Introduction
Peanut is one of many agricultural commodities that face weed management challenges in an era of herbicide resistance. Producers in the southeastern United States rely heavily upon herbicides such as flumioxazin, a protoporphyrinogen oxidase (PPO) inhibitor in the n-phenylphthalimide family, to target Palmer amaranth (Amaranthus palmeri S. Watson) and other small-seeded broadleaf weeds (UGA Extension 2025; Whitaker et al. Reference Whitaker, York, Jordan, Culpepper and Sosnoskie2011). The intensive use of herbicide chemistries to manage the diversity of troublesome weeds in peanut-producing regions threatens the long-term efficacy of an already limited pool of herbicidal options (Neve et al. Reference Neve, Norsworthy, Smith and Zelaya2011; Vencill et al. Reference Vencill, Nicholas, Webster, Soteres, Mallory-Smith, Burgos, Johnson and McClelland2012). As concerns begin to surface regarding PPO-resistant Palmer amaranth, the loss of flumioxazin as a weed control option would negatively affect peanut weed management (Culpepper and Vance Reference Culpepper and Vance2019; Randell-Singleton et al. Reference Randell-Singleton, Hand, Vance, Wright-Smith and Culpepper2024).
Integrated strategies are needed to delay the evolution of herbicidal resistance (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nicholas, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barret2012). One of the foremost strategies for combatting resistance challenges is the use of herbicide diversity through multiple modes of action (MOAs) (Hill et al. Reference Hill, Norsworthy, Barber and Gbur2016). A common practice in peanut production often includes the application of a preplant burndown of glyphosate (WSSA Group 9) + 2,4-D (WSSA Group 4), followed by a preemergence application of paraquat (WSSA Group 22) + pendimethalin (WSSA Group 3) + flumioxazin (WSSA Group 14) + diclosulam (WSSA Group 2), and a postemergence application of imazapic (WSSA Group 2) + S-metolachlor (or some other WSSA Group 15 herbicide) + 2,4-DB (WSSA Group 4) (UGA Extension 2025). (Herbicide groups are categorized by the Weed Science Society of America [WSSA].) This practice demonstrates a diverse portfolio; however, glyphosate (Culpepper et al. Reference Culpepper, Grey, Vencill, Kichler, Webster, Brown, York, Davis and Hanna2006), acetolactate synthesis (ALS), and PPO-resistant biotypes reduce confidence in their long-term efficacy. The development of MOAs for peanut is needed, thereby necessitating the need to investigate new chemistries and to repurpose other herbicides for potential uses in peanut production.
Before fluridone was registered for use on peanut crops in 2023, for several decades it was used as a systemic herbicide to selectively manage invasive submersed aquatic vegetation, and more recently, as a for use as a preemergence herbicide in cotton (Gossypium hirsutum L.) production to control Palmer amaranth, annual grasses, and other small-seeded broadleaf weeds (Anonymous 2023; Braswell et al. Reference Braswell, Cahoon, Seagroves, Jordan and York2016; Grichar et al. Reference Grichar, Dotray and McGinty2020; Miller and Carter Reference Miller and Carter1983; Rasmussen et al. Reference Rasmussen, Conrad, Green, Khanna, Wright, Hoffmann, Caudill and Gilbert2022; UGA Extension 2025). The intended purpose of adding fluridone to the peanut weed management toolbox was to target similar weed species with a new MOA. As a phytoene desaturase inhibitor (PDI) (WSSA Group 12), when applied, susceptible plants exhibit bleaching in both leaf and vegetative structures as a result of inhibition of pigment biosynthesis, which provides critical functions for photoregulation (Bartels and Watson Reference Bartels and Watson1978; Bartley and Scolnik Reference Bartley and Scolnik1995; Zou et al. Reference Zou, Zou, Zhao, Xia, Qian, Wang and Yin2018).
Recent research demonstrated acceptable peanut tolerance and weed control with preemergence applications of fluridone when used in a weed management program (Abbott et al Reference Abbott, Shay and Prostko2025a). Under normal growing conditions, regardless of cultivar, peanut was tolerant of fluridone when it was applied at rates of 128 g ha−1. Negative peanut stand and yield losses occurred when fluridone rates exceeded 252 g ha−1. Additionally, in years when growing conditions were less than ideal, including greater than normal precipitation and temperatures (>30 C), especially during early vegetative development, peanut metabolic activity was more than likely compromised, thereby increasing peanut sensitivity to fluridone.
Trifludimoxazin, a PPO inhibitor and member of the pyrimidinedione family, is registered for use on several agronomic and tree fruit crops, and nonagricultural lands (Anonymous 2021a, 2021b). Research conducted by Abbot et al. (Reference Abbott, Shay and Prostko2025b) showcased the tolerance of peanut to trifludimoxazin and noted that adding it to the peanut weed management arsenal will benefit users with lower use rates and less potential injury compared with other PPO-inhibiting herbicides. However, peanut growers in Georgia and neighboring states already use another PPO inhibitor, flumioxazin, on more than 64% of planted hectares (USDA-NASS 2024). This is concerning for a region that has recently confirmed a PPO-resistant Palmer amaranth population (Armel et al. Reference Armel, Hanzlik, Witschel, Hennigh, Bowe, Simon, Liebl and Makin2017; Randell-Singleton et al. Reference Randell-Singleton, Hand, Vance, Wright-Smith and Culpepper2024). Previous research has shown that the high bioactivity and differential binding site for trifludimoxazin targets herbicide-resistant Palmer amaranth biotypes (Armel et al. Reference Armel, Hanzlik, Witschel, Hennigh, Bowe, Simon, Liebl and Makin2017). However, Randell-Singleton et al. (Reference Randell-Singleton, Hand, Vance, Wright-Smith and Culpepper2024) have documented resistance to trifludimoxazin in Palmer amaranth.
Each year producers face challenges that impede the critical timeliness of applying soil-activated herbicides (Adcock and Banks Reference Adcock and Banks1991). The weather in Georgia during planting season, between April and May, is generally consistently wet, which limits field access and delays the application of broadcast herbicides after planting (Bosch et al. Reference Bosch, Sheridan and Davis1999). In addition to challenging weather patterns, growers commonly encounter logistical challenges, including equipment malfunctions and operational constraints that can also delay timely preemergence applications. Currently, the fluridone label recommends that its application should be restricted to within the initial 36 h after planting because delayed applications have yet to be fully explored. In addition to fluridone, no such recommendation exists yet for Trifludimoxazin has not yet been approved for use on peanut (Anonymous 2023; Johnson et al. Reference Johnson, Prostko and Mullinix2006). Previous research has shown that delaying applications of some preemergence herbicides can impede peanut development and yield (Johnson et al. Reference Johnson, Prostko and Mullinix2006). Currently, limited published information exists regarding peanut response to delayed preemergence applications of fluridone and trifludimoxazin. Therefore, the objective of this study was to compare the response of peanut to timely and delayed preemergence applications of fluridone and trifludimoxazin.
Materials and Methods
Field experiments were conducted at the University of Georgia, Ponder Research Farm near Ty Ty (31.5080°N, −83.6570° W) from 2022 through 2024. The experimental site is nearly level (<2% slope) and consisted primarily of Tifton loamy sand soil with 96% sand, 2% silt, 2% clay, 1.2% organic matter, and an average soil pH of 6.0 (USDA-NRCS 2023). Planting, application, inverting, and harvest dates are presented in Table 1. In all studies, the peanut cultivar Georgia-06G (Branch Reference Branch2007) was planted in conventionally tilled plots that were 1.8 m by 7.6 m in a twin-row configuration, including traditional single rows set to 91.4 cm with an inner twin-row offset at 22.9 cm, delivering 152,460 seeds ha−1. Plots were maintained weed-free with sequential postemergence herbicide applications including imazapic and S-metolachlor + lactofen, followed by clethodim + S-metolachlor (UGA Extension 2025). Supplemental irrigation was applied as needed to maximize crop production with a lateral-irrigation system. Irrigation and rainfall data were captured during the first 21 d after planting (DAP) (Table 2).
Table 1. Test parameters and peanut stages of growth in response to delayed timing applications of fluridone and trifludimoxazin. a,b
a Abbreviation: DAP, days after planting.
b V0 indicates peanut plants did not emerge; E indicates peanut plants did emerge, and the seed radical/root length is shown in centimeters.
Table 2. Irrigation/rainfall total during the first 21 d after peanut planting and applications of fluridone and trifludimoxazin. a
a Planting dates were May 5, 2022; May 2, 2023; and May 1, 2024.
Experiments were conducted in a randomized complete block design with treatments replicated four times. Treatments were applied using a CO2-pressurized backpack sprayer at 140 L ha. Treatments included a factorial arrangement of herbicides: fluridone at 120 g ha−1, or trifludimoxazin at 37 g ha−1 applied at 1, 3, 5, or 7 DAP, plus a nontreated control (NTC).
Documented responses to herbicide treatments included determination of growth stages at 1, 3, 5, and 7 DAP; plant population (number of plants per twin row per 1.5 m); foliar bleaching from fluridone; and foliar necrosis from trifludimoxazin according to common herbicide symptomology; plant height and width (five measurements per plot averaged prior to analysis); and peanut yield. Visible estimates of peanut injury were obtained 13, 30, 50, and 80 DAP using a subjective scale of 0% to 100% where 0% = no injury and 100% = plant death. Peanut yield was obtained by harvesting each plot individually using commercial inverting and harvesting equipment, and adjusted to 10% moisture.
Statistical Analysis
Data were subjected to the GLIMMIX procedure using SAS software (v.9.4; SAS, Cary, NC) (Littell et al. Reference Littell, Milliken, Stroup, Wolfinger and Schabenberger2006). Conditional residuals for control were used for checking assumptions of normality, independence of errors, homogeneity, and multiple covariance structures. Fixed effects included herbicide treatments (nontreated, fluridone, and trifludimoxazin) and application timings (1, 3, 5, and 7 DAP). Location, year, and replicates represented random effects. Treating year as a random effect, data were combined over years and timings. Means were compared using the LSMEANS procedure with a Tukey’s HSD test, with differences considered significant at P < 0.10.
Results and Discussion
Peanut Density
The influence of fluridone and trifludimoxazin timing on the peanut population is presented in Table 3. Photographs of peanut seed and seedlings at the various times are shown in Figure 1. Peanut density observed at 13 DAP indicated no differences compared with the NTC, averaging 25 plants/1.5 m/twin row. Plant populations of 20 to 25 plants/1.5 m of row will usually maximize yield and grade of peanut in twin-row management (Monfort Reference Monfort2022; Sarver et al. Reference Sarver, Tubbs, Beasley, Culbreath, Grey, Rowland and Smith2017).
Table 3. Peanut density 13 d after planting following fluridone and trifludimoxazin applications.a,b
a Means within a column followed by the same letter are not significantly different from each other according to Tukey’s HSD test at P < 0.10. Data were combined over 3 site-years and four application timings (1, 3, 5, 7 d after planting).
b Fluridone was applied at 120 g ha−1, trifludimoxazin was applied at 37 g ha−1.
Figure 1. Peanut seed/seedling development following fluridone and trifludimoxazin applications at 1, 3, 5, and 7 d after planting (DAP).
Peanut Bleaching/Necrosis
Observed herbicide symptomology for fluridone application at 13 and 30 DAP indicated a herbicide by application timing interaction, which is presented in Table 4. Regardless of application timing, fluridone caused a significant increase in bleaching compared with that in the NTC. Applications made at 1, 3, 5, and 7 DAP resulted in bleaching injury levels of 5%, 7%, 14%, and 21%, respectively. Bleaching increased with each delay in fluridone application once it was applied beyond 3 DAP. By 30 DAP, fluridone continued to cause an increase in herbicide symptomology with 5%, 5%, 6%, and 9% bleaching occurring after it was applied at 1, 3, 5, and 7 DAP, respectively. However, a corresponding response with the delay in application occurred when fluridone was applied 7 DAP. Trifludimoxazin did not cause peanut bleaching, and this is not considered a common injury symptom of that herbicide. However, leaf necrosis is. The level of necrosis observed was not influenced by application timing. When averaged over the four application timings, necrosis of 7% and 2% was observed at 13 and 30 DAP, respectively. This type of necrotic symptomology is common on young peanut vegetative structures, and is typically caused by soil splashing, which occurs with other PPO-inhibiting herbicides such as flumioxazin, which is used in peanut production (Abbott et al. Reference Abbott, Shay and Prostko2025b; Johnson et al. Reference Johnson, Prostko and Mullinix2006). By 80 DAP, leaf necrosis was not observable (data not reported).
Table 4. Peanut bleaching and necrosis evaluated 13 and 30 d after planting following fluridone and trifludimoxazin applications 1, 3, 5, and 7 d after planting. a b c d–e
a Abbreviation: days after planting.
b Means within a column followed by the same letter are not significantly different from each other according to Tukey’s HSD test at P < 0.10. Results indicated a fluridone by timing interaction at 13 and 30 DAP. All other treatments indicated there was no timing by herbicide interaction; therefore, data were combined over herbicide timing for each respective herbicide.
c Bleaching was not reported with trifludimoxazin use, but this is not a common herbicide symptomology; necrosis was not reported with fluridone use, but this is not a common herbicide symptomology observed at this stage of growth.
d Trifludimoxazin necrosis data were not collected for 30 DAP in 2023.
e Fluridone was applied at 120 g ha−1; trifludimoxazin was applied at 37 g ha−1.
Peanut Height, Width, and Yield
The influence of fluridone and trifludimoxazin application timing on peanut height, width, and yield is presented in Table 5. There was no year by herbicide by application timing interaction; therefore, data are combined across years and timings. Plant heights at 30 DAP were 11.2, 10.7, and 11.4 cm for fluridone and trifludimoxazin applications, and for the NTC, respectively. At 30 DAP, compared with the NTC, measurements indicated that plant height was reduced by 2% and 6% respectively, when fluridone and trifludimoxazin were applied. By 80 DAP, plant height was similar in plots that received herbicide treatments. Peanut width at 30 DAP was reduced by 6% and 10% with applications of fluridone and trifludimoxazin, respectively, a trend that was similar to that observed with height. Peanut row middles were lapped (i.e., complete canopy closure) in the NTC by 80 DAP, but width was reduced by 4% overall in plants that were treated with fluridone and trifludimoxazin. Regardless of herbicide and application timing, peanut yield was not influenced by herbicide or timing, and ranged from 5,120 to 5,300 kg ha−1. Other research has demonstrated that peanut can tolerate fluridone and trifludimoxazin when they are applied in a timely manner (Abbott et al. Reference Abbott, Shay and Prostko2025a, Reference Abbott, Shay and Prostko2025b).
a Abbreviation: DAP, days after planting.
b Means within a column followed by the same letter are not significantly different from each other according to Tukey’s HSD test at P < 0.10. Data were combined across three-site years and four application timings (1, 3, 5, 7 DAP).
For peanut width, there was an herbicide and timing interaction but no herbicide by timing interaction; therefore, data are presented separately: herbicide, combined across timing; and timing, combined across herbicide.
c Fluridone was applied at 120 g ha−1; trifludimoxazin was applied at 37 g ha−1.
Practical Implications
Fluridone and trifludimoxazin add value to peanut weed management programs by adding chemicals that target new sites of action. As the evolution of herbicide resistance continues to unfold, it is critically necessary to continue to investigate ways to diversity herbicide usage, particularly for row crops such as peanut. This study expands the current understanding of peanut response to fluridone, which has been newly registered for use on peanut crops, and trifludimoxazin, which is expected to receive registration. These observed early season responses increase our understanding of peanut establishment, vegetative growth, and maturation when herbicide applications are delayed. As a result, when peanut growers are confronted with circumstances in which a preemergence application is delayed, they can have the confidence that fluridone or trifludimoxazin will not negatively influence yield when applied up to 7 DAP.
Acknowledgments
This research could not have been conducted without the technical support of Dewayne Dales, Charlie Hilton, and Tim Richards. The support and technical input of A.S. Culpepper, R.C. Kemerait, W.S. Monfort, and T. Randell-Singleton is greatly appreciated.
Funding statement
This research was partially funded in partnership with BASF and SePro Ag.
Competing interests
The authors declare they have no competing interests.
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