Weed Resistance to Herbicides in Mexico: A Review

Abstract

Herbicide resistance in weeds has become a critical challenge in worldwide and Mexican agriculture. Many of these cases involve single, cross and multiple resistance to herbicides that inhibit Acetyl CoA Carboxylase (ACCase), Acetolactate Synthase (ALS), Hydroxyphenyl Pyruvate Dioxygenase (HPPD), and Enolpyruvyl Shikimate Phosphate Synthase (EPSPS) enzymes, as well as auxin mimic herbicides. Documented resistance mechanisms include both target-site resistance (TSR) mutations and various forms of non-target-site resistance (NTSR). In wheat and barley, biotypes with resistance to ACCase, ALS, EPSPS and auxins have been confirmed. Maize–sorghum systems show resistance to ACCase, ALS and EPSPS, and in cotton there are glyphosate-resistant populations of Amaranthus palmeri. Citrus orchards remain the focus of glyphosate resistance. Of concern is the advance of multiple resistance in cereals, exemplified by Avena fatua (ACCase + ALS) and Brassica rapa (EPSPS + ALS + auxin mimics). Unique cases, such as EPSPS resistance in Leptochloa virgata and Bidens pilosa and to HPPD in Setaria adhaerens, are unique to Mexico. These resistance patterns underline the need for robust monitoring and detailed study of molecular and physiological mechanisms, where this has not been done, to inform integrated weed management strategies and curb the spread of weeds.

1. Introduction

Herbicide resistance in weeds is the inherited condition of a biotype in a population that significantly differs in response to a herbicide compared to the average response of numerous populations of the same species [1]. Weeds can behave as sensitive (S), naturally tolerant (T), or resistant (R) to herbicides. Species and their S biotypes die when treated with the field dose of a herbicide, while tolerant species do not. On the other hand, under field conditions, R biotypes survive to that recommended. An R biotype is an individual or population of a species that differs from others by surviving the action of a herbicide applied at the recommended dose [2]. Repeated applications of a single herbicide or of different herbicides sharing the same mode of action (MoA) create intense selection pressure on weed populations. Under this pressure, the few resistant individuals that exist at very low frequency survive each treatment and manage to reproduce, while the susceptible majority are eliminated. As long as the population is large and genetically variable, these resistant individuals accumulate in each generation, and the population gradually shifts from being predominantly susceptible to resistant [3].

Two fundamental concepts to understand the phenomenon of plant resistance to herbicides are the mode of action and the mechanisms of action of herbicides. On the one hand, the mode of action is the way in which the herbicide acts on susceptible plants [4] and on the other hand, the mechanism of action is the specific biological process in which a herbicide interferes with the normal development and growth of a susceptible plant [5].

The different resistance mechanisms by which R plants survive a given herbicide are grouped into target site resistance (TSR) and non-target site resistance (NTSR) mechanisms [6]. It is assumed that the resistance mechanisms would be the same as in naturally tolerant crops. Polygenic NTSR involves several mechanisms that reduce the effective binding of the herbicide to its target enzyme, including reduced foliar retention, uptake and translocation, enhanced metabolism, vacuolar sequestration and an ABC (ATP-binding-cassette) transporter-mediated efflux, all of which can maintain herbicide concentration even at its biochemical target but below lethal levels. On the other hand, TSR mechanisms are related with changes in the MoA; that is, genetic changes reflected in the sensitivity of the MoA (enzymes) to herbicides or an overexpression of these genes, resulting in a greater production of the target enzyme [6,7] which allows the R plants to deal with an extra amount of a specific herbicide, which may or may not affect its growth.

Weeds are responsible for significant losses in global agricultural production. If left uncontrolled, they are estimated to reduce the yield of major crops by approximately 34%, and depending on the weed communities, these yield losses can be as high as 56% [8]. Herbicides are the most widely used tools for weed management, representing over 50% of all pesticides applied in agriculture [9]. However, the limited number of herbicide MoA, combined with their intensive and often inappropriate use, has exerted strong selection pressure on weed populations. As a result, the evolution of herbicide-resistant biotypes has become one of the most critical challenges facing modern agriculture, exacerbating yield losses and increasing production costs [10,11,12]. The global economic impact is estimated in billions of dollars annually. For example, in the United States and Canada, uncontrolled weeds in maize can reduce yields by up to 50%, with potential losses reaching $26.7 billion per year [13]. In Mexico, although economic studies are scarce, recent data from citrus production indicate that glyphosate-resistant weeds can raise annual control costs by 33% to 391%, significantly increasing the overall cost of weed management [14].

According to the International Herbicide-Resistant Weed Database (IHRWD), resistance has been documented in 273 weed species worldwide (156 dicotyledonous and 117 monocotyledonous), encompassing 21 of the 31 known MoAs and 168 different herbicides [15]. As of September 2025, the IHRWD reports 15 unique cases of herbicide resistance in Mexico; however, this review identifies four additional cases not yet included in that database, bringing the total to 19 confirmed cases (

Table 1. Cases of herbicide-resistant weeds in Mexico.

No.	Year	Species	Common Name	Crops	MoA 1 (HRAC Group)	Resistance Mechanisms	Reference
1	1996	Phalaris paradoxa	Awned canary-grass	Wheat/barley	ACCase; 1	Point mutations Ile-1781-Leu and Asp-2078-Gly	[15,16]
2	1996	Phalaris minor	Littleseed canarygrass	Wheat/barley	ACCase; 1	Point mutation Gly-2096-Ser	[15,17]
3	1998	Avena fatua	Wild oat	Wheat/barley	ACCase; 1	Point mutation Asp-2041-Gly, Ile–1781–Leu, Trp-1999-Cys, Trp-2027-Cys, Ile-2041-Asn, Asp-2078-Gly, Cys-2088-Arg, Gly-2096-Ala and metabolism	[18,19]
4	2009	Sorghum halepense	Johnson grass	Corn	ALS; 2	Point mutation Asp-376-Glu	[20,21,22]
5	2010	Leptochloa virgata	Tropical sprangletop	Citrus	EPSPS; 9	Reduced uptake and translocation and point mutation Pro-106-Ser	[23,24]
6	2012	Simsia amplexicaulis	Acahualillo	Barley/wheat	Auxin mimic; 4	Unknown	[25]
7	2014	Bidens pilosa	Blackjack	Citrus	EPSPS; 9	Reduced translocation and double mutation Tre-102-Ilo and Pro-106-Ser	[26]
8	2014	Ixophorus unisetus	Mexican grass	Corn	ALS; 2	Increased metabolism and point mutation Asp-376-Glu	[27,28]
9	2016	Eleusine indica	Goose grass	Citrus	EPSPS; 9	Reduced retention and translocation and point mutation Pro-106-Ser	[29]
10	2016	Amaranthus palmeri	Palmer amaranth	Cotton	EPSPS; 9	Reduced translocation and point mutation Pro-106-Ser	[30]
11	2017	Echinochloa crus-galli	Barnyard grass	Wheat	ALS; 2	Unknown	[31]
12	2017	Avena fatua	Wild oat	Wheat	ACCase and ALS; 1 and 2	Point mutation Ile-1781-Leu and Ser-653-Asn	[32]
13	2018	Parthenium hysterophorus	Parthenium weed	Citrus	EPSPS; 9	Reduced uptake and translocation and point mutation Pro-106-Ser	[33]
14	2018	Chloris barbata	Purpletop chloris	Citrus	EPSPS; 9	Reduced translocation and point mutation Pro-106-Ser	[34]
15	2019	Conyza canadensis	Canadian fleabane	Citrus	EPSPS; 9	Reduced translocation and point mutation Pro-106-Ser	[35]
16	2021	Aster squamatus	Annual saltmarsh aster	Citrus	EPSPS; 9	Reduced absorption, translocation and increased metabolism	[36]
17	2023	Brassica rapa	Field mustard	Barley/wheat	Auxin mimic, ALS and EPSPS; 2, 4 and 9	Unknown	[37,38,39]
18	2023	Setaria adhaerens	Bur bristlegrass	Corn	HPPD; 27	Increased metabolism	[40]
19	2023	Helianthus annuus	Sunflower	Sorghum	ALS; 2	Unknown	[15]

¹ Mode of Action.

). While the IHRWD provides a valuable global inventory of resistance cases, it does not address the underlying mechanisms. This review therefore compiles and analyses the resistance mechanisms that have been studied and published by the authors, although such mechanisms have not yet been investigated for all reported cases (Table 1).

The compilation of herbicide resistance cases and their underlying mechanisms in this review followed a structured and systematic approach. Although the IHRWD was used as the primary global reference, not all cases reported in Mexico are included in that database. To address this limitation, a systematic literature search was performed in the Web of Science and Scopus databases using combinations of keywords related to herbicide resistance, weed species, and Mexico. Only peer-reviewed journal articles providing experimental confirmation of resistance were considered, while reports lacking empirical evidence or focusing exclusively on management practices were excluded. In addition, proceedings from the Mexican Society of Weed Science (SOMECIMA) conferences were reviewed to capture cases first reported at the national level [41]. Postgraduate theses were also examined, although only those presenting experimental validation were incorporated. For each case, data were extracted on the resistant species, herbicide(s) involved, MoA(s), and, when available, the resistance mechanism.

Since the first documented cases in the mid 1990s, herbicide resistance in Mexico has expanded to 19 cases by 2025, involving both monocotyledonous and dicotyledonous species across diverse cropping systems. Resistance has been reported to several herbicide MoA, but the most affected is Group 9 (5-enolpyruvylshikimate-3-phosphate synthase, EPSPS inhibitors, glyphosate), which currently accounts for the largest number of resistant species, including Amaranthus palmeri, Bidens pilosa, Aster squamatus, Parthenium hysterophorus, Conyza canadensis, Leptochloa virgata, Chloris barbata, and Eleusine indica. Other important MoAs with multiple resistant species include ACCase inhibitors (Group 1) and ALS inhibitors (Group 2), affecting grasses such as Sorghum halepense, Ixophorus unisetus, Echinochloa crus-galli, Avena fatua and Helianthus annuus. This distribution reflects both the historical intensity of glyphosate use in Mexican agriculture and the recurrent selection pressure imposed by other widely used herbicides, highlighting the urgent need for integrated resistance management strategies (Figure 1).

The main objective of this work is to review for the first time the current status of herbicide resistance in Mexico, where we intend to compile and summarize all known cases of resistance and group them according to the main MoA with cases of herbicide resistance. It is also intended to update and expand the information already recorded in the IHRWD [15], since this survey does not record all cases of resistance, even when these have been published in international peer-reviewed journals. This may be because not all researchers are registered in this database, and this is one of the conditions required to register a new case.

2. Herbicide Resistance in Mexico by Mode of Action

2.1. Acetyl-CoA Carboxylase Inhibitors: Group 1

Resistance to ACCase herbicides has been documented in several grass weed species in Mexico, mainly affecting wheat and barley. Only three weed species have been reported as resistant to ACCase inhibitors in Mexico (Table 1). The first cases were reported in the 1990s in Phalaris minor, Phalaris paradoxa and A. fatua, after intensive use of post-emergent from the three families of ACCase inhibitors: aryloxyphenoxypropionates (FOPs), cyclohexanediones (DIMs) and phenylpyrazolines (DENs) [15,16,17,18,19]. In Phalaris spp. the only resistance mechanism involved is target site resistance (TSR), which is due to point mutations Ile-1781-Leu and Asp-2078-Gly in the ACCase gene and which are capable of conferring cross-resistance to all three ACCase chemical families [16,17]. In A. fatua, resistance is conferred by multiple point mutations in the ACCase gene, including Ile-1781-Leu, Asp-2078-Gly, and Gly-2096-Ala, among others, each of which confers cross-resistance to multiple chemical families [18]. In particular, several biotypes of A. fatua from Guanajuato have accumulated up to seven distinct mutations, some of which do not match in the same individual, indicating multiple independent sources of resistance rather than the spread of a single genotype [19]. An important finding in A. futua is the coexistence of target site resistance (TSR) with non-target site resistance (NTSR). Although target site mutations predominate, metabolomic analyses indicate that enhanced metabolism may also contribute to resistance in some A. fatua populations, this has not yet been fully confirmed [19].

2.2. Acetolactate Synthase Inhibitors: Group 2

Resistance to acetolactate synthase (ALS) inhibitor herbicides is one of the most widespread resistance phenomena worldwide, and Mexico is no exception. Resistance to ALS inhibitors has been confirmed in at least five weed species in Mexico: S. halepense, I. unisetus, E. crus-galli and H. annuus mainly affecting maize, sorghum and wheat production systems (Table 1) [15,20,21,22,27,28,31,32].

A common feature in these cases is the presence of mutations in the ALS gene target site, in particular Asp-376-Glu and Ser-653-Asn, which confer extensive cross-resistance to multiple chemical families within the ALS inhibitor class (sulfonylureas, imidazolinones, triazolinones, triazolopyrimidines-type 1 and pyrimidinylbenzoates). For example, populations of S. halepense from Veracruz showed resistance to five different families of ALS inhibitors, with confirmed TSR mechanisms, indicating a high level of selective pressure and limited herbicide rotation [21,22]. Likewise, biotypes of E. crus-galli with low resistance rates (2.2–3.1) to the mesosulfuron-methyl + iodosulfuron-methyl mixture have been reported from wheat in “El Bajío” (Guanajuato), representing the first confirmation of resistance to ALS inhibitors in this species in Mexico [31]. Importantly, I. unisetus, a native grass species with little global documentation of resistance, showed both TSR and NTSR mechanisms. Increased enhanced metabolism mediated by cytochrome P450 enzymes was shown to contribute to resistance, in addition to the Asp-376-Glu mutation [28]. This mechanism indicates that metabolic resistance is emerging in native Mexican species, possibly driven by the intensive and repetitive use of ALS inhibitors, in particular nicosulfuron and flucarbazone in maize. H. annuus populations were reported in the Heap database as resistant to prosulfuron in sorghum, based on field and greenhouse dose–response assays; however, peer-reviewed publications confirming the mechanism are still lacking, and this case remains only documented in the IHRWD [15].

2.3. Auxin Mimics: Group 4

2,4-D is one of the oldest and most widely used systemic herbicides worldwide since its discovery and commercialization in the 1940s, and it remains a key tool for broadleaf weed control in cereals [42]. Despite its long history of use, the number of confirmed cases of resistance to 2,4-D is lower than for other MoA, but reports have increased in recent decades. In Mexico, resistance to 2,4-D was first documented in 2012 in Simsia amplexicaulis, a weed widely distributed in wheat and barley fields of the Central Highlands. Greenhouse and laboratory dose–response assays confirmed resistance in this species, which has been subjected to over four decades of continuous exposure to auxin mimics (Table 1) [25].

2.4. EPSP Synthase Inhibitors: Group 9

Glyphosate is the most widely used herbicide in Mexico and has played a key role in weed management for decades. However, its overuse has led to the emergence of resistance in at least eight weed species: L. virgata, B. pilosa, E. indica, A. palmeri, P. hysterophorus, C. barbata, C. canadensis and A. squamatus. Most reports concentrate on citrus and cotton production systems [23,24,26,29,30,33,34,35,36]. The first confirmed glyphosate-resistant population (L. virgata, 2010, Veracruz) was followed by near-annual detections, indicating a rapid south-to-north spread from coastal citrus areas.

The main resistance mechanism in these eight weed species is the Pro-106-Ser substitution in the EPSPS gene. In some cases, such as in B. pilosa, a double mutation (Thr-102-Ile + Pro-106-Ser) has been identified [26]. In E. indica and C. barbata, where Pro-106-Ser co-exists with overexpression of the EPSPS gene, showing a stacked resistance mechanism at the target site [29,34]. NTSR mechanisms have also been described, such as reduced translocation and impaired uptake, which often overlap with TSR mechanisms. For example, L. virgata biotypes from Veracruz had reduced uptake along with the Pro-106-Ser mutation. In cotton fields in northern Mexico [24], A. palmeri shows low translocation together with the Pro-106-Ser mutation [30], while in Veracruz (Gulf of Mexico) populations of P. hysterophorus, previously confirmed as tolerant to paraquat [43,44], now show glyphosate resistance with the same Pro-106-Ser mutation and reduced translocation [33]. C. canadensis from Persian lime orchards and A. squamatus in citrus offer more diverse combinations of resistance mechanisms: the former survives with reduced translocation plus a mutation in the EPSPS gene of Pro-106-Ser, and the latter shows markedly reduced uptake and rapid metabolic detoxification, the basis of its characteristic regrowth, known as the “Phoenix Phenomenon” [35,36,45].

2.5. HPPD Inhibitors: Group 27

Resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors is much less common globally than resistance to ACCase, ALS or glyphosate. However, Mexico harbours a unique example in Setaria adhaerens. Field control failures with tembotrione and mesotrione were first recorded in 2023 in irrigated maize in “El Bajío” (Guanajuato-Querétaro-Michoacán) (Table 1), a region where post-emergence grass control relies almost exclusively on these two HPPD herbicides [40]. Pretreatment with malathion, a cytochrome P450 inhibitor, largely restored tembotrione efficacy, and metabolic profiling detected hydroxylated metabolites (M1/M1′) and their glucose conjugated (M3, M5), which are characteristic detoxification products of HPPD-inhibiting herbicides. These results point to a purely non-selective resistance mechanism (NTSR) driven by enhanced P450-mediated detoxification, with no amino acid substitutions detected at the HPPD target site [46].

S. adhaerens remains the only weed with confirmed resistance to HPPD and no similar cases have been reported elsewhere. This resistance highlights the risk of relying on a single postemergence MoA in maize, particularly when pre-emergence residual treatment programs are underutilized [47].

3. Weeds Resistant to Multiple Herbicides

Resistance to a single MoA limit weed management options in several crops, the situation becomes more complicated when multiple resistance occurs, i.e., when more than one MoA is simultaneously compromised in the same population or when one MoA is completely nullified due to cross-resistance. Two well-documented cases show how quickly Mexican weeds can develop these types of resistance.

3.1. Avena Fatua: Resistance to ACCase (Group 1) and ALS Inhibitors (Group 2)

In the wheat and barley growing region of “El Bajío”, self-pollinated A. fatua gradually accumulated resistance (Table 1). ACCase-resistant lines (Ile-1781-Leu or Asp-2078-Gly) were exposed for more than a decade to ALS inhibitory mixtures; by 2017 some biotypes had the Ile-1781-Leu mutation in ACCase together with Ser-653-Asn in ALS [32]. These dual changes in TSR mechanisms now confer resistance of biotypes to all three ACCase families (FOPs, DIMs, DENs) and major ALS chemistries (sulfonylureas, triazolopyrimidines), leaving agronomists without the main tools for weed management in these cropping systems. Metabolomic changes in the same populations suggest additional detoxification pathways, but these non-target compounds have not yet been fully characterized [32]. The presence of a dual TSR mechanism in a self-pollinated species indicates sequential selection or gene flow between lines that were previously resistant to a single MoA [3,48].

3.2. Brassica Rapa: Multiple Resistance to ALS Inhibitors (Group 2), Auxin Mimics (Group 4), and EPSPS Inhibitors (Group 9)

Studies conducted in 2021–2023 in barley, wheat and maize fields in the states of Tlaxcala and Hidalgo showed B. rapa plants surviving after application of glyphosate, 2,4-D and a commercial mixture of mesosulfuron-methyl + iodosulfuron-methyl (MES/IMS) within the same production fields [37,38,39]. Dose–response assays confirmed that these populations show cross-resistance to Group 2 herbicides (ALS inhibitors, such as MES/IMS and prosulfuron), Group 4 (auxin mimics, such as 2,4-D, dicamba and fluroxypyr) and Group 9 (glyphosate) [37,38,39]. The physiological and molecular basis of this multiple resistance is so far unknown. Until this data becomes available, the case is evidence that B. rapa can develop triple cross-resistance under intensive selection of herbicides widely used in these crops.

4. Practices and Needs in Resistance Management

In Mexico, the sustained use of herbicides has inevitably favoured the selection of resistant weed biotypes. However, the true extent of the problem remains unknown, as systematic studies and detailed field characterisations have largely not been conducted [49]. Unlike other countries, Mexico does not currently have an official register of herbicide-resistant weeds, and the few cases that have been confirmed are usually reported in local scientific forums (at SOMECIMA) rather than being consolidated in peer-reviewed publications [50]. This lack of systematic information makes it even more important to highlight the cases of resistance that have been adequately documented in Mexico, some of which are of global significance.

Herbicide-resistant weeds in Mexico exhibit resistance mechanisms already documented in the other regions [15]. However, several cases are unique to the country, in particular glyphosate resistance in L. virgata, B. pilosa, A. squamatus and C. barbata, as well as the world’s first reported case of HPPD resistance in S. adhaerens [23,26,34,36,51]. These cases highlight the need for crop-specific resistance management and control plans adapted to local conditions. In the case of resistant weeds in citrus crops, dependence on postemergence treatments with glyphosate, glufosinate and paraquat, will end up in more resistant weeds, like in the case of P. hysterophorus which is resistant to glyphosate and tolerant to paraquat or other species resistant to glyphosate could in time evolve resistance to glufosinate or other postemergence herbicides [33]. The combination of cultural practices like periodic mowing in the alleys and the use of residual preemergence herbicides under the shade area of citrus trees, could reduce pressure of postemergence herbicides [52].

The highest concentration of glyphosate-resistant weeds is found in citrus, particularly in Veracruz (Figure 1). Partial replacement of glyphosate with mixtures of glufosinate + indaziflam or glufosinate + saflufenacil reduced Leptochloa and Bidens R population densities by 75–90%, although their widespread adoption remains limited due to the cost of these combinations relative to glyphosate [14]. In citrus orchards, the introduction of live cover crops such as legumes (Arachis pintoi, Neonotonia wightii) significantly reduce weed emergence by shading the area between tree rows, as well as providing nitrogen (67–209 kg N ha⁻¹) due to the nitrogen fixation process of these plants [53]; and complemented by rotational grazing with sheep to keep the cover crop low. Another technique to be used is mulching of prunings, which delays weed emergence over time and helps to retain water in the soil [54]. These management options, combined with selective applications of residual herbicides (amicarbazone, diuron) in the drip zone, offer a way to reduce dependence on glyphosate [14].

In “El Bajío” cereals, A. fatua biotypes combine ACCase and ALS mutations, which restricts post-emergence options [32]. False sowings induce weed seed germination prior to sowing a crop, allowing for mechanical or chemical control [55]. Increased sowing density, closer spacing and altered row orientation help crops such as wheat (Triticum aestivum), barley (Hordeum vulgare), oat (Avena sativa), rye (Secale cereale), chickpea (Cyclotus aestivum), chickpea (Cicerne sativa), chickpea (Cicer arietinum), broad bean (Vicia faba), lupin (Lupinus angustifolius), field pea (Pisum sativum) and oilseed rape (Brassica napus) have increased competition with weeds by reducing their biomass and fertility [56]. While hand-rogueing of B. rapa, A. fatua or Phalaris spp. escapes prevents the exponential increase in the seed bank in cereal production systems. For B. rapa, with triple resistance (ALS + auxins + EPSPS), triasulfuron, propoxycarbazone, flucarbazone-Na and saflufenacil still offer control when integrated into diversified programmes [57,58]. The use of weed-free seeds and wheat-barley-corn rotation increases pre-emergent and post-emergent chemical alternatives, thus breaking selection pressure [59,60].

In maize, the transition from tolerance to full resistance to HPPD in S. adhaerens that has been favoured by the repeated use of tembotrione and given the limited range of post-emergent grass herbicides available for maize, makes it necessary to use pre-emergents at planting time such as dimethenamid, s-metolachlor, acetochlor, propisochlor or pethoxamid combined with a residual PS II for broadleaf management.

The relationship between resistance mechanisms and the emergence of cross and multiple resistance is becoming increasingly evident in Mexico. Target-site mutations in the ALS gene, such as Asp-376-Glu detected in S. halepense and I. unisetus, confer cross-resistance to several chemical families within Group 2 [21,27,28,61]. Similarly, I. unisetus and S. adhaerens exhibit enhanced metabolism mediated by cytochrome P450s, a non-target-site mechanism that not only explains resistance to ALS and HPPD inhibitors but also predisposes populations to broader multiple resistance [28,46]. The cases of A. fatua, resistant to both ACCase and ALS inhibitors and B. rapa resistant to ALS, EPSPS, and auxin mimics, show how resistance to multiple MoA limits postemergence control options and facilitates the persistence and spread of resistant biotypes in cereal production systems [32,37]. These findings highlight that resistance in Mexico is not limited to isolated cases but is progressing towards complex patterns shaped by the interaction of TSR and NTSR, underscoring the need for integrated weed management (IWM) strategies adapted to multiple resistance scenarios.

Comparable patterns have been documented in other regions with intensive herbicide reliance. In the United States and Brazil, the convergence of TSR and NTSR mechanisms particularly enhanced metabolic detoxification has accelerated the evolution of multiple resistance in problematic weeds such as A. palmeri and Conyza spp. [6,62,63]. This has reduced the useful life of several important herbicide MoA. In Mexico, the number of confirmed resistant cases is still lower, but recent findings of metabolic resistance in native species like I. unisetus and S. adhaerens indicate that similar processes may develop if dependence on a limited set of herbicides continues. In this context, Mexico has a great opportunity, as resistance is still in the early stages of recognition, making it essential to proactively monitor and diversify management strategies before resistance spreads as widely as in other countries [15].

The cases described in this paper confirm that prolonged reliance on a few MoA leads first to cross-resistance and then to multiple resistance, with adverse effects on the cost-effectiveness, chemical load on the environment, and sustainability of herbicides.

Programmes combining pre-emergent residual herbicides, crop rotation, competitive planting densities, live cover, mulching, grazing and early escape elimination help reduce the frequency of resistant biotypes in regional trials [53,54,55,56,57,58]. Adoption of these practices, along with routine resistance monitoring, is essential to prolong the life of herbicides and mitigate the spread of resistant biotypes in Mexico. However, the implementation of these strategies is hindered by the limited research capacity in the country, where herbicide resistance has not been prioritised and monitoring remains scarce, despite the intensive use of herbicides in Mexican agriculture. The scarcity of technical information, the limited number of specialists, and inadequate public funding have slowed progress, while most industry-funded studies focus exclusively on product efficacy. Consequently, many confirmed cases mainly glyphosate resistance in citrus have been reported by a single research group, and most other cases rely solely on dose–response assays, leaving the underlying resistance mechanisms unresolved [15]. The lack of laboratories specializing in herbicide resistance further limits national surveillance capacity, highlighting the need to increase investment in this area and improve infrastructure to promote research and strengthen resistance management. These limitations reveal that resistance in Mexico is still in an early stage of recognition compared to other countries, making it important to anticipate future challenges and develop strategies to mitigate the emergence of resistance.

5. Future Challenges

Future efforts must not only document new cases of resistance, but also address the broader challenges of prevention, monitoring, and integration of alternative practices into Mexican cropping systems.

This review provides data for the first time that should help raise awareness of the status of resistance and the threat it poses to agriculture in Mexico. This review reports 19 cases of resistance compared to the 15 reported in the Heap database [15]. The most affected MoA is group 9 (EPSPS inhibitor), with 8 cases reported in the last 15 years (Table 1), representing 42% of the total reported in this review. These data are worrying and highlight the need to take proactive measures to mitigate this trend.

In addition to these data, new foci of resistance are emerging. In rainfed cereals, multiple resistance of B. rapa to ALS and EPSPS inhibitors and auxin mimics has been confirmed, and the mechanisms involved are being investigated. In addition, the presence of unreported cases of herbicide resistance that have not yet received scientific attention is recognized. Greenhouse trials are currently underway to confirm the first prosulfuron- and 2,4-D-resistant biotype of Bidens odorata and S. amplexicaulis from Tlaxcala, Mexico and of tembotrione-resistant Hyparrhenia variabilis from Jalisco, Mexico (Dominguez-Valenzuela, personal communication). It is also expected that more cases of resistance will be found in rainfed cereals in Tlaxcala. These findings suggest that more cases will appear in rainfed cereals in the High Valleys of Mexico in the coming years, reinforcing the need for continued surveillance.

Looking ahead, a central challenge will be to reduce selection pressure by limiting the overreliance on herbicides and placing greater emphasis on IWM [64]. Nonchemical and cultural approaches should not be used merely to compensate for resistant escapes, but rather to promote long-term seedbank depletion through greater crop diversity, preventive measures, and systematic monitoring of weed populations [65]. Incorporating precision agriculture and decision-support tools can also help optimize herbicide inputs and alleviate selection pressure [66]. These approaches, combined with stronger investment in research and infrastructure, will be critical to developing sustainable and locally adapted strategies to contain the spread of resistance in Mexico.

Herbicide resistance poses an increasing threat to crop production in Mexico by constraining management options and raising production costs. Strengthening research on resistance mechanisms is essential to anticipate the evolution of new resistant biotypes and to develop effective strategies tailored to national cropping systems. Prevention remains the most cost-effective and sustainable approach, and its success relies on the adoption of integrated weed management programmes that promote crop diversity, reduce herbicide dependence, and minimise environmental impacts.