Introduction
Barnyardgrass, an allohexaploid grass, has always been considered the most troublesome weed species in rice production in China (Fang et al. Reference Fang, Zhang, Li, Chen and Dong2018, Reference Fang, Liu, Zhang, Li and Dong2019a, 2019b). Season-long competition of barnyardgrass can cause as much as 80% rice yield reduction (Ntanos and Koutroubas Reference Ntanos and Koutroubas2000; Smith Reference Smith1988). A single barnyardgrass plant growing 40 cm from a rice plant can reduce rice yield by 27% (Stauber et al. Reference Stauber, Smith and Talbert1991). The competitive ability of barnyardgrass is largely due to its extended emergence throughout the growing season, high photosynthetic capacity, rapid growth habit, and prolific seed production (Chon et al. Reference Chon, Guh and Pyon1994; Kennedy et al. Reference Kennedy, Barrett, Zee and Rumpho2010; Zhang et al. Reference Zhang, Li, Yang, Gu and Li2015). Therefore, controlling barnyardgrass is believed to be essential for optimum rice production.
Five structurally distinct chemical classes of acetolactate synthase (ALS) inhibitors, including imidazolinones, triazolopyrimidines, pyrimidinylthiobenzoates, and sulfonylamino-carbonyl-triazolinones, have been commercialized (Shaner Reference Shaner2014). These herbicides inhibit the first enzyme in the biosynthesis pathway of branched-chained amino acids including isoleucine, leucine, and valine (LaRossa and Schloss Reference LaRossa and Schloss1984). ALS-inhibiting herbicides are well known for their high efficacy, low use rate, potency, and multicrop selectivity (Shaner Reference Shaner2014). In addition, various formulations have proved to be active on both foliage and in soil, with very low mammalian toxicity (Shaner Reference Shaner2014). These positive attributes have ensured the intensive use of ALS inhibitors over huge farmland areas worldwide for weed control (Powles and Yu Reference Powles and Yu2010). ALS-inhibiting herbicides such as bispyribac, imazethapyr, and penoxsulam, are used for POST control of barnyardgrass in rice (Martini et al. Reference Martini, Burgos, Noldin, De Avila and Salas2015; Ottis et al. Reference Ottis, Chandler and Mccauley2003; Pellerin and Webster Reference Pellerin and Webster2004; Rouse et al. Reference Rouse, Roma-Burgos, Norsworthy, Tseng, Starkey and Scott2017). Unfortunately, the chance is high that barnyardgrass will become resistant to ALS inhibitors (Powles and Yu Reference Powles and Yu2010). During the last decade, resistance to ALS-inhibiting herbicides has increased drastically in various cropping systems (Fang et al. Reference Fang, Liu, Zhang, Li and Dong2019a, 2019b; Heap Reference Heap2020; McCullough et al. Reference McCullough, Yu, McElroy, Chen, Zhang, Grey and Czarnota2016a, 2016b; Powles and Yu Reference Powles and Yu2010; Yu et al. Reference Yu, McCullough, McElroy, Jespersen and Shilling2020a). To date, a total of 165 weed species worldwide have been found to be resistant to at least one of the five ALS inhibitor chemical families (Heap Reference Heap2020).
Penoxsulam is one of the commonly used ALS-inhibiting herbicides for POST control of barnyardgrass in rice fields in China (Anonymous 2020). At recommended use rates, penoxsulam demonstrated excellent safety on both indica and japonica rice varieties (Bond et al. Reference Bond, Walker, Webster, Buehring and Harrell2007) and provided broad-spectrum control of various weed species including barnyardgrass, various broadleaves, and sedges (Cyperus spp.; Ottis et al. Reference Ottis, Chandler and Mccauley2003; Khare et al. Reference Khare, Sharma, Singh and Sobhana2014; Willingham et al. Reference Willingham, McCauley, Senseman, Chandler, Richburg, Lassiter and Mann2008). Unfortunately, the extensive use of this herbicide in rice has caused resistant barnyardgrass populations in China (Chen et al. Reference Chen, Wang, Yao, Zhu and Dong2016; Fang et al. Reference Fang, Liu, Zhang, Li and Dong2019a), South Korea (Won et al. Reference Won, Lee, Min, Su, Su, Hwang, Pyon and Park2014), the United States (Norsworthy et al. Reference Norsworthy, Wilson, Scott and Gbur2014; Wilson et al. Reference Wilson, Norsworthy, Scott and Gbur2014), and Vietnam (Duy et al. Reference Duy, Chon, Mann, Kumar and Morell2018).
Herbicide resistance mechanisms can be classified to target-site resistance (TSR) and nontarget-site resistance (NTSR). Recent investigations revealed that TSR and NTSR can concurrently confer resistance to ALS inhibitors in barnyardgrass (Fang et al. Reference Fang, Liu, Zhang, Li and Dong2019a). Fang et al. (Reference Fang, Liu, Zhang, Li and Dong2019a) reported that amino acid substitution at Trp-574-Arg in the ALS was the basis of TSR mechanisms conferring penoxsulam resistance in a barnyardgrass population, while metabolism mediation by cytochrome P450 monoxygenases (P450s) and glutathione S-transferases (GSTs) also contributed to penoxsulam resistance. This population of barnyardgrass displayed cross-resistance to POST applications of other ALS-inhibiting herbicides including flucarbazone-sodium, pyroxsulam, pyribenzoxim, flucetosulfuron, and propyrisulfuron, and exhibited multiple resistance to three ACCase-inhibiting herbicides including cyhalofop-butyl, metamifop, and pinoxaden, as well as two synthetic auxin herbicides including florpyrauxifen-benzyl and quinclorac.
In a POST control scenario, applying a premix or tank-mix of herbicides with two effective modes of action are recommended to mitigate the evolution and spread of resistant weeds (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles and Burgos2012; Powles and Yu Reference Powles and Yu2010; Vencill et al. Reference Vencill, Nichols, Webster, Soteres, Mallory-Smith, Burgos, Johnson and McClelland2012; Yu et al. Reference Yu, McCullough and Czarnota2017). Although rice growers in China adopted such recommendation and applied a mixture of penoxsulam and cyhalofop-butyl or quinclorac for POST control of barnyardgrass (Anonymous 2020), multiple resistance to POST herbicides in barnyardgrass populations has increased drastically in recent years (Chen et al. Reference Chen, Wang, Yao, Zhu and Dong2016; Dong et al. Reference Dong, Gao, Fang and Chen2018; Fang et al. Reference Fang, Liu, Zhang, Li and Dong2019a, 2019b).
A barnyardgrass population in Anhui province, China, has evolved resistance to all five chemical families of ALS-inhibiting herbicides because of a target site mutation (Fang et al. Reference Fang, Zhang, Liu, Yan and Dong2019b). This population of barnyardgrass has shown resistance to multiple POST herbicides, including flucarbazone-sodium, imazapic, pyroxsulam, and propyrisulfuron, which had never been previously used for weed control in rice fields in China (Fang et al. Reference Fang, Zhang, Liu, Yan and Dong2019b). At present, rice growers have limited choices for POST control of this barnyardgrass population. Therefore, soil-applied PRE herbicides are considered for resistance management. Nevertheless, limited information exists on rice safe and efficacy using soil residual herbicides for controlling penoxsulam-resistant barnyardgrass. Thus, the objectives of this research were to evaluate rice safety and control efficacy of soil-applied herbicides for controlling penoxsulam-resistant and -susceptible barnyardgrass populations.
Materials and Methods
Plant Materials
The barnyardgrass seeds used were a known penoxsulam-resistant (AXXZ-2) population collected in 2012 in Anhui, China (110.98°E, 30.93°N), whereas the seeds of a penoxsulam-susceptible (JLGY-3) population were collected in 2012 from a recreational field in Jiangsu, China (119.12°E, 34.83°N), which has no history of having had herbicides applied. In previous research, Fang et al. (Reference Fang, Zhang, Liu, Yan and Dong2019b) confirmed that the resistant AXXZ-2 population exhibited 33-fold resistance to penoxsulam compared with the susceptible JLGY-3 population. In this paper, the penoxsulam-resistant and penoxsulam-susceptible barnyardgrass populations were renamed to be consistent with the terminology used in the previous publication (Fang et al. Reference Fang, Zhang, Liu, Yan and Dong2019b). Rice tolerance to soil-applied herbicides was evaluated with Wuyungeng32 (WY, Oryza sativa L. ssp. japonica) and Liangyou669 (LY, Oryza sativa L. ssp. indica) varieties of rice.
Barnyardgrass Control Experiments
Greenhouse experiments were established in Nanjing Agricultural University, Nanjing, China (118.46°E, 32.03N), in 2019, to evaluate the efficacy of soil-applied herbicides for controlling AXXZ-2 and JLGY-3 barnyardgrass populations. Pots with a 49-cm2 surface areas and 7-cm depth were filled a mixture of sand and potting soil (2:1 by weight). Twenty seeds from each barnyardgrass population were planted in each plastic pot and then covered with a thickness of 0.3 to 0.5 cm of soil mixture. The soil mixture contained 1.4% organic matter, pH 6.1, that had no history of herbicide use or barnyardgrass infestation.
The pots were placed in a greenhouse set for 27/23 C (day/night temperatures) with a 12-h photoperiod, 9,000 lux light intensity, and 85% relative humidity. The pots were treated with a rate titration of acetochlor, anilofos, butachlor, clomazone, oxadiazon, pendimethalin, pretilachlor, pyraclonil, and thiobencarb (Table 1). A nontreated check was included in each replication. Herbicide treatments were sprayed in a sprayer chamber (machine model 3WP-2000, Nanjing Research Institute for Agricultural Mechanization, Nanjing, National Ministry of Agriculture of China) calibrated to deliver 280 L ha−1 with an even flat-fan nozzle (Boyuan® Nozzle Technologies Inc., Guangdong, China). The pots were returned to the greenhouse at 1 h after herbicide treatment and then received 0.6 cm of irrigation. The pots were overhead-watered as needed to prevent soil moisture deficiencies. Herbicide rate, product, and manufacturer information are shown in Table 1. At 30 d after treatment (DAT), shoots were harvested, and fresh biomass was weighed.
Table 1. Herbicide rate, product, and manufacturer information.
a Weed Science Society of America group numbers listed represent the following herbicide mechanisms of action: (15) inhibitors of synthesis of very long-chain fatty acids; (13) inhibitor of 1-deoxy-D-xyulose 5-phosphate synthetase; (14) inhibitor of protoporphyrinogen oxidase (Protox, PPO); (3) inhibitor of microtubule assembly; (8) inhibitor of lipid synthesis.
b Abbreviations: EC, emulsifiable concentrates; GR, granules.
Rice Study
Greenhouse experiments were conducted in Nanjing Agricultural University in 2019 to evaluate rice safety following the application of PRE herbicides. Twenty rice seeds of WY (ssp. japonica) or LY (ssp. indica) were planted with previously described plastic pot and soil. Seeds were covered with a thickness of 0.3 to 0.5 cm soil. At 24 h after planting, pots were sprayed with a rate titration of acetochlor, anilofos, butachlor, clomazone, oxadiazon, pendimethalin, pretilachlor, pyraclonil, and thiobencarb using the aforementioned spray chamber. A nontreated check was included in each replication. Herbicide rate, product, and manufacturer information are identical to the barnyardgrass control experiments (Table 1). At 1 h after treatment, pots were returned to the greenhouse and received 0.6 cm of irrigation. Temperature, light intensity, and relative humidity of the greenhouse were described in the barnyardgrass control study. The pots were overhead-watered as needed to prevent soil moisture stress. At 30 DAT, rice shoots were harvested, and fresh shoot biomass was measured.
Experimental Design and Statistical Analysis
Each herbicide was considered to be a separate trial. The experiment designed as a randomized complete block and was a two-way factorial with six herbicide rates by two barnyardgrass populations or two rice varieties with two experimental runs and four replications in each run. All experiments were conducted twice over time.
For barnyardgrass control experiments, data were subjected to nonlinear regression analysis using SigmaPlot (version 12, Systat Software, San Jose, CA). Treatment means were plotted on figures and regressed against the following three-parameter exponential-growth function model:
$$y = {\beta _0} + {\beta _1}*\exp ( - {\beta _2}*x)$$
where y is percent barnyardgrass shoot biomass reduction from the nontreated control, x is the herbicide rate (g ai ha−1), β0 is the lower asymptote, β1 is the upper asymptote, and β2 is the slope. This equation was used because it described the relationship between the percentage of shoot mass reduction and herbicide rate. Herbicide rate that caused 50% reduction in fresh shoot biomass from the nontreated control (GR50) was calculated based on the regression curves. The 95% confidence limits for GR50 values were calculated.
For rice safety study, data were subjected to ANOVA using the MIXED procedure in SAS (version 9.3, SAS Institute, Cary, NC). Rice variety and herbicide rate were considered as the fixed factors, whereas replications and experimental runs were considered as the random factors. Treatment means were separated with Tukey adjustment means comparison at P ≤ 0.05.
Results and Discussion
The percentage of shoot biomass reduction, with standard errors of the means, is presented in Figure 1. Control of AXXZ-2 and JLGY-3 barnyardgrass populations increased with increased herbicide rate for all herbicides (Figure 1). According to 95% confidence limits, the two barnyardgrass populations were equally susceptible to acetochlor, anilofos, butachlor, oxadiazon, pretilachlor, or pyraclonil, with statistically equivalent GR50 or GR90 values (Table 2). However, JLGY-3 exhibited significantly lower GR50 or GR90 values and was more susceptible to clomazone, pendimethalin, and thiobencarb than AXXZ-2. JLGY-3 GR50 values were 61, 166, and 552 g ha−1 for clomazone, pendimethalin, and thiobencarb, whereas GR50 values for AXXZ-2 were 179, >800, and 1,798 g ha−1, respectively. GR90 values for JLGY-3 were 96, >800, and 1,988 g ha−1 for clomazone, pendimethalin, and thiobencarb; whereas GR90 values for AXXZ-2 were 624, >800, and >3,200 g ha−1, respectively.
Figure 1. Penoxsulam resistant- and penoxsulam-susceptible barnyardgrass shoot biomass reductions compared with the nontreated control at 30 d after herbicide applications in two combined greenhouse experiments, 2019, in Nanjing, Jiangsu, China. Vertical bars represent standard errors of the mean (n = 8). Results were pooled over experimental runs. AXXZ-2 represents the penoxsulam-resistant barnyardgrass population, JLGY-3 represents the penoxsulam-susceptible barnyardgrass population.
a Shoot mass reductions were regressed against a three-parameter exponential-growth function model: y = β0 + β1*exp(–β2* x), where y is percent barnyardgrass shoot biomass reduction from the nontreated control, x is the herbicide rate (g ai ha−1), β0 is the lower asymptote, β1 is the upper asymptote, and β2 is the slope.
b AXXZ-2 is the resistant barnyardgrass population, JLGY-3 is the susceptible barnyardgrass population.
c Abbreviations: CI, confidence interval; GR50, effective herbicide rate that causes 50% reduction in fresh shoot biomass from the nontreated control; GR90, effective herbicide rate that causes 90% reduction in fresh shoot biomass from the nontreated control; NA, not available; SE, standard error.
Of the herbicides included in this study, the labeled rates of acetochlor, clomazone, oxadiazon, pretilachlor, and pyraclonil were effective at controlling both barnyardgrass populations and resulted in reduced shoot biomass from the nontreated control by at least 90% (Table 2; Figure 1). However, at the labeled use rates, neither pendimethalin nor thiobencarb effectively controlled AXXZ-2. The highest rate of pendimethalin at 800 g ha−1 reduced AXXZ-2 and JLGY-3 shoot biomass by 10% and 60% compared with the nontreated, respectively, whereas thiobencarb (1,600 g ha−1) reduced AXXZ-2 and JLGY-3 shoot biomass by 65% and 89%, respectively. These findings suggest that AXXZ-2 has reduced susceptibility to pendimethalin and thiobencarb.
The effectiveness of evaluated herbicides on barnyardgrass has been well documented (Akkari et al. Reference Akkari, Talbert, Ferguson, Gilmour and Khodayari1986; Balyan et al. Reference Balyan, Malik and Dhankar1996; Juraimi et al. Reference Juraimi, Uddin, Anwar, Mahmud Tengku, Ismail and Man2013; Kirkwood and Fletcher Reference Kirkwood and Fletcher1984; Ryang Reference Ryang1998; Shaner Reference Shaner2014; Ushiguchi et al. Reference Ushiguchi, Okamoto, Takahashi, Ikeda and Sanagi2014; Wilson et al. Reference Wilson, Norsworthy, Scott and Gbur2014). Soil-applied PRE herbicides need to be applied prior to weed emergence to maximize weed control (Shaner Reference Shaner2014). However, environmental factors can influence weed seed germination and seedling emergence (Sharpe and Boyd Reference Sharpe and Boyd2019; Yu et al. Reference Yu, Sharpe and Boyd2020b). As a summer annual weed species, barnyardgrass may have several peaks of seed germination during the rice production season (Kennedy et al. Reference Kennedy, Barrett, Zee and Rumpho2010). Because of the herbicide degradation and dissipation, a single application of residual herbicides may not be able to provide season-long control of barnyardgrass (Shaner Reference Shaner2014). Field studies conducted in Arkansas revealed that sequential herbicide application programs with clomazone, thiobencarb, pendimethalin, and quinclorac applied as a delayed PRE in different combinations followed by imazethapyr early POST followed by preflood controlled ALS-resistant barnyardgrass above ≥96% in rice fields (Wilson et al. Reference Wilson, Norsworthy, Scott and Gbur2014).
The widespread resistance of barnyardgrass to POST herbicides is a concern to rice producers in China and other countries as well (Chen et al. Reference Chen, Wang, Yao, Zhu and Dong2016; Fang et al. Reference Fang, Liu, Zhang, Li and Dong2019a, 2019b; Heap Reference Heap2020; Rouse et al. Reference Rouse, Roma-Burgos, Norsworthy, Tseng, Starkey and Scott2017). Barnyardgrass with resistance to ALS-inhibiting herbicides has been frequently reported in China in recent years (Fang et al. Reference Fang, Liu, Zhang, Li and Dong2019a, 2019b; Liu et al. Reference Liu, Sun, Fu and Zhong2015; Wu et al. Reference Wu, Xu, Yang and Yang2012) and may continuously cause severe problems for rice production due to competition with crops in seasons and weed seed bank management for the following production seasons. Therefore, it is important to determine the effectiveness of soil-applied herbicides for controlling ALS-resistant barnyardgrass. In the present research, except for pendimethalin and thiobencarb, the labeled rates of the herbicides we evaluated effectively controlled both barnyardgrass populations. Those herbicides differ in their mechanisms of action with ALS inhibitors and can control penoxsulam-resistant barnyardgrass.
In addition to tank-mixture or sequential application of herbicides with different mechanisms of action, other integrated weed management strategies such as crop rotation, selection of competitive rice variety, high seedling rate; and the use of clean tillage equipment, narrow row spacing, proper weed identification, and scouting of fields, can influence the degree of herbicide resistance management (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles and Burgos2012; Vencill et al. Reference Vencill, Nichols, Webster, Soteres, Mallory-Smith, Burgos, Johnson and McClelland2012). The integration of these weed management practices in rice production may significantly slow down the herbicide resistance evolution in barnyardgrass populations (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles and Burgos2012; Vencill et al. Reference Vencill, Nichols, Webster, Soteres, Mallory-Smith, Burgos, Johnson and McClelland2012).
Except for pyraclonil, the effect of herbicide rate by rice variety interaction on rice shoot biomass was significant for all herbicides (Table 3). The effect of herbicide rate was significant for acetochlor, anilofos, clomazone, pendimethalin, and pretilachlor, while the effect of herbicide rate was not significant for butachlor, oxadiazon, and thiobencarb. For both rice varieties, acetochlor, butachlor, oxadiazon, pretilachlor, and thiobencarb at the rates tested caused ≤20% shoot biomass reduction from the nontreated control. Anilofos at 44 and 88 g ha−1, clomazone at 56 g ha−1, and pendimethalin at 50 and 100 g ha−1 caused <20% reduction in rice shoot biomass; however, these herbicides applied at higher rates caused substantial reduction in shoot biomass. At the highest rate, anilofos at 704 g ha−1, clomazone at 896 g ha−1, and pendimethalin at 800 g ha−1 caused 57%, 75%, and 18% reduction in rice shoot biomass, respectively.
Table 3. Effect of soil-applied herbicides on rice shoot biomass. a
a Means within columns followed by the same letter are not significantly different according to Tukey adjusted mean comparisons at the 0.05 probability level.
b WY represents the rice variety of Wuyungeng 32 (ssp. japonica), LY represents the rice variety of Liangyou 669 (ssp. indica).
** Significant at the 0.01 probability level.
*** Significant at the 0.001 probability level.
NS, not significant at the 0.05 probability level.
The two rice varieties, japonica and indica, differed significantly in their response to clomazone or pendimethalin (Table 3). Clomazone and pendimethalin caused significantly greater shoot biomass reduction on WY than LY. Averaged across herbicide rates, clomazone caused 52% and 34% shoot biomass reduction for WY and LY, respectively; and pendimethalin caused 25% and 9% shoot biomass reduction for WY and LY, respectively. The two rice varieties exhibited similar susceptibility to acetochlor, anilofos, butachlor, oxadiazon, pretilachlor, or thiobencarb.
The effects of herbicide rate and herbicide by rice variety interaction on rice shoot biomass were significant for pyraclonil, and as a result, data are presented across all treatment combinations (Table 4). At the rates tested, pyraclonil at rates ranging from 25 to 200 g ha−1 were safe to LY and caused ≤8% shoot biomass reduction; however, pyraclonil at 400 g ha−1 caused 32% shoot biomass reduction. At the rates tested, pyraclonil caused ≤3% shoot biomass reduction for WY.
Table 4. Effect of pyraclonil on rice shoot biomass. a
a Means within columns followed by the same letter are not significantly different according to Tukey adjusted mean comparisons at the 0.05 probability level.
b WY represents the rice variety of Wuyungeng 32 (ssp. japonica), LY represents the rice variety of Liangyou 669 (ssp. indica).
** Significant at the 0.01 probability level.
NS, not significant at the 0.05 probability level.
Rice tolerance to acetochlor, butachlor, oxadiazon, pretilachlor, and thiobencarb is consistent with previous reports (Fogleman et al. Reference Fogleman, Norsworthy, Barber and Gbur2018; Kirkwood and Fletcher Reference Kirkwood and Fletcher1984; Wilson et al. Reference Wilson, Norsworthy, Scott and Gbur2014). In the present study, however, substantial rice damage on both rice varieties was observed with anilofos and clomazone. Pyraclonil (400 g ha−1) also caused significant damage to LY. Soil-applied herbicides can offer residual weed control, but the length of control and crop safety are often dependent upon multiple factors, such as herbicide application methods, rates, soil characteristics, and environmental conditions, particularly soil moisture (Racke et al. Reference Racke, Skidmore, Hamilton, Unsworth and Cohen1999; Shaner Reference Shaner2014; Taylor-Lovell et al. Reference Taylor-Lovell, Sims and Wax2002; Wilson et al. Reference Wilson, Norsworthy, Scott and Gbur2014). In the present study, rice damage caused by clomazone may be related to the high content of sand in the soil. Although anilofos and clomazone are used for weed control in rice (Scherder et al. Reference Scherder, Talbert and Clark2004; Zhao et al. Reference Zhao, Yan, He and Han2009), caution needs to be taken when using them to avoid potential rice damage.
The two rice varieties in this study exhibited similar tolerance to all herbicides evaluated with the exception of clomazone and pendimethalin. In other studies, Zhang et al. (Reference Zhang, Webster, Blouin and Linscombe2004) found that a long-grain rice variety, ‘Drew’, exhibited significantly greater tolerance to clomazone compared with medium-grain rice varieties ‘Earl’ and ‘LL-401’. Koger et al. (Reference Koger, Walker and Krutz2006) noted differential tolerance to pendimethalin among rice varieties. The authors suggested that seeds of rice varieties that are sensitive to soil-applied herbicides should be planted more deeper, which may enable the seed to absorb soil moisture before contacting with herbicides and thus reducing potential rice injury.
In summary, penoxsulam-resistant and -susceptible barnyardgrass populations demonstrated similar susceptibility to acetochlor, anilofos, butachlor, oxadiazon, pretilachlor, or pyraclonil. However, penoxsulam-susceptible barnyardgrass exhibited greater susceptibility to clomazone, pendimethalin, and thiobencarb than penoxsulam-resistant populations. At labeled rates, acetochlor, anilofos, butachlor, clomazone, oxadiazon, pretilachlor, and pyraclonil can effectively control penoxsulam-resistant and -susceptible barnyardgrass populations; however, pendimethalin and thiobencarb are ineffective at controlling penoxsulam-resistant barnyardgrass. Except for anilofos and clomazone, the rice varieties are tolerant to the evaluated herbicides. WY is more tolerant to clomazone and pendimethalin than LY. Field research is needed to further confirm the effectiveness of these herbicides for controlling penoxsulam-resistant barnyardgrass.
Acknowledgments
Research funding was provided by the National Key R&D Program of China (No. 2018YFD0200300). The authors declare no conflict of interest.
