Control of Echinochloa spp. and Leptochloa fascicularis with the novel dihydroorotate dehydrogenase inhibitor herbicide tetflupyrolimet in California water-seeded rice

Abstract

The spread of herbicide-resistant weeds is considered a major problem for rice production in California, and there is a need for new herbicides. Tetflupyrolimet is a new herbicide with a novel dihydroorotate dehydrogenase–inhibiting site of action that has strong activity on grasses. Three field studies were conducted at the California Rice Experiment Station in Biggs, CA, in 2022 and 2023 to (1) determine control of watergrass species and bearded sprangletop with tetflupyrolimet, (2) characterize the effects of tetflupyrolimet combined with other herbicides on weed control and rice, and (3) determine the response of rice cultivars to tetflupyrolimet. In the first study, tetflupyrolimet was applied at preemergence (PRE) or at the 1- to 2-leaf stage of rice (POST) at 0.1, 0.125, or 0.15 kg ai ha⁻¹ followed by carfentrazone. Tetflupyrolimet provided ≥99% control of watergrass species and 100% bearded sprangletop control regardless of the rate or application timing, while showing no crop injury symptoms or yield reduction. In the second study, tetflupyrolimet was applied PRE or POST at 0.1 or 0.15 kg ai ha⁻¹ followed by herbicides labeled for use in California rice production. Tetflupyrolimet provided ≥98% control of watergrass species, which was better than the grower standard treatment, and ≥97% control of bearded sprangletop. In the third study, tetflupyrolimet was applied PRE or POST at 0.125, 0.15, 0.25, or 0.3 kg ai ha⁻¹ followed by carfentrazone. The six California rice cultivars evaluated, ‘M-105’, ‘M-206’, ‘M-209’, ‘M-211’, ‘L-208’, and ‘CM-203’, did not show any trend of crop injury caused by tetflupyrolimet. Overall, tetflupyrolimet provided a high level of control of watergrass species and bearded sprangletop without causing visual rice injury or yield reductions, regardless of rice cultivar, when applied alone or in combination with commonly used sedge and broadleaf herbicides in California water-seeded rice.

Introduction

The Sacramento Valley of California has nearly 200,000 ha of rice with a farm gate value of close to US$1 billion (CDFA 2021). Rice grown in California comprises mainly medium-grain cultivars. In the water-seeded rice production system, pregerminated rice seeds are aerially broadcast onto fields continuously flooded with 5 to 10 cm of water, depending on specific management strategies. More than half of the soils in California’s rice-producing region have impeded drainage with a typical infiltration rate of 1 to 5 mm d⁻¹; these fields are poorly suited to most upland crops and often are continuously planted with rice (Hill et al. 2006). These soil conditions allow growers to continuously flood their fields for suppression of highly competitive grass weeds, which also contributes to this system’s high productivity (Hill et al. 2006; Strand 2013).

The semiaquatic conditions in California rice fields have led to well-adapted weed populations of grasses [barnyardgrass, Echinochloa crus-galli (L.) P. Beauv.; early watergrass, Echinochloa oryzoides (Ard.) Fritsch; late watergrass, Echinochloa phyllopogon (Stapf) Koso-Pol.; and bearded sprangletop], sedges [ricefield bulrush, Schoenoplectus mucronatus (L.) Palla, and smallflower umbrellasedge, Cyperus difformis L.], and broadleaf weeds [ducksalad, Heteranthera limosa (Sw.) Willd., and redstem, Ammannia spp.] (Brim-DeForest et al. 2017b; Ceseski et al. 2022). The aforementioned grass weed species, if unmanaged, have especially been found to compete heavily with rice, causing a significant decrease in yields (Brim-DeForest et al. 2017a; Gibson et al. 2002; Kanter et al. 2021; Oerke 2006; Smith 1983; Stauber et al. 1991).

California’s unique crop diversity paired with strict regulatory structures has limited the number of herbicide active ingredients available to rice growers because of the potential for herbicide drift to nearby orchards as well as heightened regulations regarding environmental toxicology (Hill et al. 2006; Prather et al. 2000). Among a total of 14 active ingredients registered for use in rice in California, 6 are within the same site of action (SOA) Group 2, or acetolactate synthase–inhibiting herbicides (Espino et al. 2019). Owing to limited residual activity of these products and a narrow weed control spectrum, common weed control strategies include applying multiple herbicides to achieve effective control of grass, sedge, and broadleaf weeds in rice fields. The decades-long reliance on limited herbicidal chemistries in continuous rice has selected for weed populations that are resistant to these herbicides (Becerra-Alvarez et al. 2022; Brim-DeForest et al. 2022). Multiple cases of herbicide resistance have been detected in all aforementioned grass and sedge weed species and redstem, to various active ingredients throughout California rice fields (Abdallah et al. 2014; Becerra-Alvarez et al. 2023; Fischer et al. 2000; Hanson et al. 2014; Heap 2014; Valverde et al. 2014; Yasuor et al. 2008, 2009). The drastic rise of herbicide resistance in rice weeds has proven to be a major problem for rice growers by increasing the cost of herbicide treatment programs.

A decades-long lapse in development of novel SOAs due to the increased cost of development of new active ingredients, increased toxicological and environmental regulations, and the consolidation of the agrichemical industry to a few dominating companies has left growers overusing the same active ingredients and modes of action, therefore exacerbating the effects of herbicide resistance in agriculture (Davis and Frisvold 2017; Dayan 2019; Duke 2012). Herbicide discovery, however, has since been revived and has led to the introduction of a few new chemicals, including tetflupyrolimet, a novel dihydroorotate dehydrogenase (DHODH) inhibitor that has herbicidal activity on grasses (Dayan 2019; Satterfield et al. 2014). Tetflupyrolimet was discovered in 2014 through high-volume sourced greenhouse screening (Gaines et al. 2021; Selby et al. 2023).

Tetflupyrolimet is in the aryl pyrrolinone anilide chemical family and disrupts the de novo pyrimidine nucleotide biosynthesis pathway at the DHODH step, which is the only redox reaction of the pathway, causing both an overaccumulation of dihydroorotate (DHO) and a deficiency of uridine-5′-monophosphate (Björnberg et al. 1997; Dayan 2019; Nagy et al. 1992). The de novo pyrimidine nucleotide biosynthesis pathway is an essential process for metabolism; gene expression; and the production of substrates for DNA, RNA, and multiple biosynthesis pathways (Santoso and Thornburg 1998; Zrenner et al. 2006). There are six enzymatic steps in the pathway, and some organisms have significantly different enzymes for these steps (Doremus 1986; Nara et al. 2000; Santoso and Thornburg 1998). Most of pyrimidine biosynthesis in plants occurs in the chloroplast; however, DHODH is localized in the outer surface of the inner mitochondrial membrane (Chen and Jones 1976; Doremus and Jagendorf 1985; Kafer and Thornburg 1999; Miersch et al. 1986). The few peer-reviewed articles that mention tetflupyrolimet’s herbicidal activity note that the compound provides excellent grass control and safety on rice (Dayan 2019; Selby et al. 2023). For example, tetflupyrolimet’s activity on foxtail was reported as 10-fold greater than on rice (Dayan 2019). This may suggest that rice can metabolize tetflupyrolimet, which may result in a high level of resistance in Oryza spp.

Tetflupyrolimet was designed as a preemergence (PRE) and early postemergence (POST) granular herbicide in rice; however, weed control and crop response to tetflupyrolimet are not well studied. The objectives of this research were to (1) determine the control of watergrass species and bearded sprangletop with various rates of tetflupyrolimet applied at the PRE or POST timing, (2) characterize the effects of tetflupyrolimet on weed control and rice when applied in combination with other rice herbicides, and (3) determine the response of rice cultivars to tetflupyrolimet in a water-seeded rice system.

Materials and Methods

Site Conditions and Preparation

Three field experiments were conducted during the 2022 and 2023 growing seasons at two sites at the California Rice Experiment Station near Biggs, CA. Site 1 (39.46°N, 121.74°W) was located on the west end of the station property, and Site 2 (39.45°N, 121.72°W) was on the station’s east end. Soils at both sites are classified as Esquon-Neerdobe (fine, smectitic, thermic Xeric Epiaquerts and Duraquerts) clay. The soil at Site 1 had a pH of 5.2 and 1.9% organic matter, while soil at Site 2 had a pH of 5.1 and 1.9% organic matter. The average minimum and maximum daily temperatures in Biggs, CA, during the growing season (May to October) in 2022 were 16.6 C and 34.0 C and in 2023 were 13.0 C and 30.3 C, respectively. The sites’ weed seedbank has been described in Brim-DeForest et al. (2017a, 2017b) and contains watergrass species, bearded sprangletop, ricefield bulrush, smallflower umbrellasedge, ducksalad, and redstem.

Field preparation in both years began with a pass of a single-offset stubble disk once the winter flood was drained and the soil was dry enough to allow for equipment to pass. Prior to planting, field operations consisted of one pass with a chisel plow and two passes with a single-offset disk, followed by a land plane to smooth the soil surface. Site 1 was fertilized with 169 kg N ha⁻¹ as aqua-ammonia (20-0-0), and Site 2 was fertilized with 114 kg N ha⁻¹ as 34-17-0 in both years. Prior to flooding the field in the spring, a corrugated roller was used to pack the soil and eliminate large clods on the soil surface. Plots were 3 × 6 m and surrounded by small levees pulled by a ridger to prevent herbicide cross-contamination to other plots. Standard agronomic and pest management practices were followed based on the University of California rice production guidelines (UCANR 2023).

Plant Material

Rice seeds were soaked in water for 24 h, allowing for pregermination, then the water was drained. Depending on the study, seeds were either evenly broadcast by hand or aerially broadcast at a rate of 168 kg ha⁻¹ into a 10-cm flooded field. Rice cultivar ‘M-209’ was used in the watergrass species and bearded sprangletop control study (Study 1) as well as in the evaluation of tetflupyrolimet’s interaction with common herbicides study (Study 2). The rice varietal response to tetflupyrolimet study (Study 3) included one short-, four medium-, and one long-grain cultivar with two different maturity timings to cover an array of genetic backgrounds that are grown in California. Rice grain was harvested from each 18-m² plot with a specialized small plot combine with a swath width of 2.3 m (Almaco, Nevada, IA, USA). Rice grain yield for all experiments was adjusted to 14% moisture.

Herbicide Application

Herbicide applications were timed according to the University of California rice production guidelines (UCANR 2023). Granular herbicides were evenly broadcast by hand. Foliar herbicides were applied with a CO₂-pressurized 2-m boom equipped with six 8002VS flat-fan nozzles (TeeJet® Technologies, Springfield, IL, USA) calibrated to deliver 187 L ha⁻¹ at 193 kPa. For POST combination treatments, the spray mixture included 1.25% v/v crop oil concentrate (Agri-Dex®, Helena Agri-Enterprises, Collierville, TN, USA). Treatments including propanil or triclopyr required the 10-cm flood to be reduced to broaden the coverage of weeds below the water’s surface; therefore these plots were drained prior to the application, then reflooded to 10 cm 48 h after application.

Study 1: Watergrass Species and Bearded Sprangletop Control Study

Study 1 aimed to determine the control of barnyardgrass, early watergrass, late watergrass, and bearded sprangletop with various rates of tetflupyrolimet applied at the PRE or POST timing. Study 1 was a single-factor randomized complete block design with three replicates conducted twice in 2023 at the aforementioned sites. Before flooding, 1,000 seeds of bearded sprangletop were evenly broadcast by hand and raked into each plot to increase the population of this target weed species. Plots were seeded with rice cultivar ‘M-209’ by hand on May 20, 2023, for Site 1, and seed was aerially broadcast on June 2, 2023, for Site 2.

Tetflupyrolimet (FMC, Philadelphia, PA, USA) was applied as a 1% by weight granular formulation of the herbicide at PRE and POST and three rates followed by (fb) 0.53 kg ha⁻¹ carfentrazone (SHARK® H2O, FMC) applied as a granular at 2- to 3-leaf annual grass to control sedge and broadleaf weeds (Table 1). The same formulation of tetflupyrolimet was used in all three experiments. In addition, clomazone (CERANO® 5 MEG, Wilbur-Ellis Company, Fresno, CA, USA) and penoxsulam (Granite® GR, Corteva Agriscience, Indianapolis, IN, USA) were included as grower standard treatments for comparison. A treatment of carfentrazone alone at the same rate and timing as mentioned previously was added to compare a plot with grass weeds only, because this herbicide does not have control grasses.

Table 1. Herbicides, rates, and application timings for Study 1, at two sites near Biggs, CA, in 2023. ^a

^a Abbreviation: DOS, day of seeding.

Visual weed control ratings were conducted for watergrass species and bearded sprangletop at 14, 28, 42, and 56 days after PRE or POST tetflupyrolimet treatment (DAT) on a percent control scale ranging from 0 (no control) to 100 (complete control or no weeds present). Barnyardgrass, early watergrass, and late watergrass control ratings were grouped together as watergrass species because they are hard to differentiate at early growth stages (Fischer et al. 2000). Visual rice phytotoxicity ratings were conducted at 7, 14, and 28 DAT for general, chlorosis, bleaching, stunting, stand reduction, and necrosis on a scale ranging from 0 (no injury) to 100 (plant kill) as compared to the control plots (Frans et al. 1986). Rice plant counts were conducted at each site at the 4-leaf stage of rice (LSR) in the treated plots. Plant counts were measured in every plot by randomly laying out four 645-cm² quadrats. Rice plant counts were averaged between the four subplot counts, and density was converted to plants per square meter. The control plots were measured in the same manner for weed counts of watergrass species and bearded sprangletop. Weed counts were averaged at 7, 14, and 28 DAT for every control plot to get density in number of weeds per square meter. Additionally, the control plots were visually estimated for percent weed coverage for watergrass species, bearded sprangletop, ricefield bulrush, smallflower umbrellasedge, ducksalad, and redstem on a percent coverage scale of 0 (no weeds present) to 100 (complete plot coverage).

Study 2: Evaluation of Tetflupyrolimet’s Interaction with Common Herbicides Study

The objective of Study 2 was to characterize the effects of tetflupyrolimet on weed control and rice when applied in combination with other commonly used sedge and broadleaf rice herbicides. Study 2 was a single-factor randomized complete block design with three replicates conducted at Site 2 in 2022 and 2023. Seeds of rice cultivar ‘M-209’ were aerially broadcast on May 23, 2022, and June 1, 2023. Tetflupyrolimet was applied at two timings and two rates (Table 2).

Table 2. Herbicides, rates, and application timings for Study 2, near Biggs, CA, in 2022 and 2023. ^a

^a Abbreviation: DOS, day of seeding.

Tetflupyrolimet was applied in combination with or followed by recommended rates of carfentrazone, clomazone, thiobencarb (Bolero® UltraMax, Valent USA, San Ramon, CA, USA), propanil (SUPERWHAM!® CA, UPL NA, King of Prussia, PA, USA), triclopyr (Grandstand® CA, Corteva Agriscience), bensulfuron (LONDAX®, UPL NA), and benzobicyclon plus halosulfuron (BUTTE®, Gowan Company, Yuma, AZ, USA) at their respective recommended application timings (Table 2). These treatments were compared to a grower standard of benzobicyclon plus halosulfuron fb propanil fb a mixture of propanil and triclopyr as well as a nontreated control.

Visual ratings for weed control were conducted in the same way as in the previous study but included the weed species ricefield bulrush, smallflower umbrellasedge, ducksalad, and redstem. Visual ratings of rice response as well as rice plant counts and weed quantification were conducted in the same manner described in the previous study.

Study 3: Rice Varietal Response to Tetflupyrolimet Study

Study 3 was a two-factor split-plot design, where the cultivar was the main plot and the herbicide treatments were the subplots, with three replicates conducted in 2022 and 2023 at Site 2. Seeding dates were May 27, 2022, and May 31, 2023. Seeds of rice cultivars ‘M-105’, ‘M-206’, ‘M-209’, ‘M-211’, ‘L-208’, and ‘CM-203’ were broadcast by hand.

Tetflupyrolimet was applied as a granular formulation of 1% at two timings and four rates fb carfentrazone applied at 0.53 kg ha⁻¹ at 2 to 3 LSR to control sedge and broadleaf weeds (Table 3). In addition, a standard herbicide treatment of benzobicyclon plus halosulfuron fb propanil and triclopyr was included for a weed-free comparison along with a treatment of carfentrazone alone at the same rate and timing as in Study 1. This stand-alone carfentrazone treatment provided good suppression of broadleafs and sedges and was used in comparison to the grower standard treatment to assess the impact of grass weeds on yield. Visual ratings of rice injury, rice plant counts, and weed quantification were measured in the same manner as described in the previous studies.

Table 3. Herbicides, rates, and application timings for Study 3, near Biggs, CA, in 2022 and 2023. ^a

^a Abbreviation: DOS, day of seeding.

Statistical Analysis

The data for the three experiments were tested for homogeneity of variance and analyzed using analysis of variance (ANOVA) and linear regression in R (R Core Team 2022). Means separation was performed using Tukey–Kramer’s honestly significant difference at 95% significance level. Linear models were fit with the lme4 (Bates et al. 2015) and lmerTest (Kuznetsova et al. 2017) packages. Marginal means were estimated with the emmeans (Lenth 2023) package, and the multcomp (Hothorn et al. 2008) package was used to generate multiple comparisons among means. Control plots were excluded from the weed control and rice injury ANOVA because all values were 0. Study 1 had herbicide treatment, site, and time of rating used as fixed effects and block used as a random effect. Study 2 had herbicide treatment, year, and time of rating used as fixed effects and block used as a random effect. Study 3 had herbicide treatment, cultivar, treatment by cultivar, year, and time of rating used as fixed effects and block used as a random effect.

Results and Discussion

Study 1: Watergrass Species and Bearded Sprangletop Control Study

The weed control data showed no significant interaction between sites. The rice response data, however, showed significant treatment by site interaction for bleaching symptoms but not for chlorosis, stunting, stand reduction, necrosis, and yield. Therefore the data for bleaching were analyzed separately by site, while weed control data and all other injury symptoms and yield for both sites were combined.

Weed population composition varied at the two sites for this experiment. Site 1 had control plots dominated by sedges and broadleaf weeds, averaging 73% coverage, with watergrass species abundance averaging 2% and bearded sprangletop abundance averaging 5% based on the treated control plots (data not shown). The control plots at Site 2 had no sedge or broadleaf weeds and were dominated by watergrass species and bearded sprangletop, averaging 32% and 15%, respectively (data not shown). Weed populations could differ at these two sites because of differences in management practices, flooding times, and planting dates.

All treatments, including the grower standard, across both sites provided complete season-long watergrass species control (data not shown). The grower standard treatment had 100% control of bearded sprangletop early in the season, then decreased to 99% control by the end of the rice heading stage. Every treatment that included tetflupyrolimet had a season-long complete control of bearded sprangletop. All treatments were similar for control of bearded sprangletop, and no PRE herbicide applications showed escapes. Other experiments have made observations of bearded sprangletop escapes from PRE herbicide application due to the delayed emergence of some populations of this species, which suggests that tetflupyrolimet has longer-lasting control than other graminicides (Driver et al. 2020). Tetflupyrolimet’s season-long control of these problem grass weed species, at both PRE and POST timings, may be useful for growers who want to rotate a new SOA into their herbicide programs. Tetflupyrolimet provides an opportunity to eliminate these problem grass weeds that have resistance as outlined in a field survey of California rice weeds by Becerra-Alvarez et al. (2022, 2023). A. Becerra-Alvarez (unpublished data) also tested tetflupyrolimet against all submitted grass samples, finding complete control in all samples.

Rice injury symptoms observed were bleaching, chlorosis, stunting, and stand reduction across both sites. At both sites, moderate early bleaching was observed in the grower standard treatments (clomazone fb penoxsulam) and then subsided later in the season (Table 4). The injury is not surprising due to the extensive characterization of rice’s response to the bleaching herbicide clomazone (Jordan et al. 1998). The grower standard treatment also showed moderate stunting at 28 DAT due to the early POST application of penoxsulam, which is known to cause stunting and chlorosis in rice (Bond et al. 2007). PRE and POST tetflupyrolimet treatments did not show any bleaching throughout the season. Chlorosis, stunting, and stand reduction symptoms in all tetflupyrolimet treatments averaged across both sites were overall minimum, ranging from 0% to 8% by 28 DAT (Table 5). All symptoms observed were slight, and the rice was fully recovered by 42 DAT (data not shown). There was no significant interaction between treatments for any of these symptoms and no trend of increased symptoms with increased rate or varied application timing. The work done by Selby et al. (2023) supports the observation that tetflupyrolimet shows no significant chlorosis or necrosis in rice. These slight symptoms were not surprising and were possibly due to the cooler nights recorded around Biggs, CA, in 2023, averaging 14 C during the germination and early vegetative stages (Ceseski et al. 2022; Das 2015).

Table 4. Average rice bleaching at 14 and 28 d after treatment for Sites 1 and 2 for Study 1, near Biggs, CA, in 2023. ^a , ^b

^a Within columns, means accompanied by the same letter do not significantly differ according to Tukey’s honestly significant difference at α = 0.05.

^b Abbreviations: DAT, days after treatment; fb, followed by.

Table 5. Average rice chlorosis, stunting, stand reduction, and necrosis at 14 and 28 d after treatment for Study 1, at two sites near Biggs, CA, in 2023. ^a , ^b

^a Within columns, means accompanied by the same letter do not significantly differ according to Tukey’s honestly significant difference at α = 0.05.

^b Abbreviations: DAT, days after treatment; fb, followed by.

Yield for the grower standard treatment averaged 5,390 kg ha⁻¹, while yields of tetflupyrolimet treatments were not significantly different and ranged between 5,770 and 6,300 kg ha⁻¹. The similarities between yield data indicate that the slight early injury observed in tetflupyrolimet treatments did not affect yield. As discussed previously, the 2023 growing season experienced colder than usual temperatures, which could be a contributing factor of the lower yield measured in this study (Delerce et al. 2016). Although temperature alone could not account for this experiment’s low yields, a late planting date and rice lodging after heading also may have contributed to the low yields (Lang et al. 2012; UCANR 2023).

Tetflupyrolimet provided excellent season-long control of watergrass species and bearded sprangletop regardless of application timing. There was no trend of late-season escapes of bearded sprangletop in tetflupyrolimet treatments, unlike for other bearded sprangletop control herbicides, such as clomazone and thiobencarb.

Study 2: Evaluation of Tetflupyrolimet’s Interaction with Common Herbicides Study

There was significant Treatment × Year interaction for weed control, so the data were analyzed separately by year for watergrass species, bearded sprangletop, ricefield bulrush, smallflower umbrellasedge, ducksalad, and redstem. There was also significant Treatment × Year interaction for yield, while no significant interaction was detected for rice chlorosis, bleaching, stunting, stand reduction, and necrosis. Therefore all weed control and yield data were analyzed separately by year, while all rice injury symptom data were combined.

Weed population composition varied each year: in 2022, there was a lower density of sedge and broadleaves than in 2023; however, the dominant species for both years was watergrass species. In 2022, the untreated control plots had an abundance of watergrass species and bearded sprangletop cover, averaging 62% of the plot, while the sedges and broadleafs had an average of 23% relative cover by 4 LSR (data not shown). In 2023, the untreated control plots had a higher abundance of both grasses, averaging 74%, as well as sedges and broadleafs, averaging 45% abundance (data not shown). This shift in weed population composition is not surprising and has been observed before in previous experiments near this site due to differences in soil moisture and temperature throughout the growing season (Becerra-Alvarez et al. 2022; Brim-DeForest et al. 2017a; Lundy et al. 2014).

In 2022, the grower standard treatment showed 94% control of watergrass species throughout the season (Table 6). The grower standard treatment had significantly lower control than all treatments that included tetflupyrolimet, which had 98% to 100% control of watergrass species throughout the season. The grower standard treatment showed complete control of bearded sprangletop at 14 DAT, but this decreased to 99% control by 56 DAT. The PRE tetflupyrolimet treatments showed a similar trend with bearded sprangletop control, shifting from complete control at 14 DAT to decreasing by a few percentage points, ranging from 97% to 100%. However, treatments that included a POST application of tetflupyrolimet combined with a PRE herbicide with herbicidal activity on grasses maintained complete control throughout the season (Table 6).

Table 6. Average weed control at 14 and 56 d after treatment for Study 2 in 2022.^a,b

^a Within columns, means accompanied by the same letter do not significantly differ according to Tukey’s honestly significant difference at α = 0.05.

^b Abbreviations: DAT, days after treatment; fb, followed by.

In 2023, the grower standard treatment maintained complete control of watergrass species throughout the entire season (Table 7). Tetflupyrolimet treatments had complete or nearly complete control of watergrass species by 14 DAT, but by 56 DAT, complete control was observed in all treatments regardless of tetflupyrolimet application timing. The grower standard treatment had near-complete control of bearded sprangletop at 14 DAT, which later increased to complete control by 56 DAT. Treatments including both PRE and POST applications of tetflupyrolimet had a season-long complete control of bearded sprangletop.

Table 7. Average weed control at 14 and 56 d after treatment for Study 2 in 2023. ^a , ^b

^a Within columns, means accompanied by the same letter do not significantly differ according to Tukey’s honestly significant difference at α = 0.05.

^b Abbreviations: DAT, days after treatment; fb, followed by.

Excellent grass control by tetflupyrolimet is consistent even when combined with other herbicides with or without activity on grasses. Mixing tetflupyrolimet with graminicides does not appear to adversely affect POST graminicides, unlike the combination of other rice herbicides, such as propanil mixed with acetyl-coenzyme A carboxylase–inhibiting herbicide, reported by de Oliveira Matzenbacher et al. (2015). Because tetflupyrolimet has activity on grasses with little activity on sedge or broadleaf weeds, a combination of tetflupyrolimet with other herbicides that control sedge and broadleaf weeds is required for complete control of the weed species that are common in California rice fields.

In 2022, the grower standard treatment showed season-long complete control of ricefield bulrush (Table 6). All other treatments showed complete control of ricefield bulrush at 14 DAT, which then decreased by 56 DAT and ranged from 88% to 97% control in treatments that did not include benzobicyclon plus halosulfuron, which is an herbicide treatment with known activity on ricefield bulrush (Espino et al. 2019). The control of smallflower umbrellasedge was similar to results of ricefield bulrush control, where complete control was obtained by the grower standard and all other treatments at 14 DAT, followed by a minimal decrease in control by 56 DAT for tetflupyrolimet fb carfentrazone, tetflupyrolimet fb bensulfuron fb propanil, tetflupyrolimet fb triclopyr plus propanil, and clomazone fb tetflupyrolimet fb propanil. The only significantly different treatment for smallflower umbrellasedge control was tetflupyrolimet fb carfentrazone, which still had 72% control of smallflower umbrellasedge. The lessened control of smallflower umbrellasedge for this treatment is unlikely to be from herbicide resistance because previous greenhouse trials reported that this species did not show resistance to carfentrazone (Becerra-Alvarez et al. 2023). Complete season-long control of ducksalad was achieved by the grower standard treatment as well as all other treatments besides tetflupyrolimet fb carfentrazone, which showed 90% control at 56 DAT. Control of redstem by the grower standard treatment shifted from complete control at 14 DAT to near-complete control by 56 DAT. All other treatments not including benzobicyclon plus halosulfuron had complete season-long control of redstem. PRE tetflupyrolimet followed by POST benzobicyclon plus halosulfuron had moderate control of redstem at 82%, while PRE benzobicyclon plus halosulfuron fb POST tetflupyrolimet had minimal control of redstem at 33%. Redstem is known to emerge later in the season, missing the PRE applications (Brim-DeForest et al. 2017b; Espino et al. 2019). The delayed emergence explains why greater control is achieved when benzobicyclon plus halosulfuron is applied POST rather than PRE, because it has slight control of this species, while tetflupyrolimet has no control of redstem. Therefore herbicides like propanil and triclopyr are needed to successfully control redstem.

In 2023, control of ricefield bulrush by the grower standard at 14 DAT was 98% and 100% at 56 DAT (Table 7). There was a wide range of 40% to 100% control of ricefield bulrush for all other treatments. Ricefield bulrush control was 65%, 50%, and 40% with tetflupyrolimet fb triclopyr and propanil, clomazone fb tetflupyrolimet fb propanil, and tetflupyrolimet fb benzobicyclon plus halosulfuron, respectively. The low ricefield bulrush control by these treatments was largely because none of the PRE herbicides in these treatments have good activity on sedges (Espino et al. 2019). Furthermore, the 14 DAT rating was only 2 to 3 d after the follow-up herbicide application, which was not enough time to completely control ricefield bulrush. By 56 DAT, near-complete or complete control was observed in all treatments, except tetflupyrolimet fb carfentrazone, which gave 77% control.

In 2023, smallflower umbrellasedge control by the grower standard increased from 96% control at 14 DAT to complete control by 56 DAT. All other treatments showed either near-complete or complete control of smallflower umbrellasedge throughout the season. The grower standard showed complete control of ducksalad throughout the entire season. In all the other treatments, there was complete control of ducksalad at 14 DAT, except for tetflupyrolimet fb triclopyr (43%) and clomazone fb tetflupyrolimet fb propanil (50%). This varying control is once again because of the application timings, where the PRE herbicides do not have activity on ducksalad but, by 56 DAT, an application of propanil or triclopyr has been made. Near-complete control was shown at 56 DAT in all treatments, excluding tetflupyrolimet fb carfentrazone, which was significantly lower from most other treatments at 83%. The grower standard treatment showed no control of redstem at 14 DAT due to the weak activity of benzobicyclon plus halosulfuron on redstem (Becerra-Alvarez et al. 2023; Espino et al. 2019); however, by 56 DAT, redstem was completely controlled by the subsequent propanil and triclopyr applications. All other treatments had complete control of redstem at 14 DAT. However, by 56 DAT, control of redstem in treatments that did not include an application of propanil decreased, ranging from 63% to 73%, while the treatments that did include an application of propanil achieved near-complete control (92% to 97%). The control of grass, sedge, and broadleaf weeds by the combination of herbicides used in this experiment was acceptable in both years. There have been a multitude of both herbicide synergism and antagonism cases in rice across the world, such as the synergism of barnyardgrass and red rice (Oryza sativa f. spontanea Roshev.) control when mixing imazethapyr, propanil, and thiobencarb (Fish et al. 2015). This suggests that tetflupyrolimet may be more user-friendly for applicators and growers than other available rice herbicides; however, it is crucial to understand the weed population dynamics in a field when choosing an herbicide program to ensure the effective control of all weed species present.

The grower standard treatment showed slight rice injury symptoms of stunting and stand reduction by 28 DAT, which completely recovered by 42 DAT (data not shown). No tetflupyrolimet treatments showed any evidence of stand reduction at 14 DAT. At 28 DAT, tetflupyrolimet fb thiobencarb fb propanil showed 16% injury compared to the nontreated control. Because of the ability of thiobencarb to reduce shoot growth, the application of thiobencarb could have damaged the root system of the rice, causing the death of some plants that were not completely anchored to the seedbed (Mabbayad and Moody 1992). No chlorosis was observed for any herbicide treatment at 14 DAT; however, at 28 DAT, only three treatments—tetflupyrolimet fb benzobicyclon plus halosulfuron, tetflupyrolimet fb thiobencarb fb propanil, and benzobicyclon plus halosulfuron fb tetflupyrolimet—showed very slight chlorosis. Rice plants, however, completely recovered from chlorosis by 42 DAT. Moderate (42%) bleaching was observed in the clomazone fb tetflupyrolimet fb propanil treatment at 7 DAT but fully recovered by 14 DAT, which was not surprising, because clomazone is known to cause bleaching of rice after its application (Becerra-Alvarez et al. 2022). No bleaching symptoms were observed in any other treatments. No significant stunting was observed at 14 DAT for any treatment. At 28 DAT, tetflupyrolimet fb thiobencarb fb propanil showed slight stunting symptoms of 10% (data not shown). This response was not surprising, because thiobencarb is known to show stunting in rice (Baltazar and Smith 1994). By 42 DAT, however, the stunted rice had completely recovered. No significant or lasting necrosis symptoms were observed in this study.

The average yield for the nontreated control in 2022 was 3,690 kg ha⁻¹, which was significantly lower than all other treatments (Table 8). The grower standard treatment yielded 9,120 kg ha⁻¹, which was not significantly different from any tetflupyrolimet treatment. Of the tetflupyrolimet treatments, the lowest-yielding treatment was clomazone fb tetflupyrolimet fb propanil at 7,740 kg ha⁻¹, and the highest-yielding treatment was tetflupyrolimet fb thiobencarb fb propanil at 9,550 kg ha⁻¹. The reduced yield of the clomazone fb tetflupyrolimet fb propanil treatment could be due to early bleaching resulting from the PRE clomazone application (data not shown). The greater yield from the tetflupyrolimet fb thiobencarb fb propanil treatment could be from superior weed control throughout the season that caused a lower level of weed competition to rice. However, in 2023, the only significant difference detected was that the yield of the nontreated control (2,940 kg ha⁻¹) was significantly lower than for all other treatments (6,650 to 7,950 kg ha⁻¹). The difference of higher yields in 2022 and lower yields in 2023 could possibly be due to the cooler weather during the 2023 season compared to 2022 (Ceseski et al. 2022).

Table 8. Average rice yield in Study 2 in 2022. ^a , ^b

^a Within columns, means accompanied by the same letter do not significantly differ according to Tukey’s honestly significant difference at α = 0.05.

^b Abbreviation: DOS, day of seeding.

Introduction of tetflupyrolimet to the rice cropping system is contingent on its ability to perform well in an herbicide program in which sedge and broadleaf weeds can be controlled as well. Tetflupyrolimet applied in combination with benzobicyclon plus halosulfuron; thiobencarb fb propanil; bensulfuron fb propanil, triclopyr, and propanil; or clomazone fb propanil provided near-perfect season-long weed control. Excellent crop safety was displayed across each experiment, regardless of rate or timing. Tetflupyrolimet gave excellent grass control as both a PRE and POST herbicide incorporated into a weed management program; however, if tetflupyrolimet is applied later than day of seeding, a higher rate is likely needed for the same grass control results.

Study 3: Rice Varietal Response to Tetflupyrolimet Study

There was significant interaction between treatment and year for necrosis and yield, while there were no significant Treatment × Year interactions for bleaching, chlorosis, stunting, and stand reduction symptoms. Therefore necrosis and yield data were analyzed separately by year, and all other rice symptom data were combined.

In general, slight chlorosis symptoms in both years were observed in the grower standard treatment for ‘CM-203’, ‘M-206’, and ‘M-209’ at 14 DAT, but rice plants completely recovered by 28 DAT (data not shown). The grower standard treatment also showed minimal stunting and stand reduction symptoms in all cultivars, which fully recovered from stunting shortly thereafter. No tetflupyrolimet treatments, regardless of application timing or rate, had significant levels of chlorosis, bleaching, stunting, or stand reduction at any rating time.

In 2022, consistent necrosis on the tips of the rice leaves was observed. At 14 DAT, all treatments besides PRE tetflupyrolimet at 0.125 kg ai ha⁻¹ showed minimal necrosis; however, it does not seem to be a trend of a specific cultivar showing necrotic symptoms (Table 9). Importantly, these necrosis symptoms also were observed in treatments that did not include tetflupyrolimet, which suggests that the necrosis symptoms were not related to application of tetflupyrolimet and also reiterates the findings of Selby et al. (2023) that tetflupyrolimet does not cause injury in rice. These minimal symptoms persisted until the end of the rice heading stage, when they fully recovered before harvest. Average yield data for all tetflupyrolimet treatments were similar to the grower standard treatment within every cultivar (data not shown).

Table 9. Rice necrosis at 14 and 28 d after treatment observed in Study 3 in 2022 and 2023. ^a , ^b

^a Within columns, means accompanied by the same letter do not significantly differ according to Tukey’s honestly significant difference at α = 0.05.

^b Abbreviations: DOS, day of seeding; fb, followed by.

^c Short- (‘CM-203’), medium- (‘M-105’, ‘M-206’, ‘M-209’, and ‘M-211’), and long-grain (‘L-208’) cultivars.

In 2023, no necrosis symptoms were observed, which suggests that the necrosis symptoms observed in 2022 were not from tetflupyrolimet but were rather possibly from abiotic factors (Table 8). Furthermore, no necrosis symptoms were observed in any of the other two experiments, regardless of year or site. Genetic variations within species can contribute to differential responses of herbicides; however, no varietal response to tetflupyrolimet was observed in this study. Rice varietal response was reported in California when clomazone and triclopyr were used (Pantone and Baker 1992; Zhang et al. 2004). The yield data in 2023 for all tetflupyrolimet treatments (5,650 to 8,100 kg ha⁻¹) were comparable to the grower standard treatment (6,620 to 8,030 kg ha⁻¹) for every cultivar.

None of the six rice cultivars evaluated—‘M-105’, ‘M-206’, ‘M-209’, ‘M-211’, ‘L-208’, and ‘CM-203’—showed any trend of crop injury caused by tetflupyrolimet. Tetflupyrolimet could be a valuable addition to weed control programs in California water-seeded rice, regardless of cultivar grown.

Practical Implications

Rice growers in California have been battling herbicide-resistant weeds, especially grasses, for decades, and these pose a great threat of yield and grain quality loss. The recently developed novel SOA herbicides, like tetflupyrolimet, can be rotated in existing herbicide programs to help growers more effectively control these problem weeds. The work done in this research determined the effectiveness of tetflupyrolimet on control of watergrass species and bearded sprangletop as well as a high level of crop safety in multiple rice cultivars. The research has also shown that tetflupyrolimet can be inputted into an herbicide program, which could then obtain high levels of weed control for watergrass species, bearded sprangletop, ricefield bulrush, smallflower umbrellasedge, ducksalad, and redstem. Further research can be done to specifically identify synergistic or antagonist effects of tetflupyrolimet when applied with other herbicides to find the most effective herbicide program. Furthermore, tetflupyrolimet may also be a promising weed control tool for other crops besides rice, and this possibility should be investigated.