By Robert H. Wells
Delta Research and Extension Center

STONEVILE -- Research has determined a method to control one of the most common and troublesome weeds of Mississippi rice production -- Amazon sprangletop.

“Amazon sprangletop is an annual grass that complicates Clearfield rice production, a system designed to combat red rice,” said Jason Bond, a Mississippi State University rice weed scientist at the Delta Research and Extension Center in Stoneville. “Newpath, an herbicide labeled for use in Clearfield rice, normally provides good control of barnyardgrass and red rice, but it has performed very inconsistently on Amazon sprangletop.”

Bond said newer Clearfield varieties (CL171, CL161 and CL131) have increased tolerance to Newpath compared to older Clearfield varieties. That tolerance allows growers using the Clearfield system to apply two applications of Newpath at up to 6 ounces per acre to manage difficult grasses.

However, in research during 2006, Bond found that Clearfield rice growers can manage Amazon sprangletop with only 4 ounces per acre of Newpath if they add a residual herbicide. They must apply the combination prior to rice and weed emergence and then follow up with a second Newpath application at 4 ounces per acre when rice reaches the four-leaf development stage.

The rice weed scientist found residual herbicides Prowl H20, Prowl EC and Command worked best with this method.

“Newpath plus Prowl H20 or Prowl EC controlled Amazon sprangletop 95 percent of the time in 2006 and were the least expensive,” Bond said. “However, some injury to the rice may be observed following a Prowl application. Also, if the rice crop fails for some reason, rice cannot be replanted in a field where Prowl has been applied the same year.”

Bond said if a Clearfield rice producer has concerns about injury or the possibility of replanting, and Amazon sprangletop is not already present, then Command, which provided 85 percent control, will be the next most economical treatment.

Clearfield growers who initiate herbicide applications at pre-emergence to control Amazon sprangletop could save nearly $16 an acre on Newpath compared to efforts made after the weeds and rice emerge.

 “Sometimes factors such as sudden rains, high winds or more weed growth than expected, will leave growers no choice but to apply their herbicides postemergence,” Bond said. “If this happens, Clearfield producers should consider increasing the Newpath rate to 6 ounces per acre for Amazon sprangletop control.”

Bond said in 2006, a postemergence application of Newpath at 6 ounces per acre provided around 80 percent control of Amazon sprangletop when tank mixed with Prowl EC, Prowl H20 or Command and then followed up with a second 6 ounce per acre Newpath application at the four-leaf stage.

Bond also evaluated barnyardgrass and red rice control in the Clearfield system. The researcher achieved control of both annual grasses with all Newpath timings, rates and residual herbicides.

Mississippi rice producers planted nearly a third of the state's rice acreage to the Clearfield system in 2006.

Growers' check-off dollars from the Mississippi Rice Promotion Board sponsored Bond's research.

Nathan Buehring, the MSU Extension rice specialist based in Stoneville, said water is important when using residual herbicides in rice production.

“The key to making residuals work is water, meaning that if you apply a residual herbicide, and it does not rain within a couple days, it needs to be flushed,” Buehring said. “With no rain or water, the residual herbicide is less effective because weeds emerge before the herbicide is activated, or the residual herbicide will degrade over time. Timely rainfall events or flushing will be needed to keep residual herbicides working.”

Buehring agrees that sooner is better than later when controlling weeds.

“Grass control is cheaper, easier and results in higher yields if it is done early instead of late in the season,” Buehring said. “Every day that grass is out there competing with rice, a yield loss is occurring. It can be small or big, depending on the grass density.