A large interdisciplinary team of UC scientists conducts weed control studies at the Rice Experiment Station, in the fields of cooperating growers and in laboratories and greenhouses. This continues to be one of the most challenging areas of rice research.

This work examines new and existing herbicides, as well as combinations and sequential applications of different compounds; alternative crop establishment methods as a means of altering weed dynamics; and the underlying causes of herbicide resistance and strategies to deal with this problem.

This project is pursuing a strong emphasis on the diversification and sustainability of weed management in rice. Work is continuing on a long-term field experiment with new alternative rice stand establishment systems in order to develop novel but feasible solutions for controlling herbicide-resistant weeds – watergrasses, in particular.

Herbicide studies

Herbicide test plots in 2006 were located at two sites at the Rice Experiment Station and one site in Glenn County. These tests examined a wide range of registered and potential new herbicides for effectiveness, safety and compatibility.

Shark® (carfentrazone) has been tested for several years and has demonstrated good control of sedges and broadleaf weeds. Its manufacturer, FMC Corporation, changed the formulation to reduce potential for off-site drift and to cover large acreages for early weed control. Shark® can be applied in combination with other into-water herbicides and in sequential weed control operations. Timing of application is critical for best results. Combining Shark® with Granite® is a good management strategy to deter ALS-resistance in weeds.

Prowl® (pendimethalin) is a selective herbicide for controlling annual grasses such as barnyardgrass and sprangletop and certain broadleaf weeds as they germinate and begin to emerge. Its mode of action is to interfere with plant cellular division. A new formulation – Prowl H2O – has been developed for use in dry- and drill-seeded rice. Applied alone as a delayed pre-emergent, it initially afforded 58 percent control of watergrass and 98 percent control of sprangletop. Poor control of watergrass was attributed to many seedlings having already emerged by the time the pre-emergent application was made. Control diminished with time. Combined with foliar-active herbicides like propanil or Regiment®, control improved.

IR-5878 (orthosulfamuron) is an ALS inhibitor for broad-spectrum control of watergrass and smallflower umbrellasedge that does not harm rice. Testing was conducted on water-dispersible granules of this compound for pinpoint applications and into-the-water treatments in continuously flooded rice. The best weed control and yields were achieved when combined with Abolish® or propanil in tank mixes. A granular formulation of this compound showed best control of bulrush at the two- to four-leaf stage.

Granite® (penoxsulam) is an ALS-inhibitor for control of emerging watergrass and barnyardgrass, broadleaf weeds and sedges. It is not effective against sprangletop. A granular formulation – Granite GR – was commercially available for the first time in 2005. Most treatments – alone or in combination with Bolero®, Cerano®, propanil, Clincher® and Shark® – provided good to excellent weed control. Rice stunting can be a problem with early applications of Granite GR.

Granite SC is a liquid formulation for foliar application in pinpoint flood management against sprangletop, barnyardgrass, ricefield bulrush and has activity on smallflower umbrellasedge. It provided good to excellent broad-spectrum weed control with various combinations of Clincher®, and propanil.

Rice culture systems probed

Extensive testing in 2006 examined weed control for the three major systems of rice culture – continuous flood, pinpoint flood and dry or drill-seeded. Research is seeking the best combinations for each of these systems. A comprehensive strategy relying on various combinations of herbicides will help ensure against resistance.

Continuous flood trials were conducted at RES (Hamilton Road site) and a cooperating grower site with watergrass resistance history. Best yields in this system were obtained with at least 90 percent broad-spectrum control during the first month after seeding. Most treatments were sequential applications.

The combination of Cerano® followed by (fb.) propanil, Granite GR or Regiment®, provided excellent broad-spectrum control at the Hamilton Road site. Combinations of Granite GR fb. either propanil or Clincher® also provided broad-spectrum control at this location.

Granite® and Regiment® should not be combined, as both compounds have the same mode of action and would encourage ALS resistance. Other combinations affording excellent broad-spectrum control included Bolero® fb. propanil or Abolish® in pre-flood surface application fb. either propanil or Granite GR. Shark® applied into the water at the same time as Granite GR or fb. propanil also provided excellent broad-spectrum control.

Cerano® causes mild to severe bleaching of rice but the crop usually grows out of it. Regiment® and Granite GR may cause stunting and darkening of rice. Some root growth stunting may also occur temporarily after application. The crop seems to recover from these effects, but Granite® should not be applied earlier than at the two-leaf stage.

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Pinpoint flood system trials were conducted at RES and at the resistant watergrass site in Glenn County. Both trials were drained eight days prior to herbicide application and then reflooded two days after application. Follow-up applications of foliar herbicides require lowering of water to achieve 70 percent weed exposure for effective coverage of weed foliage. This drainage period allows weeds like sprangletop, barnyardgrass and smallflower umbrellasedge to germinate in the aerobic environment. Residual herbicides can be especially helpful in this approach.

The best broad-spectrum treatments were Granite SC fb. propanil; Clincher® fb. propanil; a tank mix of Clincher® and Granite SC fb. propanil; a tank mix of propanil and Granite SC; propanil tank mixed with Abolish®. The Regiment® and Abolish® tank mixture applied at the three- to four-leaf stage of rice continues to produce excellent watergrass control. This treatment also controlled sprangletop. Regiment® alone provides excellent control of watergrass at this location.

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M-206 rice was drill-seeded into dry ground, then flushed with water three times for establishment. Standing water will inhibit growth of rice drilled into soil, so the final, permanent flood wasn’t applied until five-leaf stage of rice. The main weeds in this system were watergrass, sprangletop, and smallflower umbrellasedge. Herbicides used in these trials provided more than 80 percent control of sedges and broadleaf weeds. Yields in this system were thus strongly influenced by watergrass control. Top yields were attained with 95 percent grass control.

The best broad-spectrum treatments were foliar-applied Shark® fb. Clincher®; Prowl® tank mixed with Clincher® in early postemergence before sedges emerged; a tank mix of Abolish® and Regiment® in early postemergence fb. Clincher®; and Granite SC in early postemergence fb. Clincher® in post-permanent flood. Abolish® in pre-emergence fb. by an early postemergence application of propanil or Regiment® were also good combinations. Control with Prowl® improved when combined with foliar-active herbicides like propanil and or Regiment®.

Alternative stand establishment

Research continued on work begun in 2004 to explore alternative rice establishment systems for their potential to shift and reduce weed species recruitment and to facilitate the use of alternative herbicides such as pendimethalin and glyphosate. The systems evaluated in this research included conventional water-seeded rice; conventional drill-seeded rice; water-seeded rice after spring tillage and a stale seedbed; water-seeded rice after a stale seedbed without spring tillage; and drill-seeded rice after a stale seedbed without spring tillage.

As in the previous two years of research, there were drastic differences in weed recruitment among the various systems. Sedge and broadleaf weeds dominated aquatic systems, while drill-seeded systems favored grasses. In the two previous years, the stale seedbed technique had been extremely useful in depleting weed populations from upper soil layers. Success with this technique depends on keeping seedbeds moist and allowing sufficient time for most weeds to emerge prior to herbicide application. The stale seedbed approach reduced total weed infestation by 50 percent in the water-seeded systems compared to a more conventional treatment. Limiting soil disturbance contributes an additional 30 percent weed reduction. Thus lowest weed infestation occurred where rice was water-seeded after a stale seedbed without spring tillage.

Rice yields did not differ among these establishment systems. Therefore, the alternative rice establishment systems evaluated in this study may be used to effectively manipulate weed species recruitment and to enable the use of herbicides that may control weed biotypes resistant to herbicides used in conventional water-seeded systems. Results from this research will be used to develop an integrated weed management program for California rice that will break weed lifecycles through rotation of stand establishment methods and alternating herbicides with different modes of action.

Rice competitiveness

Weed-competitive rice varieties would offer a low-cost and safe non-chemical addition to an integrated weed management program. Three greenhouse experiments involving different rice cultivars and late watergrass examined how well rice could suppress this weed. Rice traits conferring competitiveness, particularly those that relate to early plant vigor (e.g. tillering), were identified.

Herbicide resistance studies

Herbicide testing was conducted in a grower’s field with a heavy herbicide-resistant late watergrass (“mimic”) infestation. Best results in a permanent flood situation were obtained with Cerano® early into-the-water fb. either propanil, Shark® (foliar) or Granite SC applied just before tiller initiation of rice. Granite GR fb. propanil was also a good combination in this system. Excellent sequential combinations in pinpoint flood were propanil fb. Clincher®; Granite SC fb. propanil; and the tank mix of Regiment® and Abolish®, particularly when followed by a later application of propanil.

Resistant watergrass testing continues as a service to California rice growers. Testing this past season included Cerano®, Regiment®, Clincher®, Bolero®, Ordram®, Granite® and propanil applied at standard field rates. A new reporting method is intended to help growers interpret the results. This includes a picture showing treatment effects on samples. Various resistance patterns have been observed among the submitted samples.

Resistance studies continued with technologies such as GPS, geostatistics and molecular markers to monitor gene flow, outcrossing, mechanisms of resistance and cross-resistance in smallflower umbrellasedge. Studies on mechanisms of late watergrass resistance to Granite® and Cerano® are also in progress.

Red rice

Infestations of red or weedy rice were detected in 2005 and 2006. A project is under way to collect specimens from these locations. Seed and tissue samples are being subjected to molecular analysis. Scientists are tracking the distribution of these infestations and attempting to determine outcrossing into commercial rice.