Herbicide-resistant weedy rice significantly impacts yield potential, and the pressure intensifies in problem fields placed in continuous rice rotations. With limited technology to combat weedy rice, many farmers turn to Acetyl-CoA carboxylase (ACCase) chemistries, like the Max-Ace Cropping Solution or Provisia Rice System, as their only options.

However, weed scientists and industry experts warn of the importance of stewardship to preserve the ACCase technology and the longevity of the chemistry, because it is the only method of protection against weedy rice on the market for the foreseeable future.

Once weedy rice populations outcross into these ACCase systems, herbicide resistance sets in, and the efficacy of the technology is lost. Best management practices are key, and sometimes drastic measures are necessary to get weedy rice populations under control. 

This winter, industry and university experts from across the Midsouth rice belt participated in panel discussions at the National Conservation Systems Cotton and Rice Conference in Jonesboro, Ark. to share their advice on rice production with the Max-Ace and Provisia systems.

During the 2024 National Conservation Systems Cotton and Rice Conference, the Horizon Ag Provisia Panel discussed the importance of stewarding the ACCase inhibiting herbicide technology in rice production systems. Photo by Whitney Haigwood

Carefully plan crop rotation

Before ACCase rice cropping systems, the best option for farmers to control weedy rice was an imidazolinone (IMI) herbicide system like Clearfield or FullPage rice. In the last several years, these chemistries began to fail with weedy rice populations resistant to the IMI technology.

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In these cases, farmers were left with one option, ACCase technology, for weedy rice control. Then in 2022, reports came from Louisiana and south Arkansas of cross resistance to both IMI and ACCase herbicides in weedy rice.

For fields with heavy weedy rice populations, chemistry selection is critical and consecutive years of rice rotations with the same chemistry should be avoided. Furthermore, heavy weedy rice populations decrease the efficacy of the IMI and ACCase chemistries and increase the chance of herbicide resistance.

This resistance occurs when resistant weedy rice matures to seed and outcrosses into the rice crop. When that happens, drastic measures are necessary to control it. In these situations of herbicide failure, a farmer’s best line of defense is cropping rotation. 

The big takeaway: “If you are in a situation with heavy weedy rice infestation across the field, you do not want to go with a Max-Ace or Provisia. We have to break the cycle, and the best way to do that is with a different rotation,” said ADAMA Product Development Manager Jason Meier.

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The best fit for ACCase technology is rice fields with early weedy rice populations at only 5 to 10% infestation. You should not expect effective weedy rice control if you plant ACCase rice in fields with infestations greater than 50%. Instead, consider a soybean rotation or even leaving the ground fallow in these situations.

“Once you see outcrosses, it is over,” said Distinguished Professor and Weed Scientist Jason Norsworthy at the University of Arkansas System Division of Agriculture. “The ACCase technology will not be of value in the coming years. You now have a mess that you must try to clean up with a soybean rotation or fallow the field.”

For farmers coming out of Clearfield or FullPage systems in fields with high populations of IMI herbicide resistance, Norsworthy recommends a soybean rotation before moving to an ACCase rice cropping system.

“You are going to have to make soybean a staple rotation in a field like that to sustain the ACCase technology. If you continue with rice on rice, you do not have the long-term benefit of residual activity that was once supplied in the Clearfield and Fullpage systems, ” Norsworthy said.

Start clean and overlap residuals

Regardless of the rice system you choose, start the season off right with a clean field. Meier said do whatever it takes to get a good burndown program up front, whether it is a late fall application or early spring application, so you have residual weed control at planting to kill grasses. Otherwise, you will be chasing grass all season long.

Keep in mind that while ACCase herbicides do work to combat grasses, it is not best practice to solely rely on the chemistry for grass control. Instead, overlap residuals to control grass and devote the ACCase herbicide to weedy rice.

Tim Walker, general manager at Horizon Ag emphasized the importance of utilizing residuals. He said, “We have one bullet to kill weedy rice and that is the ACCase herbicide. Do not forget about all these other bullets we have for grasses. We need to make sure we are giving the ACCase herbicide the best opportunity to kill the weedy rice and not just using it as a one size fits all weed control program.”

Meier recommends a good pre-emerge system like Valor early preplant, Command plus glyphosate and Sharpen at planting, or tillage to start the season strong. For those who cannot get in a timely preemergent application Meier suggests a delayed preemergence application of a Prowl and Bolero mix.

For Arkansas farmers, Norsworthy recommends using clomazone to get ahead of barnyardgrass

and other grasses. Other residual herbicide options include Quinstar(quinclorac), Satellite (pendimethalin), and Bolero (thiobencarb).

In Louisiana, LSU AgCenter Weed Scientist, Connor Webster reminds all growers not to skimp on overlaying residuals, because tank mixing with ACCase herbicides will inevitably lead to antagonism.

“With the ACCase chemistry, no matter what you put with it, there is going to be some level of antagonism,” Webster said. He noted some of the worst tank mix partners for ACCase herbicides include Grasp, Regiment, Stam, and 2,4-D. 

Three-shot approach with ACCase herbicide

For ACCase herbicides, farmers are limited by label to 31 total ounces, and Norsworthy noted one way to reduce the chance of antagonism is choosing a three-shot program for ACCase applications.

Timing and rates for the three-shot approach are 11 ounces at 2-leaf stage, 10 ounces at preflood, and 10 ounces within seven days of an established flood. This is opposed to a two-shot application at 15.5 ounces each.

ACCace herbicides are also photosensitive, so cloudy, cool days increase the risk of plant injury. This is important to remember for early planted rice.

Walker added, “What I like about the three-shot split is crop safety. That way if you get a cold snap after late March or early April planting, you can kill the weedy rice with 10 to 11 ounces instead of getting out at 15.5-ounce application on sick rice that is just looking for a chance to grow.”

In zero-grade rice fields, Norsworthy recommends further suppressing weedy rice with Rogue herbicide post flood. He said, “If the weedy rice is small, I can kill it with Rogue. If it is larger and above the water, I am going to kill it with Provisia or Highcard. That is the best strategy we have seen in Arkansas in terms of insuring we make it to harvest without any escapes."

Also make sure to use good water volume per application. Meier said this is a minimum of 10 gallons per acre by ground rig or 5 gallons per acre by air.

Norsworthy added, “When I see issues of failures, a lot of those are going out at 3 gallons per acre. The minimum is 5 gallons per acre, and if a guy would put out 7 or 8 gallons per acre, it would give you much more consistent performance of the herbicide.”

You should also remember that ACCase herbicides have limited movement in the plant, meaning coverage is key. Meier reminded farmers to check their spray tips. If you are using the same tips in rice that you use for soybeans and other crops, those tips likely spray big coarse droplets. In that case, it is best to increase the volume to get better coverage in rice.

Walker noted, from a management standpoint, this technology goes beyond ‘just trying to get to next year.’ He said, “ACCase may have its own set of challenges, but it is what we have. It has worked very well, and it is important that we steward this very important technology.”