Richard T. Dunand, R. Russell Dilly, Eric P. Webster, Christopher T. Leon and Wei Zhang
Red rice, a noxious weed in rice, costs rice producers millions of dollars each year. Red rice is physiologically similar to rice. Consequently, rice and red rice are susceptible to the same herbicides, so controlling red rice with herbicides in rice fields is difficult.
Imazethapyr (NewPath) is one herbicide normally injurious and commonly fatal to both rice and red rice. A group of rice varieties has been modified through mutation and standard breeding practice to be resistant to the herbicide imazethapyr. The creation of imazethapyr-resistant rice has led to a rice production system that for the first time allows early-season herbicidal control of red rice in rice fields. The imazethapyr-resistant rice is called Clearfield rice.
The importance of Clearfield rice and use of imazethapyr in rice production in Louisiana is evident by the fact that more than 20 percent (100,000+ acres) of the acreage in 2004 was planted to CL161, a Clearfield rice variety. But as with all weed control programs, there is seldom 100 percent weed control. In weed control in general, survival of a few weeds is not a major problem. However, when the weed and crop cross-pollinate, the situation changes. Cross-pollination is a major concern. LSU AgCenter researchers have shown that rice and red rice can cross-pollinate with the potential for moving the herbicide-resistant trait into red rice through pollen exchange. Therefore, red rice that escapes control during seedling-stage applications of imazethapyr threatens the sustainability of the herbicide-resistant technology through cross-pollination (outcrossing) between red rice and Clearfield rice.
In anticipation of outcrossing, BASF Corporation, rice seed dealers and representatives of the LSU AgCenter conducted a series of meetings to introduce a stewardship program and teach rice growers how to minimize outcrossing and limit the survival of herbicide-tolerant red rice that occurs because of outcrossing. As part of this effort, AgCenter scientists are conducting research to determine if compounds with plant growth regulator activity can limit reproductive growth in red rice. Test Plot Study
In addition to herbicidal activity, imazethapyr has been shown to act as a plant growth regulator in several turfgrass species, where it suppresses growth and imparts lodging resistance to prevent plants from falling over in seed production fields. Evaluations in rice at the LSU AgCenter Rice Research Station in 2002 showed that similar results can occur in red rice. An area naturally infested with red rice was drillplanted with CL121, the first Clearfield variety released to farmers. Standard, labeled, early-season applications of imazethapyr provided more than 95 percent red rice control through direct herbicidal activity. Some red rice plants survived, providing a natural situation for evaluating the plant growth regulator effects of late-season applications of imazethapyr.
At midseason around the panicle differentiation (PD) stage of growth of red rice (when the panicle or flower is about 1/8-inch long inside the main stem approximately 70 days after planting) and later at heading, imazethapyr at a rate of 4 fluid ounces of Newpath per acre was applied in a foliar spray. Following these applications, rice and red rice (no longer seedling stage) exhibited no harmful symptoms.
At harvest, it was noted that the CL121 was unaffected by either time of application of imazethapyr. On the other hand, red rice plants treated at PD had arrested growth. Yield and milling yield of CL121 were similar between the treated and control plots. Red rice plants were short and produced no panicles in the treated plots. The application of imazethapyr at heading had minimal effects on red rice seed production. The main consequences were malformed seeds and some sterility.
With higher levels of herbicide resistance available in the newer CL161, the CLXL8 hybrid and the most recently released Clearfield variety CL131, the effects of late-season applications of imazethapyr for its plant growth regulator effects on red rice should show no ill effects on Clearfield varieties and hybrids. Evaluations of CL161 and CL131 are being conducted.
From this research, it can be concluded that the potential to selectively arrest reproductive development in red rice without affecting Clearfield rice can be done with imazethapyr, minimizing the potential for outcrossing between Clearfield rice and any red rice plants that survive the early-seasonal applications of imazethapyr. Stewardship Support
Minimizing the transfer of imazethapyr resistance from Clearfield rice to red rice requires a concerted effort. Conventional practices associated with land management, seed source, seeding method, water management and rotational crops as provided in a stewardship program will continue to play a major role in minimizing the amount of red rice in rice fields. A stewardship program will reduce the amount of red rice pressure placed on the newly developed imazethapyr-resistant technology and will serve to control the survival of any resultant herbicide-resistant red rice.
For red rice that does survive the new imazethapyr-resistant technology within a given rice cropping season, midseason applications of imazethapyr and similar growth-suppressing compounds may eliminate or interrupt reproductive development in red rice without affecting imazethapyr-resistant rice.
Used together in a planned and integrated manner, all of the measures outlined in a stewardship program, in addition to the use of growth suppressants, can reduce the opportunity for the transfer of imazethapyr resistance to red rice and lengthen the period for the usefulness of the technology. At present, imazethapyr is not labeled for lateseason application and at the standard rate of 4 fluid ounces per acre is restricted to applications no later than 45 days before harvest.
Because of this restriction, there are no recommendations for the use of plant growth regulators at this time. This research project was conducted to determine the potential of the plant growth regulator technology.
(This article appeared in the summer 2005 issue of Louisiana Agriculture.)