Weed control studies are conducted at the Rice Experiment Station, in the fields of cooperating 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; and causes of herbicide resistance and strategies to deal with this problem.

New areas of research in 2007 included characterization of red rice plants found in California fields and studies to distinguish two very similar weeds – ducksalad and monochoria.

Herbicide studies

Herbicide test plots in 2007 were located at two sites at the Rice Experiment Station and one site in Glenn County. These tests examined 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-the-water herbicides and in sequential weed control operations. Timing of application is critical for best results. Combining Shark® with Granite® is a good management practice 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 emerge. A new formulation – Prowl® H2O – has been developed for use in dry- and drill-seeded rice. It needs to be applied to moist soil without any standing water. Prowl® H2O applied as a delayed pre-emergent provided 80% watergrass/barnyardgrass control and 100% sprangletop control at 20 days after seeding. Control diminished to 58% and 93%, respectively, at 60 days after seeding. Tank mixes with foliar-active herbicides propanil, Regiment®, and Clincher® improved control. In water-seeded rice, Prowl® can be helpful to protect drained fields from weed emergence when slow or delayed reflooding is anticipated.

Strada® WG (orthosulfamuron) is an ALS inhibitor for broad-spectrum control of watergrass and smallflower umbrellasedge that does not harm rice. Testing was conducted with two formulations, a water-dispersible granular form of this compound for pinpoint applications and a granular formulation for into-the-water treatments in continuously flooded rice. In the pinpoint system, promising results were reported with an early application of Cerano® followed by Strada® applied at the one- to two-leaf stage, or with Strada® tank mixed or followed in sequence by propanil for broad-spectrum control and protection from ALS resistance. The granular Strada® GR applied into-the-water following an early application of Cerano®, or followed by propanil, was also promising.

Granite® (penoxsulam) is an ALS-inhibiting herbicide applied after flooding for control of susceptible 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. In continuously flooded rice the best Granite® treatment included a Shark® application the same day, results similar to those in 2006.

Granite® SC is a fluid formulation for foliar application in pinpoint flood management against watergrasses, sedges, and broadleafs. It was labeled for California in 2006 and was in good supply in 2007. It provided good to excellent broad-spectrum weed control. Sprangletop control failed in the absence of Clincher®. Combinations with propanil are needed to control resistant watergrass and protect against ALS resistance. Some stunting of rice can be observed in Granite®-treated fields.

Rice culture studies continue

Testing on the three major systems of rice culture – continuous flood, pinpoint flood, and dry or drill-seeded – continued in 2007. Research is seeking the best herbicide and management combinations for each of these systems. A comprehensive strategy relying on different herbicide combinations will protect against resistance.

 

Proportional weed species recruitment in conventionally water-seeded and drift-seeded rice determined 35-40 days after rice emergence in plots where conventional herbicides have not been applied. Data are averages across four years of experiments.

The first month after seeding is the critical period of competition between weeds and rice. Best yields are obtained when herbicides provide at least 90 percent broad-spectrum weed control during this period. As observed in previous years, weed infestations in drill-seeded rice tend to cause more severe yield loss, followed by those in pinpoint water-seeded systems. Continuously flooded systems afford the greatest control.

Continuous flood trials were conducted at RES (Hamilton Road site) and a cooperating grower site with watergrass resistance history. The advantage in continuously flooded rice is watergrass suppression by deep water, particularly important where resistance is a problem. Granular formulations applied early into-the-water are excellent non-drift tools for this system. Sequential applications of Cerano® followed by Granite® GR, Regiment® or propanil give excellent broad-spectrum control. Shark®, followed by Granite® GR, improved control of ricefield bulrush and provided good broadleaf and sedge control. A new granular formulation of Bolero® followed by propanil was also very effective. Rice yields for these treatments ranged from 8,900 pounds/acre to 9,700 pounds/acre.

The pinpoint system requires draining fields at the two- to four-leaf stage of rice to expose weeds to foliar herbicides. However, this technique helps weeds like sprangletop, barnyardgrass and smallflower umbrellasedge get established. Therefore, rapid reflooding – within 48 hours of herbicide application – is important.

Pinpoint flood system trials were conducted at RES and at the resistant watergrass or “mimic” site in Glenn County. Both trials were drained eight days prior to 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.

The best broad-spectrum treatments in the pinpoint system were Clincher® followed by propanil; propanil combined with Abolish®; and sequences including Granite® SC in a tank mix with Clincher® or propanil. Granite® SC applied at the three- to four-leaf stage provided excellent broad-spectrum control but needed Clincher® to control sprangletop. Regiment® applied between four leaf and the one- to two-tiller stage of rice gave good watergrass control with activity noted on ricefield bulrush but not on sprangletop and smallflower umbrellasedge. As previously noted, Strada® GR showed promising results following early application of Cerano®. Propanil following Strada was also effective.

Effect of alternative rice establishment systems on weed species that emerge with rice in plots where conventional herbicides have not been applied (except for glyphosate in the stale-seedbed treatments). Data are averages of four years.

Mimic is resistant to all available herbicides for watergrass control except propanil. In a continuously flooded system Granite® GR applied into-the-water at the two- to three-leaf stage, followed by propanil or Regiment® at the four-to five-leaf stage, controlled all weeds present (mimic, ricefield bulrush and ducksalad). Combinations of Cerano®, followed by propanil or by Regiment® at the four- to five-leaf stage, were also good treatments in this system. Yields of treatments with good weed control ranged between 7,600 pounds/acre to 10,000 pounds/acre.

In another experiment in the pinpoint system, a single application of propanil at the one-tiller stage of rice gave total control of resistant late watergrass, smallflower umbrellasedge and ducksalad, the main weeds present. Clincher® followed by propanil, Regiment® followed by propanil, or a tank mix of Regiment® plus Abolish® followed by propanil also gave excellent control.

In the drill-seeded system, a field of M-206 was flushed three times before a permanent flood was applied at five-leaf stage of rice. The main weeds in this system were watergrass and sprangletop. Residual herbicides Prowl® H2O and Abolish® in a delayed pre-emergent application were well-suited to this system to provide weed control until permanent flood. Early treatments can be followed by propanil at two- to three-leaf stage. Other good treatments included Regiment® combined with Abolish®, followed by Clincher®, and Clincher® alone followed by propanil.

Alternative stand establishment

Research also continued for a fourth year on alternative rice establishment systems for their potential to reduce weeds and to permit the use of alternative herbicides such as pendimethalin and glyphosate. The systems include drilled rice, water seeding, no-till options, and the use of a stale seedbed technique (promoting early weed emergence through irrigation flushing followed by glyphosate).

As in the three previous years of this study, these systems have demonstrated their potential for good yields while drastically altering the kinds of weed species that emerge with rice. Aquatic sedge and broadleaf weeds dominated the water-seeded systems, while the aerobic seedbeds of the drill-seeded systems favored grasses.

The lowest weed infestation occurred where rice was water-seeded after a stale seedbed without spring tillage. Alternative rice establishment systems may be used to effectively manipulate the kinds of weed species that establish with rice and to enable the use of a broader range of herbicides to control resistant weed types.

Red rice research

“Red” rice is weedy rice that has kernels with red bran. It is highly tillered, very competitive, and its seed shatters easily. It is difficult to control selectively with currently available herbicides because the same species (Oryza sativa L.) is grown in commercial rice fields.

This weedy rice could have a serious economic impact by reducing yields and quality and by increasing weed management costs. Red rice is a major problem in the Southern U.S. and elsewhere in the world. It has not been a problem in California; however, discovery of it in a few fields in Glenn and Colusa counties since 2003 necessitates monitoring.

Dealing with this potential problem requires adequate identification of this weed, scouting, and the implementation of a red rice management program. Samples of seeds of this weedy rice from California were analyzed in 2007 and found to be variable among different specimens. In general, it is a straw-hulled form that may be more closely related to weedy rice from the South than to a black-hulled strain from Missouri.

Herbicide resistance

Studies on herbicide resistance demonstrated that mimic is also resistant to the newer herbicide penoxsulam (Granite®). Its mechanism of resistance is similar to that of Regiment® and Clincher®.

Low levels of resistance to clomazone (Cerano®) were found in dose-response studies with three samples of mimic taken from Sacramento Valley fields. Studies are under way to clarify the mechanism of resistance.

Ducksalad/Monochoria identification

There is apparently widespread misidentification of one of California’s predominant rice weeds. Ducksalad (Heteranthera limosa) has been a common rice weed for decades, first recorded in 1948. It spread markedly during the early 1970s.

An aggressive blue-flowered biotype of ducksalad is gradually displacing the original white-flowered form. The blue ducksalad has apparently been misidentified as Monochoria vaginalis. Growers simply call it “monochoria.”

Morphological studies are under way to distinguish the two species and to understand weed emergence patterns. Growers of organic rice are concerned over the spread of the blue-flowered ducksalad because it is more difficult to kill with the field dry-down method of control. Its “creeping” behavior also pulls rice down during early growth stages.