Weed Control Programs for Southern Rice Production

PEST MANAGEMENT: WEEDS

Weed Control Programs for Southern Rice Production B.V. Ottis, R.E. Talbert, E.F. Scherder, M.S. Malik, and M.L. Lovelace ABSTRACT Barnyardgrass continues to be the most common weed in Arkansas rice production. Most weed control programs center around controlling barnyardgrass. In 1989, propanil-resistant barnyardgrass was discovered in Poinsett County, AR. Since then, methods have been studied to control this weed. Several new herbicides have been introduced for preemergence (PRE) and postemergence (POST) barnyardgrass control. Most recently, bispyribac-sodium and cyhalofop have been released for POST applications in rice and can adequately control barnyardgrass. However, there are limited data concerning the use of these herbicides in programs for season-long weed control. Studies were established in 2002 to evaluate early-POST tank-mixtures of cyhalofop with common residual herbicides, bispyribac-sodium herbicide programs, and a new formulation of pendimethalin. Season-long propanil-susceptible and -resistant barnyardgrass control was achieved when cyhalofop + clomazone, pendimethalin, or quinclorac was applied to 2- to 3-leaf rice. Excellent barnyardgrass control was achieved when bispyribac-sodium was applied to 4-leaf rice and/or 1 wk after flood following a delayed preemergence (DPRE) application of pendimethalin. There were no differences in barnyardgrass control between the original and new formulations of pendimethalin. When either formulation was tank-mixed with quinclorac and applied DPRE, season-long barnyardgrass control was obtained. INTRODUCTION Barnyardgrass is the most common weed in Arkansas rice production (Dowler, 1997) and is found in all production fields in the state (Carey et al., 1995). It is the

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second-most troublesome weed to control in Arkansas rice (Dowler, 1997). Broadleaf signalgrass is currently listed as the second most common weed in rice production. However it is not as difficult to control as barnyardgrass (Dowler, 1997). Propanil has been used extensively to control weeds in rice production since the early 1960s (Smith, 1965). Because of this extensive use, barnyardgrass resistant to the effects of propanil has been found in parts of Arkansas (Carey et al., 1995). Since propanil-resistant barnyardgrass was confirmed in Poinsett County in 1990 (Baltazar and Smith, 1994), it has spread to many areas of the Arkansas rice-producing region (Talbert et al., 1997). Currently, there are over 190 confirmed reports of propanil-resistant barnyardgrass populations in the state. Two populations of quinclorac-resistant barnyardgrass have also been confirmed in Arkansas (Lovelace et al., 1999). Control measures for propanil-resistant barnyardgrass have been evaluated, and many have been successful (Baltazar et al., 1994). Clomazone, cyhalofop, fenoxaprop + isoxadifen, bispyribac-sodium, imazethapyr, and quinclorac have provided excellent control of propanil-resistant barnyardgrass when applied at the correct timing with favorable environmental conditions (Scherder et al., 2001). The objectives of this research were to: 1) evaluate weed control with early postemergence tank mixtures of cyhalofop (ClincherTM) for season-long weed control; 2) evaluate weed control programs with bispyribac-sodium (RegimentTM); and 3) evaluate a new formulation of pendimethalin (ProwlTM and Prowl H20TM) alone and in tank mixtures for weed control in drill-seeded rice production. PROCEDURES All experiments were conducted at the Rice Research and Extension Center at Stuttgart, AR, on a DeWitt silt loam (fine, smectitic, thermic, Typic, Albaqualfs) with 0.9% organic matter and a pH of 5.3. Rice was dry-seeded on 16 May and 20 May 2002 in plots eight rows wide (7-in. row spacing) and 16 ft long. The rice cultivar ‘Wells’ was used in all experiments. Propanil-resistant and -susceptible barnyardgrass (Echinochloa crus-galli), broadleaf signalgrass (Brachiaria platyphylla), pitted morningglory (Ipomoea lacunosa), northern jointvetch (Aeschynomene virginica), and hemp sesbania (Sesbania exaltata) were sown in single rows perpendicular to the rows of rice. Evaluations for the control of these weeds, along with natural infestations of a propanilsusceptible (85%) and propanil-resistant (15%) barnyardgrass mix and broadleaf signalgrass, were made. Visual ratings of rice injury, rice biomass reduction, and weed control were taken 7, 14, 21, 28, 42, and 56 days after rice emergence (DAE) for soil-applied herbicides and days after treatment (DAT) for postemergence treatments. Rice was harvested with a small-plot combine from the four center rows, and yields were adjusted to 12% moisture. All data were subjected to analysis of variance, with means separated at the 0.05 level of significance using Fisher’s Protected Least Significant Difference.

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Early Postemergence Tank-Mix Programs with Cyhalofop Cyhalofop was approved for use in rice in 2002. However, it was approved late in May, therefore most applications of cyhalofop were made after flood. This study evaluated tank mixtures of cyhalofop with pendimethalin (1.0 lb ai/acre), clomazone (0.4 lb/ acre), or quinclorac (0.3 lb/acre) for residual grass control in rice. Propanil and tank mixtures thereof were also included as standard treatments. Herbicide applications were made early postemergence (2- to 3-leaf rice). Barnyardgrass, propanil-resistant barnyardgrass, and broadleaf signalgrass control were evaluated. Weed Control Programs with Bispyribac-Sodium Bispyribac-sodium was used extensively in Arkansas rice production last year and was considered a good barnyardgrass herbicide. However, limited data exist evaluating its fit in a complete herbicide program. Therefore, a study was established to evaluate bispyribac-sodium (0.02 lb ai/acre) alone, in sequential applications, and in tank mixtures with other rice herbicides. Bispyribac-sodium and tank mixtures thereof were applied to 4-leaf rice (EPOST), preflood (PREFLD), and 1 week following the flood (POFLD 1 WK). Rice injury was evaluated along with barnyardgrass, broadleaf signalgrass, pitted morningglory, and hemp sesbania control. Evaluation of a New Pendimethalin Formulation Pendimethalin (Prowl H2OTM 3.8 CS ) was evaluated for efficacy on annual grasses common to rice. Prowl H2OTM was compared to the current formulation of pendimethalin (ProwlTM 3.3 EC). Each formulation was applied alone at the labeled rate delayed preemergence (DPRE) or at the 2- to 3-leaf rice stage (EPOST). Each formulation was also evaluated in a tank-mixture with quinclorac (FacetTM) and propanil (StamTM M-4 or SuperWham) applied at standard labeled rates. RESULTS AND DISCUSSION Early Postemergence Tank-Mix Programs with Cyhalofop At 7 WAT, barnyardgrass control with propanil alone was 36%, while cyhalofop alone maintained at least 88% control (Table 1). Cyhalofop + pendimethalin or clomazone did not improve control over cyhalofop (0.25 lb/acre). However, cyhalofop (0.25 lb/acre) + quinclorac resulted in 99% control. Propanil + clomazone or pendimethalin provided less than 85% control of propanil-resistant barnyardgrass, while control with propanil + quinclorac was 99%. No significant differences were observed among cyhalofop tank mixtures, with control of propanil-resistant barnyardgrass being at least 93%, regardless of cyhalofop rate. Broadleaf signalgrass control 7 WAT was 100% with all cyhalofop treatments. With the exception of cyhalofop

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(0.19 lb/acre) + clomazone, no significant differences were observed in rice yields among cyhalofop tank mixture treatments, with yields being at least 6600 lb/acre. Weed Control Programs with Bispyribac-Sodium Rice injury and stand reduction following herbicide applications were less than 5% with all treatments (data not shown). Propanil-susceptible barnyardgrass control was at least 74% when bispyribac-sodium was applied PREFLD or POFLD 1 WK (Table 2). Propanil-resistant barnyardgrass control was similar among bispyribac treatments. Conversely, propanil + molinate resulted in