Efforts Must Be Made To Minimize Outcrossing in Clearfield Rice

Steven D. Linscombe

The Clearfield system of using herbicide-resistant rice offers for the first time the ability to selectively eliminate the weed red rice from a production rice field with the use of an herbicide. However, one major problem with the technology is the chance of outcrossing. Since red rice and rice are closely related, they can actually cross-pollinate each other. This means pollen from a rice plant can pollinate a red rice plant or vice versa. One potential outcome of this cross-pollination would be a resulting offspring plant with weedy (red rice) characteristics that is also resistant to NewPath and Beyond herbicides.

To maintain the viability of the Clearfield system, it is essential to do everything possible to minimize the potential for outcrossing and to eliminate any plants that might result from an outcross event. Unfortunately, these “outcross” plants will typically possess all the characteristics that make red rice so difficult to control and eradicate (shattering, dormancy, etc.). However, they will be immune to the activity of NewPath and Beyond herbicides.

Several actions will minimize the potential for outcrossing in a Clearfield rice field. Among the most important are these:
  1. Always use two (sequential) applications of NewPath. This will better allow for the control of any red rice plants that “escape” the first application.
  2. If red rice plants remain after the two NewPath applications, consider using Beyond, which can control larger “escaped” red rice plants.
  3. If practical, physically remove any “escaped” red rice plants.
  4. NEVER plant Clearfield rice two consecutive growing seasons in the same field.
Remember, seldom does any herbicide system provide 100 percent control of any weed. Therefore, it is probably a good policy to assume that outcrosses have occurred in any fields in which Clearfield rice has been grown. Efforts must be made to eliminate any resulting offspring from these outcrosses. How these fields are handled the season following Clearfield rice production may be the most important factor in maintaining the long-term viability of this technology. Perhaps the best approach is to plant Round-Up Ready soybeans following Clearfield rice. A weedy rice plant resistant to NewPath as the result of an outcross will still be susceptible to all other soybean herbicides that will control red rice. Many Southwest Louisiana producers may be debating the economic viability of soybeans, especially with the looming threat of Asian soybean rust. However, when one factors in the added benefit of maintaining the ability to effectively use Clearfield rice, the economics of soybean production may be seen in a different light. The worst thing one could do in a field the season after Clearfield rice is nothing. This will allow any resulting “outcross” plants to germinate and mature seed which will certainly exasperate the problem.

Clearfield technology is one of the most promising breakthroughs in Louisiana rice production in many years. It is up to all of us to make every effort to sustain the value of this technology for our rice-growing region.

Steve Linscombe, Southwest Regional Director and Rice Breeder, Rice Research Station, Crowley, La.

(This article appeared in the summer 2005 issue of Louisiana Agriculture.)
9/20/2005 9:20:09 PM
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