Introduction
Red rice is one of the most troublesome weeds in cultivated rice (Wychen Reference Wychen2017). In 1979, it was reported that red rice infestations caused a $50 million loss each year in southern US rice (Smith Reference Smith1979). The selective control of red rice using an herbicide in cultivated rice is difficult because of the genetic similarities of the two types of rice (Levy et al. Reference Levy, Bond, Webster, Griffin and Linscombe2006). Red rice is botanically classified as the same species as cultivated rice; however, there are phenotypic differences that distinguish red rice from cultivated rice (Kwon et al. Reference Kwon, Smith and Talbert1992). The phenotypic characteristics of red rice and weedy rice include dark to light green leaves, superior height, awned and/or awnless seeds, and pubescent or glabrous leaves (Rustom et al. Reference Rustom, Webster, McKnight and Bergeron2015, Reference Rustom, Webster, Blouin and McKnight2018). Red rice often has a competitive advantage over cultivated rice due to red rice’s ability to grow taller and produce more tillers than the cultivated rice (Diarra et al. Reference Diarra, Smith and Talbert1985).
In 2002, imidazolinone-resistant (IR) rice (Clearfield®; BASF, Research Triangle Park, NC 27709) was first commercialized, and it provided growers with an effective herbicide option for red rice control (Croughan Reference Croughan2003; Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018). Hybrid IR-rice (RiceTec, Inc., Houston, TX) was introduced in 2003. Soon after the adoption of the IR-rice technology, outcrosses of IR-rice with red rice were reported (Zhang et al. Reference Zhang, Linscombe, Webster, Tan and Oard2006). Research indicates gene flow from IR-rice to naturally occurring red rice has resulted in the development of IR-red rice (Rajguru et al. Reference Rajguru, Burgos, Shivrain and Stewart2005).
Hybrid IR-rice has an inherent seed dormancy characteristic with a high degree of seed shattering and often has weedy characteristics when the F2 progeny is allowed to establish in succeeding growing seasons (Burgos et al. Reference Burgos, Singh, Tseng, Black, Young, Huang, Hyma, Gealy and Caicedo2014; Rustom et al. Reference Rustom, Webster, McKnight and Bergeron2015, Reference Rustom, Webster, Blouin and McKnight2018; Sudianto et al. Reference Sudianto, Beng-Kah, Ting-Xiang, Saldain, Scott and Burgos2013). The F2 progeny can segregate, causing a weedy rice problem with different genotypes and phenotypes. This F2 progeny can also be IR if the F1 hybrid was IR (Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018). Non–IR-red rice, IR-red rice, and subsequent generations of hybrid rice are often referred to as weedy rice.
Another weed management issue in rice production is barnyardgrass. Smith (Reference Smith1968) reported that a season-long barnyardgrass infestation reduced rough rice yield up to 79%. Barnyardgrass has been confirmed in Arkansas, Mississippi, and Louisiana to be resistant to several herbicides with different sites of action (Malik et al. Reference Malik, Burgos and Talbert2010; Riar et al. Reference Riar, Norsworthy, Srivastava, Nandula, Bond and Scott2013). Propanil- and/or quinclorac-resistant barnyardgrass led many growers to the adoption of IR-rice; however, this has led to barnyardgrass resistance to imazethapyr and imazamox (Riar et al. Reference Riar, Norsworthy, Srivastava, Nandula, Bond and Scott2013). Propanil- and/or quinclorac-resistant barnyardgrass also led many growers to incorporate penoxsulam and bispyribac-Na in herbicide programs for conventional rice, which led to cross resistant populations of barnyardgrass.
In 2014, BASF announced the development of a new herbicide-resistant rice that confers resistance to acetyl co-enzyme A carboxylase (ACCase)-inhibiting herbicides to combat IR-weedy rice and herbicide-resistant barnyardgrass (Wilson Reference Wilson2014). This nontransgenic rice is resistant to quizalofop-p-ethyl, a Group 1 herbicide in the aryloxyphenoxypropionate herbicide family. The targeted single-application rate of quizalofop-p-ethyl for use in ACCase-resistant (ACCase-R) rice production is 92 to 155 g ai ha−1, not to exceed 240 g ha−1 yr−1 (Anonymous 2017). ACCase-R rice allows quizalofop-p-ethyl to be applied POST for control of annual and perennial grasses, including IR-weedy rice and barnyardgrass.
Herbicides are often applied in a mixture to broaden the weed-control spectrum, delay herbicide resistance, and save time and application costs (Gressel and Segel Reference Gressel and Segel1990; Jordan Reference Jordan1995; Zhang et al. Reference Zhang, Webster, Blouin and Leon2005). Herbicide mixture interactions may result in one of three responses: antagonistic, synergistic, or additive/neutral (Berenbaum Reference Berenbaum1981; Blackshaw et al. Reference Blackshaw, Harker, Clayton and O’Donovan2006; Blouin et al. Reference Blouin, Webster and Zhang2004, Reference Blouin, Webster and Bond2010; Drury Reference Drury1980; Fish et al. Reference Fish, Webster, Blouin and Bond2015, Reference Fish, Webster, Blouin and Bond2016; Hatzios and Penner Reference Hatzios and Penner1985; Morse Reference Morse1978; Nash Reference Nash1981; Streibig et al. Reference Streibig, Kudsk and Jensen1998). When a herbicide mixture has an observed response greater than the expected response, based on each herbicide applied separately, the interaction is considered synergistic; when the observed response is a reduction in control, the interaction is deemed antagonistic. If a herbicide mixture is said to be statistically similar as the expected value, the mixture is defined as neutral or additive (Colby Reference Colby1967).
ACCase-inhibiting herbicide antagonism has historically been observed when applied in a mixture with broadleaf or sedge herbicides (Ferreira and Coble Reference Ferreira and Coble1994; Hatzios and Penner Reference Hatzios and Penner1985; Myers and Coble Reference Myers and Coble1992; Rhodes and Coble Reference Rhodes and Coble1984; Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018, Reference Rustom, Webster, Blouin and McKnight2019; Zhang et al. Reference Zhang, Webster, Blouin and Leon2005). Rustom et al. (Reference Rustom, Webster, Blouin and McKnight2018) observed antagonism of quizalofop-p-ethyl when mixed with numerous acetolactate synthase (ALS)-inhibiting herbicides for control of either weedy rice or barnyardgrass, including bispyribac-Na, bensulfuron, halosulfuron, imazosulfuron, orthosulfuron plus halosulfuron, orthosulfuron plus quinclorac, penoxsulam, and penoxsulam plus triclopyr. Penoxsulam and bispyribac-Na proved to be the least compatible in a mixture with quizalofop-p-ethyl for control of weedy rice and barnyardgrass.
An adjuvant is a material added to a POST herbicide application or an herbicide mixture to enhance or modify the herbicide’s activity (Hazen Reference Hazen2000). Adjuvants can enhance penetration by improving spray coverage, reducing droplet surface tension, acting as a humectant, and increasing cuticle permeability (Wanamarta et al. Reference Wanamarta, Penner and Kells1989). Adjuvants are typically composed of surfactants, oils, solvents, polymers, salts, diluents, humectants, and water (Hazen Reference Hazen2000). There are two major categories of adjuvants: the utility adjuvants and the activator adjuvants. Utility adjuvants typically aid herbicide compatibility, pH buffering, spray drift reduction, and/or defoaming agents. Activator adjuvants improve herbicide efficacy by lowering surface tension, increasing adherence to the leaf surface, reducing the rate of drying, and/or eliminating the natural barriers preventing uptake.
Adjuvants are an integral component of weed management because they alter the physical and chemical properties of herbicides and modify herbicide activity (Bridges Reference Bridges1989; McWhorter Reference McWhorter and Hodgson1986). Antagonism of ACCase-inhibiting herbicides when mixed with broadleaf or sedge herbicides can be overcome by using adjuvants in the mixture (Jordan Reference Jordan1995; Jordan and York Reference Jordan and York1989; Penner Reference Penner1989). A high-concentrate crop oil concentrate (HCOC) consisting of fatty acid esters and alkoxylated alcohol phosphate ester (Dash®; BASF, Research Triangle Park, NC 27709) was patented in 1989 specifically for use with sethoxydim when mixed with other herbicides (Hazen Reference Hazen2000). The antagonism of ACCase-inhibiting herbicides when mixed with ALS-inhibiting herbicides has been associated with the reduced translocation of the graminicide, caused by the physiological effect of the ALS-inhibiting herbicide on the targeted grass species (Croon et al. Reference Croon, Ketchersid and Merkle1989; Kammler et al. Reference Kammler, Walters and Young2010). Jordan (Reference Jordan1995) observed a reduction of sethoxydim and clethodim antagonism by bentazon when applied with a nonionic surfactant in comparison with a crop oil concentrate (COC) for control of barnyardgrass, broadleaf signalgrass [Urochloa platyphylla (Munro ex. C. Wright) R.D. Webster] and johnsongrass [Sorghum halepense (L.) Pers.]. Zollinger (Reference Zollinger2005) observed an adjuvant blend of methylated seed oil and a nonionic surfactant overcame antagonism of quizalofop-p-ethyl in a mixture with tribenuron for control of yellow foxtail [Setaria pumila (Pior) Roem & Schult.].
Quizalofop-p-ethyl activity on barnyardgrass and weedy rice is often antagonized when applied in a mixture with ALS-inhibiting herbicides, including bispyribac-Na (Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018). Research has shown that adjuvants can aid in overcoming herbicide antagonism by enhancing herbicide penetration, improving spray coverage, and reducing surface tension (Penner Reference Penner1989). The objective of the current study was to evaluate the potential of different adjuvants to overcome or reduce the antagonism of quizalofop-p-ethyl when applied in a mixture with the ALS-inhibiting herbicide bispyribac-Na.
Materials and Methods
A field study was conducted in 2017 and 2018 at the Louisiana State University Agricultural Center H. Rouse Caffey Rice Research Station (30°10.812′ N, 92°20.944′ W) near Crowley, LA, to evaluate the influence of different adjuvants in overcoming the antagonism of quizalofop-p-ethyl when mixed with bispyribac-Na. The soil type at the Rice Research Station is a Midland silty clay loam with a pH of 5.7 and 3.3% organic matter. Field preparation consisted of fall and spring disking followed by two passes in the opposite direction with a two-way bed conditioner consisting of rolling baskets and s-tine harrows set at a depth of 6 cm. A preplant fertilizer of 8-24-24 (N-P2O5-K2O) was applied at 280 kg ha−1 followed by an application of 280 kg ha−1 of 46-0-0 fertilizer applied to the study area when rice was in the four-leaf to one-tiller stage, before permanent flood establishment. A permanent 10-cm flood was established when the ACCase-R rice reached the aforementioned growth stage and was maintained until 2 wk prior to harvest.
Plot size was 1.5 m by 5.1 m with eight drill-seeded rows of ACCase-R ‘PVL01’ long-grain rice (Provisia®; Horizon Ag, Memphis, TN 38125) spaced 19.5-cm apart. To simulate a weedy rice population, eight drill-seeded rows of ‘CLXL-745’ hybrid long-grain IR-rice spaced 19.5-cm apart were planted perpendicular in the front third of the plot, and eight drill-seeded rows of ‘CL-111’ long-grain IR-rice spaced 19.5-cm apart were planted perpendicular in the back third of each plot. All rice lines were planted on April 26, 2017, and April 12, 2018, at a rate of 84 kg ha−1. Awnless red rice was also broadcast across the research area at 50 kg ha−1 immediately before planting. The research area was naturally infested with barnyardgrass.
Quizalofop-p-ethyl and the respective mixtures were applied when ACCase-R rice was at the three- to four-leaf, mid-POST (MPOST) growth stage with a CO2-pressurized backpack sprayer calibrated to deliver 140 L ha−1. Red rice, CLXL-745, and CL-111 were at the three- to four-leaf growth stage and barnyardgrass was at the three- to five-leaf growth stage at the time of the herbicide application. The spray boom consisted of five flat-fan 110015 nozzles (Flat Fan AirMix Venturi Nozzle; Greenleaf Technologies, Covington, LA 70434) with 38-cm spacing.
The study was a randomized complete block with a three-factor factorial arrangement of treatments with four replications. Sources of materials are listed in Table 1. Factor A consisted of MPOST applications of quizalofop-p-ethyl at 0 or 120 g ha−1. Factor B consisted of MPOST applications of bispyribac-Na at 0 or 34 g ai ha−1. Factor C consisted of no adjuvant; a COC consisting of paraffinic oil and fatty acid esters; a silicon-based surfactant plus nitrogen source (SNS) consisting of a proprietary blend of alkanolamides, alkanoates, trisiloxane, and carbamides; and an HCOC consisting of fatty acid esters and alkoxylated alcohol phosphate ester.
Sources of materials.
a Abbreviations: COC, crop oil concentrate; EC, emulsifiable concentrate; HCOC, high-concentrate crop oil concentrate; L, liquid; SNS, silicon-based surfactant plus nitrogen source; WP, wettable powder.
b All adjuvants were applied at 1% vol/vol.
Visual evaluations for crop injury, and barnyardgrass, CL-111, CLXL-745, and red rice control were recorded at 14 and 28 d after treatment (DAT) on a scale from 0% to 100%, where 0% is no control and 100% is plant death. A second application of quizalofop-p-ethyl at 120 g ha−1 plus COC at 1% vol/vol was applied 1 wk after the 28 DAT rating date to remove non–ACCase-R rice from plots not initially treated or controlled with the initial quizalofop-p-ethyl application to prevent any rough rice yield contamination. Halosulfuron at 53 g ai ha−1 was applied 38 DAT to remove any remaining broadleaf or sedge weeds. Immediately prior to harvest, ACCase-R rice plant height was recorded, measuring from the soil surface to the tip of the extended panicle. The four center rows of ACCase-R rice were harvested with a Mitsubishi VM3 combine (Mitsubishi Corp., Tokyo, Japan) to determine the rough rice yield. Grain yield was adjusted to 12% moisture content.
Rough rice yield data were analyzed using the MIXED procedure in SAS (SAS Institute, Cary, NC). The Tukey honestly significant difference test was used to separate yield means at the 5% probability level. Control data were analyzed using the Blouin et al. (Reference Blouin, Webster and Bond2010) augmented mixed method to determine synergistic, antagonistic, or neutral responses for herbicide mixtures by comparing the expected control calculated on the basis of the activity of each herbicide applied alone with that of an observed control (Fish et al Reference Fish, Webster, Blouin and Bond2015, Reference Fish, Webster, Blouin and Bond2016; Osterholt Reference Osterholt, Webster, Blouin and McKnight2019; Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018, Reference Rustom, Webster, Blouin and McKnight2019). Herbicide treatments and evaluation timings represented the fixed effects for all models. The random effects were year, replication (nested within year), location, and all other interactions including any of these effects. The effect of different environmental conditions on herbicide activity within a year or combination of years represented the random effects of the test (Carmer et al. Reference Carmer, Nyquist and Walker1989; Hager et al. Reference Hager, Was, Bollero and Stroller2003; Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018, Reference Rustom, Webster, Blouin and McKnight2019). Normality of effects over all evaluation dates was checked with the UNIVARIATE procedure of SAS, and assumptions for normality were met.
Results and Discussion
Synergistic interactions for barnyardgrass control were observed at 14 DAT when quizalofop-p-ethyl was applied in a mixture with all adjuvants evaluated (Table 2). Antagonism of quizalofop-p-ethyl was observed for control of barnyardgrass at 14 DAT when applied in a mixture with bispyribac-Na plus no adjuvant, COC, SNS, or HCOC, with an observed control of 41%, 43%, 63%, and 86%, respectively, compared with an expected control of 95%. These results are similar to those of Rustom et al. (Reference Rustom, Webster, Blouin and McKnight2018), who observed antagonism of quizalofop-p-ethyl when mixed with bispyribac-Na plus a COC, with barnyardgrass control of 60%.
Barnyardgrass control with quizalofop-p-ethyl applied alone or mixed with bispyribac-Na and/or different adjuvants.a
a Blouin’s modified Colby’s analysis was used to determine interactions in 2017 and 2018.
b Abbreviations: COC, crop oil concentrate; DAT, days after treatment; HCOC, high-concentrate crop oil concentrate; NA, not applicable; SNS, silicon-based surfactant plus nitrogen source.
c Evaluation dates for each respective mixture component.
d Rates with a value of 1 represent a percentage of vol/vol.
e Observed means followed by − are significantly different from Blouin’s modified Colby’s expected responses at the 5% level indicating an antagonistic response. A + indicates a synergistic response. No sign indicates a neutral response.
f P < 0.05 indicates an antagonistic or synergistic response; P > 0.05 indicates a neutral response.
As with the 14-DAT evaluation, synergistic interactions for barnyardgrass control were observed at 28 DAT when treated with quizalofop-p-ethyl plus all adjuvants evaluated compared with quizalofop-p-ethyl applied alone, with 76% control of barnyardgrass (Table 2). Antagonism of quizalofop-p-ethyl was observed for control of barnyardgrass at 28 DAT when mixed with bispyribac-Na plus no adjuvant, COC, or SNS, with an observed control of 40%, 58%, and 76%, respectively, compared with an expected control of 97%. However, antagonism of quizalofop-p-ethyl mixed with bispyribac-Na plus HCOC at 14 DAT was overcome with a neutral interaction at 28 DAT for barnyardgrass control with an observed control of 91%, compared with an expected control of 97%. These data indicate that the most effective adjuvant for mixtures of quizalofop-p-ethyl and bispyribac-Na for barnyardgrass is HCOC.
The antagonism observed at 14 DAT when quizalofop-p-ethyl was mixed with bispyribac-Na plus HCOC was probably due to a delay in the absorption of quizalofop-p-ethyl by barnyardgrass and, by 28 DAT, a neutral interaction was observed. Croon et al. (Reference Croon, Ketchersid and Merkle1989) concluded that more than twice as much haloxyfop remained on the leaf surface when applied in a mixture with bentazon compared with haloxyfop applied alone. This same scenario may be occurring with the ALS- and ACCase-inhibiting herbicides evaluated in this research. Zollinger (Reference Zollinger2005) suggested that some adjuvants have an increased rate of cuticular wax solubilization compared with other adjuvants, which increases the rate of absorption of herbicides into plants. These data suggest HCOC may have a higher affinity for cuticular wax solubilization on barnyardgrass than do COC and SNS. HCOC is classified as a penetration agent, which is a material that enhances the ability of agrichemicals to penetrate a surface (Hazen Reference Hazen2000). Substituting HCOC for COC alleviated the antagonism of sethoxydim when mixed with bentazon for control of large crabgrass [Digitaria sanguinalis (L.) Scop.] (Jordan and York Reference Jordan and York1989) and provided better control of johnsongrass (Finley et al. Reference Finley, Lloyd, Gibson, Wilde and Brown1988).
Synergistic interactions were observed for CL-111 control at 14 DAT when treated with quizalofop-p-ethyl mixed with all adjuvants evaluated (Table 3). Antagonism of quizalofop-p-ethyl was observed for CL-111 control at 14 DAT when mixed with bispyribac-Na with no adjuvant, with an observed control of 61%, compared with an expected control of 86%. However, synergistic interactions were observed for CL-111 control at 14 DAT when quizalofop-p-ethyl was mixed with bispyribac-Na plus all adjuvants evaluated, with an observed control of 91% to 95%, compared with an expected control of 86%. Synergistic and/or neutral interactions were more prevalent at 14 and 28 DAT, respectively, for control of CL-111 compared with control of barnyardgrass (Table 2), due to the lack of bispyribac-Na activity on CL-111, and this influenced the expected control derived from quizalofop-p-ethyl and bispyribac-Na applied alone.
CL-111 control with quizalofop-p-ethyl applied alone or mixed with bispyribac-Na and/or different adjuvants.a
a Blouin’s modified Colby’s analysis was used to determine interactions in 2017 and 2018.
b Abbreviations: COC, crop oil concentrate; DAT, days after treatment; HCOC, high-concentrate crop oil concentrate; NA, not applicable; SNS, silicon-based surfactant plus nitrogen source.
c Evaluation dates for each respective mixture component.
d Rates with a value of 1 represent a percentage of vol/vol.
e Observed mean followed a + indicates a synergistic response. No sign indicates a neutral response.
f P < 0.05 indicates an antagonistic or synergistic response; P > 0.05 indicates a neutral response.
All mixtures at 28 DAT resulted in a neutral response for control of CL-111 with observed control ranging from 90% to 98%, compared with an expected control of 93% (Table 3). These results indicate the interaction of quizalofop-p-ethyl mixed with bispyribac-Na did not differ regardless of the adjuvant used. The neutral and synergistic interactions observed for CL-111 contrast with the antagonistic interactions observed for barnyardgrass control (Table 2). Different responses among plant species to herbicide mixture interactions may be due to genetic, physiological, or morphological differences (Zhang et al. Reference Zhang, Hamill and Weaver1995).
Synergistic interactions were observed at 14 DAT for CLXL-745 control when quizalofop-p-ethyl was mixed with all adjuvants evaluated (Table 4). Antagonism of quizalofop-p-ethyl was observed at 14 and 28 DAT when mixed with bispyribac-Na and no adjuvant, with an observed control of 73% and 86%, respectively, compared with an expected control of 88% and 91%, respectively. A synergistic interaction was observed for CLXL-745 control at 14 DAT when quizalofop-p-ethyl was mixed with bispyribac-Na plus HCOC, with an observed control of 92%, compared with an expected control of 88%. However, at 14 DAT, neutral interactions were observed for CLXL-745 control when quizalofop-p-ethyl was mixed with bispyribac-Na plus COC or SNS. Similar to CL-111, neutral and synergistic interactions were commonly observed due to the lack of bispyribac-Na activity on CLXL-745, and this directly influenced the expected control as determined using Colby’s equation (Colby Reference Colby1967).
CLXL-745 control with quizalofop-p-ethyl applied alone or mixed with bispyribac-Na and/or different adjuvants.a
a Blouin’s modified Colby’s analysis was used to determine interactions in 2017 and 2018.
b Abbreviations: COC, crop oil concentrate; DAT, days after treatment; HCOC, high-concentrate crop oil concentrate; NA, not applicable; SNS, silicon-based surfactant plus nitrogen source.
c Evaluation dates for each respective mixture component.
d Rates with a value of 1 represent a percentage of vol/vol.
e Observed means followed by a − are significantly different from Blouin’s modified Colby’s expected responses at the 5% level indicating an antagonistic response. A + indicates a synergistic response. No sign indicates a neutral response.
f P < 0.05 indicates an antagonistic or synergistic response; P > 0.05 indicates a neutral response.
Synergistic interactions were observed at 28 DAT for CLXL-745 control when quizalofop-p-ethyl was mixed with all adjuvants evaluated (Table 4). At 28 DAT, a neutral interaction was observed for CLXL-745 control when quizalofop-p-ethyl was mixed with bispyribac-Na plus COC. Synergistic interactions were observed at 28 DAT for CLXL-745 control when quizalofop-p-ethyl was mixed with bispyribac-Na plus SNS or HCOC, with an observed control of 95% and 97%, respectively, compared with an expected control of 91%. Similar to the results for barnyardgrass, HCOC proved to be the most consistent adjuvant for the mixture of quizalofop-p-ethyl plus bispyribac-Na for control of CLXL-745, with synergistic responses at 14 and 28 DAT. These results support the conclusions of Jordan and York (Reference Jordan and York1989), which were that HCOC outperformed COC when added to a mixture of sethoxydim plus bentazon in regard to large crabgrass control.
As with barnyardgrass (Table 2), CL-111 (Table 3), and CLXL-745 (Table 4), a synergistic interaction was observed at 14 DAT for red rice control with quizalofop-p-ethyl plus all adjuvants evaluated (Table 5). Antagonism of quizalofop-p-ethyl was observed at 14 DAT for red rice control when mixed with bispyribac-Na with no adjuvant, with an observed control of 67% compared with an expected control of 88%. A neutral interaction was observed at 14 DAT for red rice control when quizalofop-p-ethyl was mixed with bispyribac-Na plus COC; however, a synergistic interaction occurred with quizalofop-p-ethyl mixed with bispyribac-Na plus SNS or HCOC, with an observed control of 92% and 95%, respectively, compared with an expected control of 88%. These results are similar to those observed for CL-111 (Table 3) and CLXL-745 (Table 4) control with neutral and synergistic interactions.
Red rice control with quizalofop-p-ethyl applied alone or mixed with bispyribac-Na and/or different adjuvants.a
a Blouin’s modified Colby’s analysis was used to determine interactions in 2017 and 2018.
b Abbreviations: COC, crop oil concentrate; DAT, days after treatment; HCOC, high-concentrate crop oil concentrate; NA, not applicable; SNS, silicon-based surfactant plus nitrogen source.
c Evaluation dates for each respective mixture component.
d Rates with a value of 1 represent a percentage of vol/vol.
e Observed means followed by − are significantly different from Blouin’s modified Colby’s expected responses at the 5% level indicating an antagonistic response. A + indicates a synergistic response. No sign indicates a neutral response.
f P < 0.05 indicates an antagonistic or synergistic response; P > 0.05 indicates a neutral response.
Similar to CL-111 control (Table 3), all bispyribac-Na–containing mixtures at 28 DAT were neutral for red rice control, with an observed control of 93% to 99%, compared with an expected control of 97% (Table 5). These results indicate that regardless of the adjuvant used, neutral interactions were observed for all mixtures at 28 DAT. The results of red rice were similar to results in CL-111 (Table 3), which demonstrated synergistic interactions at 14 DAT for quizalofop-p-ethyl mixed with bispyribac-Na plus SNS or HCOC, with neutral interactions at 28 DAT.
Crop injury did not exceed 5% across all herbicide treatments and evaluation dates (data not shown). ACCase-R rice yielded 3,620 kg ha−1 when treated with quizalofop-p-ethyl plus bispyribac-Na with no adjuvant. ACCase-R rice treated with a mixture of quizalofop-p-ethyl plus bispyribac-Na plus either COC, SNS, or HCOC yielded 4,530 to 4,700 kg ha−1 (Table 6). However, ACCase-R rice treated with quizalofop-p-ethyl applied with COC, SNS, or HCOC without the addition of bispyribac-Na yielded 3,890 to 4,010 kg ha−1. The decrease in yield was due to the lack of early-season broadleaf control without the presence of bispyribac-Na. This yield reduction is a direct result of broadleaf weeds competing with the ACCase-R rice for essential growth requirements, including light, space, and nutrients, prior to the application of halosulfuron at 53 g ha−1 at 38 DAT. Similar yield reductions occurred when quizalofop-p-ethyl was not applied, because of barnyardgrass and weedy rice competing with the ACCase-R rice prior to the second application of quizalofop-p-ethyl. These decreases in yield demonstrate the necessity of herbicide mixtures for broad-spectrum weed control.
Rough rice yields of acetyl co-enzyme A carboxylase–resistant rice treated with quizalofop-p-ethyl and respective mixtures in 2017 and 2018.
a Abbreviations: COC, crop oil concentrate; HCOC, high-concentrate crop oil concentrate; SNS, silicon-based surfactant plus nitrogen source
b Respective mixture component.
c Means followed by a common letter are not significantly different at P = 0.05, Fisher protected LSD test.
In conclusion, it is essential to incorporate an adjuvant when applying quizalofop-p-ethyl alone and when applying quizalofop-p-ethyl in a mixture with bispyribac-Na. At 14 DAT, synergistic interactions were observed when quizalofop-p-ethyl was mixed with either adjuvant evaluated for barnyardgrass and all rice lines evaluated. Herbicides are often applied in a mixture to broaden the weed control spectrum, delay herbicide resistance, and save time and application costs (Gressel and Segel Reference Gressel and Segel1990; Jordan Reference Jordan1995; Zhang et al. Reference Zhang, Webster, Blouin and Leon2005). Because quizalofop-p-ethyl does not have any activity on broadleaf weeds, a herbicide with broadleaf activity may be needed in a mixture. Bispyribac-Na, a common broadleaf herbicide used in Louisiana rice production, can antagonize quizalofop-p-ethyl when applied in a mixture (Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018).
These results indicate that the antagonism of quizalofop-p-ethyl when mixed with bispyribac-Na plus HCOC at 14 DAT was overcome at 28 DAT with a neutral interaction for barnyardgrass control. The addition of COC, SNS or HCOC into a mixture of quizalofop-p-ethyl plus bispyribac-Na provided synergistic or neutral interactions at 14 and 28 DAT for CL-111, CLXL-745, and red rice control. However, HCOC most likely promotes increased uptake and translocation of quizalofop-p-ethyl in barnyardgrass and weedy rice, compared with COC and SNS (Penner Reference Penner1989). These results are similar to those of Wanamarta et al. (Reference Wanamarta, Penner and Kells1989), who found the active ingredient in HCOC overcame antagonism of sethoxydim from bentazon compared with a COC. It was reported that the active ingredient of HCOC greatly increased sethoxydim absorption when compared with more than 190 surfactants evaluated. Young et al. (Reference Young, Hart and Wax1996) concluded that using HCOC instead of COC will improve the activity of sethoxydim by increasing foliar absorption. These results suggest incorporating HCOC in a mixture of quizalofop-p-ethyl plus bispyribac-Na will offer the most beneficial mixture for broad-spectrum weed control including barnyardgrass and weedy rice in ACCase-R rice production.
Acknowledgments
Published with the approval of the director of the Louisiana Agricultural Experiment Station and the Louisiana State University Agricultural Center, Baton Rouge, LA 70803, under manuscript number 2019-306-34036. The authors would like to thank the staff of the Louisiana State University Agricultural Center Rice Research Station. The Louisiana Rice Research Board provided partial funding for this project. No conflicts of interest have been declared.
