Weed Control-81



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Project Leader and Principal UC Investigators

D.E. Bayer, Department of Botany, UC Davis

D.E. Seaman, Department of Agronomy and Range Science, UC Davis

J.E. Hill, Cooperative Extension, UC Davis


The weed control program is designed to evaluate new herbicides with a potential for use on California rice, improve the efficiency of present herbicides, determine best crop management strategies for weed control, and to learn more about the biology of serious rice weeds so critical stages in their life cycle can be better exploited.

Dr. Jim Hill, extension agronomist at UC Davis, explains a weed susceptibility chart during the annual Rice Field Day at the Rice Experiment Station.

As a result of several years of testing by the Chevron Chemical Company and in cooperative tests with the University of California, Bolero was given an emergency Section 18 registration. In 1981 this product was used on approximately 100,000 acres in California and has been extended to 200,000 acres for 1982. The herbicide provided excellent control of not only sprangletop and watergrass, but also smallflower unbrellaplant, ducksalad, waterhyssop, -and some other weeds. The Chevron Chemical Company is working on a full registration for California rice. This product will be an important tool for controlling a broad spectrum of rice weeds.

Several new experimental herbicides were studied at the Rice Experiment Station in 1981. Four of these showed considerable potential for rice weed control and will be evaluated again in 1982. Some new slow release granular formulations of experimental herbicides - as well as of molinate (Ordram), MCPA, and combinations of the two - appeared promising. In studies in Sacramento County, molinate plus Extender reduced the rate needed on preflood applications but showed no difference in postflood rates.

Area rice specialist Jack Williams shows how water depth affects rice stand, weed and rice growth, and herbicide activity in his studies in Sacramento County. Water depth plays an important role in weed control, however, sort-statured varieties and contact foliar herbicides have made water depth management complex.

The timing of Bolero, MCPA, and Basagran was tested extensively. Bolero was studied in both pre- and postplant applications. Weed control in general was poorer for the preplant treatments than for the postplant treatments. Stand reduction of the rice was most severe when Bolero was applied at the 1-leaf stage - even more severe than in preplant applications. Although the preplant treatments reduced stands modestly, surviving plants tillered vigorously and produced good yields. Preplant Bolero treatments will be investigated further in 1982.

In 1981 one formulation of MCPA received a new labeling allowing for early treatment of rice at the 3-leaf stage providing that the crop was well established and well rooted. Studies of MCPA applications to rice at 25, 35, 45, 55, and 65 days after seeding showed that although early applications (25, 35 days) stunted the rice slightly in terms of height and root growth, yields were superior to those obtained with later treatment. With smallflower umbrellaplant, in particular, control was diminished once this weed began to tiller and flower. Rice yields were highest for the 25- and 35-day treatments and declined progressively with later application dates, indicating the importance of early weed removal to reduce competition.

... only one or two barnyardgrass plants per square foot reduce yield by 10 cwt per acre.

Basagran was applied to rice 20, 25, 35, and 45 days after seeding (seedling, beginning to tiller, tillering vigorously, and tillers fully developed). The water was removed to expose the weed foliage and returned one day following herbicide application. Control of red stem and smallflowered unbrellaplant was best at the earliest two treatments. However, in the 20-day treatment some new seedlings emerged and reinfestation occurred. Early applications benefit the grower because weed competition is reduced during this critical period. After the smallflowered umbrellaplant was well-tillered and flowering, Basagran only burned the leaves but did not kill the plant. When leaf burn was severe the smallflower umbrellaplant produced new tillers.

Field studies confirmed that laboratory screening methods could be used to identify long-grain rice types with greater tolerance to molinate or thiobencarb.

Weed competition studies showed that only one or two barnyardgrass plants per square foot reduce yield by 10 cwt per acre. Barnyardgrass was much more competitive with short-statured varieties than with tall varieties. Competition was increased by high nitrogen levels. Competition by broadleaf weeds and sedges appeared to be similar for tall and short varieties. Weed control is more essential and more cost effective in short-stature rices.


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