Genetically modified maize
Genetically modified maize refers to varieties of corn (Zea mays) engineered via recombinant DNA technology primarily to confer resistance to insects, such as the European corn borer, or tolerance to herbicides like glyphosate.[1][2] Commercialization began in the mid-1990s, with the first herbicide-tolerant and insect-resistant strains approved for planting in 1996, rapidly expanding to dominate global production due to enhanced agronomic performance.[3][4] In the United States, where maize is a staple crop, adoption rates exceed 90% of planted acreage for varieties incorporating these traits, enabling higher yields—averaging 22% increases—and substantial reductions in insecticide applications by 37%, as evidenced by comprehensive meta-analyses of field trials.[5][6][2] These benefits extend to decreased mycotoxin contamination from pests and lower farmer exposure to chemicals, supporting sustainable intensification amid growing food demands.[2][7] Rigorous assessments by bodies like the National Academy of Sciences conclude that GM maize is as safe for human consumption and the environment as conventional counterparts, with no validated evidence of unique health risks or significant non-target effects after decades of scrutiny and billions of tons consumed.[8][9] Persistent controversies, often amplified by advocacy groups despite contradictory empirical data from peer-reviewed studies, center on unsubstantiated fears of allergenicity, gene flow, or monopolistic seed practices, yet regulatory approvals and long-term monitoring affirm their overall safety and efficacy.[8][7]History
Early research and development
The initial attempts to genetically transform maize (Zea mays) date to 1966, when researchers Coe and Sarkar injected genomic DNA from purple aleurone maize into immature kernels and apical meristems of white aleurone plants, observing transient pigmentation changes but no stable integration or heritability.[10] Subsequent efforts in the 1970s focused on establishing tissue culture systems essential for regeneration, with Green and Phillips in 1975 achieving plant regeneration from inbred line A188 protoplasts, laying groundwork for later genetic manipulation.[11] By 1985, Armstrong and Green developed friable, embryogenic Type II callus cultures using L-proline supplementation, which proved highly responsive to transformation due to their totipotent nature, contrasting with less regenerable Type I calli.[12] The late 1980s marked a surge in DNA delivery techniques, as maize's recalcitrance to Agrobacterium-mediated transformation—due to its monocot physiology—necessitated alternative methods. In 1986, Fromm et al. reported stable nptII gene integration via electroporation of Black Mexican Sweet suspension culture protoplasts, though plants were non-regenerable.[13] That same year, Ohta demonstrated pollen-mediated uptake of exogenous DNA, achieving reported high-efficiency transformation. Particle bombardment emerged as a breakthrough: Klein et al. in 1988 successfully delivered plasmid DNA encoding selectable (nptII) and screenable markers into intact maize suspension cells and Black Mexican Sweet protoplasts using high-velocity microprojectiles, confirming transient gene expression via GUS assays. Rhodes et al. also achieved stable transformation of protoplasts that year, regenerating calli with integrated genes.[14] Stable, fertile transgenic maize plants were first realized in 1990 through biolistic methods on embryogenic cultures. Fromm et al. bombarded SC82 and SC719 genotypes, recovering kanamycin-resistant, fertile R0 plants transmitting the bar herbicide-resistance gene to progeny at Mendelian ratios. Independently, Gordon-Kamm et al. used microprojectile bombardment on Type II calli from elite inbreds like A188 × B73, integrating selectable markers and regenerating fertile plants with stable inheritance.[11][15] These milestones overcame maize's regeneration barriers, enabling targeted trait insertion, though efficiencies remained low (typically <1%) and genotype-dependent, spurring refinements in selectable markers and promoters for broader applicability.[16]Commercial introduction in the 1990s
The commercial introduction of genetically modified maize began in the United States in 1996, following regulatory approvals for insect-resistant varieties engineered to produce Bacillus thuringiensis (Bt) δ-endotoxins targeting lepidopteran pests like the European corn borer. The U.S. Environmental Protection Agency (EPA) approved Monsanto's YieldGard corn (MON 810 event) in 1995, enabling commercial planting the following year, which marked the first widespread adoption of GM maize hybrids.[17] These varieties incorporated the cry1Ab gene from Bt kurstaki, conferring resistance without the need for broad-spectrum insecticides, and were deregulated by the U.S. Department of Agriculture (USDA) after field trials demonstrated efficacy and safety.[18] In parallel, early herbicide-tolerant maize lines emerged toward the late 1990s, with Novartis (now Syngenta) introducing LibertyLink maize tolerant to glufosinate in 1997, followed by Monsanto's glyphosate-tolerant Roundup Ready varieties approved for commercial use in 1998.[19] These traits allowed post-emergence herbicide application, simplifying weed control and integrating with no-till farming practices. Initial planting of GM maize in 1996 covered approximately 1 million hectares in the U.S., representing about 15% of total corn acreage, with rapid expansion driven by yield benefits and reduced pest management costs.[20] By the end of the decade, stacked traits combining Bt insect resistance and herbicide tolerance began entering the market, such as Monsanto's YieldGard Plus in 1999, further accelerating adoption rates to over 25% of U.S. corn acreage.[21] Commercial releases were initially confined to North America, with approvals in Canada mirroring the U.S. timeline, while European and other regions lagged due to stricter regulatory hurdles. Empirical data from early adoption showed yield increases of 5-10% in Bt maize fields under high pest pressure, supporting the technology's economic viability without evidence of widespread ecological disruption in initial assessments.[18]Expansion and adoption post-2000
The adoption of genetically modified maize accelerated markedly after 2000, as farmers in major producing regions selected varieties offering insect resistance, herbicide tolerance, and later stacked traits for improved yields and reduced production costs. In the United States, where GM maize was first commercialized, insect-resistant Bt varieties occupied 19% of corn acreage in 2000, rising to 52% by 2010 and 86% by 2024; herbicide-tolerant varieties increased from 34% in 2000 to 91% by 2023, with stacked traits dominating over 80% of plantings by the mid-2010s.[19][22] This trend extended to South America, where Argentina approved insect-resistant maize in 1998 and achieved widespread adoption equivalent to levels that conventional hybrids took 27 years to reach, within just 13 years.[23] Brazil authorized commercial GM maize planting in 2008, with adoption surpassing 80% of its maize area by 2013 and stabilizing above 88% through 2023, driven by farmer demand for traits addressing regional pest pressures and weed challenges.[24] Paraguay followed suit, reaching 80% GM maize adoption by 2023.[25] Globally, GM maize planted area expanded from approximately 10 million hectares in the early 2000s to 68.4 million hectares by 2023, representing over half of total maize cultivation in adopting countries and comprising the second-largest GM crop category after soybeans.[25] The United States accounted for the largest share at 34.8 million hectares in 2024, or 50.9% of the global total, followed by Brazil and Argentina as key contributors to hemispheric growth.[25] Stacked events integrating multiple traits became standard by the 2010s, facilitating further uptake amid rising demand for efficient crop protection without proportional increases in chemical inputs.[26] Regulatory approvals proliferated, with over 200 maize events authorized worldwide by 2023, enabling tailored varieties for diverse agroecological zones.[27]Genetic Engineering Methods
Transformation techniques for maize
Particle bombardment, or biolistics, represents the earliest and historically dominant technique for maize transformation, involving the acceleration of DNA-coated microprojectiles—typically gold or tungsten particles—into target plant cells using a gene gun device. This physical method bypasses biological barriers to DNA delivery and was first demonstrated to achieve transient gene expression in intact maize suspension culture cells in 1989, followed by stable transformation of embryogenic callus in 1990, enabling the recovery of fertile transgenic plants expressing selectable marker genes like bar for herbicide resistance.[28][29] Biolistics has facilitated the commercial development of numerous GM maize varieties, particularly through bombardment of immature embryos or friable embryogenic callus derived from elite inbred lines, though it often results in multiple transgene copies and potential rearrangements due to random integration. Transformation efficiencies via biolistics vary by genotype and protocol but typically range from 1% to 10% in optimized systems, with osmotic treatments or tissue preconditioning enhancing DNA uptake and stable event recovery.[30] Agrobacterium tumefaciens-mediated transformation, leveraging the bacterium's natural T-DNA transfer mechanism, emerged as a complementary approach for maize in the mid-1990s after overcoming monocot-specific barriers through superbinary vectors, acetosyringone induction of virulence genes, and use of immature zygotic embryos as explants. This method was first reported to yield efficient, low-copy-number transgenic events in the maize inbred A188 in 1996, with protocols co-cultivating embryos for 2-3 days to promote T-DNA integration via the host's wound-response pathways.[31][32] Agrobacterium offers advantages in producing cleaner integration sites compared to biolistics, reducing silencing risks, and has become preferred for stacking multiple traits in commercial pipelines, achieving efficiencies of 4-20% in responsive temperate genotypes like B104, though tropical lines often require further optimization such as sonication-assisted or hydroxyproline-rich glycoprotein enhancements.[33][34] Both techniques rely on subsequent tissue culture regeneration via somatic embryogenesis, with immature embryos (10-14 days post-pollination) as the standard target due to their totipotency, though direct immature embryo systems minimize somaclonal variation. Emerging refinements include baby boom (Bbm) and Wuschel2 (Wus2) morphogenic genes to enable transformation without prolonged callus phases, shortening timelines to 8-12 weeks and broadening genotype applicability beyond highly responsive lines.[35] Other physical methods like electroporation of protoplasts or pollen have been explored but remain less efficient and non-routine for stable maize transgenics. Protocol success hinges on factors including explant quality, selectable agents (e.g., phosphinothricin or hygromycin), and vector design, with peer-reviewed advancements emphasizing genotype-independent tools to accelerate trait deployment in diverse maize germplasm.[16]Key genes and traits engineered
The primary genes engineered into maize (Zea mays) confer traits such as insect resistance, herbicide tolerance, and abiotic stress tolerance, enabling targeted agronomic improvements. Insect resistance typically involves cry genes derived from Bacillus thuringiensis (Bt), which encode delta-endotoxins that disrupt insect midgut function upon ingestion, specifically targeting lepidopteran pests like the European corn borer (Ostrinia nubilalis) and fall armyworm (Spodoptera frugiperda). For instance, the cry1Ab gene in event MON810 produces Cry1Ab protein, providing protection against above-ground lepidopteran larvae.[36] Similarly, cry1F expresses Cry1F protein effective against corn earworm (Helicoverpa zea) and southwestern corn borer (Diatraea grandiosella), though efficacy against some pests like western bean cutworm has declined over time due to resistance evolution.[37][38] Herbicide tolerance is predominantly achieved through modifications to the epsps gene, which encodes 5-enolpyruvylshikimate-3-phosphate synthase, an enzyme in the shikimate pathway targeted by glyphosate. The bacterial cp4 epsps gene from Agrobacterium sp. strain CP4 produces a glyphosate-insensitive variant, as in Roundup Ready maize events like NK603, allowing post-emergence application of glyphosate for broad-spectrum weed control without crop damage.[39] Alternative variants include mepsps (maize-optimized EPSPS) in event GA21 and dgt-28 epsps from Streptomyces sviceus in events like DAS-ز625-8, which confer tolerance via prokaryotic EPSPS enzymes with altered binding affinity.[40] Overexpression of codon-optimized epsps alleles, such as TIPS-OsEPSPS under the ZmUbi promoter, has demonstrated up to threefold tolerance to recommended glyphosate doses in field trials.[41] Abiotic stress tolerance, particularly drought, involves genes like cspB from Bacillus subtilis in event MON87460 (DroughtGard), which encodes cold shock protein B to stabilize cellular processes, RNA chaperoning, and membrane protection under water-limited conditions, resulting in yield preservation during severe drought episodes.[42] This trait integrates with selectable markers like nptII (neomycin phosphotransferase II) for transformation efficiency, though the primary agronomic benefit stems from cspB expression. Stacked constructs often combine these genes, such as cry1A.105 with cry3Bb1 for dual lepidopteran and coleopteran (e.g., corn rootworm) resistance in events like MON88017, or cry1Ab/cry1Ac fusions for broadened spectrum activity.[43] These modifications are introduced via Agrobacterium-mediated transformation or particle bombardment, with expression driven by constitutive promoters like CaMV 35S or maize-specific ubi promoters to ensure tissue-specific or whole-plant efficacy.[44]Major Traits and Varieties
Herbicide-tolerant maize
Herbicide-tolerant maize expresses transgenes that confer resistance to specific broad-spectrum herbicides, permitting post-emergence applications to control weeds while minimizing damage to the crop itself.[45][46] This trait simplifies weed management by allowing farmers to use non-selective herbicides like glyphosate or glufosinate, which inhibit essential plant enzymes or metabolic pathways in susceptible weeds.[47] The primary mechanism for glyphosate tolerance involves the CP4 EPSPS gene derived from Agrobacterium species, which encodes an enzyme variant insensitive to glyphosate's inhibition of the shikimate pathway, essential for aromatic amino acid synthesis.[48] For glufosinate tolerance, the pat or bar gene from soil bacteria like Streptomyces encodes phosphinothricin acetyltransferase, which acetylates and detoxifies the herbicide, preventing its interference with glutamine synthetase and subsequent ammonia accumulation.[49][50] Commercial glyphosate-tolerant varieties, branded as Roundup Ready by Monsanto (now Bayer), include events like GA21 and NK603, with GA21 receiving U.S. regulatory approval in 1997 following petitions by Monsanto and DeKalb Genetics Corporation.[51][52] NK603 maize was approved for cultivation in the U.S. in 2000 and for import into the EU, expressing the CP4 EPSPS protein to enable glyphosate use at rates up to 3.36 kg acid equivalent per hectare without yield penalty.[53] Glufosinate-tolerant LibertyLink maize, developed by Bayer CropScience, features events such as T25, approved in the U.S. in 1998, which incorporates the pat gene for tolerance to glufosinate-ammonium applications up to 1,000 g active ingredient per hectare.[49][54] Stacked traits combining herbicide tolerance with insect resistance, such as NK603 × T25, have also gained approvals, allowing flexibility in herbicide choice.[45] Adoption of herbicide-tolerant maize has been widespread, particularly in the U.S., where it comprised over 90% of planted corn acreage by 2020, often in stacked configurations with Bt traits.[55][56] Empirical data indicate that effective weed control via these traits boosted U.S. maize yields by approximately 3,700 kg per hectare in 2005, equivalent to about $255 per hectare in added value at contemporary prices, primarily through reduced weed competition.[57] Studies attribute yield gains to simplified herbicide regimes replacing multiple narrower-spectrum applications, with pre- and post-emergence glyphosate use enabling no-till practices that preserve soil structure and moisture.[58] However, prolonged reliance on glyphosate has contributed to the evolution of resistance in over 50 weed species globally, including key maize competitors like Amaranthus spp. and Conyza canadensis, necessitating integrated management to sustain efficacy.[59][52] Regulatory assessments, including those by the EPA and EFSA, have confirmed no unintended adverse effects on maize composition or agronomic performance from these modifications, with tolerance limited to the targeted herbicide.[60][48] In regions like South America and Africa, adoption has expanded for smallholder farmers, correlating with reduced labor for manual weeding and herbicide costs, though outcomes vary with local weed pressures and management practices.[61][62] Overall, herbicide-tolerant maize has facilitated shifts toward conservation tillage, cutting fuel and erosion, but demands vigilant resistance monitoring to avoid diminished returns from over-reliance on single modes of action.[63][64]Insect-resistant Bt maize
Insect-resistant Bt maize varieties are genetically engineered to express one or more insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt), which target specific lepidopteran and coleopteran pests by disrupting their midgut epithelium upon ingestion, leading to larval paralysis and death.[65] The primary Bt toxins used include Cry1Ab or Cry1F for lepidopteran pests such as the European corn borer (Ostrinia nubilalis), and Cry3Bb1, mCry3A, or Cry34/35Ab1 for coleopteran pests like the western corn rootworm (Diabrotica virgifera virgifera).[66] These proteins are produced throughout the plant, providing season-long protection without requiring external insecticide applications.[67] The first commercial Bt maize, expressing Cry1Ab for ECB resistance, was developed by Monsanto and approved for planting in the United States in 1996 under the trade name YieldGard.[68] Subsequent varieties incorporated additional toxins, such as Cry3Bb1 introduced in 2003 for rootworm control, and pyramided stacks combining multiple Bt proteins to delay resistance evolution.[66] In Europe, MON 810 (Cry1Ab) was approved in 1998, though adoption has been limited due to regulatory and market factors.[69] Field studies demonstrate near-complete efficacy against ECB, with Bt maize reducing larval tunneling by up to 100% compared to non-Bt controls in Central European trials, resulting in yield increases of 10-15%.[70] For western corn rootworm, initial efficacy exceeded 90% post-2003 introduction, but pooled field data show a decline to about 80% by 2016 due to resistance development in some U.S. Midwest populations.[71] Pyramided traits maintain higher efficacy against resistant strains, with area-wide suppression of ECB populations observed following widespread adoption.[72] Adoption of Bt maize reached over 80% of U.S. corn acres by the mid-2010s, driven by yield gains of 5-24% over non-Bt hybrids in surveys and experiments, alongside reductions in insecticide applications by up to 37% for targeted pests.[73] Globally, Bt maize contributed to cumulative production increases equivalent to 72% from yield benefits and 28% from input savings through 2016.[74] Resistance management strategies, mandated by the U.S. EPA since 1998, include non-Bt refuges (5-20% of acreage) to preserve susceptible alleles and high-dose/refuge pyramids to slow evolutionary adaptation.[67] Despite these, field-evolved resistance to single-toxin traits has occurred in rootworm by 2009 and ECB in isolated cases, attributed to refuge non-compliance and continuous planting without rotation.[75] Scientific reviews find no verified adverse effects on human health from Bt proteins in maize, as they are degraded in the digestive tract and lack toxicity to mammals.[76] Environmentally, Bt maize shows minimal impact on non-target invertebrates compared to insecticide-sprayed conventional maize, with reduced overall pesticide use and no significant changes in soil microbial communities or biodiversity in meta-analyses.[77] However, Bt protein persistence in soil and potential toxicity to aquatic detritivores warrant ongoing monitoring.[78]Drought- and stress-tolerant varieties
Genetically modified maize varieties tolerant to drought incorporate transgenes that enhance physiological responses to water deficit, primarily by stabilizing cellular proteins and maintaining metabolic functions under stress. The leading commercial example is event MON 87460, developed by Monsanto (now Bayer), which expresses the cspB gene derived from Bacillus subtilis. This cold shock protein B protects proteins from denaturation during dehydration, enabling sustained growth and yield when irrigation or rainfall is limited.[79][80] The trait was deregulated by the U.S. Department of Agriculture in 2011 and first marketed as DroughtGard hybrids in 2013, targeting U.S. Corn Belt regions prone to variable precipitation.[81][42] Field evaluations of MON 87460 hybrids have demonstrated empirical yield benefits under managed drought conditions. In rainfed trials across multiple U.S. locations from 2009 to 2012, these varieties maintained grain yields 5 to 10 bushels per acre higher than non-transgenic comparators during severe water stress, equivalent to a 5-8% advantage, while showing no yield penalty in non-stress environments.[80] Independent assessments, including those by the European Food Safety Authority in 2012, confirmed enhanced agronomic performance without unintended compositional changes beyond the intended trait.[42] A 2024 study in sub-Saharan Africa reported MON 87460 hybrids yielding up to 24% more under high-to-severe drought compared to conventional hybrids under low-to-moderate stress, attributing gains to reduced susceptibility rather than maximal yield boosts.[82][83] These outcomes stem from randomized, replicated plots simulating reproductive-stage drought, underscoring causal links between the transgene and resilience via protein homeostasis rather than generalized vigor. Beyond the United States, MON 87460 has been integrated into African breeding programs through the Water Efficient Maize for Africa (WEMA) initiative, a public-private partnership involving Monsanto, CIMMYT, and national seed companies. This project licenses the trait royalty-free for smallholder farmers, combining it with insect resistance for stacked protection in drought-prone regions.[84] Varieties incorporating MON 87460 received regulatory approval for cultivation in South Africa in 2023 under the TELA (Tropicalization of Leading Agronomic traits for drought and Efficiency in Africa) extension of WEMA, enabling commercial release to mitigate yield losses from erratic rainfall.[85] Adoption data indicate limited but growing uptake, with DroughtGard hybrids comprising under 5% of U.S. corn acreage by 2018, reflecting targeted use in water-variable agroecologies rather than widespread replacement of conventional breeding approaches.[86] For broader abiotic stresses beyond drought, such as heat or salinity, commercial GM maize remains scarce, with most advancements confined to research-stage transgenics expressing genes like DREB or NAC transcription factors for multi-stress tolerance.[87] These experimental lines have shown promise in greenhouse and plot trials for combined drought-heat resilience, but lack large-scale field validation or market approval as of 2025, prioritizing single-trait drought engineering for regulatory and economic feasibility. Empirical data from such studies emphasize that GM stress tolerance augments, rather than supplants, agronomic practices like hybrid selection and irrigation management.[88][89]Stacked and specialty traits
Stacked traits in genetically modified maize refer to varieties incorporating multiple transgenic events, typically combining insect resistance (such as Bt toxins targeting lepidopteran and coleopteran pests) with herbicide tolerance (e.g., to glyphosate or glufosinate), achieved through conventional breeding crosses of single-event lines or direct multi-gene transformation.[90] This stacking broadens pest management efficacy and reduces reliance on single-mode interventions, with regulatory assessments confirming compositional equivalence to non-GM counterparts in most cases.[91] For instance, the variety 12-5 × IE034 stacks cry1Ab for insect resistance and epsps for glyphosate tolerance, demonstrating stable inheritance and no unintended genetic disruptions via breeding.[92] Another example is Roundup Ready YieldGard corn, derived from crossing herbicide-tolerant and Bt events, approved for commercial use since the early 2000s.[93] Adoption of stacked GM maize has risen sharply due to enhanced agronomic performance; in the United States, stacked varieties (often combining multiple Bt and herbicide-tolerance traits) comprised 83% of corn acres by 2023, reflecting farmer preferences for integrated resistance profiles amid evolving pest pressures.[56] Globally, stacked events dominate approvals, with over 90% of U.S. corn being genetically engineered overall, predominantly in multi-trait configurations that minimize cross-resistance risks through diversified Bt proteins.[94] Specialty traits in GM maize focus on output modifications altering grain composition for nutritional, feed, or industrial applications, distinct from input traits like pest resistance. Examples include lysine-enriched kernels, where transgenic expression boosts free lysine accumulation, improving swine growth rates, feed conversion, and carcass yields by 5-10% in feeding trials without affecting human safety.[95] Phytase-producing GM maize, engineered to secrete the enzyme in kernels, enhances phosphorus bioavailability in animal diets, reducing supplemental phosphate needs by up to 40% and manure phosphorus excretion, thereby mitigating environmental runoff.[34] These traits undergo rigorous compositional analysis, showing no substantive equivalence deviations beyond the targeted modification, though adoption remains niche (under 5% of total GM maize) due to market segmentation and regulatory hurdles in feed chains.[96] Industrial specialty lines, such as those optimized for starch profiles or enzyme production, support sectors like bioethanol but face limited commercialization outside major producers.[34]Commercialization and Market Dynamics
Approved and marketed products
Numerous genetically modified maize varieties featuring herbicide tolerance, insect resistance, or stacked traits have been approved for commercial marketing worldwide, with the United States hosting the largest market and highest adoption rates exceeding 90% of planted acreage as of 2024.[56][97] These products are developed primarily by companies such as Monsanto (now Bayer), Syngenta, Dow AgroSciences (now Corteva), and Pioneer, and are regulated through safety assessments by agencies including the USDA, EPA, and FDA in the US.[98] Key early commercial products include:| Event Code | Product/Trait | Developer | First Approval (Country) | Primary Markets |
|---|---|---|---|---|
| MON810 | YieldGard (Bt insect resistance against corn borer) | Monsanto | 1996 (USA) | US, Canada, Europe (limited cultivation), Argentina, Brazil |
| NK603 | Roundup Ready Corn 2 (glyphosate herbicide tolerance) | Monsanto | 2000 (USA) | US, Canada, Japan (import), EU |
| GA21 | Roundup Ready Corn (glufosinate herbicide tolerance) | Syngenta | 1998 (USA) | US, Canada, EU |
| Bt11 | Agrisure CB/LL (Bt insect resistance and glufosinate tolerance) | Syngenta | 1996 (USA) | US, Canada, South Africa |
| TC1507 | Herculex I (Bt insect resistance against corn borer and rootworm) | Dow AgroSciences/Pioneer | 2001 (USA) | US, Brazil, Canada |
Global adoption rates and trends
In 2023, genetically modified maize occupied approximately 69.3 million hectares globally, representing about 34% of the total worldwide maize planting area.[107][108] This area increased by 4.68% from the previous year, driven primarily by expansions in major producing countries such as the United States and Brazil.[107] Adoption rates vary significantly by region. In the United States, over 90% of maize acres were planted with herbicide-tolerant or stacked trait varieties in 2024.[56] Brazil and Argentina similarly exhibit high penetration rates exceeding 85% for GM maize hybrids.[107] In contrast, the European Union maintains negligible adoption due to regulatory restrictions, with commercial cultivation limited to trace amounts in a few member states like Spain and Portugal.[109]| Country/Region | Approximate GM Maize Adoption Rate (%) | Year | Source |
|---|---|---|---|
| United States | 90 | 2024 | [56] |
| Brazil | >85 | 2023 | [107] |
| Argentina | >85 | 2023 | [107] |
| South Africa | ~85 | 2023 | [107] |
| European Union | <1 | 2023 | [109] |
Economic scale and projections
In 2024, genetically modified maize occupied approximately 68.4 million hectares globally, accounting for a major share of the 210 million hectares dedicated to all GM crops.[113] This hectarage reflects high adoption rates in leading producers, where GM varieties comprised over 90% of maize plantings in the United States (covering more than 86 million acres) and similar proportions in Brazil and Argentina.[114] [19] In 2020, the most recent year with detailed farm-level data, GM maize spanned 60.77 million hectares, or 33% of total global maize area, with stacked herbicide-tolerant and insect-resistant traits driving the majority of cultivation.[26] Economic benefits at the farm level have been driven by yield gains and input cost reductions, yielding $1.55 billion in net income globally from GM maize in 2020, with the United States contributing 62% of this figure.[26] Cumulative farm income from GM maize technologies reached $20.2 billion from 1996 to 2020, primarily from insect-resistant varieties that boosted production by 47.9 million tonnes in 2020 alone through an average yield increase of 17.7% over the period.[26] These gains, equivalent to $70.50 per hectare in the United States for insect-resistant traits, underscore the technology's role in enhancing productivity without proportional increases in planted area.[26] Projections forecast sustained growth in GM maize adoption, supported by expanded approvals for stacked traits and regulatory progress in emerging markets.[107] The global GMO corn market, valued at $270.2 billion in 2024, is expected to expand to $408.4 billion by 2032, reflecting a 5.3% compound annual growth rate driven by demand for higher-yielding varieties amid population pressures.[115] Continued integration of drought-tolerant and other stress-resistant traits could further amplify these trends, particularly in Africa and Asia, where hectarage is projected to rise with improved biosafety frameworks.[26] [116]Agronomic and Economic Benefits
Yield enhancements and productivity gains
Genetically modified maize varieties engineered for insect resistance, such as Bt maize expressing Bacillus thuringiensis toxins, reduce damage from lepidopteran pests like the European corn borer and Asian corn borer, which can cause yield losses of 10-30% in susceptible conventional varieties. Empirical field trials and meta-analyses indicate that Bt maize typically delivers yield gains of 5-25% compared to non-Bt counterparts under comparable conditions, with higher benefits observed in regions with high pest pressure. For instance, a 2018 analysis of genetically engineered maize hybrids reported an average yield increase of 10.1%, equivalent to 0.7 metric tons per hectare.[2] In Vietnam, adoption of GM maize varieties resulted in yield improvements of 30.4% over conventional equivalents, based on on-farm data from 2015-2018.[117] Herbicide-tolerant maize, enabling the use of glyphosate or glufosinate for post-emergence weed control, minimizes competition from weeds that can reduce yields by 20-50% if unmanaged. Studies in the US Corn Belt have found that genetically modified maize adoption positively impacts yields, with econometric models attributing part of the observed increases to improved weed management practices. A meta-analysis of global GM crop impacts, including maize, estimated average yield enhancements of 22% attributable to GM technologies, driven by both insect resistance and herbicide tolerance traits.[118] These gains are particularly pronounced in stacked traits combining Bt and herbicide tolerance, where synergistic effects from reduced pest and weed pressures compound productivity benefits.[119] Drought- and stress-tolerant GM maize varieties, such as those incorporating the MON87460 trait, sustain yields under water-limited conditions by enhancing water use efficiency and root architecture. Field evaluations in water-stressed environments have shown yield protections of 5-15% relative to conventional maize, helping to stabilize output amid variable rainfall. Overall, while conventional breeding and agronomic improvements contribute to maize yield trends, GM traits have provided measurable incremental gains, with US corn yields rising from approximately 120 bushels per acre in 1996 to over 170 bushels per acre by 2020, partly due to widespread GM adoption exceeding 90% for key traits. Critics, such as the Union of Concerned Scientists, argue that Bt maize contributes only about 4% to US yield increases since the mid-1990s, emphasizing the role of non-GM factors, though broader peer-reviewed syntheses support higher attributions under pest-prone scenarios.[120][121]Pesticide and input reductions
Insect-resistant Bt maize varieties have substantially decreased insecticide applications by expressing Cry proteins toxic to lepidopteran pests such as the European corn borer. In the United States, adopters of insect-resistant maize used 0.013 kg/ha less insecticide on average from 1998 to 2011, representing an 11.2% decline relative to non-adopters.[122] Cumulatively, Bt maize displaced 41 million kilograms of insecticides between 1996 and 2011.[123] Globally, from 1996 to 2020, insect-resistant maize traits reduced insecticide active ingredient use by 85.4 million kilograms, a 41.2% decrease, with a 53.1% reduction observed in 2020 alone compared to conventional varieties.[26] Herbicide-tolerant maize initially contributed to modest reductions in herbicide quantities, with U.S. adopters applying 1.2% less (0.03 kg/ha) from 1998 to 2011 versus non-adopters, alongside a 9.8% lower environmental impact quotient.[122] However, herbicide use trended upward over time among adopters, exceeding non-adopters by 2011, attributable to glyphosate-resistant weeds necessitating alternative or additional applications.[122] A meta-analysis of genetically modified crops, encompassing maize, reported an overall 37% reduction in pesticide use, though herbicide-tolerant traits showed inconsistent quantity decreases.[6] These pesticide reductions have lowered associated input costs, including tractor fuel for spraying. Globally, insect-resistant maize saved 90.2 million liters of fuel from 1996 to 2020 due to fewer applications.[26] In Brazil, where GM insect-resistant maize covers 91% of acreage, farmers reduced spray runs from five to two per crop, yielding cumulative fuel savings of 369 million liters between 2008 and 2020 and a 56% drop in insecticide use.[26] Herbicide-tolerant varieties facilitate reduced tillage, further cutting fuel and labor inputs, though empirical quantification varies by region and practice adoption.[122]Farmer profitability and cost savings
Adoption of genetically modified (GM) maize, particularly insect-resistant (IR) Bt varieties and herbicide-tolerant (HT) traits, has delivered net positive profitability for farmers worldwide through a combination of yield enhancements and production cost reductions. Globally, the use of GM IR maize generated $67.8 billion in additional farm income from 1996 to 2020, equivalent to an average increase of $30 per hectare, with 72% of benefits from higher yields (averaging 17.7%) and 28% from cost savings, primarily reduced insecticide applications totaling 85.4 million kilograms cumulatively.[124][26] HT maize contributed further through lower weed control costs, including reduced tillage and fuel use, yielding additional income of approximately $12.7 billion in major markets like the United States.[26] These gains persist despite elevated seed premiums, as the return on investment averages $3.76 per dollar invested globally, rising to $5.22 in developing countries.[124] In the United States, where GM maize occupies over 80% of planted area, Bt traits provided $34.3 billion in cumulative income gains, with average per-hectare benefits of $81.5 from 7% yield increases and insecticide savings of 38 million kilograms.[26] HT and stacked traits added value via simplified weed management, saving 2,257 million liters of fuel and reducing environmental impacts from tillage. In South America, Brazilian farmers realized $7.86 billion from Bt maize (1996-2020), averaging $53.7 per hectare from yield gains of 4.7-20.1% and fuel savings of 369 million liters, while Argentine producers benefited from HT stacked traits yielding up to $102 per hectare in second-crop systems.[26] European and African contexts highlight benefits for smaller operations. In Spain, Bt maize adoption increased farm income by €50-100 per hectare through 10-20% yield gains and pesticide cost reductions of €20-40 per hectare.[124] South African smallholder farmers experienced up to 32% yield improvements and $93.6 per hectare net gains with Bt white maize, alongside 30-50% drops in pesticide use, amplifying profitability relative to larger commercial farms.[26][124] In the Philippines, Bt maize raised incomes by 20-30%, driven by 20-34% higher yields.[124]| Region/Country | Trait | Avg. Income Gain (USD/ha) | Key Cost Savings |
|---|---|---|---|
| United States | Bt | 81.5 | Insecticide: 38M kg reduced (1996-2020) |
| Brazil | Bt | 53.7 | Fuel: 369M liters saved |
| South Africa | Bt | 93.6 (small farms higher) | Pesticide use: 30-50% lower |
| Argentina | HT Stacked | 102.4 | Tillage/fuel reductions |
| Spain | Bt | ~70 (equiv.) | Pesticide: €20-40/ha |