Comparison of residual activity of pre-emergence herbicides for control and seed production of multiple herbicide-resistant Palmer amaranth in food-grade white corn

Nebraska is the number-one producer of food-grade white corn ( Zea mays L.) in the United States. Food-grade white corn has not been genetically engineered; therefore, non-selective herbicides such as glyphosate or glufosinate cannot be used. Multiple herbicide-resistant (MHR) Palmer amaranth ( Amaran-thus palmeri S. Watson) populations have been reported in multiple counties in Nebraska, and their management is a challenge, particularly for white corn producers. The objectives of this study were to evaluate the residual activity of pre-emergence (PRE) herbicides for acetolactate synthase inhibitor (halosulfuron-methyl)/atrazine/glyphosate-resistant Palmer amaranth control, density, biomass, seed production, and grain yield in food-grade white corn. Field experiments were conducted during summer 2020 and 2021 in a grower’s field infested


INTRODUCTION
Nebraska ranks first in non-genetically modified organism (non-GMO) food-grade white corn (Zea mays L.) production in the United States.It has been estimated that 5%-11% of corn area in the United States was planted with food-grade white corn in 2020 (USDA-NASS, 2020).The demand for non-GMO food products has increased in the United States from last several years (Bain & Selfa, 2017).Food-grade white corn has several nutritional benefits, such as its being a good source of fiber, vitamins B, C, and E, and potassium (Sheng et al., 2018).White corn can be roasted, grilled, steamed, or pureed in dips.Due to its strong aroma and flavor when baking or frying, it goes well in pastas and salads and is preferred for human consumption (Malvar et al., 2008).It pairs well with vegetables and meats such as basil, parsley, mint, cilantro, peas, squash, fennel, mushrooms, peppers, salty and nutty cheeses, pork, beef, poultry, and seafood (Sylvia, 2018).Food-grade white corn has not been genetically engineered; therefore, glyphosate or glufosinate cannot be used for weed management.Multiple herbicide-resistant (MHR) Palmer amaranth (Amaranthus palmeri S. Watson) is a concern for white corn growers in a no-till production system.The characteristics of Palmer amaranth include faster growth habit, a high photosynthetic rate, continued emergence throughout the crop season, and prolific seed production, making it a troublesome weed species in corn production fields (Horak & Loughin, 2000;Sellers et al., 2003;Ward et al., 2013).In addition, Palmer amaranth is a dioecious species, meaning male and female plants are separate that increase the potential for gene flow and spread of herbicide resistance (Jhala et al., 2021).A single female Palmer amaranth plant per 9-m row of cotton (Gossypium hirsutum L.) in Texas, three plants per meter row of soybean [Glycine max (L.) Merr.] in Arkansas, and 0.5 plant per meter row of corn in Kansas reduced grain yield by 13%, 17%, and 11%, respectively (Klingaman & Oliver, 1994;Massinga et al., 2001;Morgan et al., 2001).Furthermore, Palmer amaranth emerged at the density of eight plants per meter corn row reduced yield by 91% (Massinga et al., 2001).
Herbicide-resistant weeds have become widespread throughout the United States (Prince et al., 2012).The use of pre-emergence (PRE) herbicides with multiple sites of action is the cornerstone of a diversified herbicide program for management of herbicide-resistant weeds (Norsworthy et al., 2012).PRE herbicides benefit growers in several ways by reducing early season weed interference and often improving season-long Palmer amaranth control (Culpepper & York, 1998;Keeling et al., 2006;Reddy, 2001;Toler et al., 2002).Diuron, fluometuron, prometryn (PS II inhibitors), fomesafen (protoporphyrinogen oxidase inhibitors), pendimethalin (microtubule inhibitors), and pyrithiobac (ALS inhibitors) can be applied PRE in cotton for effective control of Palmer amaranth (York & Culpepper, 2009).Atrazine has been the most used herbicide for weed management in corn for many years in the United States.However, the effectiveness of ALS-and PS II-inhibiting herbicides has declined because of the presence of ALSand PS II-inhibitor-resistant weeds (Foy & Witt, 1997;Parks et al., 1996;Sprague et al., 1997;Volenberg et al., 2000).Dicamba and 2,4-D are synthetic auxin herbicides used to control emerged broadleaf weeds prior to planting broadleaf crops or applied early post-emergence (POST) in grass crops such as corn, wheat, and sorghum (Peterson et al., 2016;Vink et al., 2012).Mesotrione and isoxaflutole have been shown to be effective for controlling Amaranthus spp.(Johnson et al., 2012;Sutton et al., 2002).The evolution of Palmer amaranth resistant to PS-II and HPPD inhibitor has reduced the number of herbicide options for Palmer amaranth control in corn (Delye et al., 2013).A study in Nebraska reported that PS II-and HPPD-inhibiting herbicides combined with tank-mixture of glyphosate, dicamba plus dimethenamid-P, or pyroxasulfone were effective to control herbicide-resistant Palmer amaranth in corn (Chahal et al., 2018).However, the information for the residual activity of PRE herbicides in food-grade white corn for control of MHR Palmer amaranth was lacking in the literature.In addition, growers in Nebraska have been looking for PRE herbicide options for the effective control of MHR Palmer amaranth in food-grade white corn because POST herbicide options are limited.The objectives of this study were to compare the residual activity of PRE herbicides with different sites of action for early season control of MHR Palmer amaranth.Second, to evaluate the effect of residual PRE herbicide on Palmer amaranth density, biomass, seed production, and grain yield in food grade white corn.

Experimental design and herbicide treatments
The research site was under a continuous no-till glyphosateresistant corn-soybean rotation for the last 8 years.Food-grade white corn P1306W was no-till planted on May 12 in 2020 and May 18 in 2021 at a seeding rate of 67,500 seeds ha −1 .The experimental site was under a rainfed irrigation system, and no supplemental irrigation was applied during both years.The treatments were arranged in a randomized complete block design with four replications.The plots were 3-m wide by 9-m long, where four corn rows per plot were spaced

Core Ideas
• Acetochlor/clopyralid/mesotrione and atrazine/ bicyclopyone/mesotrione/S-metolachlor provided >90% control of Palmer amaranth at 45 and 60 days after application.• Acetochlor/clopyralid/mesotrione and atrazine/ bicyclopyone/mesotrione/S-metolachlor reduced Palmer amaranth density, biomass, and seed production and resulted in higher white corn yield.• No corn injury was noticed in any of the preemergence (PRE) herbicides tested in this study.
76 cm apart.Fifteen PRE herbicides and a nontreated control were included for comparison (Table 1).PRE herbicides were applied within 2 days of corn planting on May 14 in 2020 and May 18 in 2021.Herbicides were applied using a handheld CO 2 pressurized backpack sprayer equipped with AIXR 110015 flat fan nozzles (TeeJet Technologies; Spraying Systems) calibrated to deliver 140 L ha −1 at 276 k Pa at a constant speed of 4.8 km h −1 .

Data collection
Palmer amaranth control ratings were recorded visually at 15, 30, 45, 60, 75, and 90 days after PRE (DAPRE) herbicide was applied using a scale of 0%-100%, with 0% representing no Palmer amaranth control and 100% representing complete control (Mausbach et al., 2021).Palmer amaranth density was recorded at 15, 30, 45, 60, and 75 DAPRE by counting the Palmer amaranth plants in 0.5 m 2 quadrats placed randomly between the two center corn rows in each plot and converting into the number of plants per square meter.Above-ground biomass for Palmer amaranth plants surviving PRE herbicide treatments was collected at 30 and 60 DAPRE from randomly selected 0.5 m 2 quadrats, and the collected samples were put into paper bags, placed in an oven at 65˚C for 7 days until a constant weight was obtained, then weighed.Palmer amaranth seed production was recorded by placing a 1.0 m 2 quadrat in the center two rows of corn and collecting the seed heads of female plants from each quadrat.Palmer amaranth seed heads were stripped from the stems and separated by passing them through sieves with mesh sizes ranging from 0.50 to 3.35 mm.Material collected from the 0.50-mm sieve was processed with a seed cleaner that used air to remove the lighter floral chaff from the Palmer amaranth seeds.The seeds were thoroughly cleaned, and the T A B L E 1 Herbicides, rates, and products used for control of acetolactate synthase inhibitor/atrazine/glyphosate-resistant Palmer amaranth in food-grade white corn in field experiments conducted near Carleton, NE, in 2020 and 2021.

Herbicide program
Trade name Rate (g ai ha seed weight and number of seeds per m 2 were determined. Corn was mechanically harvested from the center two corn rows in each plot using a plot combine (Gleaner K2; AGCO), weighted, and the yield was adjusted to 15.5% moisture content and converted into kg ha −1 .

Statistical analysis
Palmer amaranth control, density, aboveground biomass, and corn yield data were subjected to analysis of variance using PROC GLIMMIX in SAS 9.4 (SAS Institute).Normality and homogeneity of error variances were confirmed by using PROC UNIVARIATE with normal Q-Q plots and the Levene test.Palmer amaranth control data were log-transformed using a beta (link = "complementary log-log") distribution.Palmer amaranth density and aboveground biomass were square-root transformed and fit to generalized linear mixed models using glmm functions gaussian (link = "identity") error distributions.Palmer amaranth seed production and corn yield data were analyzed with GLIMMIX using gaussian (link = "identity") error distributions selected for response variables based on the restricted maximum likelihood technique.Treatments and years were considered fixed effects, whereas replication was considered random effect in the model.Type III tests were used to assess fixed effects, and treatment comparisons were made based on Tukey Kramer's pairwise comparison test and Sidak adjustments.

RESULTS AND DISCUSSION
Year-by-treatment interactions for Palmer amaranth control, density, biomass, and seed production were non-significant (p > 0.05); therefore, the data are combined for both years.Below average temperature and precipitation during the PRE herbicides application were recorded, and these were 15.0˚C, 15.8˚C and 80.3 mm, 81.5 mm for 2020 and 2021, respectively, as compared to 16.4˚C and 135.4 mm in 30-year average (Table 2).There was no corn injury from any PRE herbicide applied in this study (data not shown); therefore, these herbicides are safe to use in food-grade white corn.

Palmer amaranth control
All PRE herbicides evaluated in this study, except for atrazine, provided 90%-99% control of MHR Palmer amaranth 15 and 30 DAPRE (Table 3).A study conducted in soybean by Hay (2017) in Kansas, along with Sarangi and Jhala (2019) in Nebraska, reported that saflufenacil plus dimethenamid-P provided >95% control of ALS-inhibitor-resistant Palmer amaranth 28 DAPRE.At 45 DAPRE, atrazine/bicyclopyrone/mesotrione/Smetolachlor, acetochlor/mesotrione, acetochlor/clopyralid/ mesotrione, and dimethenamid-P/saflufenacil provided 89%-95% control of MHR Palmer amaranth.These were followed by acetochlor/atrazine, acetochlor/flumetsulam/clopyralid, and saflufenacil (81%).Sarangi et al. (2017) elaborated on the effective use of very long-chain fatty acid-inhibiting herbicides for the residual control of glyphosate-resistant Amaranthus spp.The residual activity of some PRE herbicides in this study declined as the season progressed; for instance, flufenacet/isoxaflutole/thiencarbazone-methyl provided 59% control of Palmer amaranth 60 DAPRE.This might be due to lower persistence of the applied residual herbicide and late-season emergence of Palmer amaranth.Chahal et al. (2018) reported that PS-II-and HPPD-inhibitorresistant Palmer amaranth control from PRE herbicides was <26% at 6 weeks after POST in glyphosate-resistant corn.However, in this study, atrazine/bicyclopyrone/mesotrione/Smetolachlor and acetochlor/clopyralid/mesotrione controlled Palmer amaranth >96% 60 DAPRE.This demonstrated the efficacy of these residual herbicides through their persistence and by reducing interplant competition between the corn and Palmer amaranth.
Atrazine/bicyclopyrone/mesotrione/S-metolachlor, acetochlor/mesotrione, and acetochlor/clopyralid/mesotrione consistently provided 85%-92% control of MHR Palmer amaranth 75 DAPRE and 90 DAPRE (Table 3).Inman et al. (2020) reported that acetochlor plus diuron plus fomesafen applied PRE provided 79% glyphosate-resistant Palmer amaranth control at 2-3 weeks after planting.Striegel and Jhala (2022) elaborated on the use of PRE herbicides for control of glyphosate-resistant Palmer amaranth (94%-98% 35 DAPRE) in dicamba/glufosinate/glyphosate-resistant soybean and concluded that PRE herbicides have a positive effect on soybean yield.Thus, if MHR Palmer amaranth is a major weed in growers' field, the use of PRE herbicides with multiple effective sites of action is almost mandatory for early season control to avoid competition with crops (Ward et al., 2013).
At 60 DAPRE, acetochlor/clopyralid/mesotrione, dimethenamid-P/saflufenacil, and atrazine/bicyclopyone/mesotrione/S-metolachlor resulted in 5-12 g m -2 Palmer amaranth biomass compared with 134 g m -2 in the nontreated control.These were followed by saflufenacil and acetochlor/atrazine (17-18 g m -2 ); however, the remaining herbicides had biomass ranging from 22 to 66 g m -2 .This study focused only on PRE herbicides; thus, there was higher MHR Palmer amaranth biomass that indicated the importance of follow-up programs for season-long weed management.Mausbach et al. (2021) concluded that PRE herbicides with multiple sites of action followed by glufosinate provided at least 87% reduction of ALS inhibitor/glyphosate-resistant Palmer amaranth density and biomass reduction up to 60 DAPRE.

Palmer amaranth seed production
Palmer amaranth seed production was affected by PRE herbicides (Table 5).The highest MHR Palmer amaranth seed production (2,503,706 seeds from 17 plants m −2 ) resulted from atrazine and the nontreated control (2,464,016 seeds from 33 plants m −2 ).This is because Palmer amaranth in this field is highly resistant to atrazine; therefore, atrazine was not effective.Palmer amaranth density in this study was 33 plants m −2 in the nontreated control compared with all PRE herbicides (2-11 plants m −2 ) except atrazine (17 plants m −2 ).Miranda et al. (2022) concluded that the highest seed production of 376,000 seeds per plant was produced when 0.2 Palmer amaranth plants m −1 row of dry bean, and that this number decreased by 12%, 28%, 55%, and 75% when Palmer amaranth density increased to 0.3, 0.5, 1, and 2 plants m −1 row, respectively.Minimal seed production was noted in acetochlor/ clopyralid/mesotrione (32,894 seeds m −2 ) and atrazine/bicyclopyone/mesotrione/S-metolachlor (100,407 seeds m −2 ).This might be because of lower Palmer amaranth density in these treatments (2 plants m −2 ), and thus, less intraspecific competition among the MHR Palmer amaranth plants.None of the programs resulted in a 100% reduction in Palmer amaranth seed production.This might be because there was no POST herbicide applied in this study.Kaur et al. (2023) concluded that the lowest MHR Palmer seed production was observed with POST application of dicamba and atrazine/bicyclopyone/mesotrione/S-metolachlor in corn.Thus, a PRE fb a POST herbicide program has a better chance of reducing MHR Palmer amaranth seed production compared with relying only on PRE herbicide.Striegel and Jhala (2022) reported that glyphosate-resistant Palmer amaranth seed production declined to 0-325 seeds plant -1 when PRE herbicide was used compared with POST-only programs (85-4786 seeds plant -1 ) in soybeans and further reduced to 0 seeds plant -1 when a PRE herbicide was followed by a POST herbicide with residual activity.

Corn yield
Year-by-treatment interaction was significant (p ≤ 0.05); therefore, yield data were presented separately for both years (Table 5).Acetochlor/clopyralid/mesotrione recorded the highest corn yield of 12,139 and 12,093 kg ha −1 in 2020 and 2021, respectively, which was similar to isoxaflutole/thiencarbazone-methyl (11,255 kg ha −1 ) and
Abbreviations: ai, active ingredient; DA-PRE, days after pre-emergence herbicide application.a Year by treatment interaction for Palmer amaranth density and biomass was non-significant; therefore, data were combined across both years.
Data were obtained from National Oceanic and Atmospheric Administration(NOAA 2020(NOAA   & 2021)).Multiple herbicide-resistant Palmer amaranth control was affected by pre-emergence herbicides in food-grade white corn in field experiments conducted at Carleton, NE, during the 2020 and 2021 growing seasons. a

-1 )
Abbreviation: ai, active ingredient.a Year by treatment interaction for corn yield was significant; therefore, data were presented separately for both years.b Means presented within each column with no common letter(s) are significantly different as per Tukey Kramer's pairwise comparison test.