Introduction
Early-spring planting in Mississippi often involves challenges with significant rainfall and suboptimal temperatures, leaving cool and saturated soils that are less than ideal for the germination of corn (Pettit Reference Pettit2018). The Leland, MS, area received 64.3 cm of rain from March through May in 2019, ranking among the top ten wettest springs on record (NOAA 2019). Accumulation of 50 to 67 growing degree days (the threshold at which temperature is too high or too low for plant growth) is required for corn germination, but soil temperature at 4 cm must be >10 C for uniform germination (Schott et al. Reference Schott, Lagzdins, Daigh, Penderson, Brenneman and Helmers2017). However, in situations when the original corn stand does not reach the targeted density, replanting to soybean may be necessary.
High soybean yield has been directly correlated with earlier planting, but soil conditions in the midsouthern United States may often be less than optimal in early April compared with May due to precipitation and temperature (Salmerón et al. Reference Salmerón, Gbur, Bourland, Buehring, Earnest, Fritschi, Golden, Hathcoat, Lofton, Miller, Neely, Shannon, Udeigwe, Verbree, Vories, Weibold and Purcell2014). Planting density is important for achieving high yield regardless of geography. Gaspar and Conley (Reference Gaspar and Conley2014) reported that the optimal seedling density of soybean for achieving high yield is ≥247,000 plants ha−1. Oftentimes, replant situations occur outside of the optimum planting window and can ultimately compromise yield potential. Mosley et al. (Reference Mosley, Reis, Gentimis, Campos, Copes, Melanie, Egbedi, Harrell, Kongchum, Levy, Padgett, Soignier, Scroggs, Sanders, Pankey and Fic2024) reported that regardless of maturity group, the optimal planting date is April 9 in Northeast Louisiana, and planting before or after this date could potentially cause yield reductions. Carver (Reference Carver2018) reported similar soybean yield with optimum density and with 25% stand reduction; however, replanting into a stand reduced by 25% offered no yield benefit. When initial plant densities are reduced enough to warrant replanting, replant yield will be reduced by not terminating the initial stand. To maximize yield potential in this situation, it is necessary to replant at a seeding rate of at least 50% of the original seeding rate (Carver Reference Carver2018).
When planting into less-than-optimal soil conditions, the seed germination rate can vary (Hoeft et al. Reference Hoeft, Nafziger, Johnson and Aldrich2000). For example, corn yield relies on uniformity of the stand after planting, and stunted corn plants or plants that are delayed in emergence are unable to compete with healthy plants for nutrients and light because corn has a determinate growth habit. Determinate growth keeps plants from growing vegetatively throughout the growing season, so stunted plants are unable to utilize available nutrients like healthy plants can. Additionally, corn cannot compensate for missing plants with additional fruiting branches. In fields with late-emerging plants, corn yield can be decreased 10% to 65% compared with fields having uniform stands (Larson and Henry Reference Larson and Henry2015). When 25% of plants emerged 1.5 and 3 wk late, yield was reduced 6% to 8% and 10% to 22%, respectively (Terry et al. Reference Terry, Dobbles, Loux, Thomison and Johnson2012). Furthermore, when one of every six plants was two or more leaf stages behind in growth, yield was reduced 4% to 8%.
Termination of the existing corn stand before corn replant is essential (Larson and Henry Reference Larson and Henry2015). Planting corn into a clean field can result in a yield benefit of 7% versus replanting into an uncontrolled, failed stand of corn. However, information is lacking on the most effective methods of stand termination. Therefore it is imperative to identify the most effective method of termination, as well as which method allows the producer to replant their crop more rapidly.
Control of suboptimal corn stands was most consistent with clethodim, and yields were greater following replant compared with plots treated with paraquat + metribuzin or glufosinate (Terry et al. Reference Terry, Dobbles, Loux, Thomison and Johnson2012). High rates of paraquat alone provided up to 65% control, but control was increased to 97% when metribuzin was mixed with paraquat. Also, the yield of replanted corn following paraquat + metribuzin was similar to the greatest yield. Clethodim was more consistent in controlling corn (Terry et al. Reference Terry, Dobbles, Loux, Thomison and Johnson2012), but its use for terminating corn requires a 6-d preplant interval (Anonymous 2017). For this reason, if time is a concern, it may be more efficient to apply paraquat + metribuzin (Terry et al. Reference Terry, Dobbles, Loux, Thomison and Johnson2012). Edwards et al. (Reference Edwards, Lawrence, Peeples, Philips, Bond and Golden2017) reported 28% to 34% greater control of a simulated failed corn stand 7 d after treatment (DAT) with applications including paraquat compared with clethodim alone. However, only the greatest rate of clethodim (53 g ha−1) controlled a simulated failed stand of corn >75% 21 DAT (Edwards et al. Reference Edwards, Lawrence, Peeples, Philips, Bond and Golden2017). Application timing was important in controlling a failed stand of corn with greater control from applications delayed until V1 (first collared leaf) compared with the VE (emergence) growth stage.
The potential detrimental effects of failed stands on replanted corn have been documented (Larson and Henry Reference Larson and Henry2015; Terry et al. Reference Terry, Dobbles, Loux, Thomison and Johnson2012). Furthermore, herbicide treatments for control of failed corn stands have also been evaluated (Edwards et al. Reference Edwards, Lawrence, Peeples, Philips, Bond and Golden2017; Terry et al. Reference Terry, Dobbles, Loux, Thomison and Johnson2012). However, the timing of replant following termination of a failed stand of corn has not been thoroughly studied. The current research was initiated to provide data on the optimal replant timing of corn and soybean following termination of a simulated failed corn stand. Research was conducted to evaluate timing of corn or soybean replanting following application of different herbicide treatments applied to simulated failed stands of corn.
Materials and Methods
Soybean Replant Study
A study was conducted once in 2020 and twice in 2021 at the Mississippi State University Delta Research and Extension Center in Stoneville, MS, and once in 2021 at the North Mississippi Research and Extension Center in Verona, MS, to evaluate timing of soybean replant following termination of a simulated failed corn stand. Plot locations and soil information for both the Stoneville and Verona sites are presented in Table 1. Plots in Stoneville were in a conventionally tilled system with a 1:1 corn and soybean rotation. Plots in Verona were in a no-till system with a 1:1:1 corn, soybean, and cotton (Gossypium hirsutum L.) rotation. All plots received S-metolachlor (Dual II Magnum®, Syngenta Crop Protection, Greensboro, NC, USA) at 263 g ai ha−1 + paraquat (Gramoxone® SL, Syngenta Crop Protection) at 1,159 g ha−1 preemergence (PRE) to control emerged weeds and provide residual control. Glyphosate (Roundup PowerMAX® 4.5 SL, Bayer Crop Science, St. Louis, MO, USA) at 1,261 g ha−1 was applied as needed to maintain a weed-free study. Clethodim (Select Max®, Valent Biosciences, Libertyville, IL, USA) at 136 g ha−1 was applied at approximately the time the replanted soybean reached canopy closure.
Table 1. Plot coordinates and soil information for Corn and Soybean replant studies in Stoneville and Verona, MS, in 2020 and 2021.
To simulate a failed corn stand, corn hybrid Dekalb ‘DKC 70-27’ (Bayer Crop Science) was planted at a reduced population of 30,050 seeds ha−1 in Stoneville on April 9, 2020, and April 21 and 28, 2021, and in Verona on April 11, 2021. Corn was planted to a depth of 5 cm using a single-row, small-plot planter (1 John Deere PI, Moline, IL, USA) in Stoneville and a twin-row, small-plot planter (Great Plains Manufacturing, Salina, KS, USA) in Verona. Soybean cultivar Asgrow ‘AG 46X6’ (Bayer Crop Science) was planted into a simulated failed corn stand in 2020, and Asgrow ‘AG 47XF0’ was planted at both sites in 2021. Plots consisted of four rows and were either 96.5 cm in Verona or 101 cm in Stoneville and 9 m in length. Plots were bordered by a fallow alley on both ends, measuring 3 m in width. All treatments were applied with a tractor-mounted, compressed air sprayer equipped with flat-fan nozzles (TDXL 11002, Greenleaf Technologies, Covington, LA, USA) at a rate of 140 L ha−1 at the V2 corn growth stage. The average maximum/minimum temperature at the time of applications was 23/12 C with an average humidity of 96% in 2020. In 2021, average maximum/minimum air temperature at the time of applications was 24/13 C with an average humidity of 95%. Soybean insects and weeds were managed according to recommendations provided by Mississippi State Extension (Bond et al. Reference Bond, Bararpour, Dodds, Irby, Larson, Pieralisi, Reynolds and Zurweller2021; Catchot et al. Reference Catchot, Crow, Gore, Cook, Musser, Pieralisi, Layton, Dodds, Irby and Larson2021).
Treatments were arranged as a two-factor factorial in a randomized complete block design with four replications. Factor A was herbicide treatment for control of the simulated failed corn stand and consisted of no herbicide treatment, paraquat at 841 g ha−1, paraquat at 841 g ha−1 + metribuzin (TriCor™ 4F, United Phosphorous, King of Prussia, PA, USA) at 211 g ha−1, and clethodim at 51 g ha−1 + glyphosate at 1,121 g ha−1. All treatments included NIS (ACTIVATOR 90, Loveland Products, Loveland, CO, USA) at 0.5% v/v. Factor B was replant timing and consisted of soybean replanted into the simulated failed corn stand 1 and 7 DAT.
Visible weed control of the simulated failed corn stand was recorded 3, 7, 14, and 21 DAT on a scale of 0% to 100%, where 0% indicated no visible effect of herbicide and 100% indicated complete plant death. Density and mortality (%) of the simulated failed corn stand were recorded from 1 m of row to rows 2 and 3 of each plot 21 DAT. All plants with visible green tissue present were considered living for mortality evaluation. Aboveground biomass was collected from rows 2 and 3 at V4 (four trifoliate leaves) of the replanted soybean. Before being weighed, aboveground plant material was collected, bagged, and placed in a greenhouse for 4 wk until plant material was completely dry. Yield of replanted soybean was collected using a small-plot combine (Kincaid Equipment, Haven, KS, USA) and adjusted to 13% moisture.
Data were subjected to ANOVA using the PROC GLIMMIX procedure in SAS (version 9.4; SAS Institute, Cary, NC, USA) with site-year and replication (nested within site-year) set as random effect parameters (Blouin et al. Reference Blouin, Webster and Bond2011). Type III statistics were applied to test the fixed effects of herbicide treatment and replant timing for control, mortality, dry weight, and replanted soybean yield. Estimates of least squared means were utilized for mean separation (P ≤ 0.05).
Corn Replant Study
A study similar to the Soybean Replant Study was conducted once in 2020 and twice in 2021 at the Mississippi State University Delta Research and Extension Center in Stoneville, MS, and once in 2021 at the North Mississippi Research and Extension Center in Verona, MS, to evaluate timing of corn replant following termination of a simulated failed corn stand. Plot location and soil information are presented in Table 1. All plots received S-metolachlor at 1,463 g ha−1 + atrazine at 1,794 g ai ha−1 + mesotrione at 188 g ai ha−1 (Lexar® EZ, Syngenta Crop Protection) plus paraquat at 1,159 g ha−1 immediately following planting of the simulated failed stand of corn to control emerged weeds and provide residual weed control. Glyphosate at 1,261 g ha−1 was applied as needed to maintain a weed-free study. Plot dimensions, preplant maintenance, details of planting to simulate a failed corn stand, and replant information were as previously described in the Soybean Replant Study. Corn hybrid ‘DKC 70-27’ was replanted into simulated failed corn stands to a depth of 5 cm at 79,040 seeds ha−1. Corn insects and weeds were managed according to recommendations provided by Mississippi State Extension (Bond et al. Reference Bond, Bararpour, Dodds, Irby, Larson, Pieralisi, Reynolds and Zurweller2021; Catchot et al. Reference Catchot, Crow, Gore, Cook, Musser, Pieralisi, Layton, Dodds, Irby and Larson2021). Experimental design, treatment application, data collection, and analysis were as previously described in the Soybean Replant Study. Yield of replanted corn was not collected in the Corn Replant Study due to technical malfunction of the combine in both site-years. This caused the yield data in this study to be misrepresentative and misleading.
Results and Discussion
Soybean Replant Study
A main effect of herbicide treatment was detected for control of the simulated failed corn stand 3, 7, 14, and 21 DAT (Table 3). Pooled across soybean replant timing, paraquat plus metribuzin provided the greatest control 3 DAT (Table 2). Control of the simulated failed corn stand with paraquat alone never exceeded 50% regardless of evaluation timing. Clethodim + glyphosate and paraquat + metribuzin controlled the simulated failed corn stand similarly at ≥92% 7 DAT. At 14 and 21 DAT, it controlled more corn than paraquat or paraquat + metribuzin did. Even though control with clethodim + glyphosate was greater than it was with paraquat + metribuzin 14 and 21 DAT, paraquat + metribuzin controlled the simulated failed corn ≥92% at both evaluations. Chahal and Jhala (Reference Chahal and Jhala2015) reported that clethodim and additional acetyl coenzyme A carboxylase–inhibiting herbicides (sethoxydim at 350 g ai ha−1, fenoxaprop at 130 g ai ha−1, fluazifop at 210 g ai ha−1, and quizalofop at 40 g ai ha−1) applied alone controlled volunteer corn ≥93%. Steckel et al. (Reference Steckel, Thompson and Hayes2009) also reported that clethodim at 140 g ha−1 and paraquat + photosystem II–inhibiting herbicides, such as metribuzin at 210 g ha−1, atrazine at 560 g ai ha−1, 1,120 at g ai ha−1, linuron at 560 g ai ha−1, and simazine at 560 g ai ha−1, provided adequate control of a suboptimal corn stand. Additionally, increasing the paraquat rate to 840 g ha−1 controlled a suboptimal corn stand (Steckel et al. Reference Steckel, Thompson and Hayes2009), whereas control of a simulated failed corn stand with paraquat at 841 g ha−1 never exceeded 50% in the current research. This could be for many reasons, including damage to the corn before application and a difference in the nozzles used for application. In Steckel et al.’s research, air-induction nozzles were used, compared to flat-fan nozzles in the current work. Additionally, one site-year received significant frost damage before application, which might have positioned the corn plants in a more vulnerable state.
Table 3. Significance of the main effects of herbicide treatment, replant timing, and interaction among the main effects for control of a simulated failed corn stand 3, 7, 14, and 21 DAT, mortality 21 DAT, and replanted crop yield in the Soybean and Corn replant studies conducted from 2020 to 2021 in Stoneville and Verona, MS. a
a Abbreviation: DAT, days after herbicide treatment.
Table 2. Control of a simulated failed corn stand 3, 7, 14, and 21 DAT and mortality 21 DAT in the Soybean Replant Study conducted at Stoneville and Verona, MS, from 2020 to 2021.a,b,c,d
a Data were pooled across two replant timings and four experiments.
b Abbreviation: DAT, days after herbicide treatment.
c Means followed by the same letter within a column are not different at α = 0.05.
d Herbicide treatments were applied to the simulated failed corn at the V2 growth stage.
A main effect of replant timing was observed for the percent mortality of corn 21 DAT (Table 3). Pooled across soybean replant timing, corn mortality 21 DAT was 97% with clethodim + glyphosate (Table 2). This was greater than with other treatments, although paraquat + metribuzin led to 90% mortality. Mortality of the simulated failed corn stand was also influenced by a main effect of soybean replant timing. Pooled across herbicide treatments, corn mortality 21 DAT was 49% in plots replanted 7 DAT compared with 46% in plots replanted 1 DAT (Table 4).
Table 4. Mortality of simulated failed stand of corn 21 DAT in the Soybean Replant Study conducted at Stoneville and Verona, MS, from 2020 to 2021.a,b,c
a Data were pooled across four herbicide treatments and four experiments.
b Abbreviation: DAT, days after herbicide treatment.
c Means followed by the same letter are not different at α = 0.05.
Little to no research has been conducted on replant timing affecting efficacy of herbicide application targeting failed stands of crops. However, there is plenty of research done reiterating the importance of not impeding the translocation of herbicides through the target plant. For the herbicide to reach its target site and initiate phytotoxicity, it must first be thoroughly absorbed by the target plant (Sterling Reference Sterling1994). Once absorbed, herbicides need to be effectively translocated through the target plant to the target site in a concentration great enough to terminate plant growth (Shaner Reference Shaner2009). When herbicide translocation is impaired, a decrease in the amount of active ingredient reaching the target site may lead to a failed application (Menendez et al. Reference Menendez, Rojano and Prado2014). In postemergence applications, translocation via phloem and xylem becomes imperative, and if this translocation is impeded, the target plant may regrow (Hess Reference Hess2018). Putting together data in the current work with the aforementioned importance of absorption and translocation, it can easily be seen that planting too early after stand termination could cause a loss in herbicide efficacy.
The aboveground dry weight of replanted soybean at the R4 growth stage was not influenced by the treatments imposed in this study. A main effect of herbicide treatment was significant for yield of replanted soybean (Table 3). Yield in plots with no herbicide treatment to control the simulated failed corn stands was only 340 kg ha−1 (Table 5). Soybean yields in all plots receiving an herbicide treatment targeting simulated failed corn stands were similar and ≥2,150 kg ha−1. Alms et al. (Reference Alms, Moeching, Vos and Clay2016) reported no soybean yield difference between herbicide treatments targeting a failed corn stand, and corn plants left untreated at a density of 3 plants m−2 in a soybean crop reduced yield up to 71%.
Table 5. Yield of replanted soybean in the Soybean Replant Study conducted at Stoneville and Verona, MS, from 2020 to 2021.a,b,c
a Data were pooled across two replant timings and four experiments.
b Means followed by the same letter are not different at α = 0.05.
c Herbicide treatments were applied to the simulated failed corn at the V2 growth stage.
Corn Replant Study
A main effect of herbicide treatment was detected for control of a simulated failed stand of corn 3, 7, 14, and 21 DAT (Table 3). Pooled across corn replant timing, paraquat + metribuzin controlled the simulated failed corn stand 81% 3 DAT (Table 3), which was greater than with paraquat alone (48%) and clethodim + glyphosate (0%) (Table 6). Clethodim + glyphosate provided the greatest control (≥91%) 7, 14, and 21 DAT. Control with paraquat alone was ≤48% across all evaluations. Replant timing had no effect on control of the simulated failed corn stand in this study. This could potentially be due to the nonuniform nature of damage caused by the planter to the existing corn crop. The general conclusion is that planting too early has the potential to cause a loss in efficacy, but will not always do so. Paraquat alone was ineffective for control of a simulated failed stand of corn in the Soybean and Corn replant studies when all applications were applied to corn at the V2 growth stage. Across all evaluations, paraquat + metribuzin controlled a simulated failed corn stand ≥81% and 92% in the Corn and Soybean replant studies, respectively. Control 14 and 21 DAT was greatest with clethodim + glyphosate in both studies. Although yield was not collected in the Corn Replant Study, Alms et al. (Reference Alms, Moeching, Deneke and Vos2008) reported a 0% to 23% corn yield decrease due to competition from volunteer corn, and that corn yield was affected by both uncontrolled and partially controlled volunteer corn.
Table 6. Control of a simulated failed corn stand 3, 7, 14, and 21 DAT and mortality 21 DAT in the Corn Replant Study in Stoneville and Verona, MS, from 2020 to 2021.a,b,c
a Data were pooled across two replant timings and four experiments.
b Means followed by the same letter within a column are not different at α = 0.05.
c Herbicide treatments were applied to the simulated failed corn at the V2 growth stage.
A main effect of herbicide treatment was observed for percent mortality of a simulated failed corn stand (Table 3). Pooled across both corn replant timings, mortality of simulated failed corn stand 21 DAT was 96% following clethodim + glyphosate compared with 1% and 75% mortality with paraquat alone and paraquat + metribuzin, respectively (Table 6). Aboveground dry weight of the replanted corn collected 21 DAT was not influenced by the treatments imposed in this study.
Practical Implications
Clethodim + glyphosate provided the greatest control of a simulated failed corn stand at all evaluation timings except 3 DAT in the Corn Replant Study and 14 and 21 DAT in the Soybean Replant Study. Although paraquat + metribuzin controlled the simulated failed stand less than clethodim + glyphosate, control was at least 81% and 92% in the Corn and Soybean replant studies, respectively. Paraquat alone was insufficient for the control of simulated failed corn stands in both studies. This work demonstrated that replant timing following herbicide application is less important than previously believed; however, differences were detected, proving that it is important to allow adequate time between herbicide application and replanting.
Even though the yield of replanted soybean was not influenced by the main effect of replant timing or the interaction of herbicide treatment and replant timing, replanted soybean yield was decreased by ≥38% in plots with no herbicide treatment to control the simulated failed corn stand. This demonstrates the importance of terminating the original failed corn stand to maximize yield following a replant. Lack of differences in the yields of replanted soybean was attributed to the termination of all simulated failed stands of corn following completion of evaluations. If corn were allowed to grow to maturity, plants surviving herbicide treatments would have continued to compete for nutrients, possibly causing additional differences in the yield of replanted soybeans. This could have especially been observed in paraquat-treated plots, where control of the simulated failed corn stand was ≤50%.
On the basis of these results, multiple options are available for the control of a failed corn stand in replant situations. Clethodim + glyphosate effectively controls a failed corn stand before replanting but requires up to 7 d for control to be observed. If a producer’s corn is at the V2 growth stage, paraquat + metribuzin will provide sufficient control of the failed corn stand and will terminate the corn 3 DAT, compared to clethodim + glyphosate, which does not provide control until 7 DAT. If corn is at V2, paraquat + metribuzin should be utilized for termination of the failed corn stand due to its more rapid activity. Whether using paraquat + metribuzin or clethodim + glyphosate, greater target plant mortality was detected in plots replanted 7 compared to 1 DAT. Overall, data from this study do not support applying paraquat alone owing to its lack of control of the simulated failed corn stand. No differences in yields of replanted soybean were detected among the herbicide treatments, but detrimental effects to yield were present when replanting with no termination of the simulated failed corn stand.
Acknowledgments
We thank the Mississippi Soybean Promotion Board for partially funding this research. We thank personnel at the Mississippi State University Delta Research and Extension Center for their assistance.
Funding
This publication is a contribution of the Mississippi Agricultural and Forestry Experiment Station. This study was based on a larger set of research objectives that are supported by Hatch project 153300, funded by the U.S. Department of Agriculture–National Institute of Food and Agriculture.
Competing interests
The authors declare no conflicts of interest.
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