Weed Control-80



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Project Leader and Principal UC Investigators

David E. Bayer, Dept. of Botany, UC Davis

Donald E. Seaman, Dept. of Agronomy and Range Science, UC Davis


New Herbicides

New herbicides continue to show promise for controlling major rice weeds, but the extensive research and high cost associated with development continues to slow the appearance of these compounds on the market.

A major 1980 effort by the Chevron Chemical Company to secure registration of Bolero has been successful in securing an emergency registration for Bolero for 1981 use on rice. Although the current registration is for one year only, the Chevron Company hopes to obtain a full registration for the 1982 rice crop. Bolero will be an important new tool for grassy weed control, particularly as short-statured rice acreage increases. The Bolero label will specify use for grass control, but experimentally the compound has proven effective on several of the broadleaved weeds.

Dr. Don Seaman (left), UC agronomist stationed at the Rice Experiment Station, shows Cooperative Extension rice specialist, Dr. James Hill, some of his herbicide screening trials at the Rice Station. Seaman and Dr. David Bayer of UCD screened 23 new chemicals in 1980 to determine their potential for use on rice.

It appears that while the presently marketed formulation of molinate (Ordram) does not have sufficient soil residual life to control barnyardgrass over the long period during which grass germination occurs, new formulations may solve this problem.

Twenty-three new herbicides were evaluated at UCD and the Rice , Experiment Station. Eight herbicides were applied pre-flood and evaluated for their control of a broad spectrum of rice weeds. Fifteen possible new herbicides were evaluated for postflood broadleaf and sedge control. Of the eight new pre-flood surface applied herbicides, two showed promise for the control of barnyardgrass. The new herbicides tested for broadleaf and sedge control provided control only equal to MCPA.

Two new granular formulations of molinate were again evaluated. One increased the control of barnyardgrass over the commercial molinate formulation. These formulations were designed to provide a slower release of herbicide, thus, extending its residual properties.

Applications of bentazon (Basagran) plus non-phytotoxic oil again showed that timing of the application is important for the control of smallflowered umbrellaplant. Once the smallflowered umbrellaplant had established several tillers and started to flower, control was not obtained. Although severe leaf burn occurred on the smallflowered umbrellaplant, insufficient herbicide translocated into the crown, and the plants continued to grow. The addition of non-phytotoxic oil had little or no effect on bentazon activity under ideal conditions, but when the smallflowered umbrellaplants were under stress and when the plants were large, bentazon activity was enhanced by the oil. Once the smallflowered umbrellaplant began to flower, MCPA at 1.25 pounds active ingredient per acre gave the best control. This treatment caused some injury to the rice plant; however, plants recovered without a serious reduction in yield. Several new formulations of MCPA were evaluated. None were significantly better than the standard amine formulation.

The growth habits of smallflowered umbrellaplant were evaluated in the greenhouse, laboratory and in the field. As was reported last year, once smallflowered umbrellaplant developed to an advanced stage of growth, it could regenerate new growth following an application of MCPA. Vegetative buds at the crown produced this new growth.

Bolero 1OG (distributed by Chevron Chemical Company) rice herbicide trials at the rice station February 2, 1981. UC weed scientists and those of Chevron Chemical Company have secured a temporary registration of the compound for 1981. It controls barnyard grass, sprangletop, smallflowered umbrellaweed and several others.

Preflood Herbicide Experiments

In cooperation with UCD agronomists Dr. D.S. Mikkelsen and Dr. ).E. Hill, several herbicides registered in other states for weed control in drilled or transplanted rice were evaluated on peroxide coated rice seed at UCD and in Colusa County. The herbicides were applied to the dry 'soil surface after the rice had been drilled but before flooding. Poor stand establishment because the peroxide coated seed was unevenly drilled made it difficult to assess effects of the herbicide on emerging rice. However, several herbicides provided excellent broad spectrum weed control without apparent symptoms on the surviving rice plants. With further study, this stand establishment technique is likely to become a management alternative.

Thiobencarb (Bolero), oxidiazon (Ronstar), and bifenox (Modown) gave excellent barnyardgrass control with little or no injury to the rice. Although nitrofen (Tok) did not injure the rice, little or no barnyardgrass control was obtained. Applications of trifluralin (Treflan), butachlor (Machete), and MCPA caused some stand reduction and stunting of the rice.

Herbicide Use in Long-Grain Rice

This year's results confined that molinate and thiobencarb were hazardous to water-seeded variety L-201 rice at rates as low as 4 pounds active ingredient per acre, while tiocarbazil was safe at rates as high as 10 pounds active ingredient per acre. Slow-release granular molinate and thiobencarb were as safe or safer than their standard formulations when applied before flooding, but they were more hazardous after flooding. The most promising methods for safe and effective weed control in L-201 were low-rate split applications of granular molinate and the use of tiocarbazil (not yet registered).

Another solution would be the development of a herbicide resistant long-grain variety for California. Three of this year's long-grain selections have promising resistance to injury by thiobencarb.

Water-Run Herbicide Application

The performance of water-run applications of molinate or thiobencarb was improved by more rapid flooding. This enabled the herbicide-treated water to flow over the soil surface without subirrigating ahead of the water front. Both herbicides were less effective on the late form of watergrass than on the early form when applied by this method, but thiobencab was more effective than molinate on both forms. In adjacent fields, separate applications of granular molinate or thiobencarb at rates equivalent to the water-run applications gave more complete control of both watergrasses. Unregistered DOWCO 346 was the most promising alternative herbicide applied by the water-running method.

Rope-Wick Application of Glyphosate

The rope wick applicator, using glyphosate (Roundup), controlled perennial weeds on rice field levees, but Roundup has not been approved by EPA for this use. Cattail control was better when 30 percent Roundup solution was applied bidirectionally because the herbicide contacted more foliage than when it was applied in only one direction. However, this year's trials indicated that unidirectional applications may be. sufficient to control johnsongrass.

Herbicide Drift Reduction

Evaluation of young almond and female pistachio trees that were part of a cooperative 1979 MCPA drift study with Dr. N. Akesson, Department of Agricultural Engineering, indicated that pistachio trees were more susceptible to the MCPA drift than almonds. At the station 12.5 meters from the 1979 application, three young pistachio trees in a group of 12 appeared dead; however, all three sprouted from the rootstock by the spring of 1980. All of the almond trees continued to grow with only an occasional dead branch or shoot. There were no carry-over symptoms that could be detected visually at any of the more distant stations.

An evaluation was made of an airplane application of MCPA to a field of rice with and without a drift control additive. Under the conditions of this study, adding the drift control agent did not reduce the drift of MCPA as measured by symptoms produced on bean and tomato plants. Instead, the drift control additive appeared to increase the amount of MCPA drifting downwind.


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