Rice weed resistance seen to add worm woes
Byline: Harry Cline Farm Press Editorial Staff
Weed control is the overriding concern of California rice growers. It has now crossed over into insect pest management, as producers and others from the industry heard during the recent annual rice field day at the Biggs, California Rice Experiment Station.
The annual gathering at Biggs is the largest rice field day each year, and this year producers heard that broadleaf weeds are contributing to an increase in armyworm pests.
University of California Extension entomologist Larry Godfrey reported that producers are treating more often for rice feeding armyworms, primarily the yellow-stripped armyworm. And they lay eggs exclusively on broadleaf weeds.
“Weed control is an issue with the increase in armyworms,” said Godfrey.
That was just another story growers did not want to hear as part of what A.J. Fischer calls a “growing epidemic” of resistant watergrass, sedge and broadleaf weeds in California rice.
Watergrass goes to the top of the resistance list. Fischer said resistance is everywhere in California rice. The only difference between fields is the level of resistance.
Fischer said propanil is the only herbicide to which watergrass has not demonstrated resistance in California.
However, in the rest of the world where rice is planted there is propanil resistance, he noted. “The entire south has resistance to propanil…entire Latin America to barnyard grass and water grass.”
The same fate awaits California producers unless they practice resistance management by using sequential or tax mix applications of unrelated herbicides. “If you miss a weed with one application, you can catch it with the second,” he said.
Cover full spectrum
“We must use combinations and sequential applications to cover the full spectrum of control and to protect herbicides from developing resistance,” he warned.
New herbicides continue to be registered to replace those losing effectiveness or giving growers alternative modes of actions in resistance management.
Cerano is a newly registered herbicide. Labeled for the first time this year, Cerano applied into water at 12 pounds per acre from the day before seeding to the one leaf stage of rice controls resistant early watergrass and barnyard grass, according to Fischer.
However, there are populations of late-water grass (mimic) with tolerance to Cerano. Where this occurs, Fischer recommends a sequential application of propanil is required.
A success story, said Fischer, is a tank mix of Regiment with Abolish applied when rice is at the five- to six-leaf stage in a “pin-point” fashion. “This combination has a synergistic interaction that greatly enhances watergrass control and is particularly effective on resistant watergrass,” he added.
Londax and ALS inhibitor-resistant sedges and broadleaf weeds such as California arrowhead and redstem have become widespread problems.
Fortunately, Fischer said a dry flowable formulation of the herbicide Shark controls sedge and broadleaf weeds and so far no resistance has been detected to carfentrazone.
“It performs well as a direct dry application into the water at 8 ounces and rice at the two- to three-leaf growth stage,” he said. However, this is not a residual herbicide and where the rice canopy becomes debilitated, second weed emergence flushes can compromise its effectiveness.
Shark, Regiment and Cerano are all relatively new to California. Two other new compounds, Sofit and Rifit also are new. These were introduced into the market at the insistence of UC weed researchers to bring new modes of action of action to the ride herbicide portfolio.
Both are pretilachlor compounds. Sofit contains a safener.
The material is used as a pre-flood treatment applied just prior to flooding and seeding or as a foliar in a pin-point flood culture.
In the pre-flood system, Fischer said continuous flood must be maintained after application.
Both cause phytotoxicity, and Fischer said rates will have to be adjusted to reduce that. So far Sofit is the preferred formulation. Its safer minimizes phytotoxicity.
“The relevance of pretilachlor for California rice is that it controls sprangletop, herbicide-resistant watergrass and barnyardgrass as well as Londax-resistance weeds,” said Fischer, adding that it also is a promising compound for dry-seeded rice.
Direct or dry seeding rice is making a comeback because of the growing weed resistance problems. Dry seeding was common in the early days of California rice, but it was abandoned for water seeding to control barnyardgrass.
Rotating herbicides in water-seeded rice is one way growers combat herbicide resistance, but UC Extension rice specialist Jim Hill and other are joining with growers in looking at rotating the way rice is planted.
Dry seeding is coming back.
“Various forms of dry seeding and drill seeding have been used for 25 years, but there never has been an incentive to try it on a big scale when growers could aerially seed and use a couple of compounds to control weeds,” said Hill.
But easy is becoming less practical because of herbicide resistance
These old seeding methods are making a comeback because they bring new herbicides like Treflan and Prowl into the arsenal.
“Several farmers are experimenting with dry and drill seeded systems to reduce aquatic weeds such as sedges, broadleaf aquatics and watergrass,: said Hill.
Others are drilling rice into “stale seedbeds” which have been prepared in the fall and direct-seeded in the spring with no spring tillage.
Weed control costs now can be above $200 per acre because of herbicide resistance, but these dry seeded methods offer reduced costs because of improved weed control with new compounds.
“These systems also have provided diversification to the rice only cropping system through rotations of dry-and water-seeded to break the life cycle of certain pests, particularly aquatic weeds,” said Hill.
Work with growers
Sacramento Valley UC Cooperative Extension farm advisors are working with growers on refining this dry-seeding, and Hill is establishing an extensive research project on the Biggs station to looking at some issues like nutrient management and pest control. “There are potential nitrogen losses in drill seeding, and we need to look at that,” said Hill.
Hill will evaluate and compare five establishment methods on the Biggs station:
Conventional water seeding.
Conventional spring-tilled drill seeding.
Spring-tilled stale seedbed water seeding.
Fall-tilled stale seedbed water seeding.
Fall-tilled stale seedbed drilled seeding.
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