Introduction
Interference from glyphosate-resistant (GR) Italian ryegrass with the emerging rice crop can negatively affect crop height, density, and yield when left uncontrolled (Lawrence et al. Reference Lawrence, Bond, Edwards, Golden, Montgomery, Eubank and Walker2018). Rice production is directly influenced by weed management strategies (Carlson et al. Reference Carlson, Webster, Salassi, Hensley, David, Carlson, Webster, Salassi, Hensley and Blouin2011). Maximum rice yield begins with a sufficient plant population across the planted area (Buehring Reference Buehring2008), and the residue left from GR Italian ryegrass can impede planting practices in Mississippi (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014).
In most rice-growing situations in Mississippi, preplant (burndown) herbicide applications are based on glyphosate or paraquat (Bond et al. Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022). Thifensulfuron-methyl or 2,4-D are mixed with glyphosate or paraquat to improve control of broadleaf weeds, and flumioxazin may be added for residual weed control (Buehring Reference Buehring2008). Timely herbicide applications are key because these treatments typically perform better on young, actively growing weeds. Residual herbicides are the most effective and economical management strategy for controlling GR weeds (Culpepper et al. Reference Culpepper, Webster, Sosnoskie, York and Nandula2010), and fall-applied residual herbicides are often recommended for control of GR Italian ryegrass in Mississippi (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014, Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022).
Recommended residual herbicides for controlling GR Italian ryegrass in Mississippi include clomazone, dimethenamid-P, pyroxasulfone, S-metolachlor, and trifluralin (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014, Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022; Burrell et al. Reference Burrell, Bond, Allen, Dodds and Gholson2025). However, pyroxasulfone, S-metolachlor, and trifluralin are not labeled for fall application where rice is scheduled to be planted the following spring (BASF 2016; Bond et al. Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022; Gowan 2010; Lawrence et al. Reference Lawrence, Bond, Edwards, Golden, Montgomery, Eubank and Walker2018; Syngenta, 2020). Clomazone is a herbicide that inhibits 1-deoxy-D-xylulose 5-phosphate synthase that is commonly applied to rice producing areas because clomazone carries no restriction for preplant interval to rice (Bond et al. Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022; FMC 2013). Clomazone is an effective fall-applied herbicide treatment labeled for GR Italian ryegrass control where rice is scheduled to be planted the following spring (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014, Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022; FMC 2013, 2023).
Cotton (Gossypium hirsutum L.), grain sorghum (Sorghum bicolor L.), and soybean [Glycine max (L.) Merr.] may be grown in rotation with rice in the midsouthern United States (Johnson et al. Reference Johnson, Beaty, Horton, Talbert, Guy, Mattice, Lavy and Smith1995; Zhang et al. Reference Zhang, Webster and Braverman2002). Herbicide persistence and carryover in the soil are major concerns if the following season’s crop is not tolerant to the herbicide’s chemistry (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014; Johnson et al. Reference Johnson, Beaty, Horton, Talbert, Guy, Mattice, Lavy and Smith1995). Soil persistence of herbicides is directly influenced by soil properties, including pH, organic matter content, and soil texture. Carryover from the previous season’s application can cause unacceptable injury to subsequent crops (Miller and Norsworthy Reference Miller and Norsworthy2018). Carryover from herbicides commonly used in cotton, grain sorghum, and soybean production can be problematic in fields seeded with rice the following spring (Zhang et al., Reference Zhang, Webster and Braverman2000). Zhang et al. (Reference Zhang, Webster and Braverman2002) reported simulated carryover of norflurazon resulted in 20% to 56% rice injury 8 wk after planting.
Acetochlor, S-metolachlor, and dimethenamid-P are all categorized as Group 15 herbicides according to the Weed Science Society of America (BASF 2019; Bayer 2020, Syngenta, 2020). Group 15 herbicides are very-long-chain fatty acid synthesis inhibitors and include chloroacetamide, oxyacetamide, and pyrazole families (Boger et al. Reference Boger, Matthes and Schmalfu2000; Shaner Reference Shaner2003). At 130 d after application, dimethenamid-P and S-metolachlor provided 40% and 26% greater control of GR Italian ryegrass, respectively, compared to acetochlor alone (Burrell et al. Reference Burrell, Bond, Allen, Dodds and Gholson2025). However, Bond et al. (Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022) previously reported that no fall-applied treatment provides complete control, indicating the need for a sequential application for complete control of GR Italian ryegrass. Additionally, soil persistence of acetamide herbicides can be significantly affected by soil organic matter (Weber and Peter Reference Weber and Peter1982).
Flumioxazin, a Group 14 herbicide, inhibits protoporphyrinogen oxidase. Group 14 herbicides are referred to as cell membrane disruptors and are usually “burner” type herbicides (Barker et al. Reference Barker, Pawlak, Duke, Beffa, Tranel, Wuerffel, Young, Porri, Liebl, Aponte, Findley, Betz, Lerchl, Culpepper, Bradley and Dayan2023). Flumioxazin is a common fall-applied residual herbicide in Mississippi (Bond et al. Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022) and has controlled Italian ryegrass by 81% 140 d after fall application at 70 g ai ha−1 (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014). Reduced efficacy of flumioxazin has been reported in soils with high organic matter and inadequate moisture levels for plant absorption (Sebastian et al. Reference Sebastian, Nissen, Westra, Shaner and Butters2017).
Fall applications of residual herbicides often target winter annuals at early growth stages when control is more effective (Hasty et al. Reference Hasty, Sprague and Hager2004; Stougaard et al., Reference Stougaard, Kapusta and Roskamp1984). Italian ryegrass germination occurs when temperatures are between 10 and 30 C for 6 to 10 d (Hannaway et al. Reference Hannaway1999). In the midsouthern United States this temperature range can fall between September to November (NOAA 2021). The rotation interval following an application of dimethenamid-P when rice planting is scheduled for the following spring is 6 to 9 mo (BASF 2019). The S-metolachlor and acetochlor product labels list the preplant intervals to rice as the following spring (Bayer 2020; Syngenta, 2020). Bertucci et al. (Reference Bertucci, Fogleman and Norsworthy2019) reported that applying acetochlor, dimethenamid-P, and S-metolachlor 6 mo prior to planting did not reduce rice shoot density relative to the nontreated control. These results suggest that these herbicides can be safely applied in the fall after harvest with minimal risk of crop injury the following spring.
In research evaluating rice response to clomazone, pyroxasulfone, S-metolachlor, and trifluralin, Lawrence et al. (Reference Lawrence, Bond, Edwards, Golden, Montgomery, Eubank and Walker2018) reported rice injury 14 d after emergence (DAE) ranged from 29% to 37% with all fall-applied residual herbicides except clomazone. Due to widespread herbicide resistance by Italian ryegrass, fall-applied treatments of additional herbicides and herbicide mixtures should be evaluated for their effects on rice planted the following spring. Therefore, this study aimed to evaluate rice response to fall-applied residual herbicide treatments applied with and without flumioxazin.
Materials and Methods
A field study was conducted from 2021 to 2022 and 2022 to 2023 to evaluate rice performance following fall-applied treatments of residual herbicide mixtures that could be used to target GR Italian ryegrass. The soil was a commerce silty clay loam (Silty over clay, mixed, superactive, nonacidic, thermic, Fluvaquentic Endoaquepts) with a pH ranging from 5.0 to 7.6 and with approximately 2.6% organic matter. The study was established at the Delta Research and Extension center with two sites in 2021–22 (Site A, 33.4401°N, 90.9101°W; Site B, 33.4396°N, 90.9097°W), and 2022–23 (Site A, 33.4392°N, 90.9085°W; Site B, 33.4389°N, 90.9066°W). Rice cultivar CLL16 (Horizon Ag, Memphis, TN) was seeded at 83 kg ha−1 (356 seeds m−2) to a 2-cm depth in 2022 and 2023 using a Great Plains 1520 small-plot grain drill (Great Plains Manufacturing, Salina, KS). Table 1 presents the dates of treatment application, first rainfall after treatment, rice emergence, flooding, and harvest for each site-year. Each plot contained nine rows spaced approximately 20 cm apart and 4.6 m long. The plots were separated by a 3-m fallow alley and a 1.5-m fallow border on each end to minimize contamination. Saflufenacil (Sharpen 2.85 SC, 50 g ai ha−1; BASF Crop Protection, Florham Park, NJ) was applied at planting followed by a sequential application of quinclorac (Facet 1.5 L, 420 g ai ha−1; BASF) and bispyribac-sodium (Regiment, 32 g ai ha−1; Valent Biosciences, Libertyville, IL) prior to flooding to maintain experimental sites weed free. Urea (46-0-0) was also applied at 401 kg ha−1 1 d prior to establishing the permanent flood.
Key dates in the 2021–22 and 2022–23 studies.
The experimental design was a randomized complete block with a two (flumioxazin rate) × four (residual mixture) × two (application rate) factorial arrangement of treatments and four replications. The flumioxazin (Valor EZ; Valent Biosciences) rate included no flumioxazin and flumioxazin. The residual mixture included no residual mixture, dimethenamid-P (Outlook; BASF), clomazone (Command; FMC, Philadelphia, PA), and acetochlor (Warrant; Bayer Crop Science, Research Triangle Park, NC). Application rates included one (1×) and two (2×) times the recommended rates for flumioxazin and residual mixtures as fall-applied treatments in Mississippi (Table 2; Bond et al. Reference Bond, Dodds, Golden, Irby, Larson, Lawrence, Reynolds and Sarver2022). Treatments were applied using a CO2-pressurized backpack sprayer delivering 140 L ha−1 (Table 1).
Herbicide products and application rates.
Data collection included visible rice injury assessed 14 and 28 d after application on a scale of 0 to 100% where 0 indicated no injury and 100 indicated complete death. Rice density was recorded 14 and 28 DAE by counting and recording the number of plants in two randomly selected sections of one linear meter in each plot. These two counts were converted to plants per square meter. Rice plant height was recorded 28 DAE by measuring from the base of the plant to the uppermost leaf of five randomly selected plants on Rows 2 and 8 in each plot. The Greenseeker handheld crop sensor (Trimble Inc., Sunnyvale, CA) was used to obtain green normalized difference vegetation index (NDVI) 28 d after flood. Rice maturity was estimated as the number of days to 50% heading (when 50% of the panicles in an individual plot had emerged from the leaf sheath) and recorded as DAE. Plots were mechanically harvested with a small-plot combine equipped with an onboard weigh system (Zürn Harvesting GmbH, Schöntal-Westernhausen, Germany) to obtain rough rice yield. Rough rice yields were adjusted to 12% moisture for uniform statistical analysis. Total milled rice (consisting of whole and broken kernels) and whole milled rice (whole kernels) yields were determined from a cleaned 100-g subsamples of rough rice using the procedure outlined by Adair et al. (Reference Adair, Bollich, Bowman, Jordon, Johnson, Webb and Atkins1972).
All data were subjected to ANOVA using the GLIMMIX procedure in SAS software (v.9.4; SAS Institute Inc., Cary, NC) with site-year, replication (nested within site-year), and appropriate interactions containing these effects set as random effect parameters (Blouin et al. Reference Blouin, Webster and Bond2011). Type III statistics were used to test the main effects of flumioxazin, residual mixture, and application rate and interactions among main effects. The square roots of rice injury data were arcsine transformed. Arcsine transformation did not improve the homogeneity of variance; therefore, nontransformed data were used in the analysis. The means were separated using the Fisher protected LSD (P < 0.05).
Results and Discussion
No interactions containing flumioxazin, residual mixture, and/or application rate were detected for rice injury 14 and 28 DAE. However, the main effects of residual mixture were significant at 14 DAE (P ≤ 0.0001) and 28 DAE (P ≤ 0.0001). Pooled over flumioxazin rate and application rate, fall-applied acetochlor resulted in 29% injury to rice at 14 and 28 DAE (Table 3). Injury was ≤8% at both evaluations with fall-applied clomazone and dimethenamid-P. The emulsifiable concentrate form of acetochlor has been shown to cause unacceptable injury to rice (Fogleman et al. Reference Fogleman, Norsworthy, Barber and Gbur2018). However, microencapsulated acetochlor showed minimal injury following applications at 1- to 2-leaf rice growth stage. This was largely attributed to the gradual release of the microencapsulated formulation. Results from the current research demonstrate that acetochlor applied in the fall prior to rice planting can result in significant injury, indicating the lack of tolerance rice has to this herbicide in its early growth stages.
Influence of residual herbicides on rice injury 14 and 28 DAE, rice density 28 DAE, height 28 DAE, days to 50% heading, and rough rice yield.a,b
a Abbreviations: DAE, days after emergence.
b Data were pooled over two levels of flumioxazin rate, two levels of application rate, and 4 site-years. Means within a column followed by the same letter are not different according to Fisher’s protected LSD at P ≤ 0.05.
No interactions containing flumioxazin rate, residual mixture, and application rate were detected for density or height 14 DAE. However, a main effect of residual mixture was significant for both (P ≤ 0.0001). Plots treated with fall-applied clomazone exhibited rice density of 30 plants m−2 and a height of 18 cm (Table 3). Fall-applied acetochlor reduced rice density by 7 plants m−2 and height by 2 cm compared to clomazone. Similarly, Bertucci et al. (Reference Bertucci, Fogleman and Norsworthy2019) reported that acetochlor reduced rice shoot density by 15% compared to control plots. Fall-applied dimethenamid-P reduced rice density by 3 plants m−2 compared with nontreated plots when pooled over flumioxazin rate and application rate. Rice plant heights following fall-applied dimethenamid-P and acetochlor were 16 cm, which was a 1-cm reduction compared to the nontreated plants.
The NDVI has been widely used to assess abiotic and biotic plant stresses such as herbicide injury (Bell et al. Reference Bell, Martin, Kuzmic, Stone and Solie2000; Brewer et al. Reference Brewer, Willis, Rana and Askew2017). Interactions of flumioxazin rate by application rate (P = 0.0479) and flumioxazin rate by residual mixture (P = 0.0006) resulted in significant differences in NDVI (Table 4). Pooled across residual mixture, NDVI was ≥0.683 when flumioxazin was present and with 2 × application rates in the absence of flumioxazin. However, NDVI was reduced 3% from treatments that contained flumioxazin when the application rate was 1 × and flumioxazin was excluded. Pooled over application rates, the NDVI was similar with all residual mixtures when flumioxazin was present (Table 5). However, the NDVI was lower in acetochlor-treated plots compared with no residual mixture in the absence of flumioxazin. Reduced rice density likely played a part in the reduction of NDVI from acetochlor-treated plots.
a Abbreviation: NDVI, normalized difference vegetation index.
b Data were pooled over four levels of residual mixture and four site years. Means followed by the same letter are not different according to Fisher’s protected LSD at P ≤ 0.05.
Influence of flumioxazin rate by residual mixture interaction on NDVI 28 d after flood initiation.
a Abbreviation: NDVI, normalized difference vegetation index.
b Data were pooled over two levels of application rate and four site years. Means followed by the same letter are not according to Fisher’s protected LSD different at P ≤ 0.05.
A main effect of residual mixture was detected for number of days to 50% heading (P ≤ 0.0001). Fall-applied acetochlor delayed rice heading by 2 d compared with other residual mixture treatments (Table 3). Delaying rice maturity could potentially negatively affect rice producers in the mid-southern United States by increasing exposure to adverse weather conditions and reducing milling quality. Preemergence application timings that were responsible for rice injury were also responsible for the greatest delay in 50% heading (Fogleman et al. Reference Fogleman, Norsworthy, Barber and Gbur2018).
A main effect of residual mixture was significant for rough rice yield (P ≤ 0.0001). Clomazone-treated plots yielded ≥10,780 kg ha−1 rough rice when pooled over flumioxazin rate and application rate (Table 3). Fall-applied dimethenamid-P resulted in a rice yield of 670 kg ha−1 compared with yields from plots that received no residual mixture. Rough rice yield was 9,790 kg ha−1 following fall-applied acetochlor (Table 3). Similar studies by Fogleman (Reference Fogleman2018) reported a 14% to 22% reduction in yield when acetochlor was applied. Total and whole milled rice yields were not influenced by the treatments imposed in this research (data not presented).
Practical Implications
This research demonstrates that fall-applied residual herbicide mixtures can influence rice growth and yield. Rice was injured by 29% at 14 and 28 DAE with acetochlor applied in the fall. Fall-applied acetochlor resulted in a rice density of 7 plant m2 and drop in rough rice yield to ≥500 kg ha−1 compared with fall-applied clomazone. Rice injury with dimethenamid-P was ≤8% 28 DAE. Regardless of rate, applications of acetochlor caused greater crop injury and lower shoot density, which ultimately delayed maturity and decreased yield (Fogleman et al. Reference Fogleman, Norsworthy, Barber and Gbur2018). No synergistic effect was observed from the addition of flumioxazin to a fall-applied residual mixture with respect to rice injury.
The optimal window for rice planting in the midsouthern United States is when soil temperatures reach 60 F. In Mississippi, this typically occurs from April 1 to April 15 (Buehring Reference Buehring2008). Fall-applied residual herbicide mixtures should be applied in November to achieve the greatest control of GR Italian ryegrass in Mississippi (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014, Burrell et al. Reference Burrell, Bond, Allen, Dodds and Gholson2025). Given the preplant interval for dimethenamid-P (BASF 2019), the optimum planting window for rice could be missed following fall-applied dimethenamid-P. Dimethenamid-P can control GR Italian ryegrass and produce little rice injury the following spring. However, due to rice injury caused by dimethenamid-P and acetochlor, clomazone remains the only viable option when targeting GR Italian ryegrass in areas where rice is scheduled to be planted the following growing season.
Acknowledgments
We thank personnel at the Mississippi State University Delta Research and Extension Center for their assistance.
Funding
This publication is a contribution of the Mississippi Agricultural and Forestry Experiment Station. This study is based on a larger study that is supported by Hatch project 153300, which is funded by the U.S. Department of Agriculture–National Institute of Food and Agriculture. In addition, we extend gratitude to the Mississippi Rice Promotion Board for partially funding this research.
Competing Interests
The authors declare they have no competing interests.
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