Sensitivity and Recovery of Grain Sorghum to Simulated Drift Rates of Glyphosate, Glufosinate, and Paraquat

agriculture

Article Sensitivity and Recovery of Grain Sorghum to Simulated Drift Rates of Glyphosate, Glufosinate, and Paraquat

Ralph R. Hale, Taghi Bararpour, Gurpreet Kaur * , John W. Seale, Bhupinder Singh and Tessie Wilkerson

Delta Research and Extension Center, Mississippi State University, Stoneville, MS 38776, USA; [email protected] (R.R.H.); [email protected] (T.B.); [email protected] (J.W.S.); [email protected] (B.S.); [email protected] (T.W.) * Correspondence: [email protected]; Tel.: +1-662-686-9311

Received: 20 February 2019; Accepted: 27 March 2019; Published: 1 April 2019

Abstract: A field experiment was conducted in 2017 and 2018 to evaluate the sensitivity and recovery of grain sorghum to the simulated drift of glufosinate, glyphosate, and paraquat at two application timings (V6 and flag leaf growth stage). Paraquat drift caused maximum injury to sorghum plants in both years, whereas the lowest injury was caused by glyphosate in 2017. Averaged over all herbicide treatments, injury to grain sorghum from the simulated herbicide drift was 5% greater when herbicides were applied at flag leaf stage, as compared to herbicide applications at the six-leaf stage in 2017. In 2018, injury from glyphosate drift was higher when applied at the six-leaf stage than at the flag leaf stage. Paraquat and glufosinate drift caused more injury when applied at flag leaf stage than at six-leaf stage at 14 days after application in 2018. About 21% to 29% of injury from the simulated drift of paraquat led to a 31% reduction in grain sorghum yield, as compared to a nontreated check in 2017. The simulated drift of glyphosate and glufosinate did not result in any significant yield reduction compared to the nontreated check in 2017, possibly due to the recovery of sorghum plants after herbicides’ drift application.

Keywords: plant height; sorghum; injury; sorghum grain yield

1. Introduction Weed control in grain sorghum (Sorghum bicolor (L.) Moench) is mostly achieved through spraying herbicides such as atrazine, S-metolachlor, dimethenamid-P, and alachlor for preemergence weed control and 2,4-D, dicamba, prosulfuron, and bromoxynil for postemergence weed control [1] However, grain sorghum is sensitive to many herbicides, including glyphosate, glufosinate, and imazethapyr [2]. In the southern United States, including Mississippi, grain sorghum is commonly grown adjacent to corn (Zea mays L.), cotton (Gossypium hirsutum L.), or soybean (Glycine max (L.) Merr.) [2] and consequently, it becomes subjected to herbicide drift from these adjacent ﬁelds. Off-target movement of herbicides applied in corn, cotton, or soybean, is common when environmental conditions favor volatilization and/or physical drift [2]. Factors inﬂuencing the severity of the herbicide drift may include a sensitive crop, crop growth stage, environmental conditions, herbicide and formulation, droplet size, spray additive(s) and pressure, and the height of the boom [2]. Paraquat is commonly used for burndown applications prior to planting crops, but it may also be used as a harvest aid for soybean. Earlier soybean harvest allows for the possibility of grain sorghum to be exposed to paraquat. Aside from harvest aids, glyphosate and glufosinate are applied for postemergence (POST) weed control in the Roundup Ready® (Bayer CropScience LP, Research

Agriculture 2019, 9, 70; doi:10.3390/agriculture9040070 www.mdpi.com/journal/agriculture Agriculture 2019, 9, 70 2 of 10

Triangle Park, NC, USA) or LibertyLink® (BASF Corporation, Research Triangle Park, NC, USA) systems, respectively. Applications of these herbicides can expose grain sorghum in nearby fields to drift. Glyphosate is a non-selective and broad-spectrum herbicide used for POST emergence control of many annual and perennial grass and broadleaf weeds. Glyphosate causes plant death by translocating rapidly, from foliage to meristems. Glufosinate inhibits the glutamine synthetase enzyme in plants, which restricts the formation of glutamine from glutamate and ammonia and results into the rapid accumulation of ammonia causing chlorotic and necrotic leaves in plants [2]. Historically, glyphosate has been used as a standard treatment for burn-down weed control before crop planting in spring or summer [3]. Glyphosate-resistant or glufosinate-resistant varieties of corn, cotton and, soybean expand the window of their use in these crops. Injury-related to herbicide drift (from chlorslfurin, thifenslfuron, triclopyr, dicamba, glyphosate, glufosinate, etc.) has been reported in multiple crops, including sunflower (Helianthus annus L.) [4,5], cotton [6], soybean [4,7], pea (Pisum sativum L.) [8,9], canola (Brassica ssp.) [4,5], sugar beet (Beta vulgaris L.) [10], alfalfa (Medicago sativa L.) [11], grape (Vitis vinifera L.) [12], cherry (Prunus ssp.) [13], rice (Oryza sativa L.) [14], and tomato (Physalis pubescens L.) [15]. Injury symptoms from drift of herbicides (such as glufosinate, glyphosate, Imazethapyr, and sethoxydim) include chlorosis, wilting, necrosis, and distorted leaf shape [2]. For studies conducted in Illinois, Indiana, Ohio, and Ontario, Brown, et al. [16] reported that simulated glyphosate drift at 100 and 200 g ha−1 caused 11% to 61% visual injury to corn plants and reduced corn yields by 49% to 56%. In cotton, drift from 2,4-D caused higher injury than dicamba in Texas [17]. Similarly, glyphosate causes more injury to wheat plants compared to imazamox in Kansas [3]. Deeds et al. [3] reported that glyphosate drift at a 1/3x rate leads to a nearly complete kill and yield loss in wheat and there was a minor injury when applied at 1/100x rate. Visual injury to crop plants can be a good indicator of yield reduction by the herbicide drift [3]. In addition, studies have also reported greater damage from herbicide drift at later reproductive crop stages than the early growth stages of the crop [3,6,17,18]. In contrast, Lyon [19] reported that early season drift of glyphosate caused more injury to sorghum plants than later-season applications. Lyon [19] found no significant reduction in sorghum yield from glyphosate drift occurring at the bloom growth stage, as compared to the 4-inch, 12-inch, and boot growth stage drift applications. Al-Khatib et al. [2] reported grain sorghum affected by glufosinate drift exhibited symptoms of chlorosis and necrosis within two to four days after application and interveinal chlorosis and plant stunting occurred within six days of glyphosate drift. Previous work by Al-Khatib et al. [2] also reported that injury to grain sorghum increases with increasing the application rates of glyphosate from 1/100 to 1/3 of the recommended application rate and it ranges from 64% to 99% at eight weeks after the treatment application. Severe injury from herbicide drift can reduce sorghum grain yields [2]. Limited research has been conducted on the effects of glyphosate, glufosinate, and paraquat drift as well as timing of herbicide drift on grain sorghum yields in Mississippi. Therefore, the objective of this study was to evaluate the sensitivity and recovery of grain sorghum to the simulated drift of glufosinate, glyphosate, and paraquat at two application timings.

2. Materials and Methods Two-year field studies were conducted in 2017 and 2018 at the Mississippi State University Delta Research and Extension Center in Stoneville, MS. Trials were conducted on Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquerts) with 2.4% organic matter and pH 7.5. A hybrid grain sorghum variety “Pioneer ‘84P80’ (Corteva Agriscience, Indianapolis, IN, USA)” was planted in 102 cm rows at 23 seeds m−1 row length on 8 June and 31 May in 2017 and 2018, respectively. This variety was selected for its high yielding characteristic and it is commonly grown around the U.S. Experiments were set up as a randomized complete block design, with a factorial arrangement of treatments and four replications. Experiments contained two factors consisting of sorghum growth stages (V6, (sorghum plant with fully opened 6 leaves) and flag leaf (the last leaf to emerge on the sorghum plant and the panicle (head) emerges from the flag leaf sheath)) and herbicides (paraquat, Agriculture 2019, 9, 70 3 of 10 glyphosate, and glufosinate) (Table1). Each herbicide was applied at 1/10 of the recommended 1X labeled use rate. Paraquat was applied at 0.021 kg ai ha−1 + non-ionic surfactant at 0.25% (v/v), glyphosate at 0.060 kg ai ha−1, and glufosinate at 0.060 kg ai ha−1. The 1/10X rate was used to demonstrate a highly concentrated drift rate from a recommended 1X labeled rate of each herbicide used in LibertyLink® or Roundup Ready® cropping systems. In 2017, applications for the 6-leaf and flag leaf applications were made on 19 July 2017 and 1 August 2017, respectively. In 2018, the six-leaf and flag leaf applications were made on 27 June 2018 and 20 July 2018, respectively. A nontreated check was also included in the study. Herbicide applications were made using a CO2-pressurized backpack sprayer calibrated to deliver 141 L ha−1 at 276 kPa. The boom consisted of 51 cm nozzle spacing, equipped with Turbo TeeJet (TeeJet Technologies, Springfield, IL, USA) Induction (TTI) 110015 nozzles.

Table 1. Herbicide treatments, trade names, and manufacturers, used in this study in 2017 and 2018.

Herbicide Treatments Trade Name Manufacturer (Common Names) Glufosinate Liberty BASF Corporation, Research Triangle Park, NC, USA Glyphosate Roundup PowerMax Monsanto Company, St. Louis, MO, USA Paraquat Gramoxone Syngenta Crop Protection LLC, Greensboro, NC, USA

Injury ratings were assessed at 14 and 28 days after application (DAA) of herbicides. Visual injury assessments consisted of estimating treated plants that exhibited symptomologies such as stunting, necrosis, chlorosis, and/or bleaching. Ratings were based on a scale of 0% to 100%, with 0% being no injury and 100% being complete crop death, relative to the nontreated check. Plant heights were assessed at 14 DAA of herbicides. The yield was recorded at maturity using a Kincaid plot combine. Plots were not harvested in 2018 for yield due to unfavorable weather conditions. All field data were analyzed using JMP Pro 13 (SAS Institute Inc., Cary, NC, USA). The herbicide treatments and application timing were considered fixed effects. Data were analyzed separately for each year since there was a significant effect of year. Means were separated using Fisher’s protected Least Significant Difference (LSD) test (α = 0.05).

3. Results and Discussion

3.1. 2017 No signiﬁcant interaction was found between herbicide treatments and application timings in 2017. Injury was signiﬁcantly affected by the main effects of herbicide treatments and application timing. At 14 DAA, grain sorghum injury was highest from the simulated drift of paraquat (29%) and lowest from glyphosate (3%) drift in 2017, when data was averaged over application timings (Figure1). A simulated drift of paraquat (29%) caused 16% and 26% greater injury to grain sorghum plants than glufosinate (13%) and glyphosate, respectively at 14 DAA. Similar to 14 DAA, the injury to grain sorghum from herbicide drift at 28 DAA was greater with paraquat (21%) than glufosinate (7%) and glyphosate (4%) by 14 and 17%, respectively (Figure2). Injury due to glufosinate and paraquat drifts was less at 28 DAA, indicating that plants recovered from their drift. Recovery may be due to injury as a localized lesion and not translocated throughout the plant. Substantial healthy leaf tissue will allow the plant to sustain normal growth and death only occur at the herbicide contact area. When data were averaged across application timings, injury at 28 DAA was statistically similar for glyphosate and glufosinate (Figure2). Similar to our study, Al-Khatib et al. [ 2] reported 0%–10% injury to sorghum plants from glyphosate and glufosinate drift at 0.011 and 0.04 kg a.i. ha−1 rates at two weeks after application. Al-Khatib et al. [2] also observed the recovery of sorghum plants from glufosinate and showed normal growth of new leaves three weeks after application, until the end of growing season. Agriculture 2019, 9, 70 4 of 10 AgricultureAgriculture 2019 2019, 9, ,9 x, xFOR FOR PEER PEER REVIEW REVIEW 4 4

3535

3030 A A

2525

2020

1515 B B Injury (%) Injury (%) 1010

5 C 5 C 0 0 glyphosate glufosinate paraquat glyphosate glufosinate paraquat

Figure 1. Grain sorghum injury from a simulated drift of glyphosate, glufosinate, and paraquat at 14 Figure 1. Grain sorghum injury from a simulated drift of glyphosate, glufosinate, and paraquat at daysFigure after 1. Grainapplication sorghum (DAA) injury of herbicides from a simulated in 2017. driftData ofwere glyphosate, averaged glufosinate, over application and paraquat timings. at 14 14 days after application (DAA) of herbicides in 2017. Data were averaged over application timings. Samedays lettersafter applicationon bars indicate (DAA) no significant of herbicides differences in 2017. at p Data < 0.05, were according averaged to Fisher’s over leastapplication significant timings. Same letters on bars indicate no signiﬁcant differences at p < 0.05, according to Fisher’s least signiﬁcant differenceSame letters test. on bars indicate no significant differences at p < 0.05, according to Fisher’s least significant difference test. difference test. 25 25 A 20 A 20

15 15 10 Injury (%) B 10 Injury (%) 5 B B 5 B 0 glyphosate glufosinate paraquat 0 Figure 2. Grain sorghumglyphosate injury from a simulated driftglufosinate of glyphosate, glufosinate, and paraquat paraquat at 28 DAA in 2017. Data were averaged over application timings. Same letters on bars indicate no Figure 2. Grain sorghum injury from a simulated drift of glyphosate, glufosinate, and paraquat at 28 significant differences at p < 0.05, according to Fisher’s least significant difference test. DAA in 2017. Data were averaged over application timings. Same letters on bars indicate no significant differences at p < 0.05, according to Fisher’s least significant difference test. FigureAveraged 2. Grain over sorghum herbicide injury treatments, from a simulated injury to drift grain of sorghum glyphosate, from glufosinate, the simulated and paraquat drift of herbicidesat 28 DAA in 2017. Data were averaged over application timings. Same letters on bars indicate no wasAveraged 5% greater over when herbicide herbicides treatments, were applied injury at to flag grain leaf stage,sorghum as compared from the tosimulated herbicide drift applications of significant differences at p < 0.05, according to Fisher’s least significant difference test. herbicidesat the six-leaf was stage5% greater at 14 andwhen 28 herbicides DAA (Figures were3 appandlied4). The at flag injury leaf forstage, applications as compared made to herbicide at the flag leaf stage was only 17.5% and 13% at 14 and 28 DAA, respectively. Although the injury was not extreme, applicationsAveraged at the over six-leaf herbicide stage at treatments,14 and 28 DAA injury (Figures to grain 3 and sorghum4). The injury from for theapplications simulated made drift of exposure of herbicide drift at the flag leaf growth stage is critical for grain sorghum maturity and grain atherbicides the flag leaf was stage 5% greaterwas only when 17.5% herbicides and 13% wereat 14 appandlied 28 DAA, at flag respectively. leaf stage, asAlthough compared the to injury herbicide wasproduction not extreme, [2]. exposure Multiple of studies herbicide in cotton drift at and the wheatflag leaf have growth reported stage is greater critical damagefor grain fromsorghum herbicide applications at the six-leaf stage at 14 and 28 DAA (Figures 3 and 4). The injury for applications made drift at later reproductive crop stages than the early growth stages of the crop [2,5,16,17], however, at the flag leaf stage was only 17.5% and 13% at 14 and 28 DAA, respectively. Although the injury was not extreme, exposure of herbicide drift at the flag leaf growth stage is critical for grain sorghum

Agriculture 2019, 9, x FOR PEER REVIEW 5

Agriculturematurity 2019and, 9 ,grain x FOR PEERproduction REVIEW [2]. Multiple studies in cotton and wheat have reported greater5 damage from herbicide drift at later reproductive crop stages than the early growth stages of the crop Agriculture 2019, 9, 70 5 of 10 maturity[2,5,16,17], and however, grain productiona study conducted [2]. Multiple in Texas, studies USA, reportedin cotton that and early wheat season have drift reported of glyphosate greater damagecaused more from injuryherbicide to sorghumdrift at later plants reproductive than later-season crop stages applications than the early [19]. growth Lyon [19] stages found of the severe crop [2,5,16,17],injurya study to conducted sorghum however, from in a Texas, study glyphosate USA,conducted reported drift in at Texas,later that earlygrowth USA, season reported stages drift at that rates of glyphosate early of 0.2 season kg causedha drift−1 or higher. moreof glyphosate injury Lower to causedglyphosatesorghum more plants rates injury did than tonot later-season sorghum cause any plants applicationsinjury. than later-season [19]. Lyon applications [19] found severe [19]. Lyon injury [19] to sorghumfound severe from −1 injuryglyphosate to sorghum drift at from later growthglyphosate stages drift at at rates later of growth 0.2 kg ha stages−1 or at higher. rates Lowerof 0.2 kg glyphosate ha or higher. rates didLower not glyphosate rates did not cause any injury. cause any20 injury. A 20 A 15 B 15 B 10

Injury (%) 10

Injury (%) 5 5

0 6-leaf Flag leaf 0 Application Timing 6-leaf Flag leaf

Application Timing Figure 3. Grain sorghum injury from herbicide drifts at 14 DAA as affected by the application timing

inFigure 2017. 3.DataGrain were sorghum averaged injury over from different herbicide herbicides drifts used at 14 in DAA this asstudy. affected Same by letters the application on bars indicate timing Figurenoin 2017.significant 3. DataGrain weredifferences sorghum averaged injuryat p over< 0.05,from different according herbicide herbicides todrifts Fisher’s at used 14 leastDAA in this significant as study.affected Same difference by the letters application test. on bars indicatetiming inno 2017. signiﬁcant Data were differences averaged at pover< 0.05, different according herbicides to Fisher’s used leastin this signiﬁcant study. Same difference letters on test. bars indicate no15 significant differences at p < 0.05, according to Fisher’s least significant difference test. 15 A A 10 B 10 B Injury (%) 5 Injury (%) 5

0 6-leaf Flag leaf 0 Application Timing 6-leaf Flag leaf

Figure 4. Grain sorghum injury from herbicideApplication drifts at 28 Timing DAA as affected by the application timing Figurein 2017. 4. Grain Data weresorghum averaged injury overfrom herbicides herbicide drifts used at in 28 this DAA study. as affected Same letters by the on application bars indicate timing no

insigniﬁcant 2017. Data differences were averaged at p < 0.05, over according herbicides to Fisher’sused in leastthis study. signiﬁcant Same difference letters on test. bars indicate no Figuresignificant 4. Grain differences sorghum at pinjury < 0.05, from according herbicide to Fisher’s drifts at le 28ast DAA significant as affected difference by the test.application timing in 2017. Data were averaged over herbicides used in this study. Same letters on bars indicate no significant differences at p < 0.05, according to Fisher’s least significant difference test.

Agriculture 2019, 9, x FOR PEER REVIEW 6 Agriculture 2019, 9, 70 6 of 10 In 2017, sorghum plant height and yield was significantly affected only by the herbicide treatments.In 2017, Averaged sorghum over plant the height herbicide and yield application was significantly timings, affectedgrain sorghum only by theplants herbicide were 5 treatments. cm taller inAveraged glufosinate over and the paraquat herbicide treatments application than timings, the glyphosate grain sorghum treatment plants at were 14 DAA 5 cm tallerin 2017 in glufosinate(Figure 5). Thisand trend paraquat in plant treatments height thanwas thethe glyphosateopposite compar treatmented to at the 14 trend DAA observed in 2017 (Figure for injury5). This at 14 trend DAA. in Averagedplant height over was the the application opposite comparedtiming, 21% to to the 29% trend injury observed from the for simulated injury at 14 drift DAA. of paraquat Averaged (2227 over −1 −1 kgthe ha application) led to a timing,31% reduction 21% to 29% in yield, injury as from compared the simulated to the nontreated drift of paraquat check (2227(3250 kgkg haha−1)) in led 2017 to a (Figure31% reduction 6). In this in yield,study, as the compared maximum to sorghum the nontreated yield checkwas found (3250 in kg the ha −glufosinate1) in 2017 (Figure drift treatment.6). In this −1 Averagedstudy, the across maximum application sorghum timings, yield the was simulated found in drift the glufosinate of glyphosate drift (2822 treatment. kg ha ) Averagedand glufosinate across −1 (3399application kg ha timings,) did not theresult simulated in any significant drift of glyphosate yield reduct (2822ion, kg as ha compared−1) and glufosinate to the nontreated (3399 kg check. ha−1 ) Glyphosatedid not result drift in anydid not significant reduce yield reduction,significantly, as comparedin spite of toreduction the nontreated in plant check. height. Glyphosate This indicates drift thatdid plant not reduce height yield may significantly, not be a good in indicator spite of reduction of yield reduction in plant height. for glyphosate This indicates drift. thatThe plantrecovery height of sorghummay not beplants a good from indicator glufosinate of yield and reduction glyphosate for dr glyphosateift might have drift. resulted The recovery in noof yield sorghum losses. plants The simulatedfrom glufosinate drift of and paraquat glyphosate resulted drift mightin a 34% have reduction resulted inin no yield yield compared losses. The to simulated applications drift of of glufosinateparaquat resulted in 2017 in a(Figure 34% reduction 6). Although in yield glyphosa comparedteto caused applications less injury of glufosinate to sorghum in 2017 plants, (Figure as6). comparedAlthough to glyphosate paraquat, caused grain sorghum less injury yield to sorghum was not plants,significantly as compared different to between paraquat, glyphosate grain sorghum and paraquatyield was drifts not significantly in 2017. Al-Khatib different betweenet al. [2] glyphosate found no andreduction paraquat in drifts yield in due 2017. to Al-Khatib glufosinate et al.and [2] −1 glyphosatefound no reduction drift at rates in yield of 0.404 due toand glufosinate 0.011 kg ha and, glyphosaterespectively. drift However, at rates ofhigher 0.404 drift and rates 0.011 of kg these ha−1 , tworespectively. herbicides However, reduced higherthe sorghum drift rates yield of in these their two study herbicides [2]. Similar reduced to our the study, sorghum Kurtz yield and in Street their [20]study found [2]. Similarno yield to reduction our study, for Kurtz rice andfrom Street glyphosate [20] found drift no occurring yield reduction at the three- for rice to fromfour-leaf glyphosate stage. Indrift contrast, occurring Lyon at [19] the three-found toa 25% four-leaf reduction stage. in In sorg contrast,hum yield Lyon due [19] to found glyphosate a 25% reductiondrift at early in sorghum growth −1 stagesyield dueand to yield glyphosate reduced drift up atto early50% when growth glyphosate stages and was yield applied reduced at up 0.1 to and 50% 0.2 when kg ha glyphosate at the boot was andapplied bloom at 0.1growth and 0.2stage, kg harespectively.−1 at the boot Results and bloom from previous growth stage, research respectively. indicates Resultsthat glyphosate from previous drift atresearch higher indicatesrates during that reproductive glyphosate drift stages at higherresults ratesin blasted during heads reproductive and half-filled stages grains, results causing in blasted a reductionheads and in half-filled yield [19]. grains, Ellis, et causing al. [21] areported reduction glyphosate in yield [ 19drift,]. Ellis, when et applied al. [21] reportedlate in the glyphosate season at −1 0.14drift, kg when ha , appliedreduced late corn in yield the season by 33%, at 0.14whereas kg ha early−1, reduced season glyphosate corn yield by drift 33%, at whereasrates of 0.035 early to season 0.14 −1 −1 kgglyphosate ha reduced drift corn at rates yield of by 0.035 22% to to 0.14 78%. kg In ha the−1 samereduced study, corn glufosinate yield by 22% at 0.053 to 78%. kg ha In thereduced same study,corn yieldglufosinate by 13% at and 0.053 11% kg at ha the−1 reducedearly and corn late yield application by 13% timing and 11% [21]. at theIt can early be andconcluded late application that the herbicidetiming [21 drift]. It caneffects be concludedon crop yields that depend the herbicide upon driftthe crop, effects herbicide on crop type, yields application depend upon timing, the crop,and applicationherbicide type, rates. application timing, and application rates.

131

129

127 A A A 125

123

121 B 119 Sorghum Plant Height (cm) Sorghum Plant Height 117 nontreated glyphosate glufosinate paraquat

Figure 5. Grain sorghum heights for each herbicide treatment, averaged over application timing, Figure 5. Grain sorghum heights for each herbicide treatment, averaged over application timing, at at 14 DAA in 2017. Same letters on bars indicate no signiﬁcant differences at p < 0.05, according to 14 DAA in 2017. Same letters on bars indicate no significant differences at p < 0.05, according to Fisher’s least signiﬁcant difference test. Fisher’s least significant difference test.

Agriculture 2019, 9, x FOR PEER REVIEW 7

4000 3750 ) 1 - 3500 A A 3250

3000 AB 2750 2500 B 2250 2000 Grain Sorghum Yield (kg ha Yield Grain Sorghum 1750 1500 Nontreated glyphosate glufosinate paraquat

Agriculture 2019, 9, 70 7 of 10

Figure 6. Grain sorghum yield as affected by herbicide drift of glyphosate, glufosinate, and paraquat in 2017.Figure Data 6. Grain is averaged sorghum over yield application as affected timing. by herbicide Same letters drift onof barsglyphosate, indicate glufosinate, no signiﬁcant and differences paraquat at p < 0.05, according to Fisher’s least signiﬁcant difference test. in 2017. Data is averaged over application timing. Same letters on bars indicate no significant 3.2. 2018differences at p < 0.05, according to Fisher’s least significant difference test.

3.2. 2018Injury to grain sorghum at 14 and 28 DAA was affected by an interaction of application timing and herbicide treatments in 2018 (Figures7 and8). The injury to sorghum plants at 14 DAA was Injury to grain sorghum at 14 and 28 DAA was affected by an interaction of application timing 10 and 15% higher with glyphosate (25%) as compared to paraquat (15%) and glufosinate (10%), and herbicide treatments in 2018 (Figures 7 and 8). The injury to sorghum plants at 14 DAA was 10 respectively, when herbicides were sprayed at the six-leaf stage. At 14 DAA, injury due to herbicide and 15% higher with glyphosate (25%) as compared to paraquat (15%) and glufosinate (10%), drift was 19% and 38% higher with paraquat (44%) than with the glufosinate (25%) and glyphosate respectively, when herbicides were sprayed at the six-leaf stage. At 14 DAA, injury due to herbicide (6%) drift, respectively, when herbicides were sprayed at the flag-leaf stage (Figure7). At 14 DAA, drift was 19% and 38% higher with paraquat (44%) than with the glufosinate (25%) and glyphosate both glufosinate and paraquat drift cause more injury to plants when applied at the flag leaf stage as compared to the six-leaf stage, by 15% and 29%, respectively (Figure7). No significant differences were observed for injury to grain sorghum at 28 DAA between six-leaf and flat leaf application timings for glufosinate drift (Figure8). Similar to 14 DAA, paraquat drift causes 23% more injury to plants when applied at the flag-leaf stage, as compared to the six-leaf stage at 28 DAA. In contrast, glyphosate drift causes less injury when applied at the flag leaf compared to the six-leaf stage, indicating that it causes more adverse effects on early growth than the later growth stages. Averaged over application timing, grain sorghum plants were 14 and 18 cm taller in glufosinate and paraquat treatments than the glyphosate, respectively, at 14 DAA in 2018 (Figure9). Glyphosate drift caused 6% to 25% injury at different application timings, which might have resulted in poor plant growth, consequently reducing sorghum plant heights. Similar to our results, Brown et al. [16] also reported an 81% reduction in corn height due to a glyphosate drift rate of 0.1 kg ha−1. Ellis et al. [21] found that glyphosate at 0.07 and 0.14 kg ha−1 reduced corn and rice height by 6% to 87% and 51%, respectively, but glufosinate caused no decrease in plant height [21]. In 2018, yield was not recorded due to inclement weather. Excessive rainfall from August to October in 2018 resulted in wet soil conditions which prevented harvest data collection (Table2). Agriculture 2019, 9, 70 8 of 10 Agriculture 2019, 9, x FOR PEER REVIEW 8 Agriculture 2019, 9, x FOR PEER REVIEW 8

FigureFigure 7. Grain 7. sorghumGrain sorghum injury from injury a simulated from a simulateddrift at 14 DAA drift as at affected 14 DAA by as the affected interaction by theof interaction of Figureherbicides 7. Grain and application sorghum injurytiming fromin 2018. a si Samemulated letters drift on barsat 14 indicate DAA asno affectedsignificant by differences the interaction at p of herbicides and application timing in 2018. Same letters on bars indicate no signiﬁcant differences at herbicides< 0.05, according and application to Fisher’s timing least significant in 2018. Same difference letters test. on bars indicate no significant differences at p < 0.05, paccording< 0.05, according to Fisher’s to least Fisher’s significant least signiﬁcantdifference test. difference test.

Figure 8. Grain sorghum injury from a simulated drift at 28 DAA as affected by the interaction of Figure 8. Grain sorghum injury from a simulated drift at 28 DAA as affected by the interaction of herbicides and application timing in 2018. Same letters on bars indicate no signiﬁcant differences at herbicides and application timing in 2018. Same letters on bars indicate no significant differences at p p < 0.05, according to Fisher’s least signiﬁcant difference test. Figure< 0.05, according8. Grain sorghum to Fisher’s injury least significantfrom a simulated difference drift test. at 28 DAA as affected by the interaction of herbicides and application timing in 2018. Same letters on bars indicate no significant differences at p

Agriculture 2019, 9, x FOR PEER REVIEW 9 plant growth, consequently reducing sorghum plant heights. Similar to our results, Brown et al. [16] also reported an 81% reduction in corn height due to a glyphosate drift rate of 0.1 kg ha−1. Ellis et al. [21] found that glyphosate at 0.07 and 0.14 kg ha−1 reduced corn and rice height by 6% to 87% and 51%, respectively, but glufosinate caused no decrease in plant height [21]. In 2018, yield was not recorded due to inclement weather. Excessive rainfall from August to October in 2018 resulted in wet soilAgriculture conditions2019, 9 ,which 70 prevented harvest data collection (Table 2). 9 of 10

140 A AB 120 B C 100

20 Sorghum Plant Height (cm) Height Plant Sorghum 0 nontreated glyphosate glufosinate paraquat

Figure 9. Grain sorghum heights as affected by each herbicide drift treatment averaged over application Figure 9. Grain sorghum heights as affected by each herbicide drift treatment averaged over timing at 14 DAA in 2018. Same letters on bars indicate no signiﬁcant differences at p < 0.05, according application timing at 14 DAA in 2018. Same letters on bars indicate no significant differences at p < to Fisher’s least signiﬁcant difference test. 0.05, according to Fisher’s least significant difference test. Table 2. Total monthly rainfall received at the study location in 2017 and 2018. Table 2. Total monthly rainfall received at the study location in 2017 and 2018. Total Monthly Precipitation (mm) Month Total Monthly Precipitation (mm) Month 2017 2018 2017 2018 May 98 56 May June98 136 79 56 June July 136 95 68 79 July August 95 260 231 68 August September 260 22 166 231 October 43 67 September November22 46 167 166 October 43 67 November 46 167 4. Conclusions

5. ConclusionsResults from our study indicate that sensitivity and recovery of grain sorghum from herbicide drift depends upon the type of herbicide used and time of drift occurrence, whereas degree of injury Results from our study indicate that sensitivity and recovery of grain sorghum from herbicide varies by year. The level of grain sorghum physical injury from the simulated drift rates of herbicide drift depends upon the type of herbicide used and time of drift occurrence, whereas degree of injury applications was the following: Paraquat > glufosinate > glyphosate. Based on this study, there is varies by year. The level of grain sorghum physical injury from the simulated drift rates of herbicide a possibility of grain sorghum yield reduction from glyphosate drift. Grain sorghum yield was not applications was the following: Paraquat > glufosinate > glyphosate. Based on this study, there is a affected by glufosinate drift, regardless of application timing. Paraquat drift on sorghum provided the possibility of grain sorghum yield reduction from glyphosate drift. Grain sorghum yield was not most adverse effects on yield. Drift applications at the flag leaf stage of sorghum caused more damage affected by glufosinate drift, regardless of application timing. Paraquat drift on sorghum provided than the early V6 growth stage application. These results were based on a single drift event. However, the most adverse effects on yield. Drift applications at the flag leaf stage of sorghum caused more in a multiple drift event, when grain sorghum will be exposed to two or three drift events, the damage damage than the early V6 growth stage application. These results were based on a single drift event. to grain sorghum, in terms of physical injury and yield reduction, may get worse. Therefore, applicators However, in a multiple drift event, when grain sorghum will be exposed to two or three drift events, should be cautious when applying paraquat, glyphosate, or glufosinate when grain sorghum is near or the damage to grain sorghum, in terms of physical injury and yield reduction, may get worse. adjacent to the treated areas. Therefore, applicators should be cautious when applying paraquat, glyphosate, or glufosinate when grainAuthor sorghum Contributions: is near T.B.,or adjacent R.R.H., andto the J.W.S. treated were areas. involved with data collection, interpretation, manuscript preparation, and editing. G.K., B.S., and T.W. were involved in manuscript preparation, review, and editing. Acknowledgments: This publication is a contribution of the Mississippi Agricultural and Forestry Experiment Station. This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch project under accession number 230080. Conflicts of Interest: The authors declare no conflict of interest. Agriculture 2019, 9, 70 10 of 10

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