Weed Control-94


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Project Leader and Principal UC Investigators

David E. Bayer, professor, Weed Science Program, Dept. of Vegetable Crops, UC Davis

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

E. Roncoroni, staff research associate, Dept. of Vegetable Crops, UC Davis

M. Hair, postgraduate researcher, Dept. of Vegetable Crops, UC Davis

S. C. Scardaci, UC Farm Advisor, Colusa County

J.F. Williams, UC Farm Advisor, Sutter/Yuba County

M. Carriere, research assistant, Dept. of Agronomy and Range Science, UC Davis

T. Kraus, research assistant, Dept. of Agronomy and Range Science, UC Davis

D. Roush, research assistant, Weed Science Program, UC Davis

D. Brandon, research assistant, Weed Science Program, UC Davis

Weed1.jpg (144080 bytes)Research on weed control has always been one of the most important projects funded by the Rice Research Board. It has become even more important with the rapidly spreading problem of Londax resistant weeds. Three primary objectives steered scientists' work last year:
  • To develop chemical methods of weed control in rice and to improve the efficacy and safety of herbicides now in use.
  • To study the biology and physiology of rice weeds in the field, greenhouse and laboratory.
  • To study Londax-resistant weeds and develop a strategy for their control.

New Chemicals

The results of new herbicide screening indicate there are four possible new candidates for use in California waterseeded rice. Two of them - KIH-6127 and KIH-2023 - are primarily grass herbicides but have some herbicidal activity against sedges and broadleaf weeds.

Three, including the KIH herbicides, are experimental and will not be available for use by California growers in the near future. Triclopyr (Grandstand) is the most advanced and potentially could reach the California market within a year. In field trials Grandstand was most effective when applied before the sedge and broadleaf weeds had started to tiller and flower.


Research on improving the effectiveness and safety of herbicides currently registered for use in California also continues.

Experiments from last year suggest that Abolish can be effectively used in a flush-drain culture early in the season similar to the Pin-Point Flood method of southwest Louisiana. Abolish used in this manner controlled barnyardgrass, watercress, sprangletop and smallflower umbrella sedge but was less effective for the control of Richfield bulrush.

The herbicidal effect of MCP and 2,4-D amine were essentially the same. 2,4-D amine was a little more active than MCP but also slightly more injurious to rice. This year's data indicate growers could use either MCP or 2,4-D amine with essentially the same results. The most critical issue is to apply the herbicide early - about one month after seeding to well-rooted rice.

Whip provided excellent watercress and sprangletop control but rice tolerance, was marginal. Split applications provided better grass control and less injury to the rice than single higher rates. The second split provided control of weeds that emerged following the first application and the lower rates were less injurious to the rice. Applications made when the rice was in the five-leaf to one-tiller stage with 70 percent of the weed foliage exposed provided the best weed control, least rice injury and highest rice yields. Lowering the water to expose the weed foliage was as effective as draining the plot area. Uniform coverage of the rice and weed foliage was critical to minimize rice injury and maximize weed control.

Londax Monitoring

Weed2.jpg (274304 bytes)Results of a survey developed by the University of California and DuPont Co. and distributed to pest control advisers indicates that resistance to Londax has grown exponentially since it was first discovered three years ago. Total sites with suspected Londax-resistant weeds numbered 4,753 in 1994 (72 in 1993 and four in 1992). Of those sites, 43 percent were California arrowhead, 24 percent redstem, 23 percent smallflower umbrella sedge and 10 percent ricefield bulrush.

The survey data account for 82 percent of the estimated 540,000 acres of rice grown last year. Further, the survey showed that at least one resistant weed site has turned up in every county where rice is grown, including those in the San Joaquin Valley.

The survey also showed that PCAs considered smallflower umbrella sedge to be the most difficult Londax-resistant weed to control with alternative herbicides and is also the weed that contributes most to yield reduction. Redstem was considered to be the most problematic weed because of its rapid increase in resistance.

Biology and Physiology

Researchers also conducted a number of field and greenhouse studies and laboratory experiments to further knowledge of the biology and physiology of rice weeds.

In one study botanists looked at how certain weeds compete with rice. They found that the number of ricefield bulrush and smallflower umbrella sedge plants required to reduce rice yields were in the order of one plant per square foot.

Weed scientists also concluded a four-year study comparing weed abundance and competition between water-seeded and drill-seeded rice systems to determine whether cultural practices could be in continuous rice cropping systems. The study indicates that drill-seeding may provide an alternative means of controlling populations of aquatic broadleaf and sedge weeds in the absence of broadleaf herbicides. However, competitive grass species that establish during the pre-flood period are more difficult to control than under water-seeding where the continuous flood suppresses germination and growth of these weeds.

A greenhouse study of one of the new experimental herbicides, KIH-2023, determined that injury to rice roots can occur for up to two weeks following treatment. New roots initiate after this time but total root biomass lagged behind the control for an additional two weeks. This knowledge will be useful in determing optimal application time. This study also documented the ability of KIH-2023 to severely damage tomato seedlings.

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