Objective

The weed control project is a cooperative project among the Agronomy and Range Science Department and Botany Department at the University of California, Davis, and Cooperative Extension. Studies are conducted at the Rice Research Facility at Davis, the Rice Experiment Station at Biggs, and on rice growers' farms at various locations. Principal objectives are to develop new chemical methods of weed control in rice, to improve the efficacy and safety of chemicals now in use, to develop integrated rice management systems, and to investigate the biology of rice weeds and their ecological relationships with rice in the field.

Chemical Weed Control

Spraying experimental plots for herbicide studies.

Studies on the screening of new herbicides were conducted at the Rice Experiment Station and at the Rice Research Facility. Fifteen experimental herbicides were studied at the two locations.

The most promising among the experimental herbicides was Hoe33171, recently named Whip, for barnyardgrass control. Hoe-33171 was in preliminary stages of testing in 1983 but has shown enough selectivity to warrant further studies. This herbicide represents a different chemistry than the currently used herbicides molinate and thiobencarb. Most of the experimental herbicides were unsuccessful in providing adequate weed control at the rates and times of application tested.

Several studies were conducted at the Rice Experiment Station and at the Demeter Corporation on herbicide combinations and timing of herbicide applications. The MCPA timing studies confirmed previous results that the early applications at 25 to 35 days after sowing provided better weed control and less depression on yield than did later. applications at 45 or 55 days. Bolero@ and Ordram© were applied sequentially to determine if combinations of these herbicides would provide better weed control or be more injurious to rice than when applied alone.

Combinations did not injure the rice regardless of the sequence of applications as long as the growth stage requirements for rice and weeds were observed. All treatments provided good barnyardgrass control; however, Bolero@ alone or in combination also controlled sprangletop and smallflower umbrellaplant.

Management Systems for Weed Control

Test plots for evaluating new herbicides.

Studies integrating cultural practices and weed control included trials on the interaction of MCPA and bentazon with nitrogen fertilization and on herbicides and water management. Previous studies have shown that MCPA may depress leaf tissue nitrogen in the rice plant for approximately two weeks following applications. Studies in 1983 were designed to determine more precisely the period of depression and whether the application of nitrogen can overcome this effect. The leaf tissue analysis is not completed, therefore, the results will be reported later. Studies on the competition of barnyardgrass to rice conducted in 1981 and 1982 were analyzed and were used widely in establishing the need for the continued use of Ordarm® and Bolero.

Water management studies were conducted at the Rice Experiment Station and at the Demeter Corporation, Sacramento County. Preliminary data from these trials clearly indicate the importance of at least 3 to 4 inches of water to enhance weed control treatments.

Weed Biology and Ecological Relationships of Weeds and Rice

Greenhouse and laboratory studies have expanded knowledge of how the smallflower umbrellaplant germinates. In addition to a definite temperature response, smallflower umbrellaplant seed has a light requirement for germination that cannot be offset by increasing temperature. This light requirement further delays the emergence of seedlings until clods in newly-flooded fields disappear.

Germination of barnyardgrass seed occurs over a wide range of temperatures. When the germination of early and late barnyardgrass seed were compared under identical conditions, early barnyardgrass was shown to germinate quicker than late barnyardgrass. This was especially true at the cooler temperatures. Barnyardgrass germinated 70 to 80 percent after two weeks at 55°F while rice (M9) under these same conditions germinated only 14 percent.