Weed Control in Rice - 2005




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Project Leader and Principal UC Investigators

Albert Fischer, weed science professor, Dept. of Vegetable Crops, UC Davis




Weed control is one of the most important areas of research funded by rice growers. A large interdisciplinary team of UC scientists conducts studies at the Rice Experiment Station, in the fields of collaborating growers and in laboratories and greenhouses. 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.

Herbicide studies

Herbicide efficacy of currently registered and potential new herbicides has become a more crucial issue in recent years with the development of herbicide-resistant weeds. This section highlights key results from work in this area during 2005.

Cerano® (clomazone) is a pigment synthesis inhibitor that has repeatedly demonstrated good control of barnyardgrass, watergrass and sprangletop but has been less effective against herbicide-resistant watergrass. In combination with SuperWham® (propanil), Regiment® (bispyribac-sodium) or Granite® GR (penoxsulam), however, Cerano® provided very good broad-spectrum weed control.

Shark® (carfentrazone) was tested early (April 29) and late (June 3) in an into-the-water application to determine its effectiveness against ricefield bulrush. This experiment showed that best timing for Shark® is dependent on the growth of bulrush at different planting dates. Bulrush is slower to establish in the early part of the season because of cooler weather. It grows much faster in warmer weather at a later seeding date. This is important because bulrush control diminishes drastically as plants grow larger. In this study only 50 percent control was achieved by the time bulrush reached two inches in height. Therefore, Shark® should be used as early as possible with bulrush plants well below an inch tall. Rice plants should be well pegged into the soil with no less than two leaves at application.

IR-5878 is an ALS inhibitor that has worked well on continuously flooded fields in broad-spectrum weed control programs with Cerano,® Super Wham® or Clincher.® A foliar formulation of IR-5878 did not perform as well in a pinpoint flood trial as the granular formulation did in continuously flooded rice.

Granite® GR is another ALS inhibitor applied after flooding for selective broad-spectrum control. (It won’t control sprangletop and is weak against resistant late watergrass.) This formulation was available for use in 2005 and an SC foliar formulation will be available in 2006. The granular formulation worked well in continuously flooded rice. Rice plant and root stunting occurred during the period when the chemical is active in water, but this effect diminishes over time. It performed well on susceptible weeds as a sequential following Cerano® and Abolish,® or followed by SuperWham® or Clincher.® As with all ALS-inhibiting compounds, Granite® will likely fail to control certain biotypes that are resistant to the mode of action of this compound.

Regiment® is a post-emergent herbicide applied with a silicone surfactant. It is effective on watergrass, ricefield bulrush, and demonstrates good activity on California arrowhead. Regiment® was tested alone, in tank mixes and in sequences. A tank mix of Regiment® and Abolish® provided 79 percent control of resistant watergrass and 65 percent control of smallflower umbrellaplant.

Pinpoint flood systems

Pinpoint flood systems generally involve one or more foliar-active herbicides in tank mixes or sequential applications. They are called “pinpoint” because the water is drained to expose weed foliage to herbicides, and then fields are reflooded. Regiment® performed well in pinpoint systems in sequence with Clincher® (cyhalfop-butyl) and SuperWham.® Tank mixes of Regiment® and either Clincher® or SuperWham® were antagonistic—that is, their effectiveness was reduced by combining them.

Clincher® is a post-emergent ACCase inhibitor that controls watergrass and sprangletop. It is very safe on rice. Clincher® appears to work best when applied as a sequential in pinpoint systems. Very good broad-spectrum control, including sprangletop, was obtained when Clincher® followed SuperWham® at susceptible and resistant watergrass sites.

Prowl® (pendimethalin) is a meristematic inhibitor that interferes with plant cellular division and early growth. A new water-based formulation is in development for use on drill-seeded rice in California. This herbicide is active in aerobic, moist soil but appears to be rapidly inactivated in flooded rice. Prowl® H20 could be used in water-seeded, pinpoint rice where the ability to rapidly reflood after foliar herbicide treatments is diminished by limited water supply or large field checks. Prowl controls germinating weeds in preemergence. It will not control established plants.

Leathers’ Method

The Leathers’ method, so named after the grower who developed it, is a water-seeded system where the flood water is dropped after seeding in order to allow improved stand establishment. This period of extended soil exposure also encourages certain weed species to establish. Prowl® H20 in tank mix with either SuperWham® or Clincher® provided excellent control of barnyardgrass, watergrass and sprangletop. Prowl® H20 with a follow-up treatment of SuperWham® also was an excellent treatment for broad-spectrum control. Other good treatments, including sprangletop control, were Abolish® in tank mix or in sequence with SuperWham®; Clincher® in sequence with SuperWham®; and Regiment® or Granite® SC.

Drill Seeded

Drill-seeded rice is a system in which rice is seeded with a grain drill into a dry seedbed. It allows use of herbicides different than those used in water-seeded rice but also allows different weed species to flourish. Watergrass, barnyardgrass and sprangletop are three common weedy grasses found in this system. Good control can be achieved with Prowl® H20 alone or in combination with Clincher,® SuperWham® or Regiment.® Abolish® in combinations with SuperWham®, Regiment® or Clincher® were also very good, as was Clincher® followed by SuperWham.®

Alternative Establishment

Research continued on alternative rice establishment systems for their potential to shift and reduce weed species recruitment and to facilitate the use of alternative herbicides such as Prowl®, Roundup (glyphosate) or Liberty (glufosinate). This is advantageous because their different mode of action is capable of controlling weed biotypes resistant to herbicides used in conventional, water-seeded rice.

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 (no-till); and drill-seeded rice after a stale seedbed without spring tillage (no-till). The stale seedbed technique involves flushing a field with irrigation water to promote weed emergence. Weeds are then killed prior to seeding with a total non-selective herbicide like glyphosate (Roundup®).

Weed species composition was distinctly different among establishment systems. Water-seeded systems were dominated by sedge and broadleaf weeds, while the drill seeded systems were dominated by grass weed species. In the drill-seeded systems, grass control from Prowl® and Clincher® was more than 85 percent. Glyphosate used in the stale seedbed systems greatly reduced post rice emergence weed densities in the water and drill-seeded systems.

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.

Herbicide resistance

Resistant watergrass testing continues as a service to California rice growers. Testing has been done for Ordram,® Bolero,® Regiment,® Clincher® and propanil. Additional herbicides that may be added are Cerano,® Granite® and Prowl.® This service is performed during the winter in order to deliver results to growers by spring planting.


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