Farmers planted 0.55 billion hectares (1.37 billion acres) of HR corn, soybeans, and cotton from 1996 through 2011, with HR soybeans accounting for 60% of these hectares [Additional file 1: Table S7]. In terms of overall herbicide use per hectare based on NASS data, substantial increases have occurred from 1996 through 2011. In soybeans, USDA reported herbicide applications totaling 1.3 kgs/ha (1.17 pounds/acre) in 1996, and 1.6 kgs/ha (1.42 pounds/acre) in 2006, the last year soybeans were surveyed by USDA. In cotton, herbicide use has risen from 2.1 kgs/ha (1.88 pounds/acre) in 1996 to 3.0 kgs/ha (2.69 pounds/acre) in 2010, the year of the most recent USDA survey. In the case of corn, herbicide use has fallen marginally from 3.0 kgs/ha (2.66 pounds/acre) in 1996 to 2.5 kgs/ha (2.26 pounds/acre) in 2010, largely as a result of lessened reliance on older, high-rate herbicides.

Compared to herbicide use rates per hectare on non-HR hectares, HR crops increased herbicide use in the U.S. by an estimated 239 million kgs (527 million pounds) in the 1996–2011 period, with HR soybeans accounting for 70% of the total increase across the three HR crops. Rising reliance on glyphosate accounted for most of this increase.

In light of its generally favorable environmental and toxicological properties, especially compared to some of the herbicides displaced by glyphosate, the dramatic increase in glyphosate use has likely not markedly increased human health risks. Because glyphosate cannot be sprayed on most actively growing, non-GE plants, residues of glyphosate in food have been rare, at least until the expansion ~ 2006 in the number of late-season glyphosate applications on wheat and barley as a harvest aid and/or to control escaped weeds. Presumably as a result of such uses, 5.6% of 107 bread samples tested in 2010 by the U.K. Food Standards Agency contained glyphosate residues [9]. Three samples had 0.5 parts per million of glyphosate [9], a relatively high level compared to the other pesticides found in these bread samples.

Budget pressures have forced the U.S. Department of Agriculture to reduce the number of crops included in its annual NASS pesticide use survey. Soybean pesticide use has not been surveyed since 2006, about when the spread of glyphosate-resistant weeds began to significantly increase herbicide use in selected areas. Herein, total herbicide use on HR hectares is projected to rise 13.5% from 2006–2011 (about 2.7% annually), compared to a 6.6% (1.3% annually) increase on conventional soybean hectares. By way of contrast, the NASS-reported glyphosate rate of application per crop year on the average hectare of soybeans increased 8.9% per annum from 2000–2006 (see Table 1). So, despite the significant and widespread challenges inherent in managing glyphosate-resistant weeds in the 2006–2011 period, a substantial decrease is projected in the rate of increase in glyphosate applications per hectare of HR soybeans. The justification for this projected fall in the rate of increase is recognition by farmers that further increases in glyphosate use will likely not prove cost-effective, coupled with positive responses by farmers to the near-universal recommendation that corn-soybean farmers incorporate into their spray programs herbicides that work through modes of action other than glyphosate’s [10–15].

Table 1 Projected rates of change in herbicide use since the most recent USDA survey, relative to recent annual percent changes in rates Full size table

Since 1996, about 317 million trait hectares (782 million trait acres) have been planted to the three major Bt traits – Bt corn for European corn borer (ECB) and CRW, and Bt cotton. Bt corn and cotton have delivered consistent reductions in insecticide applications totaling 56 million kgs (123 million pounds) over 16 years of commercial use. Bt corn reduced insecticide use by 41 million kgs (90 million pounds), while Bt cotton displaced 15 million kgs (34 million pounds) of insecticide use.

Taking into account applications of all pesticides targeted by the traits embedded in the three major GE crops, pesticide use in the U.S. was reduced in each of the first six years of commercial use (1996–2001). But in 2002, herbicide use on HR soybeans increased 8.6 million kgs (19 million pounds), driven by a 0.2 kgs/ha (0.18 pounds/acre), increase in the glyphosate rate per crop year, a 21% increase. Overall in 2002, GE traits increased pesticide use by 6.9 million kgs (15.2 million pounds), or by about 5%. Incrementally greater annual increases in the kilograms/pounds of herbicides applied to HR hectares have continued nearly every year since, leading to progressively larger annual increases in overall pesticide use on GE hectares/acres compared to non-GE hectares/acres. The increase just in 2011 was 35.3 million kgs (77.9 million pounds), a quantity exceeding by a wide margin the cumulative, total 14 million kg (31 million pound) reduction from 1996 through 2002.

Total pesticide use has been driven upward by 183 million kgs (404 million pounds) in the U.S. since 1996 by GE crops, compared to what pesticide use would likely have been in the absence of HR and Bt cultivars. This increase represents, on average, an additional ~0.21 kgs/ha (~0.19 pounds/acre) of pesticide active ingredient for every GE-trait hectare planted. The estimated overall increase of 183 million kgs (404 million pounds) applied over the past 16 years represents about a 7% increase in total pesticide use.

There are two major factors driving the upward trend in herbicide use on HR hectares compared to hectares planted to non-HR crops: incremental reductions in the application rate of herbicides other than glyphosate applied on non-HR crop hectares, and second, the emergence and rapid spread of glyphosate-resistant weeds. The first factor is driven by progress made by the pesticide industry in discovering more potent herbicidal active ingredients effective at progressively lower rates of application.

Twenty-seven percent of U.S. soybean hectares in 1996 were treated with pendimethalin at an average rate of 1.1 kgs/ha and another 22% were sprayed with trifluralin at a rate of 0.99 kgs/ha, while the market leader (imazethapyr) was applied to 43% of hectares planted at a rate of 0.07 kgs/ha [16]. By 2002 the combined percentage of soybean hectares treated with these two high-dose herbicides had dropped from 49% to 16% [17], and just 5% were treated in 2006 [18]. Between 1996 and 2006, the number of registered soybean herbicides applied at rates below 0.11 kgs/ha increased from nine to 17. As a result, the amount of herbicides applied to conventional crops has steadily fallen since 1996. In contrast, glyphosate is a relatively high-dose herbicide that is usually applied at a rate between 0.67 to 0.9 kgs per hectare.

Resistant weeds

The emergence and spread of glyphosate-resistant weeds is the second, and by far most important factor driving up herbicide use on land planted to herbicide-resistant varieties. Glyphosate resistant (GR) weeds were practically unknown before the introduction of RR crops in 1996. The first glyphosate-resistant weed (Lolium rigidum) emerged in Australia in 1996 from canola, cereal crop, and fence line applications [19]. In the mid-1990s, as the first glyphosate-resistant crops were moving toward commercialization and gaining market share, Monsanto scientists wrote or were co-authors on several papers arguing that the evolution of GR weeds was unlikely, citing the herbicide’s long history of use (~20 years) and relative absence of resistant weeds [20, 21].

Other scientists, however, challenged this assertion [22]. Dr. Ian Heap, long-time manager of the international database on resistant weeds, warned in a 1997 conference presentation that to limit glyphosate selection pressure in Roundup Ready cropping systems, the herbicide would need to be used in conjunction with proven resistance-management practices and with non-chemical weed control methods [23]. A 1996 report by Consumers Union stated that HR crops are “custom-made” for accelerating resistance and called for the Environmental Protection Agency (EPA) to revoke approval of HR crops when and where credible evidence of resistance emerges [24].

Today, the Weed Science Society of America (WSSA) website lists 22 GR weed species in the U.S. [19]. Over two-thirds of the approximate 70 state-GR weed combinations listed by WSSA have been documented since 2005, reflecting the rapidly spreading nature of the GR-weed problem. According to the WSSA, over 5.7 million hectares (14 million acres) are now infested by GR weeds, an estimate that substantially underestimates the actual spread of resistant weeds [16, 22], [and personal communication, Dr. Ian Heap]. Dow AgroSciences carried out a recent survey on the percent of crop acres/hectares in the U.S. impacted by glyphosate-resistant weeds [25]. Findings from the survey were provided to USDA in support of Dow AgroSciences’s petition for deregulation of 2,4-D herbicide-resistant corn, and suggest that around 40 million hectares (100 million acres) are already impacted by glyphosate-resistant weeds, an estimate that Heap considers inflated [personal communication]. The true extent of spread in the U.S. likely lies around the midpoint between the WSSA and Dow AgroSciences estimates (i.e., 20–25 million hectares), and by all accounts, will continue to rise rapidly for several years.

Why have GR weeds become such a serious problem? Heavy reliance on a single herbicide – glyphosate (Roundup) -- has placed weed populations under progressively intense, and indeed unprecedented, selection pressure [10]. HR crops make it possible to extend the glyphosate application window to most of the growing season, instead of just the pre-plant and post-harvest periods. HR technology allows multiple applications of glyphosate in the same crop year. The common Midwestern rotation of HR corn-HR soybeans, or HR soybeans-HR cotton in the South, exposes weed populations to annual and repetitive glyphosate-selection pressure.

These factors trigger a perfect storm for the emergence of GR weeds. Research has traced the resistance mechanism in Palmer amaranth (Amaranthus palmeri) to 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene amplification. Resistant weed populations from Georgia contained 5-fold to 160-fold more copies of the EPSPS gene, compared to susceptible plants [26]. Moreover, EPSPS gene amplification is heritable, leading Gaines et al. to warn that the emergence of GR weeds “endangers the continued success of transgenic glyphosate-resistant crops and the sustainability of glyphosate as the world’s most important herbicide.”

Resistant Palmer amaranth (Amaranthus palmeri) has spread dramatically across southern states since the first resistant populations were confirmed in 2005, and already poses a major economic threat to U.S. cotton production. Some infestations are so severe that cotton farmers have been forced to leave some crops unharvested.

Responding to resistance

GR weed phenotypes are forcing farmers to respond by increasing herbicide application rates, making multiple applications of herbicides, applying additional herbicide active ingredients, deep tillage to bury weed seeds, and manual weeding. In recent years the first three of the above responses have been the most common. Each response increases the kilograms of herbicides applied on HR crop hectares. All five interventions increase costs. Moreover, if 2,4-D and dicamba herbicide-resistant corn and soybeans are fully deregulated by the U.S. government, there will be growing reliance on older, higher-risk herbicides for management of glyphosate-resistant weeds.

Based on an upward trajectory in the planting of 2,4-D HR corn reaching 55% of corn hectares planted by 2019, coupled with an average of 2.3 applications (the label allows three) and an average rate of 0.94 kgs/ha (0.84 pounds/acre) (the label allows 1.12 kgs/ha (1.0 pounds/acre)), 2,4-D use on corn in the U.S. would increase over 30-fold from 2010 levels [Additional file 1: Table S19]. Such a dramatic increase could pose heightened risk of birth defects [27, 28] and other reproductive problems [29], more severe impacts on aquatic ecosystems [30], and more frequent instances of off-target movement and damage to nearby crops and plants. Moreover, the efficacy of 2,4-D corn may well prove short lived, since a population of 2,4-D resistant waterhemp (Amaranthus tuberculatus) has now been confirmed in Nebraska [31], and there are already at least eight other weeds resistant to 2,4-D [19].

GR weeds typically emerge first on a few isolated fields, but their pollen, genes, and seeds can travel widely and spread quickly, especially if glyphosate continues to be relied on heavily [11]. No substantial change in the intensity of glyphosate use in the U.S. is expected in the foreseeable future; nearly all corn, soybean, and cotton cultivars now carry a RR gene. The seed industry has no plans to grow and sell more non-HR seed, and indeed is moving in the opposite direction by developing more stacked, multiple HR varieties. The share of total national corn, soybean, and cotton hectares impacted by GR weed populations is likely to grow and will, as a result, increase both the number of different herbicides applied, as well as the total kgs of herbicides applied.

As argued by many weed scientists and extension specialists, integrated weed management systems, coupled with markedly lessened reliance on RR technology are now essential to extend the useful life of RR technology [10, 12, 14, 32]. Without major change, a crisis in weed management systems is likely, triggering possibly ominous economic, public health, and environment consequences.

Higher costs triggered by resistant weeds and HR technology

Weed management costs per hectare increase by 50% to 100% or more in fields infested with glyphosate-resistant weeds, as evident in a series of case studies submitted to the USDA by Dow AgroSciences in support of its petition to the USDA seeking deregulation of 2,4-D herbicide-resistant corn [25]. In soybean production in Arkansas, for example, Dow AgroSciences compared the average cost/acre of the top-five, most popular herbicide programs in Roundup Ready soybeans in fields without resistant weeds, compared to the average of two recommended programs in fields infested with glyphosate-resistant Palmer amaranth. Herbicide costs rise 2.7-fold (from $16.29 to $44.34 per acre) [23], [Table thirty, page 93]. In Illinois soybean production, the increase in herbicide costs is estimated at 64% ($19.21 to $31.49 per acre) [23], [Table thirty-two, page 95], while in Iowa corn production, the increase is 67% ($19.23 to $32.10 per acre) [23], [Table thirty-six, page 99].

The markedly higher cost/hectare of herbicide-resistant seeds must be added to the higher herbicide costs noted above to more fully reflect the added costs associated with HR technology. The cost of a bushel of conventional, not-GE soybean seed increased during the GE-crop era from $14.80 in 1996 to $33.70 in 2010, while a bushel of GE soybean seed cost, on average, $49.60 in 2010 (all seed price data derived from USDA data) [33]. Accordingly, the cost of GE soybean seed in 2010 was 47% higher per bushel than non-GE seed. In the case of corn, conventional seed prices rose from $26.65 per acre planted in 1996 to $58.13 in 2010. The average cost of GE corn seed per acre in 2010 was $108.50, with some GE cultivars selling for over $120 per planted acre. Hence, GE corn seed costs per acre were about double the cost conventional seed.

Public health concerns

Heightened risk of public health impacts can be expected in the wake of more intensive herbicide use, especially applications later in the season on herbicide-resistant crop varieties. While current risk assessment science suggests that glyphosate is among the safer herbicides per hectare treated in terms of human health risks, both the frequency of human exposures and levels of exposure via food, drinking water, and the air have no doubt risen in the U.S. in recent years. Two-thirds to 100% of air and rainfall samples tested in Mississippi and Iowa in 2007–2008 contained glyphosate [34].

The likely approval and use of herbicide-resistant crops in the U.S. engineered to survive applications of multiple herbicides adds tricky new dimensions to herbicide-risk assessments. Applications later in the growing season will be more likely to lead to residues in silage or forage crops. As a result, herbicide residues in milk, meat, or other animal products might become more common. The jump in herbicide volumes applied during June and July will increase the risk of drift and herbicide movement via volatilization, possibly exposing people via the air, water, or crops grown in the proximity of treated fields. Risks from the drift and volatilization of 2,4-D and dicamba are of special concern, given that these two herbicides have triggered thousands of non-target crop damage episodes over the last 20 years in the U.S. Indeed, for several years, 2,4-D has been the leading cause of crop damage episodes investigated by State departments of agriculture [35].

Environmental impacts linked to HR technology

A long list of environmental effects can be triggered, or made worse, by the more intensive herbicide use required to keep pace with weeds in farming systems heavily reliant on herbicide-resistant crops. Glyphosate has been shown to impair soil microbial communities in ways that can increase plant vulnerability to pathogens [36–38], while also reducing availability of certain soil minerals and micronutrients [39]. Landscapes dominated by herbicide-resistant crops support fewer insect and bird species; e.g., a study in the American Midwest reported a 58% decline in milkweed and an 81% drop in monarch butterflies from 1999 to 2010 [40]. Heavy use of glyphosate can reduce earthworm viability [41] and water use efficiency [42]. Several studies have documented reductions in nitrogen fixation in herbicide-resistant soybean fields sprayed with glyphosate [43, 44]. Transgene flow from herbicide-resistant crops can occur via multiple mechanisms and can persist in weedy relatives [45].

Individually, these environmental impacts appear, for the most part, of the same nature and in the same ballpark as the risks associated with other herbicide-based farming systems, but collectively they raise novel concerns over long-term, possibly serious impacts on biodiversity, soil and plant health, water quality, aquatic ecosystem integrity, and human and animal health.

Bt corn and cotton impacts and prospects

While Bt-transgenic corn and cotton have displaced an estimated 56 million kgs (123 million pounds) of insecticides since 1996, every plant in a Bt corn or cotton field is manufacturing within its cells one or more forms of the natural bioinsecticide Bacillus thuringiensis. The rate of synthesis of Bt Cry protein endotoxins is roughly proportional to the rate of plant growth. As plants mature and enter senescence, Bt endotoxin expression falls.

Few published estimates are available of Bt endotoxin expression levels in contemporary corn cultivars. Nguyen et al. projected that a hectare of Bt-corn for CRW control expressing the Cry3Bb1 gene in MON88017 corn produces 905 grams of Cry3Bb1 per hectare (0.8 pounds per acre) [46]. The amount of Bt Cry proteins produced by a hectare of Bt corn for ECB and CRW control are calculated in [Additional file 1: Tables S20–S22], with key results shown in Table 2 for specific corn events, traits, and endotoxins. [Additional file 1 Tables S23–25] cover Bt cotton events. Expression level data reported by companies in regulatory documents were used to calculate per hectare production of specific endotoxins. [Additional file 1: Tables S21 and Table S24 contain the expression level data for Bt corn and cotton events, and [Additional file 1: Table S22 and Table S25] report the volumes of Bt Cry proteins produced per hectare and acre based on contemporary seeding rates.

Table 2 Bt cry protein synthesis in major GE corn cultivars Full size table

Major contemporary Bt corn events targeting the ECB synthesize nearly as much or more insecticidal Cry protein per hectare than the weighted-average rate of conventional insecticides applied on a hectare planted to Bt corn for ECB control (about 0.15 kgs insecticide per ha; 0.13 pounds/acre in 2010 [Additional file 1: Table S11]). MON810, the Cry protein in Monsanto’s original Yieldgard corn, expresses 0.2 kgs/ha of endotoxin, whereas Syngenta’s Bt 11 synthesizes 0.28 kgs/ha [Additional file 1: Table S22]. Newer events for ECB control like Monsanto’s Genuity VT Double PRO (MON 89034) produce Cry 1A.105 and Cry 2Ab2 endotoxins totaling 0.62 kgs/ha. The Dow AgroSciences-Pioneer Hi-Bred Herculex I (TC1507) event expresses the least endotoxin – 0.1 kg Bt endotoxin per hectare – just below the rate of insecticides applied.

In the case of Bt corn targeting the CRW, every hectare planted in recent years expresses substantially greater volumes of Bt endotoxins than the ~0.2 kgs of insecticides applied on the average hectare for CRW control (0.19 pounds/acre [Additional file 1: Table S12]). MON 88017 expresses 0.62 kgs/ha of Cry 3Bb1, while DAS 59122–7 expresses two Cry proteins totaling 2.8 kgs/ha, 14-fold more than the insecticides displaced [Additional file 1: Table S22]. SmartStax GE corn synthesizes six Cry proteins, three targeting the ECB, and three the CRW. Total Cry protein production is estimated at 4.2 kgs/ha (3.7 pounds/acre), 19-times the average conventional insecticide rate of application in 2010.

Should Bt endotoxins count as insecticides applied?

Entomologists are divided on the question of whether the Bt produced by transgenic plants should be counted as “insecticides applied.” The case for doing so is strong, despite the obvious differences in how Cry proteins enter corn agroecosystems. When a field of corn is sprayed with a foliar Bt insecticide, the amount of toxin sprayed per hectare should be counted when computing total insecticide use. The primary difference between the Bt Cry proteins in a Bt-transgenic plant, and a field of non-GE plants sprayed with foliar Bt, is that in the later case, the toxin is present predominantly on plant tissue surfaces, whereas in the former Bt-crop case, the toxin is inside plant cells. This distinction does not greatly matter from the perspective of the overall load of pesticides in the environment, although the presence of pesticides inside plants, as opposed to on their surface, alters relative risk profiles across non-target organisms.

It should also be noted that, in general, the systemic delivery of Bt Cry proteins poses more significant risks to animals and humans ingesting Bt crops than applications of Bt insecticides via liquid sprays. Systemic delivery also enhances a range of environmental and ecological risks [47] compared to foliar Bt use patterns that result in rapid breakdown of Bt Cry proteins as they are exposed to sunlight and rainfall.

Most corn insecticides are applied in ways that expose active ingredients to destructive abiotic and biotic forces that tend to break down the chemicals to generally less toxic forms. Granular soil insecticides applied via boxes on corn planters tend to break down within weeks as a result of soil microbial activity. Because properly applied granular insecticides are buried in the soil, exposure to non-target organisms is limited, although poorly operated or calibrated planting equipment can result in grains of insecticide remaining on the soil surface, posing a serious potential risk to some bird species. A significant portion of the foliar insecticides applied per hectare for ECB control never hit its plant target, and a portion of the insecticide that does land and lodge on plant tissues is washed off within hours, days, or weeks during rainfall events. This is why insecticide residues are rarely detected in corn grain and silage at harvest time, and why conventional insecticide applications on corn pose little or no human dietary risk.

By virtue of their altered environmental fate and risk profile, all systemic pesticides should be counted when measuring pesticide use, and hence so too should the Bt proteins manufactured in Bt- transgenic crops. If Bt-transgenic plants produced proteins that disrupted insect morphology, feeding behavior, or reproduction, the absence of a toxic mode of action would strengthen the argument that Bt Cry proteins are not functionally equivalent to insecticides, and hence should not be counted as insecticides applied. Bt-crop technology that limits Bt-endotoxin expression to only those tissues that are under active attack, and then only during times when insects are actively feeding, would also support the view that Bt crops are compatible with IPM.