Rice weed researchers exploring new species, new products to combat them

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LSU AgCenter rice weed specialist Connor Webster outlines results of his latest research experiments with yet-to-be-released herbicidal products during the Acadia Parish Rice Field Day held at the Rice Research Station’s South Farm on Aug. 15. Photo by Derek Albert/LSU AgCenter

The 2022 growing season was highlighted by warm, dry weather that provided great growing conditions for rice in Louisiana. Unfortunately, the same favorable conditions that helped rice thrive also provided great conditions for yield-robbing weeds to propagate.

AgCenter rice weed specialist Connor Webster attributes the increase in grass pressure to the warm, dry growing season that Mother Nature provided in the spring and summer months. He said the grass species that were prevalent during the growing season have a built-in defense mechanism that may have made them even harder than usual to control.

“Weather played a role, especially in grass control,” Webster said. “Whenever it’s dry and hot like that, the plant will put on a thicker cuticle to help the plant retain water. Therefore, it is a little bit more difficult to get the herbicide into the plant and get it to the site of action.”

The warm, dry growing season also contributed to what Webster called an astral alignment for Newpath carryover. Effects from prior applications of imazethapyr, the active ingredient in Newpath, were seen in several fields across the Louisiana rice belt. Webster said it was most often seen in fields where farmers planted the Clearfield system a few years ago, flooded the fields for crawfish last year and planted Provisia rice this year.

“The reason we are seeing such a long period of carryover is because they went to an anaerobic condition with the crawfish pond where there was no breakdown of that herbicide,” Webster explained. “Then, once they finished with the crawfish production, we had a really dry winter and an early spring. So, we didn't have those flushes of rain to help break down any remaining imazethapyr in the soil. The stars just kind of aligned.”

The 2022 rice weed science team, led by Webster, is rounded out by graduate student John Williams, research farm specialist Kalem Johnson, and transient student workers Miranda Arsement and Brayden Hood. Webster said Acadia Parish AgCenter agent Jeremy Hebert spent a lot of time assisting with herbicide trials at the H. Rouse Caffey Rice Research Station’s South Farm this year. Hebert said the weed research experience gave him a firsthand look at some of the same weed pressures that Acadia Parish farmers were seeing on their operations this year.

“As an agent, it helps relay the research information to the farmer a lot easier when I can see it in-person,” Hebert said of his work on the South Farm this year. “All these weeds compete for the same nutrients, the same sunlight and the same water as your rice plants. Whenever you have those weeds competing for the same space and nutrients, it’s always a problem.”

Webster said a new species seen in Louisiana rice fields is Fimbristylis littoralis, commonly called fimbry. He said this species of perennial grass was most often found in roadside ditches. Over the past couple of years, it has become more prevalent in rice fields. The plant, which often thrives in submerged conditions, is well-suited for the anaerobic, flooded environments where rice is grown. Identifying Fimbristylis can be somewhat troublesome, he said.

“One of the big problems with it is that it is often misidentified as rice flatsedge,” Webster said. “One of the big differences is how flat the Fimbristylis is, whereas rice flatsedge is three-ranked or coangular-stemmed. If you roll it between your fingers, you can feel the three edges on it.”

Webster explained that he tried 15 different types of herbicide applications in small research trials, and at least one outfield trial, to determine the best way to combat fimbry. He said contact herbicides have shown better control against the newcomer. After one week in test trials, priclopyr showed the best activity against the species. An application of Novixid — a prepackaged mixture of Loyant and Grasp — showed adequate control with signs of browning and necrosis showing up after a week. Stam also showed some efficacy, but Webster said the herbicide would show better activity if Fimbristylis was a smaller species.

Another species that has increased its prominence is Sphenoclea zeylanica, commonly called gooseweed. Webster said this flowering plant is usually controlled with Grasp, but this year some producers attested to the gooseweed resisting control with that product. He said he is considering trials to target gooseweed for the 2023 growing season.

“Gooseweed tends to come up in the areas of the field where the rice isn’t as thick,” said Webster. “It’ll pop up late in the year.”

Webster and his colleagues call the Rice Research Station’s South Farm their headquarters for rice weed research. This year at the South Farm, a new experimental herbicide produced by the FMC Corporation was the focus of several trials. The residual grass herbicide, with active ingredient tetflupyrolimet, offers a new mode of action in the fight against grass species growing in rice fields. The prepackaged mixture of tetflupyrolimet and Command can outlast the efficacy of Command on its own as soil microbes tend to develop an affinity for a particular herbicide after repeated use. Webster said the experimental product exhibited control of several weed species.

“We are getting excellent long-term residual activity on barnyard grass, probably signal grass, even late-season sprangletop emergence,” Webster told growers visiting the South Farm in June.

In field trials at the South Farm, Webster and his team tested tetflupyrolimet’s weed control alongside that of Command. Even at lower application rates than what manufacturers will likely recommend, Webster said there is significant grass control. The rice weed science team will continue testing this product ahead of its potential release in 2024, he said.

“We’re getting a lot better residual control from this new product, compared to the Gambit, by itself. We’ve got a lot of work still to do with this product to find exactly where it fits in Louisiana, but just in our second year it appears to be a pretty promising product.”

2022 was the first year Rogue SC was used commercially in rice fields. Rogue SC is based on the novel active ingredient, benzobicyclon, a herbicide that inhibits HPPD (4-hydroxyphenylpyruvate dioxygenase). According to the label, this product is only active in flood water and has no herbicidal activity when applied to soil or plant foliage. Webster used Rogue SC in several trials to test its efficacy against sprangletop and in some scenario-based tests.

“We looked at it in mixtures with other herbicides in kind of a late-season, salvage situation for rice flatsedge and sprangletop,” Webster said. “We looked at it in a water-seeded system, trying to fit it in different timings in water-seeded rice.”

Reflecting on the past year, Webster offers one piece of advice as growers prepare for the 2023 rice growing season.

“Don’t cut costs in burndown,” he cautioned. “I think there was a lot of costs cut in burndown this year with the price of Roundup. You’re going to end up spending that money later in the growing season. I know it’s tough with Roundup being $58 to $60 dollars a gallon, but that’s the cards we’re dealt.”

He added that the high cost of glyphosate may prompt trials of burndown herbicide applications that may prove to be less cost restrictive. He listed products such as First Shot, Sharpen and 2,4-D as potential alternatives to the more expensive Roundup.

11/21/2022 5:48:24 PM
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