Rice Farming

Weeds Hang Tough

Sedge management challenges California rice growers

By Brenda Carol

Weeds are the primary yield inhibitors in California rice production, and sedges are among some of the most difficult to control. Not only are herbicides limited, but the biology of the weed as well as application restrictions make sedge management one of the most challenging issues in California rice production.

“The issue with the sedges is the difficulty that their control causes because we can no longer control them with into-the-water herbicides,” says Jim Hill, rice specialist with the University of California at Davis. “Field drainage for foliar treatments is management intensive in itself, but the implications for nitrogen losses, subsequent weed seed germination and spray coverage are all related to drainage. Also, spray drift and the application restrictions imposed by using foliar materials also make life tough for growers.”

Smallflower umbrellaplant/bulrush
There are several sedges that infest California rice, but there are two that are of primary concern, according to Dave Cheetham, research and development representative with Helena Chemical Company.

“The ones that we are most concerned with are smallflower umbrellaplant and ricefield bulrush,” he says. “These weeds are extremely difficult to control depending on the stage of growth in which you try to attack them. It’s hard to get good coverage on their narrow, vertical, waxy leaves with foliar herbicides. Growers are almost totally dependent on herbicides to control them, and they don’t have many herbicides in their back pocket they can use against them.”

  Bulrush is a monocot weed. It is a true water obligate. It is a perennial that acts like an annual weed, reproduces via seed and is largely self-pollinated. Seeds can remain viable in the soil for as long as 10 years, so it can be a continuing problem.

  “It flowers about 60-70 days after the rice is flooded,” Cheetham says. “Ricefield bulrush is largely distributed through irrigation water, dirty rice seed and equipment.”

  Smallflower umbrellaplant is similar. It is also a monocot, water obligate, reproduces via seed and is largely self-pollinated. Its seed is viable in the soil for up to five years and is distributed in similar fashion as ricefield bulrush.

“Both weeds are very prolific seed producers with smallflower umbrellaplant producing as many as 20,000 seeds per plant,” Cheetham says. “Oftentimes when it’s in the seedling stage, you can see as many as 100 plants per square foot.”

In California’s rice monoculture, weeds can quickly establish and become a persistent problem for decades.
“Sedges spread rapidly in the 80s and 90s and have remained a widespread problem since that time,” Hill says.

The competition with rice can be devastating. “Bulrush has been known to reduce yields by as much as 30 percent when left uncontrolled,” Cheetham says. “ The impact of smallflower is similar.”

Threat of herbicide resistance
The more insidious problem, however, is the threat and reality of herbicide resistance.

“These weeds can be highly resistant to ALS inhibitors, which makes control even more difficult and complex,” Cheetham says. “Londax was introduced into California in 1989. Resistance was first determined in smallflower just three years later. Bulrush resistance was confirmed a year after that. By 1994, it was estimated that 96 percent of California rice acreage had some ALS resistance.”

The resistance expressed by these sedges is a cross resistance by several genes that are responsible for a target site mutation. There are five families of ALS inhibitors. Four of them are used in California rice and resistance has been documented to all four.

Complicating the management picture even further is the choice growers have among rice cultures. There are two main rice cultures in California – water seeded and dry seeded. Weed selection pressures and weed control considerations differ considerably among the two.

The picture gets even more complex depending on the various management intricacies within each culture. While that complexity gives growers a unique opportunity to manipulate the weed spectrum and fashion corresponding weed control strategies, it can also become very confusing.

Water management methods
“Some of the dynamics associated with these types of systems determine what kinds of weeds you can expect,” Cheetham says. “You can change the dynamics of the weed spectrum by the water management that you impose within your culture.”

The “Leather’s” method is a type of pinpoint flooding under a water-seeded program, but the field is drained almost immediately and then re-flooded. It is typically used where soils, or wind, make stand establishment difficult.

In general, pinpoint flood systems are drained and re-flooded and water is maintained throughout the remainder of the season. The Leather’s method, in particular, is the one pinpoint system with the earliest drainage. Water is managed to optimize herbicide programs by draining the field to expose weeds to a foliar herbicide spray.

“California’s diversity of crops limits the use of foliar active herbicide with potential drift problems,” Hill says. “Continuously flooded systems tend to dilute herbicides that are applied into the water thus requiring special properties to be effective and therefore limiting the number of herbicides that can be used by this method.

“The nice thing about into-the-water herbicides is that they can be formulated as granules and limit drift problems that are inherent with spray applications,” he adds.

Cultural control options
Cultural controls are important in California rice to boost the efficacy of integrated weed management programs. Growers must take advantage of every tool in their arsenal to get the job done.

“Seeding rates can really help,” Cheetham says. “The seeding rate in California water-seeded systems is typically about 150 pounds per acre. In some cases, you will see growers plant as much as 200 pounds of seed per acre just to try to out-compete the weeds, and that strategy does a pretty good job.”

Variety selection can also influence weed pressure. “If you can get growers to shift from less vigorous varieties such as M-206 or M-401 and rotate to M-104, you may have a denser and more vigorous rice canopy to help smother the weeds,” he says.

Rotating chemistry still remains the best defense against building resistance to the select few herbicides that rice growers can use to battle sedges as well as other weeds. Choosing herbicides with different modes of action is critical, and knowing each product’s strengths and weaknesses is essential for effective weed management.

For more information on managing weeds in rice, see http://www.ipm.ucdavis.edu/PMG/selectnewpest.rice.html.

Brenda Carol is a freelance writer based in California. Contact her at (209) 728-9226 or brenda@brendacarol.net.

Herbicide combinations look good
A number of trials were conducted to look at various herbicide combinations in different irrigation systems.

“Looking at the spectrum, we have Abolish and Bolero, which do an excellent job of controlling smallflower umbrellaplant,” Cheetham says. “Grandstand is a good bulrush material, but lacking on smallflower. Our ALS inhibitors – Granite and Londax – both offer control in cases where resistance is not expressed.

“Regiment is good on bulrush but a little weak on smallflower. Shark and propanil are mainstay technologies in rice for sedge control with Shark being an excellent into-water and foliar technology, and proponil being the standard foliar burndown herbicide in California rice.”

In the pinpoint flood management system, Regiment and Abolish looked promising.

“This is a particularly interesting mix,” Cheetham says. “A tank mix combination of Regiment and Abolish was applied at pinpoint flood right as the first tiller was initiated in rice. These two partners synergize when applied together. The combination is primarily targeting watergrass, but it does a very good job on smallflower and bulrush.

“Super Wham and Regiment in various derivations do a very good job as well,” he notes. “Into-the-water, Shark is doing a good job on ricefield bulrush, and sequential applications of Super Wham and Regiment are effective, too.”

Herbicide combinations were also evaluated in a tough situation where known resistance to ALS inhibitors was present. Researchers were trying to optimize weed control with Shark and accompany the application with various ALS inhibitors into-the-water.

“Shark is a very strong sedge material but sometimes challenged on ricefield bulrush, depending on the system, due to cold water effects and indeterminate germination of the weed,” Cheetham says. “In this trial, Shark provided excellent control on its own on smallflower, but was a little weak in this continuous flood system on bulrush. Londax was not very good on either weed on its own accord, but when applied together in the floodwater with Shark provided excellent control.

“We had similar results with Sandea and Granite,” he says. “This is something that rice growers may have available in their arsenal this year where two dry flowable formulations can be applied in the same apparatus together into the continuous flood system.”

Weed control in California rice continues to be challenging, but carefully chosen combinations of herbicides can effectively manage even the toughest of situations.