Introduction
Rice production in the United States is primarily led by Arkansas, which accounts for nearly half of the country’s total rice output, totaling almost five billion kilograms in 2023 (USDA-NASS 2024). Within the state, long- and medium-grain cultivars comprised 86% and 14% of the total rice production in 2023, respectively. In 2023, 54% of the total rice acreage was planted with the long-grain hybrid cultivars RT 7521 FP, RT XP753, RT 7321 FP, and RT 7421 FP, while 8% was allocated to the long-grain pureline cultivar DG263L (Hardke Reference Hardke2023). Additionally, 4% of the acreage was planted with the long-grain pureline cultivar CLL16 and 10% with the medium-grain pureline cultivars Jupiter and Titan.
Weed competition stands as one of the main factors limiting rice production, often resulting in more than 50% yield reductions, depending on variables such as weed density, species present, and time of emergence (King et al. Reference King, Norsworthy, Butts, Fernandes, Drescher and Avent2024; Maun and Barrett Reference Maun and Barrett1986; Ziska et al. Reference Ziska, Gealy, Burgos, Caicedo, Gressel, Lawton-Rauh, Avila, Theisen, Norsworthy, Ferrero, Vidotto, Johnson, Ferreira, Marchesan, Menezes, Cohn, Linscombe, Carmona, Tang and Merotto2015). For instance, a single Palmer amaranth [Amaranthus palmeri (S.) Watson] plant that emerges 1 wk before rice can reduce rice yield by 5% to 50% within 1.4 m to 0.4 m from the weed, respectively (King et al. Reference King, Norsworthy, Butts, Fernandes, Drescher and Avent2024). Besides decreasing yield, weeds can cause economic losses by reducing land value, primarily due to the additional costs associated with weed management and reduction in grain quality (Oerke Reference Oerke2006). Thus, effective weed control programs are essential for successful rice cultivation (Riar and Norsworthy Reference Riar and Norsworthy2011). Not surprisingly, herbicides are the most used pesticides in rice production in the United States, applied to 96% of the rice acreage (USDA-NASS 2022).
Varying degrees of herbicide tolerance have been documented among cultivars within the same crop (Beesinger et al. Reference Beesinger, Norsworthy, Butts and Roberts2022; Bond and Walker Reference Bond and Walker2011, Reference Bond and Walker2012; Griffin and Baker Reference Griffin and Baker1990; Hardcastle Reference Hardcastle1979; Montgomery et al. Reference Montgomery, Bond, Golden, Gore, Edwards, Eubank and Walker2014; Wright et al. Reference Wright, Norsworthy, Roberts, Scott, Hardke and Gbur2021). For instance, hybrid and inbred, medium-grain rice cultivars were injured more than inbred, long-grain cultivars following a postemergence application of saflufenacil or carfentrazone (Montgomery et al. Reference Montgomery, Bond, Golden, Gore, Edwards, Eubank and Walker2014). Wright et al. (Reference Wright, Norsworthy, Roberts, Scott, Hardke and Gbur2021) observed that long-grain and medium-grain pureline cultivars exhibited greater tolerance to florpyrauxifen-benzyl than a long-grain hybrid, which suffered a yield penalty when sequential applications of the herbicide were used. Additionally, rice tolerance to herbicides depends on the crop growth stage at the time of application (Bond and Walker Reference Bond and Walker2011, Reference Bond and Walker2012; Wright et al. Reference Wright, Norsworthy, Roberts, Scott, Hardke and Gbur2021). Therefore, it is crucial to assess crop tolerance to new herbicides across cultivars at multiple application timings to identify risks associated with potential crop injury and yield loss.
Fluridone is an inhibitor of phytoene desaturase, a crucial enzyme in the biosynthesis of carotenoids (Bartels and Watson Reference Bartels and Watson1978; Sandmann and Böger Reference Sandmann, Böger, Roe, Burton and Kuhr1997; Sandmann et al. Reference Sandmann, Schmidt, Linden and Böger1991). Fluridone was recently registered for use on rice starting at the 3-leaf rice stage. Fluridone offers a novel site of action in rice production, providing residual control for annual grass and broadleaf weeds (US EPA 2023). Fluridone has also been registered for use in cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) production, and has shown excellent control of Palmer amaranth (Grichar et al. Reference Grichar, Dotray and McGinty2020; Hill et al. Reference Hill, Norsworthy, Barber and Gbur2016). With the increased adoption of furrow-irrigated rice in Arkansas (Hardke Reference Hardke2023), Palmer amaranth emerged as a problematic weed in rice fields due to the favorable environmental conditions created by the nonflooded system (Bagavathiannan et al. Reference Bagavathiannan, Norsworthy and Scott2011; Butts et al. Reference Butts, Kouame, Norsworthy and Barber2022). The intensified interference from Palmer amaranth in rice fields, coupled with limited chemical options for its control due to herbicide resistance, makes this weed particularly difficult to manage. Therefore, fluridone emerges as a fundamental tool for farmers to manage this troublesome weed.
Despite its promising results in controlling Palmer amaranth, few studies have been conducted to investigate rice tolerance to fluridone. Martin et al. (Reference Martin, Norsworthy, Scott, Hardke and Lorenz2018) observed 25% injury to rice 7 wk after treatment following a preemergence application of fluridone at 224 g ai ha−1 on silt loam soil in a paddy system. Similarly, fluridone applied at 170 g ai ha−1 on clay soil following precision-leveling in a furrow-irrigation system caused more than 25% rice injury between 8 and 12 wk after treatment when applied at the 3-leaf growth stage (Butts et al. Reference Butts, Souza, Norsworthy, Barber and Hardke2024). In both studies, fluridone injury increased following irrigation initiation, likely due to greater herbicide availability.
Due to the limited effective options available, fluridone may become a significant herbicide in battling Palmer amaranth in rice systems, provided crop tolerance is acceptable. Little information is available regarding rice response to this herbicide. Further investigations are necessary to evaluate its safety across a range of rice cultivars at different application timings. Therefore, this study assessed the tolerance of 12 rice cultivars commonly grown in Arkansas to fluridone applied preemergence and at the 3-leaf growth stage in a paddy production system.
Materials and Methods
Experiment Setup
To determine the tolerance of rice to fluridone within each cultivar, preemergence and postemergence experiments were conducted by cultivar, totaling 24 experiments in 2022 and 22 in 2023. The goal was not to compare cultivars, but rather, to understand the response of each cultivar to fluridone. Cultivars were planted in independent strips and treatments were randomized within each cultivar. The experiments were organized as a randomized complete block design with four replications. All experiments were located at the Pine Tree Research Station, near Colt, AR (35.1242°N, 90.9306°W), on a Calhoun silt loam soil with 1.4% organic matter and pH of 8 and 8.1 in 2022 and 2023, respectively. Twelve rice cultivars were drill-seeded at 36, 52, and 72 seeds m−1 of row for hybrids, a pureline quizalofop-resistant cultivar, and all other pureline cultivars, respectively (Table 1). The cultivar Lynx was planted only in 2022 due to seed availability. Rice in the experiments was planted with a nine-row, small-plot drill at a 1.3-cm depth with 19 cm between rows on May 12, 2022, and April 11, 2023. The plots were 1.8 wide and 5.2 m long. The seedbed was prepared using conventional tillage in both years.
Table 1. List of rice cultivars, technology, seeding rate, description, and producer. a
a Abbreviation: UADA, University of Arkansas System Division of Agriculture.
b Supplier locations: Horizon Ag, LLC, Memphis, TN; Nutrien Ag Solutions, Inc., Saskatoon, SK, Canada; RiceTec, Inc., Alvin, TX; UADA, Stuttgart, AR.
The preemergence experiments aimed to evaluate the tolerance of each rice cultivar to fluridone when applied preemergence, and the postemergence experiments focused on rice tolerance to fluridone applied at the 3-leaf growth stage. Applications were made across all cultivars on the same date. In the preemergence experiments, the herbicide was applied to the soil surface on the day of planting. In the postemergence experiments, fluridone was sprayed on June 6, 2022, and May 16, 2023. Treatments consisted of fluridone (Brake®; SePRO Corporation, Carmel, IN) applied at 168 g ai ha−1 and at 336 g ai ha−1, which corresponds to the 1 × and 2 × label rates for the soil texture in which the experiments were conducted (US EPA 2023). A “no fluridone” treatment was included for comparison, and all experiments were conducted under weed-free conditions to avoid interference from factors other than the treatments.
Weed control management was the same for all experiments each year. Quinclorac (Facet®L; BASF, Research Triangle Park, NC) was applied preemergence in both years. Postemergence herbicides were applied to 2-leaf rice in 2022 using halosulfuron-methyl + prosulfuron (Gambit®; Gowan Company, Yuma, AZ), while in 2023, propanil + thiobencarb (Ricebeaux®; UPL Limited, King of Prussia, PA) and halosulfuron-methyl (Permit®; Gowan Company) were used. The experiments were managed following University of Arkansas System Division of Agriculture recommendations for direct-seeded, delayed-flood rice production (Henry et al. Reference Henry, Daniels, Hamilton and Hardke2021; Roberts et al. Reference Roberts, Slaton, Wilson and Norman2016). Flood establishment occurred 30 d after emergence, on June 18, 2022, and June 2, 2023, for all experiments. The herbicides were applied using a CO2-pressurized backpack sprayer equipped with four AIXR 110015 nozzles (TeeJet Technologies, Glendale Heights, IL), calibrated to deliver 140 L ha−1 at a speed of 4.8 kph. Air temperature and rainfall data were monitored via a nearby weather station.
Visible crop injury was evaluated at 2, 4, and 6 wk after emergence (WAE) in the preemergence experiments and at 2, 4, and 6 wk after treatment (WAT) in the postemergence experiments using a scale of 0 to 100, with 0 representing no injury and 100 representing plant death (Frans et al. Reference Frans, Talbert, Marx, Crowley and Camper1986). Rice shoot counts were taken in two 1-m sections of a row at 2 WAE in the preemergence experiments only, whereas all other subsequent variables were collected in both preemergence and postemergence experiments. Chlorophyll content was estimated using a soil plant analysis development (SPAD) chlorophyll meter (SPAD-502 plus Chlorophyll meter; Konica Minolta, Tokyo, Japan) at the rice panicle initiation growth stage, with readings of the uppermost fully expanded leaf of five plants per plot. A small, unmanned aerial system (DJI Mavic Air 2S; DJI Technology Co., Nanshan, Shenzhen, China) was used to capture aerial images from a height of approximately 60 m in 2022, with each image covering 12 plots in width and four plots in length. In 2023, images were captured at approximately 30 m, covering nine plots in width and four plots in length. Images were taken at 8 WAE in both years. Overhead images were analyzed using Field Analyzer (Green Research Services, Fayetteville, AR) to determine the groundcover percentage for each plot by measuring green pixel counts. Days to 50% heading were recorded for each plot and reported relative to each control. Rough rice grain yield was harvested from the center four rows of each plot using a small-plot combine, and grain moisture was adjusted to 12% when reporting yield.
Data Analysis
All data were analyzed using R statistical software version 4.2.2 (R Core Team 2023). All data were fitted to a generalized linear mixed-effect model (GLMM) (Stroup Reference Stroup2015) using the glmmTMB function within the glmmTMB package (Brooks et al. Reference Brooks, Kristensen, van Benthem, Magnusson, Berg, Nielsen, Skaug, Mächler and Bolker2017). Year was included in the model as a fixed effect, and block was treated as a random effect for the analysis of all variables. The interaction of year and fluridone rate was significant for most variables across cultivars (P > 0.05). Therefore, data were analyzed by year. Rice shoot density, chlorophyll content, groundcover, relative heading date, and yield were analyzed using a Gaussian or normal distribution. Percent visible rice injury was analyzed using a beta distribution. For injury analysis, evaluation timing (WAE or WAT) was considered a repeated-measure variable that allowed comparisons across evaluations taken on the same plot over the same interval (Gbur et al. Reference Gbur, Stroup, McCarter, Durham, Young, Christman, West and Kramer2012). An autoregressive first-order covariance structure (AR1) was applied to account for the temporal correlation between repeated measurements taken at different evaluation timings on the same plot (Hamilton Reference Hamilton1994; Kiss et al. Reference Kiss, Deák, Tóthmérész, Miglécz, Tóth, Török, Lukács, Godó, Körmöczi, Radócz and Borza2021). In the GLMM models for injury, fluridone rate and evaluation timing were considered fixed effects, and block was treated as a random effect. For models for the other variables, only fluridone rate was considered a fixed effect, and block was considered a random effect. Q-Q plots were used to check the fitness of the model, and final models were selected based on Akaike information criterion values.
Analysis of variance was performed using Type III Wald chi-square tests with the car package (Fox and Weisberg Reference Fox and Weisberg2019). Following the ANOVA, treatment-estimated marginal means (Searle et al. Reference Searle, Speed and Milliken1980) were calculated using the emmeans package (Lenth Reference Lenth2022). The multcomp package (Hothorn et al. Reference Hothorn, Bretz and Westfall2008) generated a compact letter display to distinguish significant differences among treatments. Estimated marginal means included post hoc Tukey HSD (α = 0.05) adjustments, and the compact letter display was generated via the multcomp:cld function.
Results and Discussion
Preemergence Experiments
Visible injury to rice never exceeded 24% in 2022 when fluridone was applied preemergence (Tables 2, 3, and 4). In 2023, most cultivars displayed lower injury levels at 2 and 4 WAE compared with 6 WAE (Table 4). The cultivars CLL15, CLL16, Diamond, Jupiter, RTv7231 MA, and Titan displayed less than 10% injury regardless of fluridone rate at the first two evaluation timings in 2023. At 6 WAE in 2023, injury levels of at least 30%, averaged over rates, were observed on the cultivars CLL15, DG263L, PVL02, and RT7321FP, with DG263L’displaying 58% injury. These results lead to the suggestion of increased herbicide availability with the establishment of the flood, which occurred 7 wk after planting (4 WAE), resulting in increased injury for most of the cultivars, especially in 2023. Similar results were observed by Martin et al. (Reference Martin, Norsworthy, Scott, Hardke and Lorenz2018), when rice injury increased after flood establishment following a preemergence application of fluridone at 224 g ai ha−1 on Dewitt and Calhoun silt loam soils.
Table 2. Rice cultivar injury as influenced by rate and evaluation timing interaction by year following preemergence applications of fluridone. a, b, c, d
a Abbreviations: WAE, weeks after emergence.
b Bold P-values indicate significance at α = 0.05.
c Means within a cultivar by year for the fluridone rate by evaluation timing interaction followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
d Flood establishment occurred 4 wk after emergence in both years.
Table 3. Rice cultivar injury as influenced by the main effect rate by year following preemergence applications of fluridone. a, b
a Bold P=values indicate significance at α = 0.05 based on pairwise comparisons.
b An asterisk (*) indicates a difference between fluridone rate averaged over weeks after emergence within the same column for each cultivar by year when interaction is not present.
Table 4. Rice cultivar injury as influenced by the main effect application timing by year following preemergence applications of fluridone. a,b,c,d,e
a Abbreviation: WAE, weeks after emergence.
b Bold P-values indicate significance at α = 0.05.
c Means within the same row for each cultivar by year followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
d When interaction is present, means for the main effect evaluation timing is not separated by letters.
e Flood establishment occurred 4 WAE in both years.
The lack of an increase in visible injury after the establishment of the flood in 2022 may have been influenced by environmental conditions such as higher temperatures, which may have enhanced herbicide detoxification (Figure 1). Similarly, results reported by Bond and Walker (Reference Bond and Walker2011) suggest that imazamox metabolism in treated rice plants was reduced by cooler temperatures, higher rainfall, and lower solar radiation, leading to a yield penalty. Furthermore, rice emergence occurred 7 and 20 d after planting in 2022 and 2023, respectively (Figure 1). The delayed emergence in 2023 may be attributed to lower temperatures after planting (Figure 1; Mertz et al. Reference Mertz, Henning, Soares, Baldiga, Peske and Moraes2009); however, the cool, wet conditions that existed did not increase injury prior to flood establishment that year in most cultivars. These results are different from those observed in other research with herbicides such as clomazone (Jordan et al. Reference Jordan, Bollich, Burns and Walker1998; O’Barr et al. Reference O’Barr, McCauley, Bovey, Senseman and Chandler2007).
Figure 1. Daily results of observed accumulated rainfall (mm) and air temperature (C) over a 24-h period from the day of planting until the last day of rice injury evaluation in Colt, AR, from 2022 and 2023.
Carotenoids are essential in the photosynthetic process, with one function being the protection of chlorophyll from photooxidation (Anderson and Robertson Reference Anderson and Robertson1960; Sandman et al. Reference Sandmann, Schmidt, Linden and Böger1991). When carotenoid biosynthesis is interrupted, chlorophyll undergoes photooxidative destruction. Therefore, if injury occurs following a pigment-inhibiting herbicide application such as fluridone, a decrease in chlorophyll content is likely to happen, leading to a reduction in the photosynthetic rate (Buttery and Buzzell Reference Buttery and Buzzell1977). The injury caused by fluridone treatments in 2022 did not cause a decrease in the chlorophyll content for any cultivar (Table 5). Conversely, except for the RTv7231 MA cultivar, chlorophyll content decreased in all cultivars in 2023, mostly due to applications of the 2× label rate.
Table 5. Rice shoot density, groundcover, chlorophyll content, heading date, and rough rice yield as influenced by the main effect rate by year following preemergence applications of fluridone. a,b,c,d
a SPAD is a soil plant analysis development value, an indirect estimate of chlorophyll content.
b Bold P-values indicate statistical significance at α = 0.05.
c Means within a column followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
d Dashes (–) represent a delay in heading of zero for the control.
Although most cultivars displayed injury levels below 20% in 2022 (Tables 2, 3, and 4), there was a shoot density decrease in CLL15, CLL16, Lynx, RT7321 FP, RT7521 FP, and XP753 cultivars, primarily caused by the 2× rate (Table 5). The decrease in shoot density among these cultivars was reflected in the groundcover data only for the cultivar Lynx at 8 WAE. However, Diamond, Jupiter, and Titan cultivars experienced a reduction in groundcover, even though no reduction in shoot density was detected (Table 5). In 2023, shoot density was reduced due to fluridone treatments only in the cultivars DG263L, Jupiter, PVL02, and RT7521 FP. As in 2022, cultivars that displayed a reduction in groundcover did not necessarily experience a decrease in shoot density. Groundcover reduction occurred in the cultivars CLL15, DG263L, PVL02, RT7321 FP, and XP753. Groundcover is a good predictor of crop yield (Donald Reference Donald1998); consequently, a significant reduction in groundcover would likely result in reduced rice yield. Butts et al. (Reference Butts, Souza, Norsworthy, Barber and Hardke2024) observed that fluridone applications to 3-leaf rice on a precision-leveled field resulted in a decrease of approximately a 45 percentage points in groundcover 10 wk after application at 340 g ai ha−1. The study also demonstrated that although the rice recovered and achieved a similar canopy to the nontreated by 13 wk after application, yield was still negatively affected.
The delay in reaching 50% heading was no more than 5 d relative to each control in both years (Table 5). Eight cultivars exhibited a yield decrease of at least 18% due to the 2× label rate treatment compared with their respective controls in 2022. Fluridone treatments did not affect yield in the cultivars CLL16, Diamond, PVL02, or RTv7231 MA. In 2023, rough rice yields from the CLL16, Diamond, and Titan cultivars were not affected by fluridone treatment. In contrast, there was a yield reduction ranging from 9% to 49% by all other eight cultivars compared with each control. Among the cultivars that experienced a yield loss in 2023, CLL15 had a yield penalty exclusively from the 1× label rate, DG263L and RTv7231MA had yield decreases at both rates, and the other cultivars experienced a yield reduction only from the 2× label rate treatment. Similar to the other variables analyzed, the differences in yield reduction between years were likely due to varying environmental conditions, as yield reductions at the 1× label rate occurred only in 2023.
The cultivars CLL15, DG263L, Jupiter, RT7321 FP, RT7521 FP, and XP753 suffered yield penalty in both years. Conversely, CLL16 and Diamond did not experience a yield penalty at either rate in both years, suggesting that these two cultivars are highly tolerant to fluridone when applied preemergence. Besides the cultivar PVL02 in 2022, all other cultivars in both years were negatively affected by fluridone on at least one of the variables tested (visible injury, shoot density, groundcover, chlorophyll content, or delayed heading). Among the cultivars for which there was a yield penalty, no single factor consistently contributed to the yield reduction. Therefore, none of the evaluated variables can be used individually to predict the likelihood of yield loss.
A preemergence application of fluridone leads to translocation of the herbicide to the leaves, resulting in bleaching and chlorosis in susceptible plants (Sandmann et al. Reference Sandmann, Schmidt, Linden and Böger1991; Waldrep and Taylor Reference Waldrep and Taylor1976). In nonsensitive species, fluridone tolerance is conferred by limited herbicide translocation from the roots to the shoots, as is the case with cotton (Berard et al. Reference Berard, Rainey and Lin1978). In a study exposing transplanted plants in a solution containing 14C-labeled fluridone, the herbicide translocated more rapidly to rice shoots than to those of corn (Zea mays L.), cotton, and soybean [Glycine max (L.) Merr.], indicating that rice is more susceptible than these species (Berard et al. Reference Berard, Rainey and Lin1978). Additionally, Waldrep and Taylor (Reference Waldrep and Taylor1976) reported that fluridone is more effective when applied preemergence than when applied to the foliage, and higher injury levels would be expected to occur at this application timing. Therefore, the cultivars evaluated in this study that did not exhibit yield penalty following a preemergence application of fluridone are likely tolerant to this herbicide.
Postemergence Experiments
Fluridone applied at the 3-leaf rice growth stage caused no more than 11% injury to any cultivar in 2022 (Tables 6, 7, and 8). In 2023, all cultivars experienced greater injury at 4 and 6 WAT, averaged over fluridone rate, compared with the first evaluation (Table 8). At 2 WAT, injury never exceeded 6%. However, by 6 WAT, injury levels ≥20% occurred in the cultivars DG263L, PVL02, and RTv7231 MA, with 39%, 32%, and 20% injury averaged over fluridone rates, respectively. The other cultivars exhibited no more than 15% injury on the last evaluation. As reported in the preemergence experiment, the increase in rice injury in some cultivars in 2023 may be attributed to the flood establishment, which likely enhanced herbicide availability and consequently increased rice injury. In a similar study, an increase of 19 percentage points in rice injury was observed at 4 WAT compared with 1 WAT following a fluridone application at 340 g ai ha−1 at the 3-leaf rice growth stage on a Sharkey-Steele clay soil, which was likely due to the initiation of irrigation (Butts et al. Reference Butts, Souza, Norsworthy, Barber and Hardke2024). In the same study, rice was injured 65% at 8 WAT.
Table 6. Rice cultivar injury as influenced by rate and evaluation timing interaction by year following fluridone applications at the 3-leaf stage. a,b,c,d
a Abbreviation: WAT, weeks after treatment.
b Bold P-values indicate statistical significance at α = 0.05.
c Means within a cultivar by year for the fluridone rate by evaluation timing interaction followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
d Flood establishment occurred 4 wk after emergence in both years.
Table 7. Rice cultivar injury as influenced by the main effect rate by year following fluridone applications at the 3-leaf stage. a,b
a Bold P-values indicate significance at α = 0.05 based on pairwise comparisons.
b Asterisk (*) indicates a difference between fluridone rates averaged over weeks after emeregence within the same column for each cultivar by year when interaction is not present.
Table 8. Rice cultivar injury as influenced by the main effect application timing by year following fluridone applications at the 3-leaf stage. a,b,c,d,e
a Abbreviation: WAT, weeks after treatment.
b Bold P-values indicate significance at α = 0.05.
c Means within the same row for each cultivar by year followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
d When interaction is present, means for the main effect evaluation timing is not separated by letters.
e Flood establishment occurred 4 wk after emergence in both years.
Given the minimal injury in 2022, out of the 12 cultivars tested, only Diamond exhibited a reduction in chlorophyll content (Table 9). In contrast, the only cultivar in which chlorophyll content was not decreased in 2023 was CLL15. A reduction in groundcover at 8 WAE occurred to the cultivar DG263L in both years, while groundcover in PVL02 was decreases only in 2023 (Table 9). No other cultivar’s groundcover was negatively affected by fluridone. Furthermore, a delay of no more than 4 d in reaching 50% heading was observed compared with control plants.
Table 9. Groundcover, chlorophyll content, heading date, and rough rice yield as influenced by the main effect of rate by year following fluridone applications at the 3-leaf rice stage. a,b,c,d
a SPAD is soil plant analysis development value, an indirect estimate of chlorophyll content.
b Bold P-values indicate statistical significance at α = 0.05.
c Means within a column followed by the same letter are not different according to Tukey’s HSD (α = 0.05).
d Dashes (–) represent a delay in heading of zero for the control.
Given that a fluridone application at the 3-leaf rice stage is labeled for use at the 1× label rate, which was tested in this study (US EPA 2023), no yield penalty should be expected from herbicide treatment at that rate. Minimal injury levels and few reductions in groundcover, chlorophyll content, and little or no delay in heading caused by fluridone treatment were observed in 2022 (Table 6). Consequently, no yield penalty was observed. In 2023, eight cultivars exhibited a yield penalty, primarily due to the 2× label rate of fluridone. However, only DG263L and RT7521 FP cultivars exhibited a yield reduction following the 1× label rate treatment, and language concerning the sensitivity and risk of yield loss of these cultivars should be applied to the existing label. Similarly, a yield reduction following fluridone application of 340 g ai ha−1 at the 3-leaf stage on a precision-leveled field has been reported, with the cultivar RT7521 FP showing a 21% yield penalty (Butts et al. Reference Butts, Souza, Norsworthy, Barber and Hardke2024).
Considering the high injury level displayed by DG263L (up to 32%) coupled with the reduction in groundcover and chlorophyll content at the 1× label rate, the yield loss experienced by this cultivar was expected. However, little injury occurred to RT7521 FP, and there was no negative effect in any other variable evaluated at the 1× label rate. Therefore, further research is needed to better understand the tolerance of RT7521 FP to fluridone. Yield from the cultivars CLL16, Diamond, and XP753 was not affected by either rate in either year by fluridone treatment, indicating that these cultivars are tolerant to 1× and 2× of the currently labeled fluridone rates applied at the 3-leaf growth stage.
Practical Implications
The labeling of fluridone for use in rice production offers a new site of action for growers to control annual broadleaf and grass weeds, especially Palmer amaranth. At the labeled rate fluridone can be safely applied to most cultivars of rice at the 3-leaf stage. However, yield loss occurred to DG263L and RT7521 FP when they were treated with the labeled rate of fluridone. Therefore, growers must be careful when choosing which cultivar will receive fluridone if it is going to be a part of the weed management program. Fluridone applications should be avoided in rice fields planted with DG263L and later flooded, and further research is needed to evaluate the tolerance of the cultivar RT7521 FP because it did not exhibit high injury levels (≤11%), but a yield penalty occurred at the labeled rate in one of the years.
Fluridone is highly effective when applied preemergence (Waldrep and Taylor Reference Waldrep and Taylor1976) and is expected to cause more injury to rice when applied at this time. Previous research has suggested that the growth stage at application affects rice cultivar tolerance to herbicides (Bond and Walker Reference Bond and Walker2011, Reference Bond and Walker2012; Wright et al. Reference Wright, Norsworthy, Roberts, Scott, Hardke and Gbur2021). Although direct comparisons are not statistically allowed, based on the findings presented here, rice appears to be more tolerant to fluridone when applied at the 3-leaf growth stage than preemergence. Thus, preemergence applications of fluridone to rice should be avoided. Additionally, although the label specifies a zero-day plant-back interval (US EPA 2023), rice should not be replanted in fields treated with fluridone immediately after application. Additional research is needed to determine the most appropriate rice plant-back interval following fluridone application for label clarification. Moreover, the lack of a labeled preemergence application and tolerance of the crop to fluridone applied preemergence complicates Palmer amaranth management because residual control is provided by fluridone, meaning an alternative option would be needed for weeds that have emerged by the 3-leaf stage of rice.
Furthermore, environmental conditions likely substantially affect the degree of crop response from fluridone based on visible injury, shoot density, chlorophyll content, and yield assessments reported here. Cool, wet conditions, more extreme than those tested here, may further increase the extent of injury to rice from postemergence application; however, delaying a fluridone application until the 3-leaf stage of rice should result in warmer conditions than those experienced during crop germination and emergence earlier in the growing season. Additionally, further research is needed to determine the influence of water availability on rice tolerance to fluridone under different water management techniques, such as furrow irrigation, which lacks flooding in most of the field, and alternating wetting and drying where flooding occurs intermittently.
Acknowledgments
The University of Arkansas System Division of Agriculture provided infrastructure and equipment for these experiments.
Funding
Partial support for this research was provided by SePRO Corporation and the Arkansas Rice Research and Promotion Board.
Competing Interests
The authors declare they have no competing interests.
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