Reducing Grain Sorghum (Sorghum bicolor L. Moench) Injury From Postemergence Application of Mesotrione With Dicamba

Weed management in grain sorghum is limited by the number of herbicide options. A two-year (2017-2018) field study was conducted at the Mississippi State University Delta Research and Extension Center, in Stoneville, MS to evaluate the response of grain sorghum to mesotrione application alone or when tank-mixed with dicamba at the two-leaf and four-leaf growth stage of sorghum. Mesotrione was applied at 0.07 and 0.105 kg ai ha alone or was tank-mixed with dicamba at 0.28 kg ae ha. Significant injury to grain sorghum from all herbicide treatments was observed compared with the untreated check. Increase in mesotrione application rate increased injury to grain sorghum from 14 to 19% at two-leaf and from 10 to 24% at the four-leaf stage by 4 weeks after application (WAA) in 2017. Adding dicamba to mesotrione reduced grain sorghum injury in both years. At 4-leaf sorghum application stage, mesotrione applied at 0.07 kg ha resulted in greater grain yield than all other herbicide treatments, except mesotrione (0.105 kg ha) + NIS in 2017. Our results indicate that adding dicamba to mesotrione safes grain sorghum from injury caused by mesotrione alone.


Introduction
Grain sorghum (Sorghum bicolor L. Moench) is an important cereal crop grown in the United States. In 2019, U.S. harvested 4.6 million ha of grain sorghum (USDA-NASS, 2020). Cultural practices, like crop rotation, are important for weed control in grain sorghum along with incorporating herbicide programs (Brown, Al-Khatib, Regehr, Stahlman, & Loughin, 2004). Unlike corn (Zea mays L.), cotton (Gossypium hirsutum L.), or soybean (Glycine max L. Merr.), there are limited options for weed control in grain sorghum.
Amaranthus spp. are among the most troublesome weeds in grain sorghum. In a competition study, Knezevic, Horak, and Vanderlip (1997) reported a 47% yield reduction to grain sorghum when redroot pigweed (Amaranthus retroflexus L.) emerged once grain sorghum had reached the 2.6-leaf stage. In a different study, grain sorghum yield was reduced by 48% when one Amaranthus plant per 30 cm 2 was present (Shipley & Wiese, 1969). In addition, Palmer amaranth (Amranthus palmeri S.Wats.) present in grain sorghum fields caused increased moisture in grain and other foreign materials at the time of harvest (Moore, Murray, & Westerman, 2004). Since chemical control of troublesome weeds are important in grain sorghum production, there is a need for new herbicide options for control (Abit et al., 2009).
Preemergence weed control in sorghum can be achieved by using atrazine, S-metolachlor, dimethenamid-p, or alachlor and postemergence with 2,4-D, dicamba, or bromoxynil (Smith & Scott, 2010). Mesotrione is labeled to use in corn to control annual broadleaf and some grass weeds in corn (Janak & Grichar, 2016;Mitchell et al., 2001), including triazine-resistant Palmer amaranth, common waterhemp (Amranthus rudis Sauer), common lambsquarters (Chenopodium album L.), and black nightshade (Solanum nigrum L.), and ALS resistant weeds including Amaranthus ssp., common cocklebur (Xanthium strumarium L.), and annual sowthistle (Sonchus oleraceus L.) (Abit et al., 2009;Sutton, Richards, Buren, & Glasgow, 2002). Mesotrione is labeled to use as preplant and preemergence for weed control in grain sorghum (Abit et al., 2009). However, its postemergence applications have been reported to cause injury to sorghum plants. Mesotrione applied at 70.5 g ai ha -1 resulted in 20% chlorosis injury in grain sorghum but provided consistent weed control (Horky & Martin, 2004). In another study, Miller and Regehr (2002) reported early postemergence applications of mesotrione to grain sorghum exhibited 40-60% bleaching, but late postemergence applications exhibited less grain sorghum injury. Therefore, it is possible that mixing mesotrione with other herbicides will alleviate the injury caused on sorghum plants by mesotrione.
Limited research has indicated that the use of some growth regulators, like 2,4-D or dicamba, may save some herbicides in causing injury. Brown et al. (2004) reported 2,4-D and dicamba in combination with metsulfuron or fluroxypyr exhibited less injury to grain sorghum compared to metsulfuron applied alone. Bararpour, Norsworthy, Hale, and Jones (2017) reported the lowest grain sorghum injury (23%) when mesotrione was applied with dicamba postemergence compared to the combination of mesotrione and atrazine (54%) at 2 wk after application (WAA); furthermore, grain sorghum recovered by 4 WAA with only 2% injury with mesotrione and dicamba and 7% injury with mesotrione and atrazine. A reduction in the injury level observed with mesotrione when added to dicamba may indicate that the addition of dicamba will safen mesotrione, but more research is needed to validate it for sorghum planted in Mid-South United States. Therefore, the objective of this research was to validate the claim that the addition of dicamba to applications of mesotrione will safen grain sorghum against injury when applied postemergence.

Materials and Methods
Field studies were conducted in 2017 and 2018 at the Mississippi State University Delta Research and Extension Center in Stoneville, MS. Experiments 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) was planted on June 8 and May 31 in 2017 and 2018, respectively, at a seeding rate of 23 seeds m -1 row length. This variety was selected for its high yielding characteristic and is commonly grown around the U.S. The plot size was 4 m wide by 6 m long. Row spacing was 102 cm and there were four rows of sorghum in each plot.
The experimental design was a randomized complete block with a factorial arrangement of treatments. Experiments contained two factors including the two-application timings of herbicides (2-leaf stage, 4-leaf stage) and nine herbicide treatments ( Table 1). The trade names, site of action, chemical group, and manufacturer information for herbicides used in this study are listed in Table 2. Mesotrione was applied at two rates at 0.07 and 0.105 kg ai ha -1 at 2-and 4-leaf stage of grain sorghum either alone or tank-mix with dicamba at 0.28 kg ae ha -1 . Herbicide treatments were applied on June 26, 2017 and June 28, 2018 for the 2-leaf stage of grain sorghum. For 4-leaf stage application timing, the herbicides were applied on July 7, 2017 and July 10, 2018. Environmental data at each application timing in 2017 and 2018 are listed in Table 3. A non-treated control was also included in the study. Herbicide applications were made using a CO 2 -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) Induction (TTI) 110015 nozzles.   Visual injury assessments consisted of estimating treated plants that exhibited symptomologies such as bleaching (which is general symptomology of mesotrione application), stunting, necrosis, and/or chlorosis. 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. Injury ratings were taken thrice after herbicide application and dates for injury rating assessment is provided in Tables 4 and 5. Data on plant height and percent heading in sorghum was also collected afer final injury assessment. Grain sorghum was harvested using a Kincaid plot combine on October 17, 2017. Plots were not harvested in 2018 for yield due to untimely rain and high-intensity winds during harvesting season which led to grain loss.
All the collected data were analyzed using SAS statistical software (SAS Institute Inc., Cary, NC) using the GLIMMIX procedure. Prior to analysis, all data was analyzed for normality using the UNIVARIATE procedure in SAS. All of the data was normally distributed. The herbicide treatments and application timing were considered fixed effects, whereas treatment replication was considered as random. Data were analyzed separately for each year since there was a significant effect of the year. Means were separated using T-grouping (α = 0.05).

Year 2017
In 2017, the injury to grain sorghum assessed on 1 August and 17 August was affected by the interaction of application timing and herbicide treatments as well as by the main effects of application timing and herbicide treatments (Table 4). First grain sorghum physical injury was evaluated four weeks after application (WAA). Averaged over herbicide treatments, grain sorghum showed greater injury to mesotrione application at 4-leaf growth stage (15%) than at 2-leaf growth stage (10%) ( Figure 1A). This result may again indicate that grain sorghum is less sensitive to mesotrione application at 2-leaf than 4-leaf. Grain sorghum injury increased from 14% to 19% as mesotrione (alone) application rate at 2-leaf growth stage increased from 0.07 kg ha -1 to 0.105 kg ha -1 ( Figure 1A). Similarly, increasing mesotrione (alone) application rate from 0.07 kg ha -1 to 0.105 kg ha -1 at 4leaf growth stage increased injury to sorghum plants from 10% to 24%. Grain sorghum injury level was 16 and 15% from 0.07 and 0.105 kg ha -1 mesotrione alone + NIS applications at 2-leaf growth stage, respectively. However, the application of dicamba with mesotrione to the tank at 2-leaf growth stage reduced grain sorghum injury 4 WAA ( Figure 1A). This indicates that dicamba had safening effect on grain sorghum from mesotrione.    (Figure 2) de treatments in Abbreviations ha -1 ; NIS, Nonming and inter application tim ha -1 greater so esotrione appli esotrione (0.10 reased grain so ced injury) eff Vol. 12,No. 12; n effect of herb treatments or ).

Year 2
There wer from meso sorghum in Mesotrion the mesotr dicamba in dicamba a significant