Introduction
Tetflupyrolimet [CAS: 2053901-33-8; (3S,4S)-N-(2-fluorophenyl)-1-methyl-2-oxo-4-[3-(trifluoromethyl)phenyl]pyrrolidine-3-carboxamide] is an aryl pyrrolidinone anilide herbicide that was developed for selective control of grass, sedge, and broadleaf weeds in rice cropping systems. As the first novel mode-of-action Group 28 herbicide (as categorized by the Herbicide Resistance Action Committee and Weed Science Society of America) in the last three decades, tetflupyrolimet is a potent inhibitor of dihydroorotate dehydrogenase (DHODH), which is involved in the de novo pyrimidine nucleotide biosynthesis (Maienfisch and Mangelinckx Reference Maienfisch, Mangelinckx, Maienfisch and Mangelinckx2021). The DHODH is the fourth enzyme of the pyrimidine de novo biosynthesis pathway that is localized to the mitochondria and catalyzes the conversion of dihydroorotate to orotate (Zrenner et al. Reference Zrenner, Stitt, Sonnewald and Boldt2006). In plants, the de novo pyrimidine nucleotide biosynthesis pathway is a vital process for metabolism; gene expression; and the production of substrates for DNA, RNA, and multiple biosynthesis pathways such as polysaccharides, glycoproteins, and phospholipids (Kang et al. Reference Kang, Emptage, Kim and Gutteridge2023; Zrenner et al. Reference Zrenner, Stitt, Sonnewald and Boldt2006).
Owing to the central role of nucleotides, inhibition of DHODH is lethal to most organisms (Dayan Reference Dayan2019). Tetflupyrolimet has been shown to have high levels of activity against grass species such as watergrasses (Echinochloa P. Beauv.), sprangletops (Leptochloa P. Beauv.) (Lombardi and Al-Khatib Reference Lombardi and Al-Khatib2024), giant foxtail (Setaria faberi Herrm.), and hairy crabgrass [Digitaria sanguinalis (L.) Scop.] (Selby et al. Reference Selby, Satterfield, Puri, Stevenson, Travis, Campbell, Taggi, Hughes and Bereznak2023). Moreover, tetflupyrolimet’s activity is at least 10-fold greater on weeds such as foxtail than rice, which suggests it is selective in rice because the crop can metabolize tetflupyrolimet (Dayan Reference Dayan2019; Selby et al. Reference Selby, Satterfield, Puri, Stevenson, Travis, Campbell, Taggi, Hughes and Bereznak2023). Therefore, the discovery of tetflupyrolimet is important and promising for weed management in rice cropping systems.
California is the second largest rice producer in the United States with approximately 220,000 ha under cultivation (Galvin et al. Reference Galvin, Inci, Mesgaran, Brim-DeForest and Al-Khatib2022), which accounts for more than US$1 billion in farmgate value (CDFA 2024). The primary rice production area is in the Sacramento and Northern San Joaquin valleys, and the crop typically is water-seeded and grown under continuously flooded conditions during the growing season (Inci et al. Reference Inci, Leinfelder-Miles and Al-Khatib2024d). Competitive grass weeds in rice systems such as barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], early watergrass [E. oryzoides (Ard.) Fritsch], late watergrass [E. phyllopogon (Stapf) Koso-Pol.], and bearded sprangletop [Leptochloa fusca (L.) Kunth ssp. fascicularis (Lam.) N. Snow] can significantly reduce rice yields (Brim-DeForest et al. Reference Brim-DeForest, Al-Khatib and Fischer2017). Alongside cultural management methods such as planting certified weed-free rice seed, high seeding rates, and continuous water management, herbicides are crucial for weed management in rice (Inci and Al-Khatib Reference Inci and Al-Khatib2024).
Beyond rice, California is the primary producer of many specialty crop commodities, including production of more than 99% of the nation’s almonds, nectarines, peaches, pistachios, plums, raisin grapes, and walnuts (CDFA 2024). Among those, grape is the most valued crop in California, generating more than US$5.5 billion in cash receipts from 350,000 ha of wine, table, and raisin grapes. Tree nuts are produced on 1 million ha in the Sacramento and San Joaquin valleys, and these regions account for 85% of the world’s almond production. Collectively, grapes, stone fruits, and tree nuts are significant crops grown on ∼1.5 million ha with a value of more than US$12 billion. With such diverse cropping systems in California, rice is often grown adjacent to orchards and vineyards (Inci et al. Reference Inci, Hanson and Al-Khatib2024b, Reference Inci, Hanson and Al-Khatib2024c).
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 off-target herbicide drift to nearby orchards and vineyards (Hill et al. Reference Hill, Williams, Mutters and Greer2006). Research on tetflupyrolimet in the complex herbicidal programs of California rice suggests its utility as both a preemergence and early postemergence herbicide (Lombardi and Al-Khatib Reference Lombardi and Al-Khatib2024). The application timing of tetflupyrolimet is from the day-of-seeding to the 2-leaf rice growth stage. This herbicide application timing usually occurs from May to June depending on the planting date, rice variety, and environmental conditions.
During May and June, orchard crops and grapes are in relatively vulnerable growth stages for off-target herbicide exposures. Almond, pistachio, and walnut trees are in vigorous growth periods and actively developing terminal and lateral buds, leaves, spurs, and shoots and accumulating assimilates for kernels (Galla et al. Reference Galla, Al-Khatib and Hanson2018a, Reference Galla, Al-Khatib and Hanson2018b). Grapevine phenological stages at this time range from bloom to veraison (Bettiga Reference Bettiga2013), and stone fruits are at a stage when the endocarp (pit) hardening process begins, and the fruit size increases (Strand Reference Strand1999).
Herbicide drift is the physical movement of spray droplets through the air at the time of application or soon thereafter to any site other than the intended target (Whithaus Reference Whithaus2016). Under most circumstances, off-target herbicide exposures are similar to herbicide fractional rates from below 1/100× up to 1/33× of the field application rates of herbicides (Al-Khatib and Peterson Reference Al-Khatib and Peterson1999). Significant drift events are most frequently associated with relatively high air temperature and wind speed, low relative humidity, small spray droplet size, and short distances to nearby nontarget crops (Whithaus Reference Whithaus2016). The concerns of rice herbicide drift to off-target crops in the Sacramento and Northern San Joaquin valleys are common among growers, crop consultants, and researchers.
Tetflupyrolimet is anticipated to be a useful and widely used herbicide in rice cropping systems (Lombardi and Al-Khatib Reference Lombardi and Al-Khatib2024). It is important to understand the relative sensitivity of crops subjected to fractional rates of tetflupyrolimet, particularly considering the economic impact of California grape, stone fruit, and tree nut industries. To steward tetflupyrolimet prior to its registration, this research was conducted to compare the onset of foliar symptoms from fractional tetflupyrolimet rates onto six tree and vine crops. We analyzed the growth response of almond, peach, pistachio, plum, and walnut trees to tetflupyrolimet at different rates, and evaluated grapevine yield in response to tetflupyrolimet exposure.
Materials and Methods
Study Sites
Six herbicide experiments were conducted in 2022 and 2023 in a 3- to 4-yr-old almond (38.539°N, 121.794°W), peach (38.539°N, 121.794°W), pistachio (38.539°N, 121.793°W), plum (38.538°N, 121.794°W), and walnut (38.539°N, 121.794°W) orchards (elev. 18 m asl) at the University of California–Davis Plant Sciences Field Facility orchards; and in a 25- to 26-yr-old wine grape vineyard (38.525°N, 121.788°W) at the University of California–Davis Department of Viticulture and Enology Tyree Vineyard. The orchards were established in March 2020 with ‘Nonpareil’ almond on ‘Empyrean 1’, ‘Coralstar’ peach on ‘Krymsk 86’, ‘Kerman’ pistachio on ‘UCB 1’, ‘French Improved’ plum on ‘Krymsk 86’, and ‘Chandler’ walnut on ‘clonal RX1’. All almond, peach, plum, and walnut trees were planted 6 m apart within rows and 4.2 m apart between rows, while pistachio trees were 6 m apart within rows and 7 m apart between rows. The vineyard was established in 1998 with a bilateral double-cordon-trained ‘Grenache’ wine grape planted 1.8 m apart within rows and 3.6 m apart between rows.
The soil was classified as a Yolo silt loam with NO3-N 57 ppm, Olsen-P 26 ppm, K 351 ppm, Na 21 ppm, Ca 8 meq 100 g–1, Mg 10 meq 100 g–1, cation exchange capacity (CEC) 19 meq 100 g–1, organic matter 2.7%, and pH 6.7 in the orchards; and NO3-N 23 ppm, Olsen-P 12 ppm, K 288 ppm, Na 12 ppm, Ca 11 meq 100 g–1, Mg 9 meq 100 g–1, CEC 21 meq 100 g–1, organic matter 2.5%, and pH 7.1 in the vineyard (soils were analyzed at the University of California–Davis Analytical Laboratory). Irrigation was applied in all crops through a single-line drip irrigation system with emitters spaced every 30 cm during the growing seasons. All trees and vines were maintained free of diseases and insects following standard commercial practices (Bettiga Reference Bettiga2013; Strand Reference Strand1999). In all experiments, weeds between rows were managed with regular mowing and within rows with a mixture of rimsulfuron at 70 g ai ha–1, indaziflam at 50 g ai ha–1, oxyfluorfen at 560 g ai ha–1, and glufosinate-ammonium at 450 g ai ha–1 plus manufacturer recommended surfactants.
Herbicide Applications
Tetflupyrolimet (Dodhylex™, 400 g ai L–1; FMC Corporation, Philadelphia, PA) herbicide as a suspension concentrate (SC) formulation was applied on June 6, 2022, in all experiments. Tetflupyrolimet was applied at concentrations of 1/200×, 1/100×, 1/33×, 1/10×, and 1× the use rate for rice of 125 g ai ha–1 on almond, pistachio, and walnut trees. The concentrations actually represent percentages of the use rate as 0.5%, 1%, 3%, 10%, and 100% (Inci et al. Reference Inci, Hanson and Al-Khatib2024b). Due to a limited number of trees and vines, the 1× tetflupyrolimet treatment was not included for grapevines, or peach and plum trees. Plots with nontreated trees or vines were also included for comparison in each experiment.
All herbicide treatments were applied to one side of the tree or vine canopy as one pass (top to bottom for trees and side to side for cordon-trained vines) with a handheld, carbon dioxide-propelled backpack sprayer calibrated to deliver 187 L ha–1 at 206 kPa pressure through XR 8004-VS nozzle tips (TeeJet Technologies, Glendale Heights, IL). The sprayer boom had two nozzles spaced 50 cm apart and spray was delivered based on a 3-s pass per tree or vine. Plots were sprayed early in the morning when winds were calm to avoid herbicide cross-contamination to adjacent trees or vines. At the time of the orchard and vineyard applications on June 6, 2022, the air temperature was 16 C, with 58% relative humidity (RH) and 0.4 m s–1 wind speed. All experiments were repeated on May 31, 2023, with a different set of trees or vines in the same orchards and vineyard. At the time of second-year application the air temperature was 18 C, with 50% RH and 0.6 m s–1 wind speed. Because the trees were relatively young, no yield data were taken for orchard crops; however, grapevines had 5–10 mm berries present at the time of herbicide application in both 2022 and 2023.
Data Collection and Experimental Design
Experiments were arranged in a randomized complete block design with four replications, where an individual tree or vine was an experimental unit. Trees and vines were observed for visible injury symptoms at 6, 12, 24, 48, and 72 h after herbicide treatment; and 7, 14, 21, 28, 35, 42, and 90 d after treatment (DAT). Visible injury was rated on a scale on which 0 = no injury and 100 = plant death.
Trees and vines treated with tetflupyrolimet were compared with nontreated control plants at each observation. Furthermore, trunk diameters of almond, peach, pistachio, plum, and walnut trees were measured using a digital caliper with ±25 µm accuracy at approximately 25 cm above the ground on April 15 and October 20, 2022, and on April 23 and October 20, 2023. The timing of these measurements correspond with the beginning of spring growth, which starts in April (pretreatment), and after the growing season, which ends in October (posttreatment) (Inci et al. Reference Inci, Hanson and Al-Khatib2024b, Reference Inci, Hanson and Al-Khatib2024c). To maintain the consistency of assessments, the trunk diameter of all trees was measured four times regardless of whether they were treated in 2022 or 2023 experiments. Tree growth was expressed through trunk diameter growth as a percentage increase based on the following formula:
$$y=\{[x_{n+1}/x_n)-1] \times100\}+100$$
where y is the percent relative change of trunk diameter, x n = trunk diameter at pretreatment measurements in spring, and x n+1 = trunk diameter at posttreatment measurements approximately 140 DAT in fall. Thereby, the relative change in trunk diameter of herbicide-treated trees was compared with the relative change in the diameter of nontreated control trees.
Grapes were hand-harvested when berries on the vines of nontreated control plants reached ∼20°Bx (1% soluble solids), a common practice for the Northern San Joaquin and Sacramento Valleys grapevine industry (Bettiga Reference Bettiga2013). Grape clusters were harvested from all treated vines and from nontreated control vines, and weighed for total fruit yield and sugar content from a fruit subsample determined with a handheld refractometer.
Statistical Analysis
Trunk diameter data were subjected to analysis of covariance using agricolae (de Mendiburu Reference de Mendiburu2024) and dplyr (Wickham et al. Reference Wickham, Çetinkaya-Rundel and Grolemund2024a, Reference Wickham, François, Henry, Müller and Vaughan2024b) packages to characterize the growth of the orchard crops with equation Y = A + BX, where Y is the predicted value, A is the y-intercept; B is the slope of the line, and X is the observation time. These analyses were conducted using RStudio v. 2024.09.1+394 (R Core Team 2024). Means were separated using Tukey’s honestly significant difference post hoc test at significance level of P ≤ 0.05, when applicable. The multcomp (Bretz et al. Reference Bretz, Hothorn and Westfall2010) package was used to generate multiple comparisons among means. Tetflupyrolimet fractional rates were considered fixed factors, while crop, year, and replication were considered random factors. Grape yield and brix were analyzed with ANOVA at α = 0.05 (Kniss and Streibig Reference Kniss and Streibig2018). The Type II Wald F-tests with the Kenward-Roger degrees-of-freedom method and Type III with the Satterthwaite method were used when the confidence level was 0.95 for both ANOVA types. Graphical illustration was generated using the ggplot2 package v. 3.5.1 in RStudio (Wickham et al. Reference Wickham, Navarro and Pedersen2024c).
Results and Discussion
Tetflupyrolimet did not cause any distinguishable injury symptoms to any crop at any rating time or at any herbicide treatment, including up to the 1× rate of 125 g ai ha–1 on almond, pistachio, and walnut (data not shown). The lack of any observed injury could be because established plants are not dependent on the pyrimidine nucleotide biosynthesis as much as developing plants because the pyrimidine nucleotide biosynthesis is energetically expensive, cells utilize pyrimidine nucleotides only if they are rapidly growing and dividing. In established plants, mature cells can meet their metabolic needs through a salvage pathway, a reutilization mechanism, to break down cellular components that are no longer needed and that do not metabolize the DHODH enzyme (Zrenner et al. Reference Zrenner, Stitt, Sonnewald and Boldt2006). Tetflupyrolimet, as a DHODH inhibitor, is therefore most active against weed seedlings and did not cause any injury symptoms to established trees and vines. No fruit yield data were taken in the orchard crops, fruit that were present appeared normal and consistent among treatments.
Tree trunk diameter change is a common parameter for interpreting orchard crop growth (Inci et al. Reference Inci, Hanson and Al-Khatib2024b, Reference Inci, Hanson and Al-Khatib2024c). The percent of relative change data for 2022 and 2023 were combined (n = 8) for tree crops because there were no significant interactions between year and treatment (Wickham et al. Reference Wickham, Çetinkaya-Rundel and Grolemund2024a). In all orchard crops, tree trunk diameter change was variable for almond, peach, pistachio, plum, and walnut trees. From April 2022 to October 2023, the relative trunk diameter of all tested crops increased substantially, and the growth was not different (P < 0.05) from that of nontreated control trees (Figures 1 and 2). Almond, peach, and plum trees exhibited an average of ∼30 mm trunk diameter increase across all treatments in 2022, whereas the average diameter of walnut and pistachio trees increased by 22 mm and 12 mm, respectively (data not shown). In 2023, the diameter of almond increased by 17 mm, 10 mm in peach and pistachio, 5 mm in plum, and 7 mm in walnut. At the fall 2023 observations, the total of two seasons of growth, almond, peach, pistachio, plum, and walnut trees showed cumulative diameter increases of 70 mm, 50 mm, 35 mm, 50 mm, and 45 mm, respectively. Together, these data corresponded to an average trunk diameter of almonds increased by 230%, pistachios by 292%, walnuts by 210% (Figure 1), peaches by 220%, and plums by 241% (Figure 2) at the end of two seasons.
Trunk diameter measurements (n = 8) of almond (top), pistachio (middle), and walnut (bottom) trees before initiation of tetflupyrolimet treatments (April 2022 and 2023) and after treatments (October 2022 and 2023). Tree trunk diameter change was expressed as percent relative change with a linear model Y = A + BX, where Y is the predicted value, A is the y-intercept; B is the slope of the line, and X is the observation time. Tetflupyrolimet simulated drift rates were expressed as a fraction of rice use rate of 125 g ai ha–1.
Trunk diameter measurements (n = 8) of peach (top) and plum (bottom) trees before initiation of tetflupyrolimet treatments (April 2022 and 2023) and after treatments (October 2022 and 2023). Tree trunk diameter change was expressed as percent relative change with a linear model Y = A + BX, where Y is the predicted value, A is the y-intercept; B is the slope of the line, and X is the observation time. Tetflupyrolimet simulated drift rates were expressed as a fraction of rice use rate of 125 g ai ha–1.
The yield response of grapevines treated with tetflupyrolimet fractional rates did not differ (P < 0.05) from the nontreated control vines, which was approximately 23.6 kg vine–1 in 2022 and 16.6 kg vine–1 in 2023 (Table 1). Grape yield was approximately 6.85 kg vine–1 lower overall in 2023 than 2022, including nontreated control vines, possibly related to a cooler (average air temperature ∼1.5 C lower) season in 2023. Likewise, grape sugar content was similar among all treatments and ranged at approximately 20–21°Bx in 2022 and 2023 harvest (Table 1). The insignificant differences overall indicate that tetflupyrolimet drift events did not cause meaningful damage to grape yield or brix levels.
Grape yield and sugar concentration response to tetflupyrolimet simulated drift rates in 2022 and 2023 growing seasons near Davis, California.a,b
a Abbreviation: NTC, nontreated control treatment.
b There were no significant differences within each column at P < 0.05 according to Tukey’s honestly significant difference post hoc test.
c Tetflupyrolimet rate is expressed as a fraction of the rice use rate, 125 g ai ha–1.
d Yield is reported as average mean, where parentheses represent se.
e One degree °Bx is 1 g of sucrose in 100 g of solution, where parentheses represent SE.
Under most conditions, realistic herbicide drift rates range from below 1/100× up to 1/33× of the field use rates of herbicides (Al-Khatib and Peterson Reference Al-Khatib and Peterson1999; Inci et al. Reference Inci, Hanson and Al-Khatib2024a). This research included tetflupyrolimet at rates up to 1/10× or 1× in these crops to evaluate a worst-case scenario such as consecutive drift events, an accidental herbicide application, or herbicide-contaminated tank, events that are unlikely to happen in a typical drift situation. Even at these exaggerated rates, almond, grape, peach, pistachio, plum, and walnut crops were not injured by tetflupyrolimet exposure. This simulated drift research was conducted using a constant spray volume with variable rates including field use rate on tree nut crops, which is different from actual drift scenarios where both concentration and volume change as herbicides move off target. Banks and Schroeder (Reference Banks and Schroeder2002) suggested that droplet concentration can affect crop injury; however, understanding the relative sensitivity of the stone fruit, tree nut, and vine crops species grown near California rice fields is highly relevant to stewardship of tetflupyrolimet in the Sacramento and San Joaquin valleys (Inci et al. Reference Inci, Hanson and Al-Khatib2024b). Anticipated application advisories for tetflupyrolimet in rice systems should also help prevent the off-target movement of herbicides to nontarget crops (Anonymous 2024). Together, these datasets suggest that tetflupyrolimet can be safely used for early-season weed management in rice fields with normal off-target herbicide drift precautions, as noted on the herbicide labels.
Practical Implications
Owing to its outstanding grass activity and crop safety on rice, tetflupyrolimet is expected to be widely used in rice cropping systems (Lombardi and Al-Khatib Reference Lombardi and Al-Khatib2024). Anticipated rice field applications for tetflupyrolimet are as pre-plant, preemergence, or early postemergence up to the 3-leaf stage for grasses (Anonymous 2024; Lombardi and Al-Khatib Reference Lombardi and Al-Khatib2024). This research suggests that tetflupyrolimet is not likely to cause significant injury to nearby tree and vine crops if off-target drift occurs. Given the apparent low risk of crop injury to tree crops, tetflupyrolimet might be of interest for future registration for use on these crops. As a new site of action, tetflupyrolimet could help manage weeds in orchards and vineyards that are difficult to control, such as glyphosate-resistant grasses. Further studies should investigate the use of tetflupyrolimet in floor management of orchard and vineyard production systems.
Acknowledgments
We thank Seth Watkins and the University of California–Davis weed science graduate students for their assistance with the fieldwork.
Funding
This research was funded by the California Rice Research Board, the University of California Melvin D. Androus Endowment and the University of California, Davis Henry A. Jastro–Shields Graduate Research Award.
Competing Interests
The authors declare they have no competing interests.
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