Submitted to: Symposium Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: September 9, 2005
Publication Date: August 9, 2005
Citation: Gealy, D.R. 2005. Gene flow between red rice and rice in herbicide resistant rice fields: evaluating risks and management options. Proceedings Fourth Brazilian Rice Congress, August 9-12, 2005, Santa Maria, RS, Brazil. 2:610. Technical Abstract: Imidazolinone (IMI)-resistant rice cultivars have been increasingly adopted in the southern U.S. since their initial introduction in 2002, largely due to the newly acquired control of red rice that is now possible in these systems. It is estimated that IMI rice is being grown on 20 to 25% of the acres for the 2005 rice crop in the mid south, consisting primarily of the variety, CL 161, but also including a small percentage of IMI resistant hybrid varieties such as CL-XL8. Much of the IMI rice is being planted in fields with heavy red rice infestations. Although most reports indicate that control of red rice has been good to excellent in IMI rice fields over the past three years, weed control failures can and have occurred, leading to varying degrees of outcrossing and the formation of some populations of IMI-resistant rice-red rice hybrids. These hybrids are easily identified in the first generation. They produce very large plants with pubescent leaves, red seeds, and are herbicide-resistant. Those derived from crosses with awnless strawhull red rice are further distinguishable because of their extremely late flowering pattern (some do not complete flowering during the normal growing season), while those derived from crosses with awned or blackhull red rice are distinguishable because of their reddish lower stems and long, pink awns. SSR DNA marker analysis can be used to further confirm that DNA was contributed from both rice and red rice parents, and to help establish the precise parentage of the hybrids. Subsequent segregating generations from these crosses are more difficult to identify precisely. Inadequate red rice control in IMI rice fields can result from incomplete aerial herbicide coverage when trees or power lines interfere with a pilot’s ability to fly safely over the entire rice field. Suboptimal weather or untimely irrigation activities can also affect herbicide performance adversely. Historical studies of outcrossing rates between red rice and herbicide-resistant or non-resistant rice have yielded varying results. Over many studies, outcrossing rates have averaged about 0.2% and were usually less than 0.5%. Recent reports from Louisiana indicate that outcrossing rates in IMI rice fields in which heavy red rice infestations were poorly controlled, can be as high as 3%. Outcrossing depends on a number of factors, including red rice ecotype, rice cultivar, vertical and/or horizontal distances between panicles, synchronization of flowering periods, flower morphology, and seed production, as well as environmental conditions that may extend or reduce pollen viability. Subsequent incorporation of herbicide resistance or other commercial rice traits into red rice populations can be lessened by additional factors including very late flowering and low seed set in hybrids. Commercial rice traits apparently have been incorporated into Arkansas red rice populations over a long period of time via outcrossing as evidenced by the presence of low levels (<1%) of semi-dwarf-sized red rice plants on farm fields sampled across the state. Sustainability of IMI rice technology may be maximized if growers commit to several practices: plant only commercially certified seed (do not replant harvested seed); ensure early, accurate identification of hybrid plant types; consider spot spraying suspected hybrid plants with non-selective herbicides or other non-IMI-based herbicides, and/or rouge fields if necessary; avoid growing IMI rice in back-to-back crop seasons and plant rotational crops where feasible; and actively monitor and manage areas of poor herbicide performance, especially where coverage is poor near power lines, trees, or sprayer skips. One of the unsolved dilemmas facing U.S. growers is that the greatest use of IMI rice tends to be in the areas with the heaviest red rice infestations which have the greatest potential for herbicide failure and/or red rice survival, which leads to the greatest pressure for outcrossing. Integrated strategies that address both short- and long-term economic and agronomic challenges of red rice control and resistance management in IMI rice systems will be necessary to extend the viability of these systems in the U.S.