Introduction
As of 2006, Arkansas accounted for approximately half of the total rice production in the United States (NASS 2023; Talbert and Burgos Reference Talbert and Burgos2007), signifying the economic importance of the crop to the state. Currently Arkansas remains the number one producer of U.S. rice, planting a hectarage of rice almost equivalent to that of California, Louisiana, Mississippi, Missouri, and Texas combined (NASS 2023). About 82% of the rice hectares are produced within the midsouthern U.S. region, where herbicide-resistant Echinochloa species have been identified as the most difficult-to-manage grass weeds (Butts et al. Reference Butts, Kouame, Norsworthy and Barber2022; Fischer et al. Reference Fischer, Ateh, Bayer and Hill2000; Silva et al. Reference Silva, Streck, Zanon, Ribas, Fruet and Ulguim2022; Van Wychen Reference Van Wychen2020) and can reduce grain yields up to 79% from season-long infestations (Norsworthy et al. Reference Norsworthy, Bond and Scott2013).
Barnyardgrass has historically been successful as a weedy pest in cultivated rice for centuries (King Reference King1966) and likely migrated to other geographies from contaminated seed stock (Barrett Reference Barrett1983). Before the extensive use of pesticides, morphological similarities between early Echinochloa crus-galli biotypes and rice aided the competitive nature of the weed. The abilities of certain barnyardgrass biotypes [e.g., Echinochloa crus-galli (L.) P. Beauv. var. oryzicola (Vasinger) Ohwi] to germinate in anaerobic conditions, thrive in flooded rice culture, and mimic rice phenotypes are all evolutionary mechanisms responsible for the success of barnyardgrass in rice. However, the extensive use of pesticides in rice following the commercialization of propanil in 1959 has placed less selection pressure on similar morphological and physiological characteristics and instead placed a greater emphasis on herbicide resistance (Barrett Reference Barrett1983). Since introducing chemical weed management strategies in rice, barnyardgrass has evolved resistance to six sites of action (SOAs) in Arkansas (Herbicide Resistance Action Committee [HRAC]/Weed Science Society of America [WSSA] Groups 1, 2, 4, 5, 13, and 29), with some biotypes displaying multiple resistances (Heap Reference Heap2024). Given the current resistance status, it is apparent that rice-producing states have a need for novel chemical or management strategies.
Tetflupyrolimet will be the first herbicide with a novel SOA to be commercialized for use in agronomic crops over the last 30 yr (HRAC/WSSA Group 28). Tetflupyrolimet is anticipated to provide effective control of the most challenging grass weeds in rice (FMC Corporation 2023). To date, internal testing conducted by FMC has shown that tetflupyrolimet provides season-long control of grass weeds, and it continues to be evaluated in other crops with other analogs of the molecule, including in corn (Zea mays L.), soybean [Glycine max (L.) Merr.], sugarcane (Saccharum officinarum L.), and wheat (Triticum aestivum L.). However, the novelty of tetflupyrolimet and limited published research create challenges in defining the true scope of the weed control spectrum, especially in its infancy prior to commercialization. Selby et al. (Reference Selby, Satterfield, Puri, Stevenson, Travis, Campbell, Taggi, Hughes and Bereznak2023) and Lombardi and Al-Khatib (Reference Lombardi and Al-Khatib2024) provided some insight into a portion of the expected weed control spectrum from tetflupyrolimet due to the success of the herbicide in controlling Echinochloa spp., Leptochloa spp., and Monochoria spp. in field trials conducted on direct-seeded and transplanted rice in Japan, Indonesia, India, Vietnam, Brazil, and the United States.
Tetflupyrolimet is classified as an aryl pyrrolinone anilide chemistry, discovered in 2014 through high-volume greenhouse screenings (Gaines et al. Reference Gaines, Busi and Küpper2021; Selby et al. Reference Selby, Satterfield, Puri, Stevenson, Travis, Campbell, Taggi, Hughes and Bereznak2023). The novel SOA targets de novo pyrimidine biosynthesis, which is one of the oldest and most essential metabolic pathways in plants and animals (Nara et al. Reference Nara, Hshimoto and Aoki2000). Pyrimidines can be synthesized through salvage or de novo pathways, although the latter are more advantageous in eukaryotic organisms. When applied to sensitive species, tetflupyrolimet inhibits the functionality of dihydroorotate dehydrogenase (DHODH), an enzyme in the fourth step of de novo pyrimidine biosynthesis that facilitates the only redox reaction in the pathway. Disruption of DHODH leads to a lethal accumulation of dihydroorotate and a downstream deficiency of uridine-5′-monophosphate, which deprives plants of pyrimidine bases needed for metabolism, gene expression, and deoxyribonucleic and ribonucleic acid biosynthesis (Dayan Reference Dayan2019; Nagy et al. Reference Nagy, Lacroute and Thomas1992; Zrenner et al. Reference Zrenner, Stitt, Sonnewald and Boldt2006). In terms of selectivity of tetflupyrolimet between weeds and crops, inhibition of DHODH on Setaria spp. from tetflupyrolimet was 10-fold greater than for rice, but tolerance of the crop appeared to be much greater than a magnitude of 10, suggesting that differential metabolism may be responsible for increased tolerance (Dayan Reference Dayan2019). Differential selectivity among various weed species or crops could also be attributed to organisms having significantly different enzymes in the six-step de novo pyrimidine biosynthesis pathway (Santoso and Thornburg Reference Santoso and Thornburg1998).
Tolerance to a specific herbicide can be variable and is dependent on the ability of a crop to metabolize and detoxify the compound (Cole Reference Cole1994). Differential tolerance has also been documented among hybrid and inbred imidazolinone-resistant rice cultivars in response to applications of imazamox (Bond and Walker Reference Bond and Walker2012). Recently, rice cultivars have been observed to have differing levels of sensitivity to florpyrauxifen-benzyl, particularly when applied to medium-grain and hybrid cultivars (Wright et al. Reference Wright, Norsworthy, Roberts, Scott, Hardke and Gbur2021). Environmental conditions (temperature and soil moisture), herbicide rate, and growth stage are all parameters that can influence the degree of crop tolerance (Bond and Walker Reference Bond and Walker2012; Burt and Akinsorotan Reference Burt and Akinsorotan1976; Reick and Wright Reference Reick and Wright1973).
With the arrival of tetflupyrolimet as the first novel SOA for use in agronomic crops in three decades, it is important to address the utility of the herbicide in all available rice production systems while maintaining a high degree of crop safety. The recent issues surrounding the commercialization of florpyrauxifen-benzyl in rice, specifically the variation in barnyardgrass efficacy and unforeseen injury to hybrid cultivars (Wright et al. Reference Wright, Norsworthy, Roberts, Scott, Hardke and Gbur2021), emphasize the importance of extensive testing in those capacities. The objectives of these field experiments were (1) to evaluate the weed control efficacy of tetflupyrolimet and clomazone mixtures on medium- and fine-textured soils as residual herbicides in conventional, imidazolinone-resistant, and quizalofop-resistant rice systems and (2) to evaluate rice response to tetflupyrolimet applied PRE and POST to commonly planted rice cultivars in the midsouthern region.
Materials and Methods
Optimization of Tetflupyrolimet in Different Rice Production Systems
Eight field experiments were arranged as single-factor randomized complete-block designs with four replicates, focusing on weed control programs that included tetflupyrolimet on silt loam– and clay-textured soils. Herbicide treatments are shown in Tables 1 to 6. Each field experiment was conducted in 2021 and repeated in 2022. All silt loam paddy rice experiments were conducted near Stuttgart, AR, at the Rice Research and Extension Center (34.464°N, 91.404°W) on a Dewitt silt loam soil (19% sand, 64% silt, and 17% clay, with 1.1% organic matter) with pH 5.7. Furrow-irrigated silt loam experiments were conducted near Colt, AR, at the Pine Tree Research Station (35.117°N, 90.924°W) on a Calloway silt loam soil (17% sand, 68% silt, and 15% clay, with 1.4% organic matter) with pH 6.7. All fine-textured field experiments were conducted near Keiser, AR, at the Northeast Research and Extension Center (35.662°N, 90.082°W) on a clay (41% sand, 1% silt, and 58% clay, with 2.8% organic matter) with pH 5.5.
Sources of materials for cultivar response, conventional, furrow-irrigated, imidazolinone-resistant, quizalofop-resistant rice field experiments.
The rice cultivars ‘Diamond’ (conventional) (University of Arkansas System Division of Agriculture, Little Rock, AR, USA), ‘FullPage 7521’ (imidazolinone resistant) (RiceTec, Alvin, TX, USA), and ‘PVL02’ (quizalofop resistant) (Horizon Ag, Memphis, TN, USA) were planted at the seeding rates found in Table 7 for the direct-seeded, delayed continuous flood experiments. ‘FullPage 7521’ was also used to plant all furrow-irrigated rice (FIR) experiments. Rice was planted on May 14 (‘Diamond’), May 15 (‘FullPage 7521’), and May 15 (‘PVL02’) in 2021 and on April 30, 2022 (all cultivars), at the silt loam location near Stuttgart, AR. FIR was planted at the silt loam location near Colt, AR, on May 14, 2021, and May 17, 2022. At the fine-textured soil location, all paddy rice was planted on May 20, 2021, and May 10, 2022. FIR at the clay location was planted on June 1, 2021, and May 10, 2022. Paddy rice experiments were conventionally drilled with 19-cm spacing into plots measuring 1.8 × 5.2 m with 1-m alleys. Each treatment for the FIR experiments consisted of two tilled and bedded rows with 97-cm spacing, conventionally drilled with the same 19-cm drill into plots measuring 1.9 × 6.1 m. All herbicide applications were made using a CO2-pressurized backpack sprayer equipped with TeeJet® AIXR 110015 flat-fan nozzles (TeeJet® Technologies, Glendale Heights, IL, USA) calibrated to deliver a spray volume of 140 L ha−1 at 4.8 km h−1. Visible rice injury ratings were assessed at 7, 14, and 28 d after (DA) the most recent herbicide application. In addition to visible injury, weed control was visually evaluated at 14, 28, 42, and 56 DA the most recent application, with an emphasis on barnyardgrass. Visible injury and weed control ratings were assessed on a scale ranging from 0% to 100%, with 0% and 100% representing no injury or control and crop death or complete control, respectively (Frans and Talbert Reference Frans and Talbert1977).
Except for FIR, all experiments were maintained as conventional paddy rice with the establishment of a permanent flood at the 5-leaf growth stage. Soil fertility was addressed specifically to each production system, soil texture, and rice cultivar planted according to the current Arkansas Rice Production Handbook (Roberts et al. Reference Roberts, Slaton, Wilson and Norman2016). Nontarget broadleaf and sedge weeds were controlled with halosulfuron at 53 g ai ha−1, halosulfuron at 70 g ai ha−1 + prosulfuron at 40 g ai ha−1, or 2,4-D at 1,120 g ai ha−1 prior to flood establishment. Unless otherwise specified, all methodology is the same.
All distributions were analyzed using the JMP PRO (version 17.1; SAS Institute, Cary, NC, USA) distribution platform, and all data assumed a normal distribution (Avent et al. 2022). Data were analyzed in JMP PRO 17.1 and subjected to analysis of variance (ANOVA) using the fit model platform. Means were separated using Tukey’s honestly significant difference (HSD) (α = 0.05). Herbicide program, soil texture, and site-year were included in the initial model as fixed effects, with block considered as random, to determine if barnyardgrass control and rice visible injury were different on a silt loam and a clay soil. An interaction of the herbicide program and soil texture or herbicide program and site-year was not observed. Therefore all data were averaged over site-year (random effect) and soil texture (random effect). FIR experiments were analyzed by soil texture due to differences in herbicide programs. Because there were no interactions between the herbicide program and soil texture for the paddy rice systems, the rate adjustments from silt loam to clay soil were assumed to be sufficient. In the final model for the conventional, imidazolinone-resistant, and quizalofop-resistant paddy rice systems, the herbicide program was considered as the only fixed effect, with site-year, soil texture, and block considered random effects. The final model for FIR systems included the herbicide program as a fixed effect by soil texture, with site-year and block as random effects.
Rice Tolerance to PRE- and POST-Applied Tetflupyrolimet
To determine the response of 12 genetically different and commonly planted rice cultivars in Arkansas to a single PRE or POST application of tetflupyrolimet, field experiments were conducted at the Pine Tree Research Station near Colt, AR, on a Calloway silt loam soil in 2021, 2022, and 2023. Before planting, each field was subjected to conventional tillage events for preparation of the seedbed. The experiment was arranged as a two-factor randomized complete-block design with four replicates for each respective cultivar, and each plot measured 1.8 × 5.2 m (Table 7). All 12 cultivars were planted and treated on the same dates for each year with tetflupyrolimet at 0, 200, or 400 g ai ha−1 PRE or POST (2-to3-leaf rice). Rice was planted and PRE applications were made on April 12, 2021, May 12, 2022, and April 12, 2023. POST applications were made on May 26, 2021, June 6, 2022, and May 17, 2023. All applications were applied with a hand-held backpack sprayer equipped with TeeJet® AIXR 110015 nozzles (TeeJet® Technologies) calibrated to deliver 140 L ha−1 at 4.8 km h−1, and non-ionic surfactant at 0.25% v/v was included in all POST herbicide treatments.
Before designated plots received a PRE application of tetflupyrolimet, all plots, including the nontreated control for each cultivar, received a broadcast application of clomazone + quinclorac (Obey®) (FMC, Philadelphia, PA, USA) at 900 g ai ha−1 to ensure that the experiment remained weed-free. Immediately following the broadcast PRE application of clomazone + quinclorac, the appropriate tetflupyrolimet-containing herbicide treatments were applied. Throughout the growing season, additional maintenance herbicide applications were made for the presence of any broadleaf weeds, grasses, or sedges. Depending on the weed species present, florpyrauxifen-benzyl (Loyant®) (Corteva Agriscience, Indianapolis, IN, USA) at 15 g ae ha−1, halosulfuron (Permit®) (Gowan, Yuma, AZ, USA) at 70 g ai ha−1, or propanil (STAM® M4) (RiceCo, Memphis, TN, USA) at 4,500 g ai ha−1 was applied from early POST until the permanent flood was established.
Soil test potassium and phosphorus concentrations were determined from samples collected in the fall before the start of each growing season, and soils were amended prior to planting for each site-year. The field also received a total of 150 kg ha−1 of nitrogen throughout the growing season, with 105 kg ha−1 urea (46-0-0) applied preflood and the remaining when rice reached 1.3 cm internode elongation (Roberts et al. Reference Roberts, Slaton, Wilson and Norman2016). Once each experiment reached the 5-leaf growth stage or tillering, a permanent flood was established until harvest maturity.
Visible rice injury ratings were collected at 7, 14, 21, and 28 DA treatment for the PRE and POST applications and rice stand counts at 14 d after PRE application (DAPRE). Aerial images were captured with a 4K RGB camera mounted on a Mavic Air II (DJI Innovations, Los Angeles, CA, USA) drone to assess the percent canopy growth at 12, 7, and 13 wk after planting (WAP) in 2021, 2022, and 2023, respectively. For each aerial image at the respective collection date, drone altitude was maintained at 30 m to minimize variability in image resolution for percent canopy growth analysis. Aerial images were analyzed for percent canopy growth and made relative to the nontreated control for each rice cultivar using FieldAnalyzer (Green Research Services, Fayetteville, AR, USA). Rice maturity was assessed by recording 50% heading dates for each cultivar before harvest, the timing of which was determined when approximately 50% of the rice in each plot exhibited a panicle. At full maturity, a 1.5-m-wide swath out of the 1.8-m-wide plot was harvested using a small-plot combine (ALMACO, Nevada, IA, USA), and grain yield was determined by adjusting the harvested weights to 12% moisture.
Site-year, tetflupyrolimet rate, and application timing were included in the ANOVA model using JMP Pro 17.1 to determine the presence of significant interactions or main effects (α = 0.05). Percent canopy growth was analyzed by site-year because the aerial images were collected at different evaluation timings (12, 7, and 13 WAP in 2021, 2022, and 2023, respectively) relative to the PRE application. Site-year was the only significant main effect in the model (4 out of 12 cultivars for relative grain yield). Dunnett’s procedure was used when differences occurred between herbicide treatments and the nontreated control (α = 0.05).
Results and Discussion
Optimization of Tetflupyrolimet in Different Rice Production Systems
All conventional paddy rice programs maintained 96% barnyardgrass control with ≤10% visible phytotoxicity to rice at all evaluation dates in a two-pass system (two total herbicide applications) when averaged over site-year and soil texture (Table 2), which indicates that the rate adjustment for tetflupyrolimet was appropriate for fine-textured soils. Visible injury varied among herbicide programs due to bleaching from clomazone at 7 DA PRE but was overall minimal and transient in the weeks following the last application (data not shown). At 56 DA (the 3- to 4-leaf rice application), all herbicide programs exhibited ≥98% barnyardgrass control, and tetflupyrolimet-containing treatments did not display any advantage owing to the high performance of all treatments.
Rice injury and barnyardgrass control of tetflupyrolimet applied with other herbicides preemergence and at 3- to 4-leaf-stage rice in a conventional paddy rice system averaged across the silt loam and clay soil locations and across 2021 and 2022.a,b,c
a Means within a column and crop followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
b Abbreviations: DA, days after; ECHCG, barnyardgrass.
c Silt loam location, Rice Research and Extension Center near Stuttgart, AR; clay location, Northeast Research and Extension Center near Keiser, AR.
d Silt loam rate, clay rate.
The barnyardgrass population at each location likely did not exhibit resistance to HRAC/WSSA Groups 1, 2, 4, 13, or 29, which may explain the high efficacy of all programs that did not include tetflupyrolimet PRE but utilized clomazone as an alternative. In future field experiments, herbicide-resistant barnyardgrass needs to be overseeded to determine if there are advantages when utilizing tetflupyrolimet in a program approach. The addition of tetflupyrolimet would potentially aid in managing herbicide-resistant Echinochloa crus-galli biotypes owing to its novelty and lack of prior exposure to the herbicide. A screening conducted in California on suspected herbicide-resistant grass weed populations collected from rice fields confirmed that tetflupyrolimet controlled all suspected herbicide-resistant samples (A. Becerra-Alvarez, unpublished data; Lombardi and Al-Khatib Reference Lombardi and Al-Khatib2024). Lombardi and Al-Khatib mention that tetflupyrolimet provided effective control of bearded sprangletop [Leptochloa fascicularis (Lam.) A. Gray]. Bearded sprangletop and Amazon sprangletop [Leptochloa panicoides (J. Presl) Hitchc.] are the most prevalent Leptochloa species in the midsouthern United States and can be highly competitive and difficult to control in rice fields (Stauber et al. Reference Stauber, Nastasi, Smith, Baltazar and Talbert1991; Tehranchian et al. Reference Tehranchian, Norsworthy, Korres, McElroy, Chen and Scott2016), reducing grain yield up to 36% (Smith Reference Smith1988). In the midsouthern region, tetflupyrolimet will be marketed as a copack with clomazone (R. Edmund, FMC Corporation, personal communication, 2023), which would increase the spectrum of grass control and introduce an effective and novel SOA into weed control programs. Tetflupyrolimet did effectively control Amazon sprangletop when present in plots, but the density was not sufficient to evaluate in multiple site-years; therefore the data are not presented.
In the imidazolinone- and quizalofop-resistant paddy rice systems, each evaluated herbicide program offered highly effective season-long control of barnyardgrass regardless of whether tetflupyrolimet was included (Tables 3 and 4). Neither technology had an advantage when utilizing a two- or three-pass (three independent herbicide applications) system, although the latter approach would be recommended to mitigate the evolution of herbicide resistance (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). It is important to note that a two-pass system was used to determine if programs that included tetflupyrolimet could be comparable to three separate herbicide applications without the herbicide. Still, all treatments provided exceptional barnyardgrass control (99% control at 56 DA the preflood treatment for imidazolinone- and quizalofop-resistant systems). Additionally, clomazone served as the PRE foundation in each herbicide program and is still considered a highly effective residual herbicide for barnyardgrass, despite cases of confirmed resistance to the chemical in the Midsouth (Heap Reference Heap2024). Other research has demonstrated that clomazone can control barnyardgrass resistant to HRAC/WSSA Groups 2, 4, and 5, so it is not surprising that all herbicide programs in these experiments were successful (Wilson et al. Reference Wilson, Norsworthy, Scott and Gbur2014).
Rice injury and barnyardgrass control of tetflupyrolimet applied with other herbicides preemergence and at 2- to 3-leaf-, 3- to 4-leaf-, and preflood-stage rice in an imidazolinone-resistant paddy rice system averaged across the silt loam and clay soil locations and across 2021 and 2022.a,b
a Abbreviations: DA, days after; ECHCG, barnyardgrass.
b Silt loam location, Rice Research and Extension Center near Stuttgart, AR; clay location, Northeast Research and Extension Center near Keiser, AR.
c Silt loam rate, clay rate.
Rice injury and barnyardgrass control of tetflupyrolimet applied with other herbicides preemergence and at 2- to 3-leaf-, 3- to 4-leaf-, and preflood-stage rice in a quizalofop-resistant paddy rice system averaged across the silt loam and clay soil locations and across 2021 and 2022.a,b,c
a Means within a column and crop followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
b Abbreviations: DA, days after; ECHCG, barnyardgrass.
c Silt loam location, Rice Research and Extension Center near Stuttgart, AR; clay location, Northeast Research and Extension Center near Keiser, AR.
d Silt loam rate, clay rate.
Traditional paddy rice is predominate in much of the midsouthern U.S. rice-growing region, but furrow-irrigated production systems have become increasingly popular to simplify crop rotation and various management strategies (Hardke Reference Hardke2022). As of 2022, FIR accounts for approximately 18% of rice hectares in Arkansas. It is important to ensure that tetflupyrolimet-containing herbicide programs maintain consistent efficacy in the presence of aerobic and anaerobic conditions that exist at the same time in FIR systems. Weed management can be especially challenging in FIR due to an extended period of emergence and regrowth of escapes (Norsworthy et al. Reference Norsworthy, Griffith and Scott2008). Visible barnyardgrass control averaged 99% at the silt loam and clay locations over the 2021 and 2022 site-years (Tables 5 and 6). Visible injury was not compared between the paddy rice and FIR systems, but the magnitude of early-season damage to rice appeared to be more extensive in FIR at the silt loam location than in other sites based on visual observations. Rice on top of beds exhibited more vigor, but plants in furrows were more prone to bleaching and necrosis, potentially resulting from standing water in the furrow. Rainfall or irrigation can reactivate clomazone and elicit crop symptomology (Anonymous 2021). At the 3-leaf application, the maximum observed visible injury was 9%; injury caused by clomazone was transient. FIR at the clay soil location displayed minimal early- or mid-season injury (at 7% and 6%, respectively).
Rice injury and barnyardgrass control of tetflupyrolimet applied with other herbicides preemergence and on 3-leaf and tillering rice in a conventional furrow-irrigated rice system averaged across 2021 and 2022 at the silt loam location.a,b
a Abbreviations: DA, days after; ECHCG, barnyardgrass.
b Silt loam location, Pine Tree Research and Extension Center near Colt, AR.
Rice injury and barnyardgrass control of tetflupyrolimet applied with other herbicides preemergence and on 3-leaf and tillering rice in a conventional furrow-irrigated rice system averaged across 2021 and 2022 at the clay location.a,b
a Abbreviations: DA, days after; ECHCG, barnyardgrass.
b Silt loam location, Pine Tree Research and Extension Center near Colt, AR.
Rice Response to PRE- and POST-Applied Tetflupyrolimet
None of the rice cultivars displayed any symptoms associated with a single PRE or POST application of tetflupyrolimet at 200 or 400 g ai ha−1, and therefore visible injury data were not subject to statistical analysis (Table 8). In addition to the lack of visible symptomology from a tetflupyrolimet application, none of the evaluated parameters (excluding percent canopy growth), such as rice stand, relative maturity, and relative grain yield, were reduced by the herbicide (Table 7). The percent canopy growth was different than the nontreated control in only two instances. One of those instances involved the imidazolinone-resistant, inbred, long-grain cultivar ‘CLL16’, where percent canopy growth was greater than the nontreated control by 4 percentage points at 7 WAP in the 2022 site-year. Percent canopy growth was reduced by 7 percentage points at 12 WAP in the 2021 site-year for ‘PVL02’ (quizalofop-resistant, inbred, long-grain cultivar) but was comparable to the nontreated control for all other parameters.
Rice cultivars selected to determine respective response to a single preemergence or postemergence application of tetflupyrolimet at a 200 or 400 g ai ha−1 rate for the 2021, 2022, and 2023 site-years.a
a The ‘Lynx’ cultivar was not available for the 2023 site-year.
Rice stand counts, percent canopy growth, rice maturity, and relative grain yield after a preemergence or postemergence application of tetflupyrolimet, averaged in 2021, 2022, and 2023, at the Pine Tree Research Station near Colt, AR.a,b,c,d,e,f
a All data, excluding canopy growth, were averaged over site-year.
b An asterisk denotes significance from the nontreated control using Dunnett’s procedure (α = 0.05). For rice canopy growth, any difference observed is only within the respective site-year.
c Abbreviation: DAPRE, days after PRE application.
d Percent canopy growth was collected at 12, 7, and 13 wk after PRE treatment in 2021, 2022, and 2023, respectively.
e Maturity was measured in days relative to the nontreated control when 50% of rice in each plot exhibited panicles.
f Grain yield was not collected for ‘Lynx’ and ‘PVL02’ cultivars in the 2023 site-year; data were averaged over 2021 and 2022.
g Grain yield of the nontreated control is presented in parentheses in kg ha−1.
Rice cultivars are known to respond differently to some herbicides, such as benzobicyclon and florpyrauxifen-benzyl, requiring thorough testing before commercialization of new herbicides. In the case of benzobicyclon, tolerance was conferred based on the presence of a functioning HIS1 gene, and the level of expression was often dictated by the rice growth stage (Brabham et al. Reference Brabham, Norsworthy, Sha, Varanasi and Gonzalez-Torralva2022). Similar studies were conducted prior to the commercialization of benzobicyclon, where tropical japonica cultivars maintained excellent crop safety, while two indica cultivars (‘Purple Marker’ and ‘Rondo’) expressed severe phytotoxicity from the absence of a functioning HIS1 gene (Kato et al. Reference Kato, Maeda, Sunohara, Ando, Oshima, Kawata, Yoshida, Hirose, Kawagishi, Taniguchi, Murata, Maeda, Yamada, Sekino and Yamakazi2015; Maeda et al. Reference Maeda, Murata, Sakuma, Takei, Yamazaki, Karim, Kawata, Hirose, Kawagishi-Kobayashi, Taniguchi, Suzuki, Sekino, Oshima, Kato, Yoshida and Tozawa2019; Young et al. Reference Young, Norsworthy, Scott and Barber2017). It would not be surprising if there were differential tolerance between the two subspecies of rice, but the research reported by Selby et al. (Reference Selby, Satterfield, Puri, Stevenson, Travis, Campbell, Taggi, Hughes and Bereznak2023) indicates that tolerance to tetflupyrolimet is conferred in each subspecies. In additional support of these data collected from 12 midsouthern rice cultivars, a high level of tolerance with no impact to grain yield was also confirmed in six rice cultivars common to California rice production with different genetic backgrounds (one short grain, four medium grain, and one long grain) (Lombardi and Al-Khatib Reference Lombardi and Al-Khatib2024). Considering that no other differences were found among all other evaluated parameters for each cultivar, it is concluded that the evaluated midsouthern rice cultivars have a high degree of tolerance to tetflupyrolimet, similarly to those documented by Selby et al. (Reference Selby, Satterfield, Puri, Stevenson, Travis, Campbell, Taggi, Hughes and Bereznak2023).
Following the commercial launch of florpyrauxifen-benzyl, it was determined that some cultivars differed in sensitivity to the herbicide based on grain size and genotype (inbred and hybrid) (Anonymous 2023; Wright et al. Reference Wright, Norsworthy, Roberts, Scott, Hardke and Gbur2021), although the mechanism responsible is not well understood. For this reason, the authors selected rice cultivars that encompassed a variety of differing genotypes for evaluation. Dihydroorotate dehydrogenase present in rice appears to confer broad tolerance to tetflupyrolimet, suggesting that the evaluated cultivars may carry a similar form of the enzyme or are able to effectively metabolize the herbicide.
Practical Implications
Tetflupyrolimet will provide rice producers with an alternative soil-applied herbicide SOA for control of barnyardgrass populations in the midsouthern United States. Results from these experiments have demonstrated the overall effectiveness and versatility of tetflupyrolimet as a soil-applied herbicide on silt loam and clay soils for the management of barnyardgrass in conventional, imidazolinone-resistant, and quizalofop-resistant rice production systems, which also include FIR. There is minimal injury from an individual PRE or POST application of tetflupyrolimet to the rice cultivars evaluated in a paddy rice system across 3 site-years. Visible injury to rice was observed only when tetflupyrolimet was mixed with other herbicides known to cause injury, such as clomazone, imidazolinone herbicides, penoxsulam, quinclorac, or quizalofop. Mixing clomazone and tetflupyrolimet will provide two effective SOAs to manage barnyardgrass and mitigate selection pressure placed on the already limited POST grass herbicides.
Acknowledgments
The staff at the Rice Research and Extension Center and the Northeast Research and Extension Center are appreciated for their assistance with plot establishment and maintenance.
Funding
FMC Corporation provided funding for this research.
Competing interests
MCC is currently an employee of FMC Corporation.
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