Weed control systems with imazethapyr-tolerant rice (Oryza sativa L.)

Red rice is the number one production problem for Arkansas rice producers. Development of a mutated rice line tolerant to imazethapyr by scientists with Louisiana State University in 1993 presented an opportunity to potentially control red rice in dry-seeded rice. Studies evaluated efficacy of imazethapyr applied at various rates and timings to imazethapyr-tolerant rice. Crop rotation studies evaluated imazethapyr carryover potential. Since rice and red rice are the same species, possibility of gene flow was evaluated and presumed hybrids phenotypically characterized. Red rice was consistently controlled with sequential applications of 70 g/ha imazethapyr soil-applied followed by postemergence or sequential postemergence applications. Single and other rates of sequential applications were less consistent.;Single applications (70 g/ha) were effective on broadleaf signalgrass, but sequential applications provided improved control of propanil-resistant and susceptible barnyardgrass and yellow nutsedge. Yellow nutsedge control was optimized when halo sulfuron-methyl plus imazethapyr followed a soil application. Hemp sesbania was not controlled by imazethapyr; however, tank-mixtures of imazethapyr with propanil and halosulfuron-methyl provided effective control. Rice injury occasionally occurred with some postemergence imazethapyr applications.;Rotational crop studies revealed that imazethapyr residues in the soil varied from year to year. Rotational crops of wheat, oat, or susceptible rice were not injured when planted following applications to imazethapyr-tolerant rice in one study, but oat and rice were significantly injured in another.;Red rice seed collected from an area where simultaneous flowering between red rice and imazethapyr-tolerant rice occurred were screened for imazethapyr resistance. Three tolerant plants exhibited red rice characteristics. When planted in the field, seed from one plant germinated, and progeny segregated phenotypically and by imazethapyr tolerance. The field area was screened using imazethapyr applications. Surviving individuals possessed characteristics of red rice as well as mixed characteristics between red rice and imazethapyr-tolerant rice. This diverse population likely reflects F1 populations plus segregating individuals from earlier hybridization events.;Although red rice and other hard-to-control weeds can be effectively controlled by imazethapyr in imazethapyr-tolerant rice, every effort should be made to achieve 100% control or eradicate red rice survivors to preserve the technology.