Introduction
Weeds can cause significant yield losses by competing with crops for critical resources (Oerke Reference Oerke2006). Species in the genus Echinochloa are among the most problematic weeds in rice production worldwide. Traits such as high seed production potential, seed dormancy, and genetic diversity enable Echinochloa plants to adapt to a wide range of environmental conditions (Lopez-Martinez et al. Reference Lopez-Martinez, Salva, Finch, Marshall and De Prado1999; Maun and Barrett Reference Maun and Barrett1986). In Texas rice fields, junglerice [Echinochloa colona (L.) Link.] is reported to be the most predominant Echinochloa species, followed by barnyardgrass [E. crus-galli (L.) P. Beauv.] and rough barnyardgrass [E. muricata (P. Beauv.) Fernald] (Liu et al. Reference Liu, Singh, Abugho, Lin, Zhou and Bagavathiannan2021b). Junglerice and barnyardgrass exhibit similar growth characteristics and are often difficult to distinguish at the seedling stage, leading to their names being used interchangeably by farmers. Echinochloa spp. can cause yield losses of up to 80% in rice if not adequately controlled (Smith Reference Smith1983). When emerging with the rice crop, barnyardgrass can produce up to 39,000 seeds plant−1, although seed production can vary greatly across environments (Bagavathiannan et al. Reference Bagavathiannan, Norsworthy, Smith and Neve2011).
Rice production in Texas and other southern U.S. states is highly mechanized, and the direct-seeded system (dry-seeded, with delayed flooding at the 5- to 6-leaf stage) is common (Hill et al. Reference Hill, Bayer, Bocchi and Clampett1991; Rao et al. Reference Rao, Johnson, Sivaprasad, Ladha and Mortimer2007). The phenology of Echinochloa seedlings closely resemble rice seedlings, and selective management through cultural/mechanical approaches is a challenge (Barrett Reference Barrett1987). The use of herbicides has been the most preferred method to control Echinochloa spp. in rice. Although Echinochloa can be selectively controlled in rice by propanil, repeated use of this herbicide over time has led to the evolution of propanil-resistant Echinochloa biotypes. Barnyardgrass was first reported to have evolved resistance to propanil in 1990 in Arkansas (Carey et al. Reference Carey, Hoagland and Talbert1995). Subsequently, propanil-resistant barnyardgrass was reported in Texas, Missouri, and Louisiana in 1991, 1994, and 1995, respectively (Heap Reference Heap2025). Quinclorac was introduced in 1992 to control propanil-resistant barnyardgrass and soon became a widely used substitute for propanil (Malik et al. Reference Malik, Burgos and Talbert2010). However, only a few years later, barnyardgrass resistance to quinclorac was reported in Louisiana in 1998 (Heap Reference Heap2025). Barnyardgrass resistance to more than one herbicide site of action (SOA) then appeared. The first case of multiple herbicide–resistant barnyardgrass was reported in 1999 in Arkansas, with resistance to both propanil and quinclorac (Lovelace et al. Reference Lovelace, Talbert, Hoagland and Scherder2007). In 2000, a barnyardgrass population was reported being resistant to inhibitors of acetolactate synthase (ALS) and lipid synthesis in California (Fischer et al. Reference Fischer, Ateh, Bayer and Hill2000). Under current herbicide use scenarios to control Echinochloa spp. in rice, the risk of the development of resistance to multiple herbicides is high (Bagavathiannan et al. Reference Bagavathiannan, Norsworthy, Smith and Neve2014). A barnyardgrass population with multiple resistance to four herbicide SOAs (including inhibitors of acetyl-CoA carboxylase [ACCase], ALS, photosystem II [PS II], and auxin mimics) was documented in 2011 in Mississippi (Heap Reference Heap2025). The number of effective herbicide options available for weed control in rice has been reduced due to cases of multiple resistance.
Diversifying the SOA of herbicide programs used for weed management is key to delaying the evolution of herbicide resistance in weed populations (Jutsum and Graham Reference Jutsum and Graham1995; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos and Witt2012). Understanding weed response to herbicides offers valuable insights for developing effective management strategies. The herbicide resistance status of Echinochloa spp. in Texas rice production is not well understood. This study aimed to assess the resistance status to clomazone (WSSA Group 13), fenoxaprop (Group 1), propanil (Group 5), imazethapyr (Group 2), and quinclorac (Group 4) in Echinochloa populations collected from rice production fields in Texas. (Herbicides are categorized into groups by the Herbicide Resistance Action Committee and the Weed Science Society of America [WSSA].)
Materials and Methods
Plant Material
A total of 60 Echinochloa populations were collected during a rice field survey across the rice-growing counties in Texas (Figure 1) from late July to early August 2015 and 2016. The survey specifically targeted late-season weeds prior to rice harvest that had escaped the in-season management program. The field survey used a semistratified survey methodology, and the survey sites were randomly selected on a Google map using ITN Converter software (v.1.88; Benichou Software, https://itn-converter.software.informer.com/download/) without prior information regarding the distribution of weed escapes in each field or their resistance status. Approximately 20 mature Echinochloa spp. seed heads from each field, including samples from inside of the field and field edges, were collected and pooled into a single composite sample. More details of the survey methodology can be found in Liu et al. (Reference Liu, Singh, Zhou and Bagavathiannan2021a). Table 1 in Liu et al. (Reference Liu, Singh, Zhou and Bagavathiannan2021a) provides the GPS coordinates for all the sample collection locations. Harvested samples were bagged and dried in a hot-air oven at 50 C for 48 h, then hand thrashed, cleaned, and stored prior to use in herbicide assays. A subsequent investigation identified all but two populations as E. colona (Liu et al. Reference Liu, Singh, Abugho, Lin, Zhou and Bagavathiannan2021b). Consequently, this manuscript focuses exclusively on characterizing the herbicide resistance status of the 58 E. colona populations.
Echinochloa sampling sites (blue dots) in Texas rice-producing counties (highlighted in green). Rice fields in all counties were randomly surveyed at crop maturity (preharvest), but Echinochloa plants were available in harvestable quantities only at the sites indicated by blue dots.
Details of the herbicides used in resistance evaluationsa.
a Abbreviations: ACCase, acetyl CoA carboxylase; ALS, acetolactate synthase; COC, crop oil concentrate; MSO, methylated seed oil; PS II, photosystem II.
b Herbicides groups are assigned by the Weed Science Society of America.
c Manufacturer locations: BASF Corporation, Research Triangle Park, NC; FMC Corporation, Philadelphia, PA; Gowan Company, Yuma, AZ; RiceCo LLC, Memphis, TN.
d Applied preemergence.
Herbicide Assays
Whole-plant herbicide assays were conducted at the Norman Borlaug Center for Southern Crop Improvement Greenhouse Research Facility at Texas A&M University, College Station, Texas, in 2016 and 2017. Seedlings were grown in a 30/26 C day/night temperature regime with a 14-h photoperiod. Screenings were conducted for two preemergence herbicides: clomazone (Command 3ME; WSSA Group 13) and quinclorac (Facet 75 DF; WSSA Group 4); and four postemergence herbicides: propanil (RiceShot; WSSA Group 5), fenoxaprop (Ricestar HT; WSSA Group 1), imazethapyr (Newpath; WSSA Group 2), and quinclorac (Facet 75 DF; WSSA Group 4). The application rates followed the recommended label rate (1×) for each herbicide, along with recommended adjuvants, as outlined in Table 1. The actual number of E. colona populations screened for each herbicide varied from 48 to 58 (Table 2), depending on seed availability. The experimental units were arranged in a completely randomized design, with four replications and two independent experimental runs.
Herbicide resistance profiling of the junglerice populations collected from Texas rice fields.
a Herbicide application rates are the labeled rates listed in Table 1.
b Junglerice was considered resistant when 0% to 50% injury was observed; less sensitive indicates 51% to 90% injury; susceptible indicates 91% to 100% injury. A population is considered resistant even if only one survivor showed ≤50% injury. Injury scoring was carried out as an overall rating on all surviving plants within a pot.
c Total number of populations evaluated for each herbicide. The numbers varied due to seed availability or germination issues.
For postemergence herbicide evaluations, seeds of each population were broadcast-planted into pots (10 cm diam, 9 cm deep) containing a commercial potting medium (Sun Gro Sunshine LC1 Grower Mix with RESiLIENCE). Approximately 15 to 20 seeds were planted in each pot, and the emerged seedlings were thinned to five per pot at the 1- to 2-leaf stage. Treatments were applied at the 2- to 3-leaf growth stage using a track-sprayer equipped with an XR8002VS nozzle (Teejet Spraying Systems, Glendale Heights, IL), delivering 140 L ha−1 of herbicide solution at 276 Kpa and 4.8 km h−1. A nontreated control and a susceptible check were included for comparison. At 21 d after application (DAA), the number of plants that survived in each pot (out of the five total plants) was recorded. For this purpose, any plant with ≤90% injury compared with the nontreated check was considered alive. The survival frequency (i.e., the percentage calculated from the number of plants that survived out of the total 40 plants treated in two runs) reflects the progression of resistance within a population; a low frequency indicates that resistance is detectable through assays but is still in the early stages of evolution and may not be easily noticed by growers. Furthermore, the visible injury was assessed on surviving seedlings at the pot level on a scale of 0% to 99% compared with the nontreated pots, where 0% indicated no injury and 99% represented near plant death. If all plants in a pot were completely killed, then it was scored as 100% injury. Injury scoring was carried out as an overall rating on all surviving plants within a pot.
For preemergence herbicide evaluations, soil in an Echinochloa-free area with no herbicide application in the previous 2 yr was collected from the Texas A&M field research farm (30.46°N, 96.43°W). The soil type was Weswood silty clay loam (thermic Udifluventic Haplustepts); 8.0 pH; and with 29% sand, 42% silt, and 29% clay. Soil was kept moist for 2 to 3 wk prior to use in the preemergence assays to ensure that the soil was void of an Echinochloa seedbank. The field soil was then filled into individual pots (10 cm diam, 9 cm deep), with exactly 25 seeds planted in each pot. A nontreated control for each population, along with a susceptible check, was included for comparison. Herbicides were applied using a track sprayer, with application parameters identical to those of the postemergence applications. Immediately after application, the pots were watered to activate the preemergence herbicides. At 21 DAA, the number of successfully emerged seedlings and visible injury (%) of the emerged seedlings compared with the nontreated check were recorded in each pot. Percent control was calculated based on the number of seedlings that emerged in the herbicide-treated pots compared with seedling numbers in the nontreated pots for the same population.
Dose-Response Assays
Dose-response assays were conducted on the junglerice populations that individually exhibited the highest resistance to the postemergence applications of fenoxaprop, imazethapyr, and propanil. A dose-response assay could not be conducted for quinclorac due to a limited seed supply. A total of three resistant and three susceptible populations were selected, one for each herbicide. Seedlings of these populations were grown in pots (10 cm diam, 9 cm deep) in the Texas A&M University greenhouse using the same procedures outlined above for the postemergence herbicide screening experiment. The resistant populations were treated with eight rates (0×, 0.5×, 1×, 2×, 4×, 8×, 16×, and 32× the labeled rate) of each of the test postemergence herbicides, whereas the susceptible populations were treated with seven rates (0×, 0.0625×, 0.125×, 0.25×, 0.5×, 1×, and 2× the labeled rate) of each herbicide. Four pots (five seedlings per pot) were included for each herbicide dose, and the treatments were arranged in a completely randomized design. The experiment was repeated in time. Survival (number of plants alive in each pot) and visible injury ratings on the survivors (0% to 99%) were recorded at 21 DAA as described above.
Data Analysis
Based on the visible injury ratings for each herbicide, the populations were grouped into being either resistant (0% to 50% injury), less sensitive (51% to 90% injury), or susceptible (91% to 100% injury). A population was considered resistant even if only one survivor showed ≤50% injury. The survival frequency distribution for each herbicide is illustrated using a boxplot (Figure 2). Dose-response curves were fit to the injury data using a three-parameter log-logistic function (Equation 1) using SigmaPlot software (v.13; Systat Software, Inc., San Jose, CA):
$${\rm{Y}} = {\rm{c}}/\left\{ {1 + {{\rm{e}}^{[ - {\rm{a}}({\rm{x}} - {\rm{b}})]}}} \right\}$$
where c is the upper asymptote, b is the inflection point, a is the slope or rate parameter, and x is the dose. The effective herbicide dose that caused a 50% injury/control (ED50) was estimated from the regression equation. Resistance ratios (R/S) were calculated by dividing the respective ED50 values of the resistant population by the ED50 of the susceptible control.
Within-population survival frequency distribution of junglerice to the postemergence herbicides fenoxaprop, imazethapyr, propanil, and quinclorac. The survival frequency reflects the progression of resistance within a population. For example, a 50% survival indicates that half of the individuals in the given population are resistant to the herbicide, and resistance is highly noticeable in the production field. A low frequency indicates that resistance is detectable through assays but is still in the early stages of evolution and may not be easily noticed by growers. The application rates for each herbicide are the labeled rates listed in Table 1.
Results and Discussion
Distribution of Junglerice in Texas Rice Production
Most of the junglerice populations were collected from Chambers, Colorado, Jackson, Jefferson, Lavaca, and Wharton counties (Figure 1). Colorado and Wharton counties accounted for the greatest number of populations. Field surveys conducted by Liu et al. (Reference Liu, Singh, Zhou and Bagavathiannan2021a) reported that junglerice was the top late-season weed escape (28% frequency of occurrence) with the highest infestation (13% average field area infestation) among all the weed species found in Texas rice production.
Junglerice Response to Clomazone
All collected junglerice populations were susceptible to clomazone at the 1× label rate (Table 2). Clomazone was originally introduced as an alternative herbicide to control barnyardgrass populations that were resistant to propanil and quinclorac (Talbert and Burgos Reference Talbert and Burgos2007). Clomazone is a preemergence herbicide option, often followed by other postemergence herbicides in weed management programs to achieve annual grass control and to broaden the weed control spectrum (Willingham et al. Reference Willingham, Falkenberg, McCauley and Chandler2008; Zhang et al. Reference Zhang, Webster and Blouin2005). Clomazone is listed as the most recommended preemergence herbicide by crop consultants in Arkansas (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos and Witt2012) and Texas (Liu et al. Reference Liu, Singh, Zhou and Bagavathiannan2021a). As a preemergence herbicide, it provides excellent control of grass weeds such as Echinochloa spp., Urochloa spp., and Leptochloa spp. However, barnyardgrass resistance to clomazone was already confirmed in Arkansas rice production in 2008 (Norsworthy et al. 2008). Rouse et al. (Reference Rouse, Burgos, Norsworthy, Tseng, Starkey and Scott2018) also reported that approximately 2% of the 450 Echinochloa accessions collected in Arkansas from 2006 to 2016 were resistant to clomazone. Although resistance to clomazone has not been found among the junglerice populations in Texas, the risk of resistance should not be overlooked when making weed control decisions.
Junglerice Response to Quinclorac
None of the 58 populations were classified as being resistant to quinclorac when it was applied preemergence, although 62% of the populations were less sensitive at the 1× rate, and 38% were susceptible (Table 2). When quinclorac was applied postemergence, 45% of the 54 tested populations were found to be resistant (0% to 50% injury), 32% were less sensitive (51% to 90% injury), and 23% were susceptible (91% to 100% injury). Furthermore, the postemergence resistant populations exhibited a high survival frequency (median ∼40%) (Figure 2), indicating that resistance is in the advanced stages of evolution in many rice fields and is readily observable under field conditions. This finding suggests that junglerice is generally more sensitive to preemergence applications of quinclorac than to postemergence applications, suggesting that preemergence applications remain largely effective for managing populations with postemergence quinclorac resistance. Similarly, Randell-Singleton et al. (Reference Randell-Singleton, Hand, Vance, Wright-Smith and Culpepper2024) reported PPO-inhibitor herbicides applied preemergence were more effective than those applied postemergence to control a Palmer amaranth population in Georgia.
Quinclorac has been known to provide excellent control of Echinochloa spp. that are resistant to propanil; this has been reported since 1990 in Arkansas, Louisiana, Missouri, and Texas. However, reports of resistance to quinclorac soon became common after the first resistant population was identified in Louisiana in 1998 (Heap Reference Heap2025). In the following year, the barnyardgrass population collected in Arkansas was found to be resistant to both quinclorac and propanil at 16× the recommended rates (Lovelace Reference Lovelace2003). Quinclorac resistance was identified as the second most prevalent issue in Echinochloa spp., following propanil resistance, in rice in Arkansas (Rouse et al. Reference Rouse, Burgos, Norsworthy, Tseng, Starkey and Scott2018). Among the 450 Echinochloa populations collected from Arkansas from 2006 to 2016, 23% exhibited resistance to quinclorac (Rouse et al. Reference Rouse, Burgos, Norsworthy, Tseng, Starkey and Scott2018). These findings suggest that quinclorac resistance is widespread in Echinochloa spp. in U.S. rice production regions. According to a survey conducted among Texas rice growers, quinclorac ranked second after clomazone as the preemergence herbicide most often applied (52% frequency), and the postemergence herbicide most often applied (72% frequency). Rice growers in Texas should exercise caution when relying solely on quinclorac for postemergence control of junglerice, because resistance may develop over time.
Junglerice Response to Fenoxaprop
Of the 58 populations evaluated, 7% were resistant (0% to 50% injury) and 30% were less sensitive (51% to 90% injury) to fenoxaprop (Table 2). Among the postemergence herbicides, resistance to fenoxaprop was the least commonly detected in this study. To date, fenoxaprop resistance in Echinochloa spp. has been reported in Bolivia, Costa Rica, Nicaragua, China, and South Korea (Heap Reference Heap2025). In the United States, multiple resistance involving fenoxaprop by Echinochloa spp. has been reported in Arkansas and California (Fischer et al. Reference Fischer, Ateh, Bayer and Hill2000; Norsworthy et al. Reference Norsworthy, Bond and Scott2013). The low frequency of survival to fenoxaprop in resistant populations observed in the current study indicates that these populations are in the early stages of developing resistance (Figure 2).
Out of the four populations classified as being resistant, one was further characterized in a dose-response experiment. The ED50 values from the dose-response assays for fenoxaprop-resistant and -susceptible populations were 132 and 43 g ai ha−1, respectively (Table 3). The R/S values derived from the ED50 values of the resistant and susceptible populations indicated that the resistant population was 3-fold more resistant to fenoxaprop than the susceptible standard (Table 3; Figure 3A), which is considered low. A population of E. phyllopogon from California was confirmed to show resistance to fenoxaprop (ED50 110 g ai ha−1), which was above the label rate (86 g ai ha−1), but 10-fold less sensitive to this herbicide compared with the susceptible standard (ED50 11 g ai ha−1) (Bakkali et al. Reference Bakkali, Ruiz-Santaella, Osuna, Wagner, Fischer and De Prado2007). Fenoxaprop is still considered an effective herbicide for selective grass control in rice (Stoltenberg et al. Reference Stoltenberg, Gronwald, Wyse, Burton, Somers and Gengenbach1989). Rice growers in Texas should be good stewards of this chemistry to limit the spread of resistance, thereby maintaining its effectiveness in controlling weeds.
ED50 values and resistance ratios for the highly resistant junglerice populations sampled in Texas rice fieldsa.
a Abbreviations: ED50, the effective herbicide rate that caused 50% plant injury; R, the population that exhibited the highest resistance (i.e., the lowest injury) in the initial 1× screening; S, susceptible standard; R/S is the resistance ratio, which was derived based on the ED50 values of the resistant population relative to the susceptible standard; RMSE, root mean square error.
b Resistant and susceptible populations for each herbicide are different.
c The R/S ratio could not be developed for junglerice populations that were resistant to imazethapyr and propanil because 100% injury could not be achieved at the highest rate (32×) tested.
Dose-response analyses showing the percent control (i.e., injury %) of the highly resistant and susceptible junglerice populations to A) fenoxaprop, B) imazethapyr, and C) propanil, at the recommended field rates of 86, 105, and 4,484 g ai ha−1, respectively. The resistant and susceptible populations for each herbicide differ.
Junglerice Response to Imazethapyr
Junglerice resistance to imazethapyr was high, with 60% of the 48 tested populations classified as being resistant and 28% exhibiting reduced sensitivity at the 1× recommended label rate (Table 2). Additionally, the frequency of survival was also high within the resistant populations (median ∼42%; Figure 2), indicating an advanced state of resistance evolution to this herbicide. Clearfield rice is widely grown in Texas (Liu et al. Reference Liu, Singh, Zhou and Bagavathiannan2021a). The evolution of imazethapyr resistance has coincided with the widespread use of the Clearfield rice technology since it was commercialized in 2002. Rouse et al. (Reference Rouse, Burgos, Norsworthy, Tseng, Starkey and Scott2018) have found Echinochloa resistance to ALS inhibitors in 114 accessions from 20 counties in Arkansas. The authors have also reported that <10% of the Echinochloa populations tested in Arkansas were confirmed to be resistant to one or more ALS-inhibitor herbicides before 2011, but from 2013 to 2016, the number of resistant populations had increased by more than 20%.
The ED50 values for the junglerice population could not be determined from the dose-response curves. The populations exhibited high resistance to imazethapyr because 50% control was not achieved even at the highest rate tested (i.e. 32×, Figure 3). The ED50 value of the imazethapyr-susceptible population was 48 g ai ha−1 (Table 3). The calculated R/S ratio revealed that the resistant population was >70-fold more resistant to imazethapyr than the susceptible check (Table 3). A similar case was reported in four Echinochloa populations from Italy, which exhibited cross-resistance to the ALS-inhibitor herbicides penoxsulam and imazamox (Panozzo et al. Reference Panozzo, Scarabel, Tranel and Sattin2013). The highest applied herbicide dose failed to achieve 50% control compared with the untreated standard, with R/S (ED50) ratios exceeding 25 for penoxsulam and 56 for imazamox. The high resistance levels observed in the present study suggest that resistance is robust even at very high application rates. The high level of resistance expressed in this population indicates a possible target site resistance mechanism may be at work.
Junglerice Response to Propanil
Propanil has been one of the most effective herbicides for controlling grass weeds in rice production since its commercialization nearly 30 yr ago (Smith and Hill Reference Smith and Hill1990). It is still used in rice production, but its effectiveness has decreased drastically due to the widespread and frequent evolution of propanil resistance in weeds such as Echinochloa spp. All 50 junglerice populations evaluated in this study exhibited resistance to propanil (0% to 50% injury), with high frequencies (70% to 100%) of plant survival within a population (Figure 2). These results corroborate the findings reported by Rouse et al. (Reference Rouse, Burgos, Norsworthy, Tseng, Starkey and Scott2018) in Arkansas, where the long history of propanil use has led to a high incidence of Echinochloa resistance. Among the 450 populations collected from 2006 to 2016, 57% were found to be resistant to propanil. Additionally, a stakeholder survey carried out by Norsworthy et al. (Reference Norsworthy, Bond and Scott2013) in Arkansas reported that 58% of respondents identified propanil-resistant barnyardgrass as the most prevalent weed in their rice fields.
The ED50 value could not be calculated for the propanil-resistant population identified in this study because 50% control was not achieved even at the highest application rate (i.e., 32×; Figure 3). The ED50 value of the propanil-susceptible population was 2,428 g ai ha−1 (Table 3). The R/S ratio, calculated based on the ED50 of the susceptible population, indicated that the resistant population was more than 15-fold resistant than the susceptible check (Table 3; Figure 3).
Junglerice Resistance to Multiple Herbicide Sites of Action
Multiple resistance to more than one herbicide SOA is potentially present in a few accessions documented in this study (Table 4). Among the 58 junglerice populations tested, 5% exhibited resistance to all four postemergence herbicides evaluated. Fifteen percent of the populations exhibited three-way resistance to propanil, imazethapyr, and quinclorac. Forty percent exhibited multiple resistance to two herbicide SOAs, with 23% demonstrating resistance to propanil and imazethapyr, 15% to propanil and quinclorac, and 2% to propanil and fenoxaprop (Table 4). Only 40% of the populations showed a single SOA resistance (propanil, quinclorac, or imazethapyr); the fenoxaprop-resistant populations were also resistant to propanil (Table 4).
Resistance to multiple or single herbicide sites of action by junglerice populations collected from Texas rice fieldsa.
a Abbreviations: R, resistant; SOA, site of action.
b Proportion of a specific resistance type among all resistance cases confirmed in the study (total 100%). No population exhibited resistance to fenoxaprop only.
Multiple herbicide resistance by Echinochloa has been widely documented in rice production worldwide (Heap Reference Heap2025), with reports of two-way resistance in 11 cases, three-way resistance in six cases, and four-way resistance in one case. The four-way resistant population documented in Mississippi has exhibited resistance to fenoxaprop, imazamox, imazethapyr, quinclorac, and propanil. The scenario of multiple herbicide resistance by junglerice in Texas rice fields closely mirrors the situation in the Mississippi Delta region. Malik et al. (Reference Malik, Burgos and Talbert2010) documented barnyardgrass resistance to propanil and quinclorac in Arkansas. These populations were collected from farms with a history of propanil use exceeding 20 yr and quinclorac use for more than 5 yr. Propanil resistance, first reported in the early 1990s (Carey et al. Reference Carey, Hoagland and Talbert1995), was attributed to enhanced detoxification by the enzyme arylacylamidase (Carey et al. Reference Carey, Hoagland and Talbert1997). By the time growers adopted quinclorac, many Echinochloa populations were likely already resistant to propanil. This trend continued, including in Texas rice production, leading to multiple resistance to several effective herbicides. The emergence of multiple herbicide resistance in Echinochloa poses a significant threat to the sustainability of rice production in the region because it accelerates the loss of effective control options for this species.
Practical Implications
This study has highlighted the widespread occurrence of herbicide resistance by junglerice in Texas rice production. As reported by Liu et al. (Reference Liu, Singh, Zhou and Bagavathiannan2021a, Reference Liu, Singh, Abugho, Lin, Zhou and Bagavathiannan2021b) through both field and stakeholder surveys and taxonomic confirmation, junglerice is the most predominant and problematic weed species in Texas rice production. The surveys indicated that clomazone was the most widely used preemergence herbicide; however, resistance was not documented in any of the populations studied. Similarly, use of quinclorac as a preemergence herbicide in Texas rice production is common, but no resistance was detected in the current study. It should be noted that the present resistance reporting is based on seeds collected approximately 10 yr ago (2015–2016), and the current resistance situation may be different. This highlights the need for follow-up surveys, with the current report serving as a historical trend and reference point.
Field and farmer surveys have also indicated that junglerice is the most common late-season weed escape, infesting an average of about 13% of an individual rice field area (Liu et al. Reference Liu, Singh, Zhou and Bagavathiannan2021a). The resistance screening data from the current study support these observations, showing a high frequency of survivors within each tested population, particularly for the herbicides imazethapyr, propanil, and quinclorac (Figure 2). These results suggest that resistance is already at an advanced stage of evolution, and a significant portion of the late-season junglerice escapes are likely to be resistant to herbicides.
The observed resistance of junglerice to commonly used herbicides such as fenoxaprop, imazethapyr, propanil, and quinclorac, and the presence of multiple herbicide resistance in several populations, suggest the critical need for the development and implementation of best resistance management practices. Given that multiple resistance to herbicides with different SOAs is already a significant problem, rice producers should prioritize the use of herbicides with alternative SOAs, which will help reduce the selection pressure exerted by any single herbicide SOA.
Use of preemergence herbicides should be integrated into the management programs, as was highlighted by Norsworthy et al. (Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos and Witt2012), as a key best management practice for mitigating resistance. Preemergence application of herbicides such as clomazone, which showed no resistance by junglerice populations, should be considered. Even though clomazone resistance has not yet been detected in Texas, the possibility of future resistance should not be overlooked, and its use should be rotated with other herbicide SOAs to prevent resistance from evolving. Clomazone-resistant Echinochloa spp. has already been reported to occur in Arkansas (Norsworthy et al. Reference Norsworthy, Scott and Smith2007). Integrating quinclorac with a focus on preemergence applications can be an effective component of junglerice management, as preemergence quinclorac applications provided a generally high level of E. colona control in this study, even on the populations that showed postemergence resistance. However, the increasing number of populations exhibiting reduced sensitivity to preemergence quinclorac applications warrants caution and further investigation. This study also highlights the low within-population frequency of resistance to fenoxaprop, emphasizing the importance of timely management interventions to prevent further spread.
Crop rotation and the use of nonchemical control measures are additional tools that should be incorporated into management practices. Diversifying weed control methods can reduce reliance on herbicides, thereby slowing the rate at which resistance develops. There is also a vital need to implement resistance monitoring practices, which involve regularly testing weed populations for resistance to the herbicides they use. This will enable the early detection of resistance, allowing for timely adjustments to management strategies before resistance becomes widespread. Furthermore, understanding the biology and ecology of junglerice populations can aid in the development of robust management interventions. Education and outreach programs should be implemented to keep growers informed about the growing issue of herbicide resistance and to promote the adoption of integrated weed management strategies. Taking a proactive approach to herbicide resistance can help ensure the long-term sustainability of rice production in the region, maintain effective control of junglerice and other problematic weeds, and reduce the economic burden caused by herbicide-resistant weed populations on the rice industry.
Funding
Funding for this study was provided by the Texas Rice Research Foundation.
Competing Interests
The authors declare they have no competing interests.
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