Weed Control - 93


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Project Leader and Principal UC Investigators

David E. Bayer, professor, Plant Biology, UC Davis

James E. Hill, extension agronomist, Department of Agronomy & Range Science, UC Davis

J. Webster, staff research associate, Department of Agronomy & Range Science, UC Davis

E. Roncoroni, staff research associate, Section of Plant Biology, UC Davis

S.C. Scardaci, farm advisor, Colusa County Cooperative Extension

J.F. Williams, farm advisor, Sutter/Yuba Cooperative Extension

C.M. Wick, farm advisor, Butte County Cooperative Extension

M. Carriere, research assistant, Department of Agronomy & Range Science, UC Davis

T. Karus, research assistant, Department of Agronomy & Range Science, UC Davis

This ongoing project continues to focus on the biology of weeds important to rice culture and the effectiveness of new and existing herbicides. However, research on weed control has been overshadowed by the evidence of spreading weed resistance to the grower's most popular herbicide, Londax®.

Hill-Londax.jpg (243107 bytes)Londax® Resistance

Londax® resistance was first noted late in the 1992 growing season. Last year DuPont Co. representatives, University of California scientists, and pest control advisers conducted an investigation to find out just how widespread the resistance had become. Their findings are not encouraging.

Londax® resistance was observed at 72 sites from Fresno County to Butte County. Of these the species showing resistance were as follows: 68 percent California arrowhead, 19 percent smallflower umbrellaplant, 7 percent ricefield bulrush and 6 percent redstern. (See accompanying table.) Laboratory tests by DuPont confirmed that ricefield bulrush and redstem are resistant to Londax®. "It is quite clear that weed resistance is an emerging problem for California rice growers and will require a carefully planned strategy to prevent the obsolescence of Londax®," researchers concluded.

County Arrowhead Umbrellaplant Bulrush Redstem Total
Butte 11 1 1 0 13
Colusa 5 1 1 0 7
Fresno 0 0 0 0 0
Glenn 8 1 0 0 9
Placer 1 0 1 1 3
Sacramento 3 1 0 1 5
San Joaquin 1 0 0 0 1
Sutter 13 8 2 2 25
Yuba 7 2 0 0 9
Total 49 14 5 4 72

New Chemicals

Weed scientists conducted two sets of trials on four experimental herbicides - MON-0139, KIH-2023, R-104992 and KIH-6127. MON-0139 provided good watergrass control when applied while rice was at the first filler stage. This herbicide also provided excellent ricefield bulrush control when rice was at the late filtering or panicle initiation stage. Unfortunately, all treatments of this experimental herbicide severely injured rice.

KIH-2023 provided acceptable watergrass control at the four- to six-leaf stage, but plots treated in the earlier stages of growth were invaded with late watergrass following the treatments. KIH-2023 does not have any soil residual activity.

R-104992 showed similar results to KIH-2023 in that applications controlled watergrass was at the four- to six-leaf stage. Earlier treatments, however, resulted in a subsequent watergrass invasion.

KIH-6127 provided early watergrass control when applied preemergence but postemergence treatments were unsuccessful -whether applied in shallow water or deeper water.

Existing Herbicides

Grandstand®, a broadleaf herbicide now used on rice grown in the Southern U.S., may offer a substitute for MCPA and also offers an additional tool to control Londax®-resistant weeds. Researchers found that it provides good broadleaf rice weed control but poorer control of sedges. Although sedge control was poor, smallflower umbrellaplant was more susceptible to Grandstand® than ricefield bulrush. Grandstand® (triclopyr) did not appear to injure the rice at these timings of applications (35 days after flooding).

Two formulations of Whip® were evaluated for watergrass control. Both formulations provided equivalent control with little or no rice injury when applied at the five-leaf to one-tiller growth stage.

Two formulations of Ordram® were tested for eficacy. Both formulations provided approximately equal grass control. The application of preflood treatments of Ordram® in combinations with Londax® was successful for the fourth consecutive year. Both preplant incorporated (PPI) Ordram® and preflood surface (PFS) Abolish® 8E provided good watergrass control.

Studies to minimize phenoxy herbicide injury to rice showed that both MCPA and 2,4-D provided the best ricefield bulrush control with the least effect on rice when applied 30 days after seeding. Later applications of these herbicides provided less ricefield bulrush control and yields were lower.

Drill Seeding

Drill-seeding is being explored for its potential to keep Londax®-resistant broadleaf and sedge weeds in check. For the third year, water-seeded and drill-seeded rice were compared with respect to weed type and abundance. Grass weeds were shown to be highest in drill-seeded rice and aquatic weeds were highest in water-seeded rice. In drill- seeded rice, competition from grass weeds was very intense, requiring a grass herbicide to achieve a reasonable yield. Thus, while drill-seeding does minimize broadleaf and sedge weeds, much more pressure from grass weeds will occur.

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