especially in preserving a full stand of rice tillers. While control of highly competitive weeds such as redstem is critical to ensuring good rice yields, cosmetically distracting weeds such as annual arrowhead may be annoying but are not as crucial. In fact, researchers say that control of annual arrowhead in good rice stands is "probably not worth the cost."

In the future this work should lead to a canopy-dynamic model that can be used as a management tool for predicting rice growth and competition with weeds.

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