**1. Introduction**

During the past 20 years, the use of glyphosate‐resistant crop production systems has been adopted and used extensively in various regions of the USA [1]. In 2009, nearly 61 million ha of soybean [*Glycine max* (L.) Merr.], cotton (*Gossypium hirsutum* L), and corn (*Zea mays* L.) con‐ tained a modified 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS) gene that confers resistance to glyphosate [2]. The wide use of row crops with glyphosate‐resistance, the reduc‐ tion of traditional herbicide and cultivation practices, and the use of intense management of weeds using glyphosate as the predominant control strategy has caused a shift in weed popu‐ lations and created a selective advantage for glyphosate‐resistant weeds [3, 4].

The development of herbicide‐resistant crops allows weed control by nonselective postemer‐ gence (POST) herbicides, such as glyphosate and glufosinate, widening the array of weed management programs available to producers [5–7]. Both glyphosate and glufosinate con‐ trol a wide range of weeds in herbicide‐resistant crops [7] with little, if any, crop injury [8, 9]. POST applications of glyphosate or glufosinate provide consistent and greater control of large‐seeded broadleaf weed species including velvetleaf (*Abutilon theophrasti* Medik.), giant ragweed (*Ambrosia trifida* L.), common cocklebur (*Xanthium strumarium* L.), and morningglory spp. (*Ipomoea* spp.) compared with preemergence (PRE) herbicides [9]. Even though the per‐ formance of glyphosate and glufosinate is similar, glufosinate is less likely to succeed in a single POST application program since glufosinate is less effective on larger weeds, needs an increased spray volume, and a need for high humidity at application [7].

Glyphosate‐resistant weeds, specifically Amaranthus species, have become an issue across all the USA corn and cotton‐producing areas [10]. Estimates are that more than 1.2 million ha of cropland in the USA are now affected by glyphosate‐resistant Amaranthus species [10]. In cotton, Palmer amaranth (*Amaranthus palmeri* S. Wats.) has been shown to reduce lint yield by 57% when growing at a density of 10 plants per 9.1 m of row [11]. Additionally, with Palmer amaranth growing at densities greater than six plants per 9.1 m of row, cotton may not be harvestable due to the potential for damage to harvest equipment [11]. A study by Smith et al. [12] found that Palmer amaranth densities of 650–3260 plants ha−1 in dryland stripper‐har‐ vested cotton increased harvesting time by 2‐ to 3.5‐fold.

Weed resistance to photosystem II (PSII)‐inhibiting herbicides, such as atrazine, has also been documented across many corn‐growing areas of the USA [10]. Resistance to PSII inhibi‐ tors has been documented in 7 monocot and 17 dicot species in the corn‐producing regions [13]. Also, populations of tall waterhemp [*Amaranthus tuberculatus* (Moq.) Sauer] have been identified with resistance to herbicides that inhibit acetolactate synthase (ALS), PSII, proto‐ porphyrinogen oxidase (PPO), 5‐enolpyruvylshikimate‐3‐phosphate‐synthase (EPSPS), and 4‐hydroxyphenyl‐pyruvate‐dioxygenase (HPPD) in Illinois and Iowa, and Palmer amaranth populations resistant to ALS, PSII, and HPPD inhibitors have been identified in Kansas [13], indicating the continued need for alternative modes of action in corn to reduce the chance of herbicide resistance. The HPPD‐inhibiting herbicides have become popular among corn pro‐ ducers because of their broad‐spectrum weed control, flexible application timings, tank‐mix compatibilities, and crop safety [14–16].

Cotton growers have experienced more problems with weed resistance because of cotton's slower emergence after planting and fewer registered herbicides compared with other major crops [17]. The first documented cases of glyphosate‐resistant (GR) Palmer amaranth in cotton occurred in 2000 in Lauderdale County, TN [18] and in 2003 in Edgecombe County, NC [19]. The first confirmed case of GR Palmer amaranth was documented in a biotype of Palmer ama‐ ranth growing in a Macon County, GA cotton field, where six‐ to eightfold levels of resistance to glyphosate were observed [3].

**1. Introduction**

64 Herbicide Resistance in Weeds and Crops

During the past 20 years, the use of glyphosate‐resistant crop production systems has been adopted and used extensively in various regions of the USA [1]. In 2009, nearly 61 million ha of soybean [*Glycine max* (L.) Merr.], cotton (*Gossypium hirsutum* L), and corn (*Zea mays* L.) con‐ tained a modified 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS) gene that confers resistance to glyphosate [2]. The wide use of row crops with glyphosate‐resistance, the reduc‐ tion of traditional herbicide and cultivation practices, and the use of intense management of weeds using glyphosate as the predominant control strategy has caused a shift in weed popu‐

The development of herbicide‐resistant crops allows weed control by nonselective postemer‐ gence (POST) herbicides, such as glyphosate and glufosinate, widening the array of weed management programs available to producers [5–7]. Both glyphosate and glufosinate con‐ trol a wide range of weeds in herbicide‐resistant crops [7] with little, if any, crop injury [8, 9]. POST applications of glyphosate or glufosinate provide consistent and greater control of large‐seeded broadleaf weed species including velvetleaf (*Abutilon theophrasti* Medik.), giant ragweed (*Ambrosia trifida* L.), common cocklebur (*Xanthium strumarium* L.), and morningglory spp. (*Ipomoea* spp.) compared with preemergence (PRE) herbicides [9]. Even though the per‐ formance of glyphosate and glufosinate is similar, glufosinate is less likely to succeed in a single POST application program since glufosinate is less effective on larger weeds, needs an

Glyphosate‐resistant weeds, specifically Amaranthus species, have become an issue across all the USA corn and cotton‐producing areas [10]. Estimates are that more than 1.2 million ha of cropland in the USA are now affected by glyphosate‐resistant Amaranthus species [10]. In cotton, Palmer amaranth (*Amaranthus palmeri* S. Wats.) has been shown to reduce lint yield by 57% when growing at a density of 10 plants per 9.1 m of row [11]. Additionally, with Palmer amaranth growing at densities greater than six plants per 9.1 m of row, cotton may not be harvestable due to the potential for damage to harvest equipment [11]. A study by Smith et al. [12] found that Palmer amaranth densities of 650–3260 plants ha−1 in dryland stripper‐har‐

Weed resistance to photosystem II (PSII)‐inhibiting herbicides, such as atrazine, has also been documented across many corn‐growing areas of the USA [10]. Resistance to PSII inhibi‐ tors has been documented in 7 monocot and 17 dicot species in the corn‐producing regions [13]. Also, populations of tall waterhemp [*Amaranthus tuberculatus* (Moq.) Sauer] have been identified with resistance to herbicides that inhibit acetolactate synthase (ALS), PSII, proto‐ porphyrinogen oxidase (PPO), 5‐enolpyruvylshikimate‐3‐phosphate‐synthase (EPSPS), and 4‐hydroxyphenyl‐pyruvate‐dioxygenase (HPPD) in Illinois and Iowa, and Palmer amaranth populations resistant to ALS, PSII, and HPPD inhibitors have been identified in Kansas [13], indicating the continued need for alternative modes of action in corn to reduce the chance of herbicide resistance. The HPPD‐inhibiting herbicides have become popular among corn pro‐ ducers because of their broad‐spectrum weed control, flexible application timings, tank‐mix

lations and created a selective advantage for glyphosate‐resistant weeds [3, 4].

increased spray volume, and a need for high humidity at application [7].

vested cotton increased harvesting time by 2‐ to 3.5‐fold.

compatibilities, and crop safety [14–16].

With the widespread adoption of glyphosate‐resistant cotton after its introduction in 1997, cotton weed management practices largely shifted away from the use of soil‐applied resid‐ ual herbicides to POST herbicide programs based on glyphosate [20]. Studies conducted in 2006 and 2007 by Legleiter and Bradley [21] confirmed glyphosate resistance in a bio‐ type of common waterhemp (*Amaranthus rudis* Sauer) found in a Missouri soybean field following multiple glyphosate applications. Currently, glyphosate‐resistant Palmer ama‐ ranth and common waterhemp have been reported in 27 and 18 USA states, respectively [10]. Through surveys sent to weed scientists across the USA, Culpepper [3] revealed that 50% of respondents indicated that weeds of the genus Amaranthus had increased sig‐ nificantly in cotton. The respondents also provided the following four recommendations for managing glyphosate‐induced weed species shifts: tank‐mix combinations of other herbicides with glyphosate for POST applications, rotating with non‐glyphosate‐resistant crops (though there was some disagreement among respondents), use of POST herbi‐ cides other than glyphosate, and using preplant‐incorporated (PPI) or (PRE) soil‐applied herbicides.

Amaranthus species are some of the most common weed species found in annual crop pro‐ duction throughout the USA [22]. Palmer amaranth is now ranked as the most troublesome weed found in the USA [23]. It is a common weed in many major crops around the world and is found in all areas of Texas [24]. Up until the 1990s, its distribution in North America was the southern half of the USA [24]; however, since then, it has become established in every state with the exception of the northwestern USA, including Washington, Oregon, Montana, and North Dakota [25]. In Texas, Palmer amaranth can be found in all areas of the state [26] and is one of the two Amaranthus species with confirmed resistant to glyphosate across Texas (common waterhemp is the other) [27]. It is a dioecious, summer‐annual spe‐ cies that is native to the desert southwest region of the USA [28, 29]. Plants of the genus Amaranthus are often very problematic weeds in agronomic crops due to their ability to ger‐ minate under a wide range of conditions, grow rapidly, and produce large numbers of seed, all while competing with the crop for sunlight, moisture, and nutrients. Despite its origin, Palmer amaranth is able to survive in many diverse environments because of its biological characteristics [6, 30]. It has a lengthy germination window, robust growth habit, and is a prolific seed producer [31–33], and these characteristics make control of this weed difficult. Common waterhemp is an obligate outcrossing annual broadleaf weed that is capable of long‐distance pollen dispersal [34]. It germinates optimally between 20/25 and 30/35°C [35], has an aggressive growth habit and may grow 1.6 mm per growing degree day [32], and is capable of producing more than 250,000 seeds per plant [30]. These factors make it a strong competitor with most crops.

Traditional corn and cotton weed management programs have relied on PRE applications of a broadleaf and grass herbicide for residual season‐long weed control [36–41]. In corn, these PRE programs usually have included atrazine in combination for broad‐spectrum weed con‐ trol. Atrazine is used in over 60% of the USA corn, and its doses have gotten lower with most doses of no more than 1.12 kg ha−1 with some growers applying no more than 0.84 kg ha−1 [42]. Atrazine and 4‐hydroxylphenylpyruvate dioxygenase (HPPD)‐inhibiting herbicides are commonly used for weed control in corn and are effective in controlling glyphosate‐resistant weeds, including Palmer amaranth [36, 43, 44]. Atrazine can be applied PRE or POST alone or in tank‐mixtures with several herbicides [16].

Since POST herbicides are applied after the weed species and severity are known, this allows growers to assess the problem before making a herbicide application; therefore, POST herbi‐ cides are an essential component of an integrated weed management system to combat the herbicide‐resistant weeds [45]. In addition, POST herbicides typically do not require rainfall for herbicide activation, making performance less dependent on environmental conditions [45]. Also, POST herbicides can reduce the potential for water pollution [46]. A Minnesota study showed reduced atrazine concentrations in runoff water when applied POST compared with soil‐applied applications because of the increased plant residue and cover, limiting the amount of herbicide reaching the soil [47].

Two new herbicide systems have recently become important in POST weed control in cotton [48–53]. Dicamba (3,6‐dichloro‐2‐methoxybenzoic acid) is synthetic auxin herbicide that controls glyphosate‐resistant Palmer amaranth and other broadleaf weeds alone or in sequential combina‐ tions with glyphosate or glufosinate [48]. An enzyme, dicamba O‐demethylase, was discovered in a soil bacterium (*Pseudomonas maltophilia*) that converts dicamba to 3,6‐dichlorosalicylic acid (DCSA) [49]. The enzyme DCSA has no significant herbicidal properties. The gene responsible for this enzyme is known as DMO (dicamba monooxygenase). This gene was successfully inserted into mouse‐ear cress [*Arabidopsis thaliana (L). Heynh*.], tomato (*Solanum lycopersicum* L.), and tobacco (*Nicotiana tabacum* L.) and showed to provide these plants with effective tolerance to foliar applications of dicamba [49]. Dicamba‐tolerant cotton, coupled with existing glyphosate‐ and glu‐ fosinate‐tolerant traits, was deregulated in the USA in 2015 and has since become significant por‐ tion of the cotton planted in the USA, comprising over 40% of the crop planted in 2016 [50, 51].

Enlist Duo herbicide, a premix formulation containing 195 g ae L−1 of 2,4‐D choline and 205 g ae L−1 of glyphosate dimethylamine, was developed for use in Enlist corn, cotton, and soy‐ bean. Resistance to 2,4‐D is conferred by the insertion of a gene that codes for the enzyme ary‐ loxyalkanoate dioxygenase. Plants transformed to include this gene can metabolize 2,4‐D to a nonlethal form [52]. Developed during World War II, 2,4‐D was the first selective herbicide widely used in agriculture [53]. Since that time, researchers have demonstrated control of a large number of dicotyledonous weed species with 2,4‐D [54–57].

The adoption of 2,4‐D in Enlist crops will be influenced by yield potential of the crop, weed species infesting fields, and, most notably, the ability of growers to mitigate off‐target movement of 2,4‐D [58–60]. Although Enlist cotton is resistant to 2,4‐D [61], all other cotton cultivars, including cotton resistant to dicamba, are extremely sensitive to the herbicide, with reports of cotton injury due to 2,4‐D drift dating back to the time of development [62]. Multiple studies showed that exposure to 2,4‐D resulted in cotton injury with sensitivity increasing at earlier growth stages and higher herbicide concentrations [63–65].

The prime strategy for managing herbicide resistance in weeds is to reduce the selection pressure for resistance evolution by any one selecting agent, while managing adequate weed control [66]. Selection pressure has the greatest impact on herbicide‐resistance evolution and is a factor that growers can control. Selection pressure imposed by an herbicide is the prod‐ uct of efficacy and persistence in the soil [67]. Herbicides applied in crop generally result in the greatest selection pressure compared with other application timings. Selection pressure against a weed population over time, resulting in increasing frequency of resistant individu‐ als that collectively possess one or more resistance mechanisms, is a function of frequency of application [66]. Herbicide sequences, rotations, or mixtures generally have the greatest effect in delaying resistance when the mechanism conferring resistance is target‐based, the weed species are highly self‐pollinated, and seed spread is restricted [68, 69] and herbicide mixtures may delay resistance longer than rotations [70].

The rapid increase in resistant weeds in corn and cotton and the concerns pertaining to the overuse of atrazine in corn, including detection in surface and groundwater, rotational crop injury, and the development of triazine‐resistant weeds, calls for the development of appropri‐ ate and effective management techniques. Also, growing questions about the renewed use of PRE and POST herbicides for early season and possibly season‐long weed control in corn and cotton have also become a major topic of discussion. Therefore, the objective of this research was to evaluate the effect of various PRE and POST herbicides alone and in combinations for crop tolerance and weed control efficacy in the Texas corn and cotton‐producing regions. In cotton, several herbicide programs in glyphosate‐, glufosinate‐, and dicamba‐tolerant cotton were evaluated for their efficacy on both Palmer amaranth and common waterhemp.
