Red rice is the most troublesome weed in Louisiana rice production. Red rice and commercial rice are closely related and actually belong to the same taxonomic genus and species. Red rice thrives under the same conditions as rice and competes for nutrients, sunlight, water, etc. However, red rice has several weedy characteristics that make it extremely undesirable in a rice production field. One characteristic is red rice shatters. Seed drops off the plant to the ground before harvest. Those grains that do make it into the combine will have a red bran layer and red bran streaks after milling. Red rice also typically breaks up much more than commercial rice during the milling process. These characteristics can cause significant quality reductions, and samples of harvested rice with excessive amounts of red rice can render it unmarketable. Red rice seed also have a characteristic called dormancy, which allows the seed to remain viable in the soil for extended periods of time (30 plus years). This makes it extremely difficult to eradicate from a field once it has become established.

Because the weed is so closely related to the crop plant, it has been impossible to develop a conventional rice herbicide that will control red rice in a rice crop without causing severe damage to the commercial crop. Therefore, research efforts at the LSU AgCenter’s Rice Research Station were undertaken to develop an “unconventional” approach to red rice control. This long-term research led to the successful development of the Clearfield Rice Production System.

The Clearfield System offers, for the first time, the ability to selectively eliminate red rice from a production rice field with the use of an herbicide. The technology is based on an induced mutant resistant to imidazalinone herbicides. This mutant was developed by subjecting a large number of conventional rice seed to a chemical that can cause changes (mutations) in the genetic makeup of the seed. These seed were then planted and the resulting plants were sprayed with the herbicide. Naturally, most of the plants were susceptible and were killed by the herbicide. However, one plant survived the herbicide treatment and was resistant to the herbicide. This plant was resistant because the chemical had caused a subtle change in one enzyme system in that plant which allowed that plant to be resistant to the “imi” herbicides.

Several rice varieties (and hybrids) have been developed from this mutant line. These varieties have a high level of resistance to NewPath and Beyond – the “imi” herbicides used with this system. Both of these herbicides do a good job of controlling red rice, as well as a number of other troublesome rice weeds.

This technology has been rapidly adopted by Louisiana rice producers and was used on more than 100,000 acres (or more than 20% of Louisiana rice acres) in 2004. Indications are that usage will increase in the 2005 crop, which affirms the value of this technology for the state’s rice farmers.

However, one major problem with the technology is the possibility of “outcrossing.” Because 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 the “imi” 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 result from an “outcross event.” Remember that these outcross plants will typically possess all the characteristics that make red rice so difficult to control and eradicate (shattering, dormancy, etc.) and will be immune to the activity of NewPath and Beyond herbicides.

The following recommendations will minimize the potential for outcrossing in a Clearfield rice field:

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.

Seldom does any herbicide system provide 100% 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, and efforts must be made to eliminate any resulting offspring from these outcrosses. How these fields are handled during the season following Clearfield rice production may be the most important factor in maintaining the long-term viability of this technology on your farm. Perhaps the best approach is to plant Round-Up Ready soybeans following Clearfield rice. Remember that a weedy rice plant resistant to NewPath as the result of an outcross will still be susceptible to all other soybean herbicides. 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.

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.