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California Weed Control Strategies

Experts discuss herbicide options for various production systems

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University of California – UC Davis – weed specialists shared their findings on the efficacy of various herbicides in water-seeded continuously flooded rice, pinpoint systems and drill-seeded systems during the 2010 Rice Field Day at the Rice Experiment Station in Biggs, Calif.

Continuous, Flooded Rice
This system intends to maximize weed suppression by flooding, notably the elimination of barnyardgrass and sprangletop as problems. After seeding into a flooded field, water depth is maintained at four inches throughout the season. When late post-emergence applications are needed, water is lowered to expose about 70 percent of weed foliage to the herbicide spray, but fields are never drained.

Watergrasses (early and late) were the predominant weeds, followed by ricefield bulrush, ducksalad, monochoria and waterhyssop. A couple of herbicide programs provided early and sustained weed control. They were Cerano followed by Granite GR and Granite GR followed by Ultra Stam 4SC.

Herbicide-Resistant Weed Management Utilizing A Stale Seedbed Technique

A stale seedbed technique was implemented prior to seeding rice at a cooperating grower’s field in Glenn County. The field was spring tilled and irrigation was applied in early May (shallow flood) during 17 days to encourage weed emergence. The field was drained for five days to allow ground rig travel.

Glyphosate (2.25 lb ai/A) was applied over the main field and designated plots in a replicated experiment within the field. Despite lengthy stale seedbed, population of grasses was low.

The field was flooded four days later without any tillage, then seeded two days later with 180 pounds of M104 per acre.

Later emergence of weeds that were not exposed to the Roundup spray (ricefield bulrush, smallflower umbrellasedge and ducksalad) was controlled when rice was at the 3-4 least significant ratio (lsr) with either Super Wham (4 qt/A + 1.25% COC), Granite SC (2oz/A + 2.5% COC) or Regiment (0.79 oz/A + 0.125% silicon-based surfactant) in the replicated test plots. The main field was treated at this same time with Granite SC (2 oz/A) + 2.5% COC) to control the same spectrum of weeds.

A neighbor also implemented the spring-tilled stale seedbed technique on a field with resistant late watergrass. It appears that this field is very clean in terms of watergrass infestation.

Source: Rice Experiment Station, Biggs, Calif.

An additional early treatment combination with sustained weed control was Cerano followed by Shark and Londax followed by Ultra Stam 4SC, although this is likely a very expensive combination. Cerano generally provides good-to-excellent control of grasses. A follow up treatment of Sandea added bulrush control and improved watergrass control where resistant strains existed.

Bolero Ultramax followed by either Super Wham or Regiment applied at the 1-3 tiller stage of rice generally provided broad spectrum control of weeds, although weed competition between applications may impact yields. Shark followed by Super wham had broad spectrum control, but watergrass control was less than expected due to the late timing of the propanil application. Super Wham plus Grandstand applied at the one- tiller stage of rice provided good broad spectrum control.

Additionally, we have test plots where we are testing a new product called Bombard. This product has the same concentration of active ingredient as Cerano. The formulation is a prill instead of the extruded granule of Cerano. Injury to rice and efficacy on grasses appear to be the same as Cerano.

Pinpoint Flood Rice
Often, cold weather or windy conditions in spring require early field drainage to favor rice establishment, which precludes the use of into-the-water treatments. In this experiment, weeds were controlled by foliar herbicide treatments applied during a period of field drainage for good weed exposure to the herbicides. Prevailing weeds were early and late watergrass, sprangletop, smallflower umbrellasedge, ducksalad and waterhyssop.

Excellent overall treatments in this non-resistant site were Regiment plus Abolish; a tankmix of Granite SC and clincher followed by either Ultra Stam SC or Super Wham; Granite SC; Regiment applied alone or followed by Super Wham at the 1-2 tiller stage of rice. Late broad spectrum control is achieved with applications of Ultra Stam 4SC, Ultra Stam 80EFD, Super Wham or Rice Shot applied at the one- tiller stage of rice, although weed control is lower than earlier applications and weed competition occurred for a longer period of time.

Granite SC tankmixed with Ultra Stam 80EDF followed by Clincher generally had good control. the addition of clincher to Super Wham in a tankmix gave excellent watergrass control with activity of ricefield bulrush and sprangletop.

Drill-Seeded Rice
This is the system that offers flexibility for herbicide use when proximity to sensitive crops impose restrictions to aerial applications. Drill-seeding favors weeds adapted to dryland seedbeds (sprangletop is typically problematic) and is less favorable for aquatic species (ricefield bulrush, ducksalad, redstem) and is thus useful for alternation with water-seeded systems when the pressures of aquatic weeds becomes problematic.

Prowl is a pre-emergence herbicide that can protect from weed emergence after seeding rice during the period prior to the permanent flood. It controls watergrass, barnyardgrass, sprangletop and has some activity on smallflower umbrellasedge. Other good sprangletop herbicides for this system are Abolish and Clincher. Main weeds in the experiment were Echinochloa complex and sprangletop.

Overall best control was achieved with a tankmix of Granite SC, Prowl H2O plus Clincher. This is the second year that this three-way tankmix has had the best broad spectrum weed control.

Other good treatments were: a three-way mix of Prowl H2O, Super Wham and Clincher; Regiment plus Abolish followed by Super Wham plus Clincher; Granite SC followed by Clincher; Granite SC tankmixed with Clincher followed by Super Wham post permanent flood; Clincher followed by Super Wham.

Prowl H2O applied alone at either delayed pre-emergent or 2-3 least significant ratio (lsr) provides about 50 percent watergrass control and is thus an excellent partner for follow-up foliar applications.

For specific rates and application timing, please contact your local Farm Advisor.

A.J. Fischer, Associate Professor, Weed Science Program, Department of Plant Sciences; J.W. Eckert, Staff Research Associate IV; J.E. Hill, Rice Extension Specialist; H. Yasuor, Post Doctoral Researcher; L. Boddy, PhD. Student; R. Pedroso MS Student; A Garcia, PhD. Student; W. Brim-DeForest, MS Student; R. Alarcon-Reverte, Post Doctoral Student; S. Johnson, SRA 1; C. Greer, Farm Advisor, Yuba-Sutter Co; L. Espino, Farm Advisor Colusa-Glenn Co.; R. G. Mutters, Farm Advisor, Butte Co. contributed information for this article.

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