John F. "Jack" Williams - UC Farm Advisor, Sutter and Yuba counties

Randall G. "Cass" Mutters - UC Farm Advisor, Butte County

• To integrate new and existing herbicides and cultural practices for rice weed control.
• To study the biology and physiology of rice weeds in the field, greenhouse and laboratory.
• To study the phytotoxic effects of herbicide movement from rice fields.
• To study Londax®-resistant weeds and to develop strategies for their control.

This section of the report summarizes significant developments in weed control research - including a promising new chemical scheduled for registration within two years; results from studies reexamining older herbicides; a project analyzing aerial drift; and a study confirming fears of watergrass resistance to several common herbicides.

Hopeful Developments in Chemical Controls

Carfentrazone, an experimental herbicide known as F-8426, exhibited good control of Londax®-resistant weeds in experiments with foliar applications at three on-farm sites and at the Rice Experiment Station. Researchers report "excellent" control of ricefield bulrush, California arrowhead and redstem, and very good control of smallflower umbrella sedge. It also provided control of water plantain at one on-farm site but did not provide any control of ducksalad. In a preflood surface application and direct into-the-water application, carfentrazone did not injure rice. In applications directly to rice foliage, however, some initial but temporary injury appeared as reddish-brown spotting. Granular formulations of carfentrazone are in early testing stages. This FMC product is slated for full registration in 1999.

Weed scientists also carried out additional studies at the Rice Experiment Station and in UC Davis greenhouses on V-10029, a promising watergrass/ barnyardgrass herbicide, both alone and in combinations with other herbicides It is used as a mid- to late postemergence foliar application with a surfactant. Results varied greatly depending on timing of application, concentration of active ingredients and other herbicides in the mix. In general, however, V-10029 alone and in combination showed good action against barnyardgrass, watergrass, ricefield bulrush, smallflower umbrella sedge and monochoria with little or no injury to rice. Effectiveness decreased with later timing.

Other new rice weed herbicides under scrutiny by researchers include broad spectrum, non-selective herbicides, such as glufosinate, to be used with genetically resistant rice cultivars. Glufosinate, marketed by AgrEvo USA as Liberty, is a broad-spectrum herbicide previously used on other crops but not in rice. This chemical is highly toxic to most plants, including rice, but through genetic engineering, resistance to the herbicide can be introduced to rice. This is a relatively new technology only now being commercialized in a few major crops. Researchers examined the action of a postemergence application of glufosinate on a genetically engineered sample of Koshihikari, a Japanese variety. Single applications of glufosinate did not adequately control ricefield bulrush; however, split applications provided very good control at higher rates. All plots with herbicide-resistant rice showed some injury from the herbicide. Despite these shortcomings, researchers are optimistic that glufosinate-resistant rice holds considerable promise in well-adapted California varieties.

Existing Herbicides and Combinations

Several "old" herbicides - MCPA, 2,4-D, trichlopyr (Grandstand) and Whip - were tested to support re-registration or to develop needed performance information.

Four formulations of 2,4-D Amine were evaluated in rice for control of ricefield bulrush, smallflower umbrella sedge and monochoria and for rice injury. Granular formulations applied to floodwater were not effective against broadleaf or sedge weeds at the four-leaf and first-tiller rice stage. However, spray solutions of 2,4-D completely controlled ricefield bulrush and gave satisfactory control of smallflower umbrella sedge and monochoria at the first tiller stage; some injury occurred to the roots of rice plants. All rates of Hi-Dep formulations gave excellent control of ricefield bulrush and 90 percent or better control of the other two weeds. At higher rates - up to 1.5 pounds active ingredient per acre - both the Solutions and standard formulations of 2,4-D provided 100 percent control of ricefield bulrush.

Grandstand was evaluated at three locations at the Rice Experiment Station to determine its efficacy as well as its potential for rice injury. The best control occurred at the four-leaf rice stage when weeds were small. Researchers also note that weed control was better with the use of a surfactant such as L-77. In a second, weed-free location Grandstand in combination with low rates of 2,4-D did not adversely affect rice yields. At the third site, where weed infestations were limited, successive applications at first through third tiller stages reduced rice yields.

Researchers also looked at applications of thiobenearb (Abolish or Bolero) alone or in combination with molinate (Ordram). Several combinations provided satisfactory watergrass control. A preflood surface application of Abolish showed slight injury to rice. Successive applications of Ordram applied preflood and postflood at the two-leaf stage of rice increased watergrass control over similar applications of Ordram followed by Bolero.

Researchers also examined fenoxaprop (Whip) alone or with surfactants at the first and third tiller stages of rice. just as the rice was starting to tiller, researchers lowered the floodwater two to three inches to expose watergrass and spranglet6p foliage. Whip applications were made and flood water returned to a depth of five to six inches 48 hours later. Various combinations of Whip with the adjuvants Kenetic, X-77 or crop oil concentrate provided enhanced levels of watergrass or sprangletop control in most tests. Although researchers observed leaf tip burn on rice at the three tiller stage in all treatments, the plants soon recovered.

In another experiment to simulate the pin-point flood production practice, researchers examined postemergence applications of Abolish alone or in combination with other chemicals. Late spring rains just prior to flooding and seeding allowed watergrass, sprangletop and some bamyardgrass to become established and was too large for the Abolish treatment to be effective. The combination of V-10029 plus Abolish looks very promising when grass weeds get too large for Abolish. V-10029 gave control of on-site weeds and Abolish provided the soil residual. The combination of Abolish and Whip controlled sprangletop but not watergrass. The combinations including Londax® or the experimental herbicide F-8426 controlled broadleaf and sedge weeds. A combination including MCPA and 2,4-D amine applied at first tiller stage gave complete control of the broadleaf and sedge weeds. Other combinations including 2,4-D or Grandstand also gave satisfactory control of the broadleaf and sedge weeds.

How would similar applications compare to a postflood treatment? Applied at the three-leaf stage various combinations provided satisfactory watergrass control and good to excellent broad-spectrum sedge and broadleaf control. The experimental herbicide V-10029 provided excellent control of established watergrass and barnyard- grass and also provided residual protection against late emerging watergrass. Combinations with the experimental P-8426 provided nearly equivalent control of ricefield bulrush, smallflower umbrella sedge and monochoria, as did the combination with Londax®.

Herbicide Drift Studies

Project leaders also cooperated in three drift studies, two with new formulations of propanil and one on 2,4-D. These studies were needed to assist in the expansion of propanil use zones.

In the propanil trials, researchers measured off-target movement during application and for a three-day period following treatment. High-volume air samplers and cotton plants used as living monitors of drift showed symptoms up to 150 feet downwind of the treated area. Beyond that distance fewer symptoms were noted. Researchers concluded that small amounts of propanil move off-target during application and far less is moved off-target as the so-called lift-off.

In the 2,4-D trial, researchers also measured off-target movement during application and for a three-day period following treatment. This trial also sought to compare drift potential from fixed-wing aircraft and helicopter applications. Drift was considerably higher with fixed wing, which researchers believe is related to the type of nozzles used and the forward velocity of fixed-wing aircraft.

Watergrass Resistance

Concern that watergrass may be developing resistance to Whip, Ordram, Abolish and Bolero, prompted researchers to test field samples against these herbicides. Greenhouse tests confirmed their concerns that some watergrass populations may be developing partial resistance. Researchers identified a resistant watergrass biotype that is tolerant to Whip, Abolish and Ordram.

In a related study on the biology and physiology of rice weeds, researchers determined that there are at least three different levels of Londax® resistance in smallflower umbrella sedge. The biotype shown to be the most resistant also appears to be the most vigorous.

Londax® Resistant Weeds

Collections of Londax®-resistant weeds tested in a greenhouse study showed variable resistance to Londax®, indicating that more than one mechanism may confer resistance.

Greenhouse studies were also conducted to evaluate possible herbicides for Londax®-resistant weeds. Pursuit shows promise for selective control of smallflower umbrella sedge but application needs to be made early when weeds are small and most vulnerable.