Waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] has become one of the most problematic weed species in midwestern United States crop production. Reductions in tillage, greater reliance on herbicides for weed management, and the evolution of resistance to multiple herbicide modes of action (MOA) have contributed to the rapid increase of waterhemp in agricultural cropping systems [1] [2]. Waterhemp has been reported in 19 states of the USA and three provinces in Canada where it interferes with corn and soybean production [3] [4]. The rapid movement of waterhemp and evolution of herbicide resistance among individuals and populations is facilitated by its dioecism, rapid growth rate, high reproductive rate, delayed emergence, and extended emergence pattern [5] [6]. Resistance to photosystem II (PS II)-, acetolactate synthase (ALS)-, and protoporphyrinogen oxidase (PPO)-inhibiting herbicides was identified in waterhemp in 1990, 1993, and 2001, respectively [4] [7] [8] [9]. Waterhemp resistant to 5-enolpyruyl shikimate-3-phosphate synthase (EPSPS)-inhibitors was first reported in the USA in 2005 and Ontario, Canada in 2014 [4]. More recent reports from Ontario have identified multiple-herbicide-resistant (MHR) waterhemp populations resistant to ALS-, PS II-, EPSPS-, and PPO-inhibiting herbicides. Waterhemp continues to evolve resistance to currently used MOA and is the first weed species to develop resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides [4] [7] [8] [9]. The first MHR waterhemp population with six-way resistance to synthetic auxins and ALS-, PS II-, EPSPS-, PPO-, and HPPD-inhibiting herbicides was identified in Missouri in 2015 [10]. Resistance to very-long-chain fatty-acid (VLCFA)-inhibiting herbicides has since been detected within a MHR waterhemp population resistant to 2,4-D and ALS-, PS II-, PPO-, and HPPD-inhibiting herbicides [11]. The ability of MHR waterhemp to rapidly evolve and accumulate traits that confer resistance to multiple MOA makes it difficult to manage in agricultural cropping systems [12].

Weed interference must be prevented during the early stages of corn (Zea mays L.) growth and development to prevent yield loss [13]. The relative time of crop and weed emergence has a greater effect on corn yield than weed density and biomass [13]. Weeds that emerge with the crop have the greatest impact on corn yield [13]. Steckel and Sprague [2] reported MHR waterhemp emerging at VE corn growth stage reduced grain yield 74% compared to only 2% yield loss when waterhemp emerged at V8 and was left uncontrolled for the remainder of the growing season. In contrast, Cordes et al. [14] reported corn yield loss was dependent on waterhemp density. When competing with corn, waterhemp can be placed at a disadvantage due to its characteristic late emergence; however, corn yield losses of up to 17% have been reported when densities of 369 to 445 plants m⁻² emerge and compete up to V7 corn growth stage [14] [15]. Steckel and Sprague [2] reported corn yield reductions when waterhemp emerged before the V8 corn growth stage. In Ontario, corn yield losses of up to 48% have been reported when waterhemp populations are left uncontrolled [16]. The critical period of weed control in corn to prevent yield loss varies with the relative time of weed and crop emergence, weed density, species, and environment [17] [18]. It is recommended that corn remain waterhemp-free from emergence to V6 to maximize grain yield [2]. Early-season control of MHR waterhemp is imperative to reduce early-season weed interference, prevent corn yield loss, and reduce weed escapes.

Current herbicide-based MHR waterhemp management strategies include preemergence (PRE), postemergence (POST), and PRE followed-by (fb) POST herbicide applications that utilize multiple effective MOA [16] [19] [20] [21]. The HPPD-inhibitors isoxaflutole, mesotrione, and tolpyralate are often applied in combination with a PS II-inhibitor such as atrazine and result in excellent control of MHR waterhemp [22] [23] [24] [25] [26]. Complementary activity between HPPD-inhibitors and atrazine has been reported for the control of triazine-susceptible and triazine-resistant redroot pigweed (Amaranthus retroflexus L.), waterhemp, and Palmer amaranth (Amaranthus palmeri S. Watson) [25] [26] [27]. HPPD-inhibitors inhibit the production of carotenoids, α-tocopherols, and plastoquinone, and atrazine increases the production of reactive oxygen species [28] [29]. The enhanced weed control efficacy when a HPPD-inhibitor is co-applied with a PS II-inhibitor is due to 1) increased binding efficiency of atrazine to the D1 protein of PS II-inhibitor caused by the shortage of plastoquinone, and 2) enhanced reactive oxygen species (ROS) levels due to the lack of quenching carotenoids, tocopherols, and plastoquinone. Synthetic auxin herbicides are another effective MOA for MHR waterhemp control; however, current literature reports variable responses [16] [30] [31]. Synthetic auxin herbicides provide control of broadleaf weeds by mimicking plant growth hormones which causes unregulated plant growth and death in some plants [32]. Superior MHR waterhemp control with dicamba/atrazine compared to other POST tank-mixtures has been reported [33]. Anderson et al. [30], Soltani et al. [16] and Vyn et al. [31] reported dicamba/atrazine provided ≥86% control of herbicide-resistant waterhemp. Benoit et al. [33] and Schryver et al. [34] found that POST applications of dicamba are more effective than PRE applications. The application of a new glyphosate/2,4-D choline formulation registered for application to ENLIST^TM (Corteva Agriscience, Wilmington, DE) corn hybrids allows for a second synthetic auxin herbicide for MHR waterhemp control in corn [34] [35] [36]. ENLIST^TM corn hybrids contain transgenes that confer resistance to glyphosate and glufosinate plus the aryloxyalkanoate dioxygenase-1 (AAD-1) transgene which enables them to exhibit resistance to glyphosate, glufosinate, and greater tolerance to 2,4-D and the arlyloxyphenoxy propionates than traditional glyphosate (RoundupReady^®) (Bayer CropScience Inc., 160 Quarry Park Boulevard SE, Calgary, AB) and glufosinate (LibertyLink^®) (BASF Canada Inc., 100 Milverton Drive, Mississauga, ON) resistant hybrids [35] [37] [38]. Robinson et al. [36] reported up to 94% control of common waterhemp (Amaranthus rudis Sauer) with 2,4-D (1120 g ae) and 99% control with 2,4-D + glyphosate (280 + 840 g ae); however, it is important to note glyphosate (840 g ae) alone provided 100% control 4 WAA in that study. Similarly, Miller and Norsworthy [39] obtained ≥87% control of glyphosate-resistant Palmer amaranth in soybean. Numerous PRE, POST, and PRE fb POST herbicide applications in corn have been developed for the control of MHR waterhemp with varying resistance profiles [3] [19] [31] [33] [40] [41].

Waterhemp control is affected by herbicide application timing. Previous studies have shown that PRE applications can control MHR waterhemp better than POST applications [33] [42] [43]. Hager et al. [44] reported 8% to 9% greater control of ALS-resistant-waterhemp 1, 2, and 3 WAA when herbicides were applied to 5 cm waterhemp early POST (EPOST) compared to 10 cm waterhemp (POST) in soybean. Similarly, Hedges et al. [43] observed a 20% reduction in waterhemp control as POST applications were delayed from 5 to 25 cm tall waterhemp. These studies suggest differences between EPOST and POST can be attributed to slower herbicide activity on larger waterhemp plants and reduced interception due to shading of younger plants caused by the extended emergence pattern [33] [42] [43]. Corn producers should eliminate MHR waterhemp interference from VE to V6 corn growth stage and control it before it exceeds 10 cm in height [2] [44] [45].

Delayed POST herbicide applications can result in reduced control due to larger weed size at application and decreased corn yield due to early-season waterhemp interference. To achieve season-long control of MHR waterhemp, it is imperative that herbicide applications include effective MOA, provide soil residual and target small weed size (≤10 cm). We hypothesized that EPOST herbicide tank mixtures made to 5 cm MHR waterhemp will provide season-long control of MHR waterhemp in corn. The objective of this research was to identify effective EPOST herbicide tank-mixtures that provide control of emerged MHR waterhemp and season-long residual control in corn while stewarding currently available herbicide MOA.

Most EPOST herbicide tank-mixtures provided greater than 90% control of MHR waterhemp. The density, biomass, and population resistance profile are reflected in the differences in control between sites. At 4, 8, and 12 WAA, control of MHR waterhemp ranged from 61% to 100% across sites and was lower at S1, S3, and S4 due to greater density and biomass compared to S2 and S5 (Table 1, Tables 3-5). Density and biomass of MHR waterhemp in the nontreated control at S1, S3, and S4 averaged 263 to 962 plants m⁻² and 70.2 to 259.4 g·m⁻², respectively, compared to 60 plants m⁻² and 72.2 g·m⁻² at S2 and S5. Vyn et al. [31] reported similar site differences in POST MHR waterhemp control which they attributed to plant density and site-specific MHR waterhemp resistance profiles. In that study, one waterhemp population exhibited resistance to ALS-inhibiting herbicides and the other to both ALS- and PS II-inhibiting herbicides [31]. All sites contained waterhemp resistant to ALS-, PS II-, EPSPS- and PPO-inhibitors; however, the proportion of individuals resistant to each MOA varied by site. The MHR waterhemp population at S1, S3, and S4 contained a greater number of individuals exhibiting resistance to ALS-, PS II-, and PPO-inhibitors than that of S2 and S5.

Note: Herbicide treatments glyphosate + mesotrione + atrazine and glyphosate/S-metolachlor/mesotrione + atrazine included Agral^® 90 (Syngenta Canada Inc., 140 Research Lane, Research Park, Guelph, ON.) (0.2% v/v); and glyphosate + tolpyralate + atrazine included methylated seed oil (MSO Concentrate^®) (Loveland Products Inc., 3005 Rocky Mountain Ave., Loveland, CO.) (0.5% v/v), and urea ammonium nitrate (UAN 28-0-0) (Sylvite, 3221 North Service Road, Burlington, ON.) (2.5% v/v). a-c: Means followed by the same letter are not significantly different (P < 0.05).

Note: Herbicide treatments glyphosate + mesotrione + atrazine and glyphosate/S-metolachlor/mesotrione + atrazine included Agral^® 90 (Syngenta Canada Inc., 140 Research Lane, Research Park, Guelph, ON.) (0.2% v/v); and glyphosate + tolpyralate + atrazine included methylated seed oil (MSO Concentrate^®) (Loveland Products Inc., 3005 Rocky Mountain Ave., Loveland, CO.) (0.5% v/v), and urea ammonium nitrate (UAN 28-0-0) (Sylvite, 3221 North Service Road, Burlington, ON.) (2.5% v/v). a-d: Means followed by the same letter are not significantly different (P < 0.05).

Note: Herbicide treatments glyphosate + mesotrione + atrazine and glyphosate/S-metolachlor/mesotrione + atrazine included Agral^® 90 (Syngenta Canada Inc., 140 Research Lane, Research Park, Guelph, ON.) (0.2% v/v); and glyphosate + tolpyralate + atrazine included methylated seed oil (MSO Concentrate^®) (Loveland Products Inc., 3005 Rocky Mountain Ave., Loveland, CO) (0.5% v/v), and urea ammonium nitrate (UAN 28-0-0) (Sylvite, 3221 North Service Road, Burlington, ON.) (2.5% v/v). a-b: Means followed by the same letter are not significantly different (P < 0.05).

At S1, S3, and S4, glyphosate + S-metolachlor/mesotrione/bicyclopyrone/ atrazine, glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine, glyphosate + S-metolachlor/atrazine/mesotrione, and glyphosate + mesotrione + atrazine controlled MHR waterhemp 95% to 100% at 4, 8, and 12 WAA and reduced plant density and biomass by 97% to 100%. Glyphosate/S-metolachlor/ mesotrione + atrazine, glyphosate + S-metolachlor/mesotrione/bicyclopyrone, glyphosate/2,4-D choline + rimsulfuron + mesotrione, and glyphosate + pyroxasulfone + dicamba/atrazine controlled MHR waterhemp 90% to 99% at 4, 8, and 12 WAA and reduced plant density and biomass 96% to 100%. Control of MHR waterhemp ranged from 99% to 100% and plant density and biomass were reduced 100% with all herbicides at S2 and S5. These findings are consistent with Sarangi and Jhala [46] who reported excellent control of MHR waterhemp with S-metolachlor/mesotrione/bicyclopyrone/atrazine applied POST to 8 to 10 cm and 15 to 18 cm waterhemp. Benoit et al. [3] [19] [33] and Vyn et al. [31] reported comparable control of MHR waterhemp with mesotrione + atrazine applied POST providing ≥ 90% control 4, 8 and 12 WAA in multiple trials. Vyn et al. [31] reported similar control of MHR waterhemp with 2,4-D/atrazine (1404 g·ha⁻¹) of 84% to 100% 4 and 10 WAA. Additionally, Sarangi et al. [47] and Chahal et al. [45] reported excellent control of MHR waterhemp with glyphosate + 2,4-D choline (840 + 800 g·ha⁻¹) applied POST, however, Chahal et al. [45] suggested that plants should be less than 10 cm in height at application. At S1, S3, and S4, glyphosate + topramezone/dimethenamid-P + dicamba/atrazine, glyphosate + isoxaflutole + atrazine, and glyphosate + tolpyralate + atrazine controlled MHR waterhemp 79% to 99% at 4, 8 and 12 WAA and reduced plant density and biomass 92% to 100%. These results complement another Ontario study that reported ≥96% MHR waterhemp control from 4 to 12 WAA with glyphosate + tolpyralate + atrazine (900 + 30 + 560 g·ha⁻¹) applied POST [26]. Moreover, MHR waterhemp control with isoxaflutole + atrazine applied EPOST was similar to PRE applications that provided 82% to 100% control in other studies [16] [19] [31] [48]. Control of MHR waterhemp was similar for all herbicide tank-mixtures except glyphosate + S-metolachlor/atrazine and glyphosate + dicamba/atrazine at S4, S1 and S3 which resulted in the lowest level of control ranging from 73% to 75% and 76% to 83%, respectively. Glyphosate + S-metolachlor/atrazine resulted in lower MHR waterhemp control than glyphosate + dicamba/atrazine at S1 and S3; however, control was similar at S4. This is consistent with Benoit et al. [19] who reported 84% and 87% MHR waterhemp control 4 and 8 WAA with dicamba/atrazine POST. In contrast, Soltani et al. [16] reported greater MHR waterhemp control and reductions in density and biomass with dicamba/atrazine (1800 g·ha⁻¹) POST that were similar to mesotrione + atrazine POST and isoxaflutole + atrazine PRE in that study. Schryver et al. [34] reported MHR waterhemp control with dicamba (600 g·ha⁻¹) is greatest when applied POST resulting in 91% to 100% control compared to 60% to 65% at 10 WAA when applied PRE. Lower control with glyphosate + dicamba/atrazine may be due to lower dicamba application rate (504 g·ha⁻¹). Control of MHR waterhemp from 4 to 12 WAA increased at S1 and S3 and decreased at S4 for most herbicide tank-mixtures evaluated. Increasing MHR waterhemp control over the course of the growing season could be due to natural thinning of waterhemp populations reported by Benoit et al. [19] and [49]; in contrast, late emerging cohorts have been reported to reduce end-of-season control as well [2].

Corn injury was ≤10% for all herbicide treatments 1, 2, and 4 WAA at all sites except S2 (Table 5). Glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine caused 28%, 31%, and 31% corn injury 1, 2, and 4 WAA , respectively at S2; symptoms included brace root malformation and lodging which resulted in reduced corn stand. Applications of glyphosate/2,4-D choline to non-ENLIST^TM hybrids can cause stalk brittleness, leaning, malformed brace roots, and leaf rolling in the whorl [50] [51]. Ruen et al. [38] reported similar leaf necrosis and leaning of ENLIST^TM corn hybrids treated with single applications of glyphosate/2,4-D choline at V4 and V7 corn growth stages and sequential applications at V4 fb V7. Interestingly, the addition of atrazine to glyphosate/2,4-D choline + rimsulfuron + mesotrione increased corn injury 25%, 25%, and 26% at 1, 2, and 4 WAA, respectively. We do not have an explanation for this observation; the response should be evaluated in future studies to determine if this is a real response. Tolerance of conventional corn hybrids to 2,4-D varies with hybrid, corn growth stage at application, soil characteristics, and weather conditions [38]. It is recommended that glyphosate/2,4-D choline (ENLIST DUO) only be applied to ENLIST^TM field corn hybrids that contain the AAD-1 transgene [35]. Glyphosate/2,4-D choline applications can also be made up to the V8 corn growth stage; in this study, herbicides were applied to V4 corn (data not shown). Corn injury caused by glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine resulted in lower corn yield than glyphosate + S-metolachlor/atrazine; however, yield was similar to the weed-free control. When waterhemp was left uncontrolled, corn yield was reduced 39% at S4 and was similar to another Ontario study that reported a corn yield reduction of 48% [16]. Relative to the weed-free control, corn yield was not reduced at S1, S2, S3, and S5 which could again be the result of comparatively lower MHR waterhemp density and biomass. Cordes et al. [14] reported corn yield reductions due to the late removal of waterhemp when plants reached 15 cm. This result supports previous research that suggests EPOST herbicide applications reduce early season weed interference and minimize corn yield loss [13].

Most of the EPOST herbicide tank-mixtures evaluated in this study resulted in greater than 90% control of MHR waterhemp. The top four herbicides treatments were glyphosate + S-metolachlor/mesotrione/bicyclopyrone/atrazine, glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine, glyphosate + S-metolachlor/atrazine/mesotrione, and glyphosate + mesotrione + atrazine which controlled MHR waterhemp 95% to 100% throughout the growing season at all sites. Glyphosate/S-metolachlor/mesotrione + atrazine, glyphosate + S-metolachlor/mesotrione/bicyclopyrone, glyphosate/2,4-D choline + rimsulfuron + mesotrione, and glyphosate + pyroxasulfone + dicamba/atrazine provided 90% to 100% MHR waterhemp control 4, 8, and 12 WAA. Control of MHR waterhemp was similar amongst all herbicide tank-mixtures, except glyphosate + S-metolachlor/atrazine and glyphosate + dicamba/atrazine which resulted in lower control than all other treatments, control ranged from 61% to 100% and 63% to 100%, respectively. Reductions in MHR waterhemp control with these herbicides resulted in greater MHR waterhemp density and biomass 4 WAA at three of five sites. Differences in MHR waterhemp control were the result of variation in population resistance profiles, competitiveness of individual populations, and comparatively lower MHR waterhemp density and biomass at two of five sites. Furthermore, herbicide tank-mixtures reduced MHR waterhemp density and biomass > 91% and 97%, respectively, except glyphosate + S-metolachlor/ atrazine and glyphosate + dicamba/atrazine (67% to 82% and 74% to 92%). Weed interference caused by MHR waterhemp did not reduce corn yield in this study at four out of five sites. This study identifies EPOST herbicide tank-mixtures that expand POST MHR waterhemp management and have a wide margin of crop safety. Corn producers should incorporate the use of strategic tillage, cover crops, crop row spacing, crop density, PRE herbicides, EPOST herbicides, POST herbicides, and PRE fb POST herbicide programs to steward the use of currently effective MOA and reduce the evolution of herbicide resistance. Strategies to manage MHR waterhemp should be implemented early in the growing season, herbicide applications should be made before MHR waterhemp reaches 10 cm in height to prevent corn yield loss.

The authors acknowledge Chris Kramer for his technical support, Dr. Michelle Edwards for her statistical support, the University of Guelph, Ridgetown Campus summer staff for their field support and the Grain Farmers of Ontario and herbicide manufacturers for providing funding for this research.

The authors declare no conflicts of interest regarding the publication of this paper.

Willemse, C., Soltani, N., Benoit, L., Jhala, A.J., Hooker, D.C., Robinson, D.E. and Sikkema, P.H. (2021) Early Postemergence Herbicide Tank-Mixtures for Control of Waterhemp Resistant to Four Herbicide Modes of Action in Corn. Agricultural Sciences, 12, 354-369. https://doi.org/10.4236/as.2021.124023