Introduction
In response to herbicide-resistant (HR) weeds, new HR soybean traits have been commercialized. Soybean resistant to various combinations of 2,4-D choline, dicamba, glufosinate, glyphosate, isoxaflutole, and mesotrione are or will be available to help manage HR weeds. Soybean resistant to dicamba and glyphosate (Xtend® soybean) and 2,4-D choline, glufosinate, and glyphosate (Enlist E3® soybean) were commercialized in 2017 and 2019, respectively. In 2021, soybean resistant to dicamba, glufosinate, and glyphosate (XtendFlex® soybean) were available for use by producers. A plan to manage HR weeds is critical; therefore, new HT technologies can provide producers with additional options for their HR weed management program (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012).
In 2019, soybean resistant to glufosinate, glyphosate, and isoxaflutole (LibertyLink® GT27® soybean), which is the first soybean resistant to a 4-hyroxyphenylpyruvate (HPPD)-inhibiting herbicide, became commercially available. Isoxaflutole is labeled for use in LibertyLink® GT27® soybean only as a preemergence treatment (Anonymous 2022a). In addition, a formulation of mesotrione was recently labeled for preemergence use in LibertyLink® GT27® Soybean as well (Anonymous 2022b). Also, Bowers (Reference Bowers2018) and Lindenmayer et al. (Reference Lindenmayer, Bowers, Franssen and Schraer2018) reported the development of a novel HPPD-inhibitor stack, SYHT0H2, which will provide tolerance to isoxaflutole and mesotrione preemergence and glufosinate postemergence.
Weed management programs should utilize residual herbicides to mitigate HR weeds (Jha and Norsworthy Reference Jha and Norsworthy2009; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). The introduction of HPPD inhibitor-resistant soybean provides additional options for weed management with herbicides that provide residual control. Isoxaflutole and mesotrione control Amaranthus spp. when applied preemergence (Johnson et al. Reference Johnson, Chahal and Regeh2012; Sutton et al. Reference Sutton, Richards, Buren and Glasgow2002). Meyer et al. (Reference Meyer, Norsworthy, Young, Steckel, Bradley, Johnson, Loux, Davis, Kruger, Bararpour, Ikley, Spaunhorst and Butts2016) reported 90% and 97% Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [Amaranthus tuberculatus (Moq.) Sauer] control, respectively, 4 wk following isoxaflutole at 105 g ai ha−1 preemergence. Isoxaflutole at 105 g ha−1 preemergence controlled glyphosate-resistant (GR) waterhemp 73% and 42% at 2 and 12 wk after treatment (WAT), respectively (Schryver et al. Reference Schryver, Soltani, Hooker, Robinson, Tranel and Sikkema2017). These authors also observed 56% and 18% control of GR waterhemp at 2 and 12 WAT following mesotrione at 140 g ai ha−1.
These technologies offer soybean producers more options to manage HR weeds, but producers should avoid sensitive crop injury from off-target movement from herbicides such as 2,4-D choline, dicamba, isoxaflutole, or mesotrione. Physical drift, volatility, and equipment contamination are examples of off-target movement that can be problematic when an HR technology in a crop is in close proximity to a sensitive crop (Culpepper et al. Reference Culpepper, Sosnoskie, Shugart, Leifheit, Curry and Gray2018). Off-target movement of pesticides ranges from 1% to 8% and 20% to 35% for ground and aerial applications, respectively (Maybank et al. Reference Maybank, Yoshida and Grover1978; Wolf et al. Reference Wolf, Grover, Wallace, Shewchuk and Maybank1992). Others have evaluated simulated herbicide off-target movement on corn (Zea mays L.), cotton (Gossypium hirsutum L.), grain sorghum [Sorghum bicolor (L.) Moench], rice (Oryza sativa L.), and soybean (Al-Khatib et al. Reference Al-Khatib, Claassen, Stahlman, Geier, Regehr, Duncan and Heer2003; Bailey and Kapusta Reference Bailey and Kapusta1993; Ellis and Griffin Reference Ellis and Griffin2002; Ellis et al. Reference Ellis, Griffin and Jones2002, Reference Ellis, Griffin, Linscombe and Webster2003; Matocha and Jones Reference Matocha and Jones2015; Sperry et al. Reference Sperry, Lawrence, Bond, Reynolds, Golden and Edwards2019; Stephenson et al. Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019a, Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019b; Steppig et al. Reference Steppig, Norsworthy, Scott and Lorenz2018).
The availability of multiple HR soybean technologies provides producers with additional tools for managing HR weeds, but the probability that a herbicide from one HR technology will move off-target, resulting in soybean injury and yield reduction, is a concern. Off-target movement of dicamba from dicamba-resistant soybean to dicamba-susceptible soybean is an example of the deleterious effect of off-target movement (Bradley Reference Bradley2017; Kniss Reference Kniss2018). Therefore, injury and yield reductions following off-target movement of isoxaflutole or mesotrione to HPPD inhibitor-susceptible soybean is possible. Little information is available concerning the effect of these herbicides applied at doses chosen to simulate physical drift or misapplication to early vegetative soybean on injury and subsequent growth, yield, and potential economic impact.
Materials and Methods
Concurrent field studies (isoxaflutole study and mesotrione study) were conducted at the Louisiana State University AgCenter Dean Lee Research and Extension Center near Alexandria, LA (31.18°N, 92.41°W), in 2017, 2018, and 2019; the Louisiana State University AgCenter Northeast Research Station in St Joseph, LA (31.95°N, 91.23°W), in 2018 and 2019; and the Mississippi State University Delta Research and Extension Center in Stoneville, MS (33.42°N, 90.91°W), in 2018 and 2019. Soil at Alexandria was a Coushatta silt loam (fine-silty, mixed, superactive, thermic Fluventic Entrudepts), with a pH of 8.0 and 1.5% organic matter; and soil at St Joseph and Stoneville was a Commerce silt loam (fine-silty, mixed, superactive, nonacid, thermic Fluvaquentic Endoaquepts), with a pH of 6.5 to 6.8 and 0.3% to 0.6% organic matter.
The experimental design for both studies was a randomized complete block with nine treatments in a factorial arrangement replicated four times each year at all sites. Factor 1 was application timing, with treatments applied to unifoliate (V1), 2-trifoliate (V2), or 4-trifoliate (V4) soybean (Fehr et al. Reference Fehr, Caviness, Burmond and Pennington1971). These soybean growth stages were selected to evaluate the effect of isoxaflutole and mesotrione on early vegetative soybean. Factor 2 was isoxaflutole (Balance® Bean, Bayer CropScience LP, 2 T.W. Alexander Drive, Research Triangle Park, NC 27709; Alite 27™, BASF, 26 Davis Drive, Research Triangle Park, NC 27709) or mesotrione (Callisto®, Syngenta Crop Protection, P.O. Box 18300, Greensboro, NC 27419) dose. Isoxaflutole doses in the isoxaflutole study were 7, 13, and 105 g ha−1; and mesotrione doses in the mesotrione study were 11, 22, and 176 g ha−1. These doses represented 6.25%, 12.5%, and 100% of the field use doses of isoxaflutole (Anonymous 2022a) and mesotrione (Anonymous 2022b). The 100% field use dose was based upon information from Syngenta Crop Protection, which reported that mesotrione at 140 to 210 g ha−1, depending upon soil texture, is currently being evaluated for weed control in mesotrione-resistant soybean (SR Moore, Syngenta Crop Protection, personal communication). Doses were chosen to represent those observed with physical drift (6.25% and 12.5%) or misapplication (100%). Maybank et al. (Reference Maybank, Yoshida and Grover1978) reported off-target droplet drift varies between 1% and 8% for ground applications. However, Wolf et al. (Reference Wolf, Grover, Wallace, Shewchuk and Maybank1992) reported drift from nonshielded sprayers to range from 2% to 16%. The two physical drift rates evaluated in our studies, 6.25% and 12.5%, were chosen to reflect these ranges. A nontreated control was included for comparison in each study.
Plots were four 9-m rows spaced 0.97 m apart at Alexandria and four 9-m rows spaced 1 m apart at both St. Joseph and Stoneville, with only the center two rows receiving prescribed treatments at all locations. All study areas were maintained weed-free throughout the season with as-needed applications of glyphosate (Roundup PowerMax®, Monsanto Company, 800 N. Lindbergh Blvd., St Louis, MO 63167) at 1,260 g ae ha−1 and hand weeding. At all locations, treatments were applied with a CO2-pressurized backpack sprayer in a constant carrier volume of 140 L ha−1. Others have simulated off-target movement of herbicides utilizing constant carrier volume (Al-Khatib et al. Reference Al-Khatib, Claassen, Stahlman, Geier, Regehr, Duncan and Heer2003; Bailey and Kapusta Reference Bailey and Kapusta1993; Ellis and Griffin Reference Ellis and Griffin2002; Ellis et al. Reference Ellis, Griffin, Linscombe and Webster2003; Sperry et al. Reference Sperry, Lawrence, Bond, Reynolds, Golden and Edwards2019; Stephenson et al. Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019a, Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019b; Steppig et al. Reference Steppig, Norsworthy, Scott and Lorenz2018). At Alexandria, all treatments were applied at 190 kPa using four flat-fan 11002 nozzles (AIXR TeeJet®, TeeJet Technologies Southeast, P.O. Box 832, Tifton, GA 31794). At St Joseph, treatments were applied at 214 kPa using four flat-fan 11002 nozzles (Turbo TeeJet®, Teejet Memphis). At Stoneville, treatments were applied at 234 kPa using four flat-fan 110015 nozzles (Airmix Low Pressure Nozzle, Greenleaf Technologies, P.O. Box 1767, Covington, LA 70434). Soybean variety and dates of planting, treatment application, and harvest for each location and year are presented in Table 1. Although, Young et al. (Reference Young, Young and Matthews2003) reported differential susceptibility among soybean varieties to mesotrione postemergence, determining differences in susceptibility among soybean varieties was not an objective of these studies. Table 2 lists environmental conditions at the time of application for each location in all years.
Table 1. Soybean variety and dates of planting, V1, V2, and V4 application timings, and harvest in research conducted from 2017 to 2019 at Alexandria and St Joseph, LA, and Stoneville, MS.
a Soybean yield not collected in 2018 at Stoneville, MS.
Table 2. Air and soil temperatures and relative humidity (RH) at V1, V2, and V4 application timings in research conducted from 2017 to 2019 at Alexandria and St Joseph, LA, and Stoneville, MS.
a Soil temperature was not collected in Stoneville in either year at all application timings.
Visible estimates of soybean injury were recorded at 3, 7, 14, 28, and 42 d after each application timing (DAT) using a 0% to 100% scale (0% representing no injury, and 100% representing complete soybean death). Visible chlorosis, necrosis, and height reduction were accounted for in the visible injury rating, which is consistent with Frans et al. (Reference Frans, Talbert, Marx, Crowley and Camper1986), who described using visible discoloration and stunting to determine visible injury. Soybean heights were recorded at 14, 28, and 42 DAT by measuring 10 randomly selected plants in each plot from the soil to the apical terminal of each plant (Rana et al. Reference Rana, Norsworthy and Scott2014; Sperry et al. Reference Sperry, Lawrence, Bond, Reynolds, Golden and Edwards2019; Stephenson et al. Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019a, Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019b). Soybean heights were collected to provide a quantitative measurement of potential height reduction caused by the treatments. Soybean heights at 14, 28, and 42 DAT were converted to a percentage of that observed for the nontreated within each application timing before analysis to allow comparison among application timings. Yield, adjusted to 13% moisture, was determined by harvesting treated rows using conventional harvesting equipment.
The economic impact following applications of isoxaflutole and mesotrione doses were determined by a loss calculation based on U.S. dollars per kilogram ($US kg−1). Revenue losses were calculated by multiplying the difference between the nontreated and treated soybean yield by the average soybean prices received in Louisiana and Mississippi in 2017, 2018, and 2019, which was U.S.$0.34 kg−1 (USDA-NASS 2021). A 3-yr average price was used, because it corresponds to the years the studies were conducted.
Before analysis, data were subjected to PROC UNIVARIATE analysis in SAS® release 9.4 (SAS Institute, 100 SAS Campus Dr., Cary, NC 27513) to test for normality and homogeneity of variance. ANOVA was performed on all data utilizing PROC MIXED in SAS with isoxaflutole or mesotrione application timings, doses, evaluation dates, and all interactions as fixed effects. Random effects were year, locations, and replications within year; the interaction of location, replication, and application timing nested within year; and the interaction of location, replication, application timing, and herbicide dose nested within year. Least-square means were calculated, and means were separated using Tukey’s honest significant difference test at P ≤ 0.05. In the presence of a significant interaction involving application timing, dose, and evaluation date for visible injury, data were subjected to regression procedures using PROC REG in SAS, testing linear and quadratic functions against evaluation date. Model fit was evaluated using the goodness-of-fit parameters root mean-square error (RMSE) (Wilmott Reference Willmott1981) and the coefficient of determination (R2) (Legates and McCabe Reference Legates and McCabe1999). RMSE was utilized to measure goodness of fit in addition to R2, as Willmott (Reference Willmott1981) and Willmott and Matsura (Reference Willmott and Matsuura2006) suggest RMSE provides a better parameter to estimate the accuracy of a model to be utilized for predictive purposes. A smaller RMSE value represents a better fit.
Results and Discussion
Isoxaflutole Study
Analysis indicated the interaction of isoxaflutole timing and dose and evaluation date was significant (P = 0.0302); therefore, visible injury data were subjected to regression procedures. Soybean visible injury RMSE values ranged from 2.1 to 9.8 for all significant regressions, indicating a good fit for all models. Main effects for application timing and isoxaflutole dose were significant for soybean height as a percent of the nontreated at 14 DAT (data not shown); however, their interaction was significant for heights at 28 and 42 DAT (Table 3). Furthermore, analysis of soybean yield, yield loss, and revenue loss indicated that only main effects were significant (Table 4).
Table 3. Soybean height as a percent of the nontreated as influenced by the interaction of application timing and isoxaflutole dose in the isoxaflutole study conducted from 2017 to 2019 at Alexandria and St Joseph, LA, and Stoneville, MS. a
a Means followed by the same letter for each evaluation date are not different according to Tukey’s honest significant difference test at P ≤ 0.05.
Table 4. Soybean yields, yield loss, and revenue loss in the isoxaflutole study and mesotrione study conducted from 2017 to 2019 at Alexandria and St Joseph, LA, and Stoneville, MS. a
a Means for yield or revenue loss followed by same letter for each parameter in each study are not different according to Tukey’s honest significant difference test at P ≤ 0.05.
b Data pooled across isoxaflutole doses of 6.25%, 12.5%, and 100% of the field use dose of 105 g ai ha−1.
Data pooled across mesotrione doses of 6.25%, 12.5%, and 100% of the field use dose of 176 g ai ha−1.
c Yield loss calculated by yield loss = nontreated yield − treated yield.
d Estimated soybean price was $0.34 kg−1 based upon USDA reported 3-yr average price received by Louisiana and Mississippi soybean producers in 2017, 2018, and 2019. Revenue loss calculated by revenue loss = (nontreated yield − treated yield) × $0.34 kg−1.
e Data pooled across application timings of V1, V2, and V4 soybean growth stages.
Isoxaflutole injury was characterized by chlorosis with slight necrosis at 3 and 7 DAT, progressing to necrosis and height reduction at 14 and 28 DAT, with visible height reduction the only symptom at 42 DAT. Overall, visible injury correlated with isoxaflutole dose, with injury increasing at 3 to 21 DAT and decreasing at 28 DAT and after (Figure 1A–C). The only exception was following the 6.25% dose, with visible injury of V4 soybean decreasing linearly. Isoxaflutole at the 6.25% and 12.5% doses injured V2 soybean more than V1 or V4 soybean at all evaluation dates (Figure 1A and B). Environmental conditions at the time of application were similar in all years and locations across application timings (Table 2) and thus could not explain the greater injury to V2 soybean. Visible injury at 21 to 48 DAT was predominantly visible height reduction, and soybean height as a percent of the nontreated supports the greater visible injury observed following the V2 application. Soybean height at 14 d after the V2 application was 80% of the nontreated, but V1 and V4 heights were 89% and 85% of the nontreated, respectively (data not shown). At 28 and 42 DAT, soybean heights following the V2 application of both low doses were 77% to 85%, which were less than V1 and V4 heights following the 6.25% dose at 28 DAT and the 12.5% dose at 42 DAT (Table 3). Therefore, a reduction in height following the V2 application is what led to greater observed injury. Similarly, Stephenson et al. (Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019b) reported that metribuzin postemergence at 39.5 and 79 g ai ha−1, which is 12.5% and 25% of the field use rate, injured V2 soybean greater than unifoliate and V4 soybean at 14, 28, and 42 DAT. Also, fomesafen postemergence at 12.5% of the labeled use rate injured V5 and V7 corn greater than V1, V3, or V9 corn (Sperry et al. Reference Sperry, Lawrence, Bond, Reynolds, Golden and Edwards2019).
Figure 1. Visible soybean injury in the isoxaflutole study following treatments applied to V1, V2, or V4 soybean as a function of days after treatment (DAT) at (A) 6.25%, (B) 12.5%, and (C) 100% of the field use dose of 105 g ai ha−1 at Alexandria and St Joseph, LA, and Stoneville, MS, in 2017, 2018, and 2019. Sample means represent 3, 7, 14, 28, and 42 DAT. Error bars represent ±1 SE.
Unifoliate (V1) soybean was injured greater than V2 and V4 soybean following the 100% isoxaflutole dose at all evaluation dates (Figure 1C). Maximum injury observed occurred at 28 DAT, with V1, V2, and V4 soybean injured 90%, 85%, and 72%, respectively, but injury began to decline by 42 DAT. Soybean heights as a percent of the nontreated support visible injury observations. At 14 DAT, heights following the 100% dose were 73% of the nontreated, which was only different from the 6.25% dose (data not shown). At 28 and 42 DAT, soybean height was 53% and 43% of the nontreated, respectively, (Table 3). As for the two low doses, no literature is available reporting the effect of isoxaflutole postemergence on different growth stages of crops or weeds. However, our findings are in agreement with those of Young et al. (Reference Young, Young and Matthews2003), who reported that soybean injury decreased as soybean growth stage increased following mesotrione postemergence. Similarly, soybean and cotton injury following low-dose flumioxazin decreased as growth stage increased (Stephenson et al. Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019a, Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019b).
No differences in soybean yield, yield loss, or revenue loss were observed among application timings (Table 4). Soybean yields were 3,830, 3,850, and 3,770 kg ha−1 following the V1, V2, and V4 applications, respectively. Yield losses ranged from 690 to 740 kg ha−1, resulting in revenue losses of US$226 ha−1 to US$257 ha−1. Following low-dose flumioxazin applications to V1, V2, or V4 soybean, yield and revenue losses ranged from US$196 ha−1 to US$393 kg ha−1 and US$71 ha−1 to US$141 ha−1, respectively (Stephenson et al. Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019b). Regardless of application, all doses of isoxaflutole reduced soybean yield (Table 4). Nontreated soybean yield was 4,530 kg ha−1, but yields following the 6.25%, 12.5%, and 100% doses were 4,220, 4,020, and 2,490 kg ha−1, respectively. Although differences in visible injury and soybean height were observed between the two low doses, no yield difference was observed, which indicates that soybean can recover from early-season injury following low doses of isoxaflutole. Similarly, Stephenson et al. (Reference Stephenson, Spivey, Deliberto, Blouin, Woolam and Buck2019b) reported that low-dose metribuzin injured soybean and reduced plant height, but only reduced yield 2% to 4%. However, revenue loss and isoxaflutole dose were positively correlated. Losses of US$147 ha−1, US$202 ha−1, and US$623 ha−1 were calculated for the 6.25%, 12.5%, and 100% doses, respectively (Table 4). Soybean yield and revenue losses highlight the detrimental effect of an errant isoxaflutole application on HPPD inhibitor-susceptible soybean.
Mesotrione Study
Analysis indicated the interaction of mesotrione timing and dose and evaluation date was significant (P = 0.0009); therefore, visible injury data were subjected to regression procedures. Soybean visible injury RMSE values ranged from 2.8 to 10.7 for all significant regressions, indicating a good fit for all models. Also, the interaction of application timing and mesotrione dose for soybean height as a percent of the nontreated was significant at all evaluation dates (Table 5). However, only the main effects of application timing and mesotrione dose were significant for soybean yield, yield loss, and revenue loss (Table 4).
Table 5. Soybean height as a percent of the nontreated as influenced by the interaction of application timing and mesotrione dose in the mesotrione study conducted from 2017 to 2019 at Alexandria and St Joseph, LA, and Stoneville, MS. a
a Means followed by the same letter for each evaluation date are not different according to Tukey’s honest significant difference test at P ≤ 0.05.
As in the isoxaflutole study, soybean injury in the mesotrione study was chlorosis, followed by necrosis, and culminating with visible height reduction. However, chlorosis was observed until 28 DAT, which differed from observations in the isoxaflutole study. Others observed stunting, leaf bleaching and malformation, and necrosis following low doses of mesotrione postemergence (Brown et al. Reference Brown, Robinson, Chandler, Swanton, Nurse and Sikkema2009; Young et al. Reference Young, Young and Matthews2003). Following the 6.25% dose, visible injury decreased linearly and ranged from 20% to 27% and did not differ among application timings (Figure 2A). Also, application timings were similar, and visible injury ranged from 27% to 36% following the 12.5% dose; however, visible injury followed a quadratic trend, increasing at 3 to 14 DAT and beginning to decrease between 14 and 28 DAT (Figure 2B). Injury of V1 soybean following mesotrione postemergence at 11 g ha−1 (which is equal to our 6.25% dose) was 35% and 49% at 7 and 14 DAT, respectively, but decreased to 10% by 28 DAT (Young et al. Reference Young, Young and Matthews2003). Similarly, Brown et al. (Reference Brown, Robinson, Chandler, Swanton, Nurse and Sikkema2009) observed 26%, 27%, and 11% injury of V2 soybean at 7, 14, and 28 DAT, respectively, following 10 g ha−1 of mesotrione postemergence. The level of injury reported by Young et al. (Reference Young, Young and Matthews2003) was more like the injury data following the 12.5% dose, but the injury described by Brown et al. (Reference Brown, Robinson, Chandler, Swanton, Nurse and Sikkema2009) supports our findings.
Figure 2. Visible soybean injury in the mesotrione study following treatments applied to V1, V2, or V4 soybean as a function of days after treatment (DAT) at (A) 6.25%, (B) 12.5%, and (C) 100% of the field use dose of 176 g ai ha−1 at Alexandria and St Joseph, LA, and Stoneville, MS, in 2017, 2018, and 2019. Sample means represent 3, 7, 14, 28, and 42 DAT. Error bars represent ±1 SE.
Unifoliate (V1), V2, and V4 soybean was injured 62%, 43%, and 35% at 3 DAT, respectively, with no difference between V2 and V4 soybean following the 100% mesotrione dose. Visible injury followed a quadratic trend, with injury increasing until 14 DAT and beginning to decrease at approximately 21 DAT (Figure 2C). Visible injury of V1, V2, and V4 soybean was 69%, 53%, and 45% at 7 DAT and 77%, 65%, and 57% at 14 DAT, respectively, indicating that mesotrione sensitivity decreased as application was delayed to later growth stages. Young et al. (Reference Young, Young and Matthews2003) reported that VC soybean was more sensitive than V1 or V2 soybean to mesotrione postemergence, which supports our findings following the 100% dose, but not our data for the two lower doses. Soybean heights at 14 DAT following the V1, V2, and V4 applications did not differ and ranged from 60% to 66% (Table 5). Therefore, differences in observed visible injury can be attributed to increased chlorosis and necrosis at 3 through 14 DAT. However, little to no difference was observed among application timings for visible injury at 28 and 42 DAT, with injury decreasing with time (Figure 2C). This is supported by soybean height data at the later evaluation dates, in which only V1 and V2 soybean differed at 28 DAT following the 100% mesotrione dose (Table 5).
No differences among application timings were observed for soybean yield, which ranged from 3,220 to 3,380 kg ha−1 (Table 4). However, yield losses following the V1, V2, and V4 application timings were 320, 240, and 420 kg ha−1, respectively, with differences between the V2 and V4 applications. The difference in yield loss resulted in a revenue loss of US$134 ha−1, US$93 ha−1, and US$157 ha−1 following the V1, V2, and V4 applications, respectively, with differences between V2 and V4 soybean, which mirrored yield loss (Table 4). These differences may be due to greater visible injury, with soybean injury at V4 being greater than at the V1 and V2 timings following both low doses of mesotrione at 28 and 42 DAT and greater than at the V1 timing at 42 DAT following the 100% dose (Figure 2A–C). As observed in the isoxaflutole study, yield and revenue losses highlight the impact mesotrione will have on HPPD inhibitor-susceptible soybean.
Regardless of application timing, soybean yield decreased with increasing mesotrione dose (Table 4). Yields for the nontreated and following the V1 application timing did not differ greatly at 3,700 and 3,620 kg ha−1, respectively, but a revenue loss of US$60 ha−1 for V1 soybean illustrates that even low doses of mesotrione can negatively impact producers. Young et al. (Reference Young, Young and Matthews2003) reported that 35 and 105 g ha−1 reduced soybean yield 11% and 22%, respectively, but yield was not reduced following mesotrione at doses less than 35 g ha−1. This contrasts with our finding that the 12.5% dose, 22 g ha−1, reduced soybean yield 270 kg ha−1, resulting in a revenue loss of US$103 ha−1 (Table 4). However, Brown et al. (Reference Brown, Robinson, Chandler, Swanton, Nurse and Sikkema2009) found that 20 g ha−1 of mesotrione reduced soybean seed yield to 84% of the nontreated, which was greater than our results. Following the 100% dose, soybean yielded 2,540 kg ha−1, which meant a revenue loss of US$373 ha−1. These data indicate that off-target mesotrione postemergence can have a detrimental effect on HPPD inhibitor-susceptible soybean yield, leading to revenue loss.
Isoxaflutole and mesotrione doses simulating physical drift and misapplication will injure HPPD inhibitor-susceptible soybean and reduce height and yield, which will result in a revenue loss. Visible injury decreased with time for both herbicides regardless of dose, which could lead a producer to assume the soybean is recovering, but decreasing visible injury could be misleading. Our data indicate that isoxaflutole and mesotrione visible injury should not be used as the only indicator of potential yield and revenue loss in HPPD inhibitor-susceptible soybean. Producers are cautioned to adhere to proper cleanout procedures, to consider the HPPD inhibitor-susceptible soybean planted in close proximity, and use best management practices to reduce the chance of off-target movement or application.
Acknowledgments
The authors thank the support staff at the Louisiana State University Agricultural Center Dean Lee Research and Extension Center and the Northeast Research Station and the Mississippi State University Delta Research and Extension Center for their help with this research. No conflicts of interest have been declared. Approved by publication as manuscript no. 2022-263-37216 of the Louisiana State University Agricultural Center.
