The recent discovery in Arkansas of out-crossing between herbicide-resistant Clearfield rice and the nuisance red rice demonstrates the need for farmers to follow the stewardship guidelines established for Clearfield, LSU AgCenter officials stress.

Clearfield rice was developed by researchers at the LSU AgCenter's Rice Research Station at Crowley, La. Its advantage is that its breeding allows Newpath herbicide, which doesn't damage the rice plants, to be used to control the nuisance red rice.

The field where the outcrossing — unintended cross-breeding in the field between Clearfield and red rice plants — occurred was planted in Clearfield for two consecutive growing seasons.

“Of all the stewardship recommendations, the one that suggests not planting Clearfield rice two growing seasons in a row is probably the most important for maintaining the longevity of this technology,” said Steve Linscombe, the LSU AgCenter's regional director for southwestern Louisiana, who also is an expert in rice breeding. “We must assume that there will be a low level of out-crossing in many fields, and it will be critical to try to eliminate any resulting plants during the following growing season.”

Linscombe said the Clearfield crop in the Arkansas field was sprayed with two applications of Newpath herbicide, followed by an application of Beyond herbicide, but a large number of uncontrolled plants remained in the field.

Genetic testing confirmed the red rice plants contained the resistant gene that prevents Newpath from damaging rice plants, Linscombe said.

Red rice is a weedy relative of rice that has been an enormous problem in U.S. rice production for many years. It causes great losses in both yield and quality of a commercial rice crop.

The close relationship of red rice to white rice has made it difficult to develop a conventional herbicide capable of controlling red rice without harming the commercial white rice plants in a field. Clearfield rice was developed at the LSU AgCenter Rice Research Station by mutation breeding, a widely accepted plant breeding tool that has been used for decades.

The potential for out-crossing stems from the possibility that pollen from Clearfield rice plants can fertilize a red rice plant — resulting in offspring that have the weedy characteristics of red rice but also the resistance to Newpath herbicide.

“No herbicide program provides 100 percent control,” Linscombe said. “Some red rice plants are likely to survive after treatment with Newpath herbicide.”

Other stewardship practices developed by BASF, the manufacturer of Newpath, with input from the LSU AgCenter, include:

  • Application of the herbicide in two separate treatments helps insure that any plants not killed by the first application will be killed by the second.
  • Physical removal of any surviving red rice plants.
  • Always planting certified Clearfield seed.
  • Following Clearfield rice planting with a sound rotational program the following year. This may include planting of a Roundup Ready crop under which red rice plants will be killed by the glyphosate application.

Officials say another good practice is to fallow plow the field as often as needed the following year to destroy a number of crops that may out-cross red rice plants.

LSU AgCenter rice specialist Johnny Saichuk stressed following the stewardship practices is important for the entire rice farming industry, not just for individual farmers.

“This is one of those times you need to think for the good of the industry rather than yourself,” he said. “If you know you can't abide by the guidelines, then you shouldn't use this technology.”

Evidence of out-crossing also has been found in Louisiana. James Oard, an LSU AgCenter geneticist, said he has documented out-crossing in Louisiana with two years of field research.

In Oard's research, only four of 24 fields sampled in southwest Louisiana over the two years had no red rice out-crossing, he said.

“Whenever red rice grows alongside Clearfield, there is a chance for out-crossing,” Oard said, explaining that although most of the out-crossed red rice he found matured later than Clearfield, some of it did mature on the same schedule.


Bruce Schultz writes for the LSU AgCenter. e-mail: bschultz@agcenter.lsu.edu.