Imazethapyr-based weed control treatments were evaluated in water-seeded imidazolinone-resistant (IR) rice in Louisiana in 2000 and 2001. Treatments included imazethapyr applied to soil before seedling flood (surface [SURF] applied) or no soil application (no SURF), followed by imazethapyr applied postemergence (POST). Averaged over SURF application treatments (SURF, no SURF), all imazethapyr mixtures increased control of barnyardgrass at 14 and 42 d after POST treatment (DAT) compared with imazethapyr alone. Treatments containing imazethapyr-applied SURF followed by POST treatments controlled red rice 90 to 96% at 21 DAT. Alligatorweed and hemp sesbania control increased with POST mixtures compared with imazethapyr-only treatments at 35 DAT. Rice yield increased with imazethapyr applied SURF compared with no SURF application, averaged over POST applications. Other herbicides will be required to control troublesome weeds such as hemp sesbania and alligatorweed in an IR rice production system.

A study was conducted to evaluate weed control with imazethapyr applied at different timings or a combination of timings with or without an early POST application of bensulfuron, bentazon plus aciflurofen, bispyribac, carfentrazone, halosulfuron, propanil plus molinate, or triclopyr in drill-seeded imidazolinone-resistant rice. Control of barnyardgrass, red rice, and alligatorweed was insufficient with imazethapyr at 87 g/ha PRE or at 53 g/ha late POST alone. Imazethapyr at 87 g/ha PRE followed by imazethapyr at 53 g/ha late POST controlled red rice 86% or greater but did not control hemp sesbania. The imazethapyr PRE followed by late POST combination was also weak on barnyardgrass and alligatorweed early in the season. The addition of an early POST application of the other herbicides to the imazethapyr combination improved overall weed control, especially hemp sesbania control. Among the early POST herbicides, bispyribac, carfentrazone, or propanil plus molinate were more effective in helping improve the overall weed control, resulting in better rice grain yield.

Field studies were conducted in Crowley, LA, and Stoneville, MS, in drill-seeded rice to evaluate economical returns of weed control with imazethapyr. Red rice and barnyardgrass control was evaluated with imazethapyr alone at various rates and application timings. Imazethapyr, averaged across rate, controlled red rice 89% and barnyardgrass 90% when the initial application of imazethapyr was applied at emergence followed by a second application of imazethapyr 2 wk later. No difference in red rice and barnyardgrass control was observed with imazethapyr, averaged across timing. Yield and economical returns were maximized when the initial application of imazethapyr was applied at rice emergence followed by a second application of imazethapyr 2 wk later.

A study was established to evaluate weed control and crop response of imidazolinone-tolerant (IMI-tolerant) rice in water-seeded culture with imazethapyr at 70, 105, and 140 g ai/ha at four different soil application timings with or without 70 g/ha imazethapyr postemergence (POST). Application timings included preplant incorporated (PPI), surface application prior to seeding (SURFACE), following seeding (SEED), and at pegging (PEG). The PEG treatments were applied when green leaf tissue had emerged from the seed and the root had begun to extend downward into the soil. Response of barnyardgrass, red rice, and rice injury was not affected by the addition of an imazethapyr POST application. Barnyardgrass control was above 90% at 28 d after treatment (DAT) for both years. At 42 DAT, barnyardgrass control decreased to 88% in 1998, compared with 94% control in 1999. Averaged over years, Indian jointvetch control ranged from 44 to 74% at 28 DAT and 41 to 71% at 42 DAT. Indian jointvetch control was inconsistent and lacked uniformity over all factors. Rice injury increased as application timing was delayed from PPI to PEG in 1998 at both rating dates. In 1999, injury was not observed with imazethapyr at 28 DAT, and 1% injury was observed at 42 DAT with all imazethapyr application timings.

Field experiments were conducted in 2002 and 2003 in Beaumont, TX, to evaluate the effect of flood timing on red rice control with imazethapyr applied at different cultivated rice growth stages. Treatments included flood establishment at 1, 7, 14, and 21 d after postemergence (POST) herbicide treatment (DAT). Imazethapyr was applied preemergence at 70 g ai/ha followed by 70 g/ ha POST when imidazolinone-tolerant rice cultivar ‘CL-161’ had three- to four-leaf stage (EPOST) or five-leaf stage (LPOST). Flood needed to be established within 14 DAT to achieve at least 95% red rice control when imazethapyr was applied EPOST. However, flood needed to be established within 7 DAT to provide at least 95% red rice control when imazethapyr was applied LPOST. Delaying the flood up to 21 DAT reduced rice grain yield for both application timings.

A study was conducted in 1999 to evaluate weed control and imidazolinone-tolerant (IMI-tolerant) rice response to 140 g ai/ha imazethapyr at different application timings and to flood depths of 5, 10, and 20 cm at the Rice Research Station near Crowley, LA, and at the Northeast Research Station near St. Joseph, LA. Barnyardgrass control, rice injury, and rice yield were not influenced by flood depth; therefore, data were averaged over flood depths. Season-long barnyardgrass control was greater than 80% at both locations with imazethapyr applied preplant incorporated (PPI), early postemergence (EPOST), and mid-postemergence (MPOST). IMI-tolerant rice injury at 7 d after late postemergence (LPOST) treatment (DAT) was less than 5% for all imazethapyr treatments at Crowley and 1 to 8% for PPI, preemergence (PRE), delayed preemergence (DPRE), and EPOST timings at St. Joseph. However, IMI-tolerant rice injury was 5% or less at 42 DAT for all treatments at Crowley and St. Joseph. IMI-tolerant rice yield at Crowley was 2,780 to 3,000 kg/ha following applications of imazethapyr at PPI, PRE, DPRE, and EPOST, and these yields were higher than the yields of the MPOST and LPOST treatments. At St. Joseph, IMI-tolerant rice yields were equal for imazethapyr applied PPI, DPRE, and EPOST treatments compared with Crowley.

Field studies were conducted in 2008 and 2009 near Crowley, Louisiana, to evaluate the addition of different propanil formulations in mixture with a standard imazethapyr program of 70 g ai ha−1 early postemergence followed by (fb) 70 g ha−1 late postemergence. Weeds evaluated included red rice, barnyardgrass, Texasweed, and alligatorweed. Control of all species with treatments, including a propanil formulation applied at 3,400 g ai ha−1, was equivalent to, or greater than, the standard imazethapyr program. Rough rice yield and economic returns were maximized when the propanil formulations of Propanil 1 or Propanil 3 were mixed with imazethapyr in the early postemergence applications. The addition of propanil to imazethapyr increased rough rice yield and economic returns because of improved weed control.

Field studies were conducted in 2008 and 2009 near Crowley, LA to evaluate the addition of a herbicide with soil-residual activity in mixture with imazethapyr applied very early POST followed by an application of imazethapyr or imazamox 2 wk after the initial application. Weeds evaluated included red rice, barnyardgrass, and alligatorweed. Weed control with treatments including a herbicide with soil-residual activity was equivalent to or higher than imazethapyr applied alone followed by imazethapyr or imazamox. Yield and economical returns were maximized with quinclorac or penoxsulam mixed with imazethapyr followed by imazethapyr or imazamox. The addition of quinclorac or penoxsulam proved to be beneficial in a total weed management program.

Imazethapyr applied to the soil at 0, 35, 53, 70, 87, 105, and 140 g ai/ha followed by (fb) early-postemergence (EPOST) or late-postemergence (LPOST) applications at 140, 105, 87, 70, 53, 35, and 0 g ai/ha, respectively, was evaluated for weed control, crop tolerance, and grain yield in imidazolinone-tolerant rice. In drill- and water-seeded rice, imazethapyr effectively controlled barnyardgrass 91 to 97% with a soil fb EPOST or LPOST application except when applied at 105 fb 35 g/ha EPOST at 49 d after late-postemergence treatment. Rice yields were 5,920 and 3,920 kg/ ha in the drill-seeded study with a single application of imazethapyr at 140 g/ha EPOST in 2000 and 2001, respectively. These yields were greater than or equal to those from treatments with two imazethapyr applications.