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"There are non-chemical ways to grow crops and control pests." 

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"There are non-chemical ways to grow crops and control pests." 

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In this portion of the video an expert was referring to organic farming. This sector of farming has chosen not to use chemical fertilizers and GM seeds. Many consumers prefer organic products because they have the peace of mind that those products are up to standards to be considered organic and are even willing to pay more for the food. The British Food Journal reported that with consumer preferences changing and favoring organic food more the agricultural industry will have to adjust to meet those preferences (Beharrell, 1991).

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The date on your computer is in the past. If your computer's clock shows a date before 1 Jan 1970, the browser will automatically forget the cookie. To fix this, set the correct time and date on your computer.

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"The properties that made DDT work so well also made it a poison."  

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"The properties that made DDT work so well also made it a poison."  

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DDT was originally used to help control the spread of lice and other insects. It worked so well that farmers would spray it on their crops without knowing the potential health risks. This is a perfect example of why we should be a little more careful of farm technology and innovations when the long term effects are difficult to measure. The European Network of Scientists for Social and Environmental Responsibility reports that there still is no consensus on the effects of GMOs on human health. Since there is no verdict on GMOs the EU has banned them until they are proven to be safe (Hilbeck, 2015).

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Regarding the safety of GM crops and foods for human and animal health, a comprehensive review of animal feeding studies of GM crops found 'An equilibrium in the number [of] research groups suggesting, on the basis of their studies, that a number of varieties of GM products (mainly maize and soybeans) are as safe and nutritious as the respective conventional non-GM plant, and those raising still serious concerns'. The review also found that most studies concluding that GM foods were as safe and nutritious as those obtained by conventional breeding were 'performed by biotechnology companies or associates, which are also responsible [for] commercializing these GM plants' [].

A separate review of animal feeding studies that is often cited as showing that GM foods are safe included studies that found significant differences in the GM-fed animals. While the review authors dismissed these findings as not biologically significant [], the interpretation of these differences is the subject of continuing scientific debate [,-] and no consensus exists on the topic.

Rigorous studies investigating the safety of GM crops and foods would normally involve, inter alia, animal feeding studies in which one group of animals is fed GM food and another group is fed an equivalent non-GM diet. Independent studies of this type are rare, but when such studies have been performed, some have revealed toxic effects or signs of toxicity in the GM-fed animals [,,-]. The concerns raised by these studies have not been followed up by targeted research that could confirm or refute the initial findings.

The lack of scientific consensus on the safety of GM foods and crops is underlined by the recent research calls of the European Union and the French government to investigate the long-term health impacts of GM food consumption in the light of uncertainties raised by animal feeding studies [,]. These official calls imply recognition of the inadequacy of the relevant existing scientific research protocols. They call into question the claim that existing research can be deemed conclusive and the scientific debate on biosafety closed.

It is often claimed that 'trillions of GM meals' have been eaten in the US with no ill effects. However, no epidemiological studies in human populations have been carried out to establish whether there are any health effects associated with GM food consumption. As GM foods and other products are not monitored or labelled after release in North America, a major producer and consumer of GM crops, it is scientifically impossible to trace, let alone study, patterns of consumption and their impacts. Therefore, claims that GM foods are safe for human health based on the experience of North American populations have no scientific basis.

Claims that there is a consensus among scientific and governmental bodies that GM foods are safe, or that they are no more risky than non-GM foods [,], are false. For instance, an expert panel of the Royal Society of Canada issued a report that was highly critical of the regulatory system for GM foods and crops in that country. The report declared that it is 'scientifically unjustifiable' to presume that GM foods are safe without rigorous scientific testing and that the 'default prediction' for every GM food should be that the introduction of a new gene will cause 'unanticipated changes' in the expression of other genes, the pattern of proteins produced, and/or metabolic activities. Possible outcomes of these changes identified in the report included the presence of new or unexpected allergens [].

A report by the British Medical Association concluded that with regard to the long-term effects of GM foods on human health and the environment, 'many unanswered questions remain' and that 'safety concerns cannot, as yet, be dismissed completely on the basis of information currently available'. The report called for more research, especially on potential impacts on human health and the environment [].

Moreover, the positions taken by other organizations have frequently been highly qualified, acknowledging data gaps and potential risks, as well as potential benefits, of GM technology. For example, a statement by the American Medical Association's Council on Science and Public Health acknowledged 'a small potential for adverse events ... due mainly to horizontal gene transfer, allergenicity, and toxicity' and recommended that the current voluntary notification procedure practised in the US prior to market release of GM crops be made mandatory []. It should be noted that even a 'small potential for adverse events' may turn out to be significant, given the widespread exposure of human and animal populations to GM crops.

A statement by the board of directors of the American Association for the Advancement of Science (AAAS) affirming the safety of GM crops and opposing labelling [] cannot be assumed to represent the view of AAAS members as a whole and was challenged in an open letter by a group of 21 scientists, including many long-standing members of the AAAS []. This episode underlined the lack of consensus among scientists about GMO safety.

An EU research project [] has been cited internationally as providing evidence for GM crop and food safety. However, the report based on this project, 'A Decade of EU-Funded GMO Research', presents no data that could provide such evidence from long-term feeding studies in animals.

Indeed, the project was not designed to test the safety of any single GM food but to focus on 'the development of safety assessment approaches' []. Only five published animal feeding studies are referenced in the SAFOTEST section of the report, which is dedicated to GM food safety []. None of these studies tested a commercialized GM food; none tested the GM food for long-term effects beyond the subchronic period of 90 days; all found differences in the GM-fed animals, which in some cases were statistically significant; and none concluded on the safety of the GM food tested, let alone on the safety of GM foods in general. Therefore, the EU research project provides no evidence for sweeping claims about the safety of any single GM food or of GM crops in general.

A frequently cited claim published on an Internet website that several hundred studies 'document the general safety and nutritional wholesomeness of GM foods and feeds' [] is misleading. Examination of the studies listed reveals that many do not provide evidence of GM food safety and, in fact, some provide evidence of a lack of safety. For example:

We conclude that these studies, taken as a whole, are misrepresented on the Internet website as they do not 'document the general safety and nutritional wholesomeness of GM foods and feeds'. Rather, some of the studies give serious cause for concern and should be followed up by more detailed investigations over an extended period of time.

Environmental risks posed by GM crops include the effects of insecticidal Bt (a bacterial toxin from Bacillus thuringiensis engineered into crops) crops on non-target organisms and the effects of the herbicides used in tandem with herbicide-tolerant GM crops.

As with GM food safety, no scientific consensus exists regarding the environmental risks of GM crops. A review of environmental risk assessment approaches for GM crops identified shortcomings in the procedures used and found 'no consensus' globally on the methodologies that should be applied, let alone on standardized testing procedures []. Some reviews of the published data on Bt crops have found that they can have adverse effects on non-target and beneficial organisms [-] - effects that are widely neglected in regulatory assessments and by some scientific commentators. Resistance to Bt toxins has emerged in target pests [], and problems with secondary (non-target) pests have been noted, for example, in Bt cotton in China [,].

Herbicide-tolerant GM crops have proved equally controversial. Some reviews and individual studies have associated them with increased herbicide use [,], the rapid spread of herbicide-resistant weeds [], and adverse health effects in human and animal populations exposed to Roundup, the herbicide used on the majority of GM crops [-].

As with GM food safety, disagreement among scientists on the environmental risks of GM crops may be correlated with funding sources. A peer-reviewed survey of the views of 62 life scientists on the environmental risks of GM crops found that funding and disciplinary training had a significant effect on attitudes. Scientists with industry funding and/or those trained in molecular biology were very likely to have a positive attitude to GM crops and to hold that they do not represent any unique risks, while publicly-funded scientists working independently of GM crop developer companies and/or those trained in ecology were more likely to hold a 'moderately negative' attitude to GM crop safety and to emphasize the uncertainty and ignorance involved. The review authors concluded 'The strong effects of training and funding might justify certain institutional changes concerning how we organize science and how we make public decisions when new technologies are to be evaluated' [].

The Cartagena Protocol on Biosafety was negotiated over many years and implemented in 2003. The Cartagena Protocol is an international agreement ratified by 166 governments worldwide that seeks to protect biological diversity from the risks posed by GM technology. It embodies the Precautionary Principle in that it allows signatory states to take precautionary measures to protect themselves against threats of damage from GM crops and foods, even in case of a lack of scientific certainty [].

Another international body, the UN's Codex Alimentarius, worked with scientific experts for 7 years to develop international guidelines for the assessment of GM foods and crops because of concerns about the risks they pose. These guidelines were adopted by the Codex Alimentarius Commission, of which over 160 nations are members, including major GM crop producers such as the United States [].

The Cartagena Protocol and Codex share a precautionary approach to GM crops and foods, in that they agree that genetic engineering differs from conventional breeding and that safety assessments should be required before GM organisms are used in food or released into the environment.

These agreements would never have been negotiated, and the implementation processes elaborating how such safety assessments should be conducted would not currently be happening, without widespread international recognition of the risks posed by GM crops and foods and the unresolved state of existing scientific understanding. Concerns about risks are well founded, as has been demonstrated by studies on some GM crops and foods that have shown adverse effects on animal health and non-target organisms, indicated above. Many of these studies have, in fact, fed into the negotiation and/or implementation processes of the Cartagena Protocol and the Codex. We support the application of the Precautionary Principle with regard to the release and transboundary movement of GM crops and foods.

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Regarding the safety of GM crops and foods for human and animal health, a comprehensive review of animal feeding studies of GM crops found 'An equilibrium in the number [of] research groups suggesting, on the basis of their studies, that a number of varieties of GM products (mainly maize and soybeans) are as safe and nutritious as the respective conventional non-GM plant, and those raising still serious concerns'.

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"In the years after World War I and the introduction of inexpensive fertilizers the yields tripled." 

# data[].attributes.subject
"In the years after World War I and the introduction of inexpensive fertilizers the yields tripled." 

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After World War I companies who were producing synthetic nitrate for explosives switched to producing nitrogen fertilizer. At first not all farmers were on board with using fertilizers, but the results were unmatched, and it became clear that if you did not use fertilizers you would be left behind not producing as much or as healthy of a crop. The Center for Soil and Environmental Quality reports that too much fertilizer can be a bad thing though. It is stated that excessive nitrogen fertilization, or fertilization at the wrong time, can lead to leaching and can potentially ruin the soil for future growing (Di, Cameron, 2002).

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Abstract

Nitrate (NO ₃−) leaching from agriculturalproduction systems is blamed for the rising concentrations ofNO ₃− in ground- and surface-waters around the world.This paper reviews the evidence of NO ₃− leachinglosses from various land use systems, including cut grassland, grazed pastures,arable cropping, mixed cropping with pasture leys, organic farming,horticultural systems, and forest ecosystems. Soil, climatic and managementfactors which affect NO ₃− leaching are discussed.Nitrate leaching occurs when there is an accumulation ofNO ₃− in the soil profile that coincides with or isfollowed by a period of high drainage. Therefore, excessive nitrogen (N)fertilizer or waste effluent application rates or N applications at the wrongtime (e.g. late autumn) of the year, ploughing pasture leys early in the autumn,or long periods of fallow ground, can all potentially lead to highNO ₃− leaching losses. N returns in animal urine havea major impact on NO ₃− leaching in grazed pastures.Of the land use systems considered in this paper, the potential for causingNO ₃− leaching typically follow the order: forest< cut grassland < grazed pastures, arable cropping < ploughing ofpasture < market gardens. A range ofmanagement options to mitigate NO ₃− leaching isdescribed, including reducing N application rates, synchronizing N supply toplant demand, use of cover crops, better timing of ploughing pasture leys,improved stock management, precision farming, and regulatory measures. This isfollowed by a discussion of future research needs to improve our ability topredict and mitigate NO ₃− leaching.

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References

1. Ledgard S.F., Smith O.S. and Sprossen M.S. 1992. Fate of ¹⁵N-labelled nitrogen fertiliser applied to kiwifruit ( Actimidia deliciosa). 1. ¹⁵ N recovery in plant and soil. Plant Soil 147: 49-57.

2. Smith C.T., Dyck W.J., Beets P.N., Hodgkiss P.D. and Lowe A.T. 1994. Nutrition and productivity of Pinus radiata following harvest disturbance and fertilization of coastal sand dunes. Forest Ecol. Manag. 66: 5-38.

© Kluwer Academic Publishers 2002

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Nitrate leaching in temperate agroecosystems: sources, factors and mitigating strategies

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Nitrate (NO 3 −) leaching from agriculturalproduction systems is blamed for the rising concentrations ofNO 3 − in ground- and surface-waters around the world.This paper reviews the evidence of NO 3 − leachinglosses from various land use systems, including cut grassland, grazed pastures,arable cropping, mixed cropping with pasture leys, organic farming,horticultural systems, and forest ecosystems. Soil, climatic and managementfactors which affect NO 3 − leaching are discussed.Nitrate leaching occurs when there is an accumulation ofNO 3 − in the soil profile that coincides with or isfollowed by a period of high drainage.

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"Seeds are just as technological but a lot more controversial." 

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"Seeds are just as technological but a lot more controversial." 

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Farm technology has improved in all aspects from machinery to fertilizers to seeds. However, there is much more controversy about seed, insecticide, and herbicide technology than anything else. Consumers don’t like the idea of "frankenfoods" and putting chemicals in their bodies without knowing the effects. These GM seeds and field treatments though are beneficial to farmers and help produce the best crop. The Public Library of Science reports that the main reason for the controversy about GMOs is the fact that there is so much uncertainty, the government has approved these products relying on information from the companies that make them and it is impossible to know the lasting effects since they are still relatively new (Klumper, Qiam, 2014).

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Wilhelm Klümper,

Matin Qaim

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Citation: Klümper W, Qaim M (2014) A Meta-Analysis of the Impacts of Genetically Modified Crops. PLoS ONE 9(11): e111629. https://doi.org/10.1371/journal.pone.0111629

Editor: emidio albertini, University of Perugia, Italy

Received: June 23, 2014; Accepted: October 3, 2014; Published: November 3, 2014

Copyright: © 2014 Klümper, Qaim. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files.

Funding: This research was financially supported by the German Federal Ministry of Economic Cooperation and Development (BMZ) and the European Union's Seventh Framework Programme (FP7/2007-2011) under Grant Agreement 290693 FOODSECURE. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Neither BMZ nor FOODSECURE and any of its partner organizations, any organization of the European Union or the European Commission are accountable for the content of this article.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Despite the rapid adoption of genetically modified (GM) crops by farmers in many countries, public controversies about the risks and benefits continue [1]-[4]. Numerous independent science academies and regulatory bodies have reviewed the evidence about risks, concluding that commercialized GM crops are safe for human consumption and the environment [5]-[7]. There are also plenty of studies showing that GM crops cause benefits in terms of higher yields and cost savings in agricultural production [8]-[12], and welfare gains among adopting farm households [13]-[15]. However, some argue that the evidence about impacts is mixed and that studies showing large benefits may have problems with the data and methods used [16]-[18]. Uncertainty about GM crop impacts is one reason for the widespread public suspicion towards this technology. We have carried out a meta-analysis that may help to consolidate the evidence.

While earlier reviews of GM crop impacts exist [19]-[22], our approach adds to the knowledge in two important ways. First, we include more recent studies into the meta-analysis. In the emerging literature on GM crop impacts, new studies are published continuously, broadening the geographical area covered, the methods used, and the type of outcome variables considered. For instance, in addition to other impacts we analyze effects of GM crop adoption on pesticide quantity, which previous meta-analyses could not because of the limited number of observations for this particular outcome variable. Second, we go beyond average impacts and use meta-regressions to explain impact heterogeneity and test for possible biases.

Our meta-analysis concentrates on the most important GM crops, including herbicide-tolerant (HT) soybean, maize, and cotton, as well as insect-resistant (IR) maize and cotton. For these crops, a sufficiently large number of original impact studies have been published to estimate meaningful average effect sizes. We estimate mean impacts of GM crop adoption on crop yield, pesticide quantity, pesticide cost, total production cost, and farmer profit. Furthermore, we analyze several factors that may influence outcomes, such as geographic location, modified crop trait, and type of data and methods used in the original studies.

Materials and Methods

Literature search

Original studies for inclusion in this meta-analysis were identified through keyword searches in relevant literature databanks. Studies were searched in the ISI Web of Knowledge, Google Scholar, EconLit, and AgEcon Search. We searched for studies in the English language that were published after 1995. We did not extend the review to earlier years, because the commercial adoption of GM crops started only in the mid-1990s [23]. The search was performed for combinations of keywords related to GM technology and related to the outcome of interest. Concrete keywords used related to GM technology were (an asterisk is a replacement for any ending of the respective term; quotation marks indicate that the term was used as a whole, not each word alone): GM*, "genetically engineered", "genetically modified", transgenic, "agricultural biotechnology", HT, "herbicide tolerant", Roundup, Bt, "insect resistant". Concrete keywords used related to outcome variables were: impact*, effect*, benefit*, yield*, economic*, income*, cost*, soci*, pesticide*, herbicide*, insecticide*, productivity*, margin*, profit*. The search was completed in March 2014.

Most of the publications in the ISI Web of Knowledge are articles in academic journals, while Google Scholar, EconLit, and AgEcon Search also comprise book chapters and grey literature such as conference papers, working papers, and reports in institutional series. Articles published in academic journals have usually passed a rigorous peer-review process. Most papers presented at academic conferences have also passed a peer-review process, which is often less strict than that of good journals though. Some of the other publications are peer reviewed, while many are not. Some of the working papers and reports are published by research institutes or government organizations, while others are NGO publications. Unlike previous reviews of GM crop impacts, we did not limit the sample to peer-reviewed studies but included all publications for two reasons. First, a clear-cut distinction between studies with and without peer review is not always possible, especially when dealing with papers that were not published in a journal or presented at an academic conference [24]. Second, studies without peer review also influence the public and policy debate on GM crops; ignoring them completely would be short-sighted.

Of the studies identified through the keyword searches, not all reported original impact results. We classified studies by screening titles, abstracts, and full texts. Studies had to fulfill the following criteria to be included:

The study is an empirical investigation of the agronomic and/or economic impacts of GM soybean, GM maize, or GM cotton using micro-level data from individual plots and/or farms. Other GM crops such as GM rapeseed, GM sugarbeet, and GM papaya were commercialized in selected countries [23], but the number of impact studies available for these other crops is very small.

The study reports GM crop impacts in terms of one or more of the following outcome variables: yield, pesticide quantity (especially insecticides and herbicides), pesticide costs, total variable costs, gross margins, farmer profits. If only the number of pesticide sprays was reported, this was used as a proxy for pesticide quantity.

The study analyzes the performance of GM crops by either reporting mean outcomes for GM and non-GM, absolute or percentage differences, or estimated coefficients of regression models that can be used to calculate percentage differences between GM and non-GM crops.

The study contains original results and is not only a review of previous studies.

In some cases, the same results were reported in different publications; in these cases, only one of the publications was included to avoid double counting. On the other hand, several publications involve more than one impact observation, even for a single outcome variable, for instance when reporting results for different geographical regions or derived with different methods (e.g., comparison of mean outcomes of GM and non-GM crops plus regression model estimates). In those cases, all observations were included. Moreover, the same primary dataset was sometimes used for different publications without reporting identical results (e.g., analysis of different outcome variables, different waves of panel data, use of different methods). Hence, the number of impact observations in our sample is larger than the number of publications and primary datasets ( Data S1). The number of studies selected at various stages is shown in the flow diagram in Figure 1. The number of publications finally included in the meta-analysis is 147 ( Table S1).

Effect sizes and influencing factors

Effect sizes are measures of outcome variables. We chose the percentage difference between GM and non-GM crops for five different outcome variables, namely yield, pesticide quantity, pesticide cost, total production cost, and farmer profits per unit area. Most studies that analyze production costs focus on variable costs, which are the costs primarily affected through GM technology adoption. Accordingly, profits are calculated as revenues minus variable production costs (profits calculated in this way are also referred to as gross margins). These production costs also take into account the higher prices charged by private companies for GM seeds. Hence, the percentage differences in profits considered here are net economic benefits for farmers using GM technology. Percentage differences, when not reported in the original studies, were calculated from mean value comparisons between GM and non-GM or from estimated regression coefficients.

Since we look at different types of GM technologies (different modified traits) that are used in different countries and regions, we do not expect that effect sizes are homogenous across studies. Hence, our approach of combining effect sizes corresponds to a random-effects model in meta-analysis [25]. To explain impact heterogeneity and test for possible biases, we also compiled data on a number of study descriptors that may influence the reported effect sizes. These influencing factors include information on the type of GM technology (modified trait), the region studied, the type of data and method used, the source of funding, and the type of publication. All influencing factors are defined as dummy variables. The exact definition of these dummy variables is given in Table 1. Variable distributions of the study descriptors are shown in Table S2.

Statistical analysis

In a first step, we estimate average effect sizes for each outcome variable. To test whether these mean impacts are significantly different from zero, we regress each outcome variable on a constant with cluster correction of standard errors by primary dataset. Thus, the test for significance is valid also when observations from the same dataset are correlated. We estimate average effect sizes for all GM crops combined. However, we expect that the results may differ by modified trait, so that we also analyze mean effects for HT crops and IR crops separately.

Meta-analyses often weight impact estimates by their variances; estimates with low variance are considered more reliable and receive a higher weight [26]. In our case, several of the original studies do not report measures of variance, so that weighting by variance is not possible. Alternatively, weighting by sample size is common, but sample sizes are also not reported in all studies considered, especially not in some of the grey literature publications. To test the robustness of the results, we employ a different weighting procedure, using the inverse of the number of impact observations per dataset as weights. This procedure avoids that individual datasets that were used in several publications dominate the calculation of average effect sizes.

In a second step, we use meta-regressions to explain impact heterogeneity and test for possible biases. Linear regression models are estimated separately for all of the five outcome variables: is the effect size (percentage difference between GM and non-GM) of each outcome variable h for observation i in publication j, and is a vector of influencing factors. is a coefficient and a vector of coefficients to be estimated; is a random error term. Influencing factors used in the regressions are defined in Table 1.

Results and Discussion

Average effect sizes

Distributions of all five outcome variables are shown in Figure S1. Table 2 presents unweighted mean impacts. As a robustness check, we weighted by the inverse of the number of impact observations per dataset. Comparing unweighted results ( Table 2) with weighted results ( Table S3) we find only very small differences. This comparison suggests that the unweighted results are robust.

On average, GM technology has increased crop yields by 21% ( Figure 2). These yield increases are not due to higher genetic yield potential, but to more effective pest control and thus lower crop damage [27]. At the same time, GM crops have reduced pesticide quantity by 37% and pesticide cost by 39%. The effect on the cost of production is not significant. GM seeds are more expensive than non-GM seeds, but the additional seed costs are compensated through savings in chemical and mechanical pest control. Average profit gains for GM-adopting farmers are 69%.

Average percentage differences between GM and non-GM crops are shown. Results refer to all GM crops, including herbicide-tolerant and insect-resistant traits. The number of observations varies by outcome variable; yield: 451; pesticide quantity: 121; pesticide cost: 193; total production cost: 115; farmer profit: 136. *** indicates statistical significance at the 1% level.

Results of Cochran's test [25], which are reported in Figure S1, confirm that there is significant heterogeneity across study observations for all five outcome variables. Hence it is useful to further disaggregate the results. Table 2 shows a breakdown by modified crop trait. While significant reductions in pesticide costs are observed for both HT and IR crops, only IR crops cause a consistent reduction in pesticide quantity. Such disparities are expected, because the two technologies are quite different. IR crops protect themselves against certain insect pests, so that spraying can be reduced. HT crops, on the other hand, are not protected against pests but against a broad-spectrum chemical herbicide (mostly glyphosate), use of which facilitates weed control. While HT crops have reduced herbicide quantity in some situations, they have contributed to increases in the use of broad-spectrum herbicides elsewhere [2], [11], [19]. The savings in pesticide costs for HT crops in spite of higher quantities can be explained by the fact that broad-spectrum herbicides are often much cheaper than the selective herbicides that were used before. The average farmer profit effect for HT crops is large and positive, but not statistically significant because of considerable variation and a relatively small number of observations for this outcome variable.

Impact heterogeneity and possible biases

Table 3 shows the estimation results from the meta-regressions that explain how different factors influence impact heterogeneity. Controlling for other factors, yield gains of IR crops are almost 7 percentage points higher than those of HT crops (column 1). Furthermore, yield gains of GM crops are 14 percentage points higher in developing countries than in developed countries. Especially smallholder farmers in the tropics and subtropics suffer from considerable pest damage that can be reduced through GM crop adoption [27].

Most original studies in this meta-analysis build on farm surveys, although some are based on field-trial data. Field-trial results are often criticized to overestimate impacts, because farmers may not be able to replicate experimental conditions. However, results in Table 3 (column 1) show that field-trial data do not overestimate the yield effects of GM crops. Reported yield gains from field trials are even lower than those from farm surveys. This is plausible, because pest damage in non-GM crops is often more severe in farmers' fields than on well-managed experimental plots.

Another concern often voiced in the public debate is that studies funded by industry money might report inflated benefits. Our results show that the source of funding does not significantly influence the impact estimates. We also analyzed whether the statistical method plays a role. Many of the earlier studies just compared yields of GM and non-GM crops without considering possible differences in other inputs and conditions that may also affect the outcome. Net impacts of GM technology can be estimated with regression-based production function models that control for other factors. Interestingly, results derived from regression analysis report higher average yield effects.

Finally, we examined whether the type of publication matters. Controlling for other factors, the regression coefficient for journal publications in column (1) of Table 3 implies that studies published in peer-reviewed journals show 12 percentage points higher yield gains than studies published elsewhere. Indeed, when only including observations from studies that were published in journals, the mean effect size is larger than if all observations are included ( Figure S2). On first sight, one might suspect publication bias, meaning that only studies that report substantial effects are accepted for publication in a journal. A common way to assess possible publication bias in meta-analysis is through funnel plots [25], which we show in Figure S3. However, in our case these funnel plots should not be over-interpreted. First, only studies that report variance measures can be included in the funnel plots, which holds true only for a subset of the original studies used here. Second, even if there were publication bias, our mean results would be estimated correctly, because we do include studies that were not published in peer-reviewed journals.

Further analysis suggests that the journal review process does not systematically filter out studies with small effect sizes. The journal articles in the sample report a wide range of yield effects, even including negative estimates in some cases. Moreover, when combining journal articles with papers presented at academic conferences, average yield gains are even higher ( Table 3, column 2). Studies that were neither published in a journal nor presented at an academic conference encompass a diverse set of papers, including reports by NGOs and outspoken biotechnology critics. These reports show lower GM yield effects on average, but not all meet common scientific standards. Hence, rather than indicating publication bias, the positive and significant journal coefficient may be the result of a negative NGO bias in some of the grey literature.

Concerning other outcome variables, IR crops have much stronger reducing effects on pesticide quantity than HT crops ( Table 3, column 3), as already discussed above. In terms of pesticide costs, the difference between IR and HT is less pronounced and not statistically significant (column 4). The profit gains of GM crops are 60 percentage points higher in developing countries than in developed countries (column 6). This large difference is due to higher GM yield gains and stronger pesticide cost savings in developing countries. Moreover, most GM crops are not patented in developing countries, so that GM seed prices are lower [19]. Like for yields, studies published in peer-reviewed journals report higher profit gains than studies published elsewhere, but again we do not find evidence of publication bias (column 7).

Conclusion

This meta-analysis confirms that - in spite of impact heterogeneity - the average agronomic and economic benefits of GM crops are large and significant. Impacts vary especially by modified crop trait and geographic region. Yield gains and pesticide reductions are larger for IR crops than for HT crops. Yield and farmer profit gains are higher in developing countries than in developed countries. Recent impact studies used better data and methods than earlier studies, but these improvements in study design did not reduce the estimates of GM crop advantages. Rather, NGO reports and other publications without scientific peer review seem to bias the impact estimates downward. But even with such biased estimates included, mean effects remain sizeable.

One limitation is that not all of the original studies included in this meta-analysis reported sample sizes and measures of variance. This is not untypical for analyses in the social sciences, especially when studies from the grey literature are also included. Future impact studies with primary data should follow more standardized reporting procedures. Nevertheless, our findings reveal that there is robust evidence of GM crop benefits. Such evidence may help to gradually increase public trust in this promising technology.

Acknowledgments

We thank Sinja Buri and Tingting Xu for assistance in compiling the dataset. We also thank Joachim von Braun and three reviewers of this journal for useful comments.

Author Contributions

Conceived and designed the research: WK MQ. Analyzed the data: WK MQ. Contributed to the writing of the manuscript: WK MQ. Compiled the data: WK.

References

14. Kathage J, Qaim M (2012) Economic impacts and impact dynamics of Bt (Bacillus thuringiensis) cotton in India. Proceedings of the National Academy of Sciences USA 109: 11652-11656.

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A Meta-Analysis of the Impacts of Genetically Modified Crops

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Background Despite the rapid adoption of genetically modified (GM) crops by farmers in many countries, controversies about this technology continue. Uncertainty about GM crop impacts is one reason for widespread public suspicion. Objective We carry out a meta-analysis of the agronomic and economic impacts of GM crops to consolidate the evidence.

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"Tractors meant larger farms and fewer farmers." 

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"Tractors meant larger farms and fewer farmers." 

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Before the 1940s a large portion of the population worked on farms and now only 1% of the American population farm. This is partly because technology has evolved so much and has become so efficient that not as many people are required to get the job done even though farms have grown in size. An article from the National Review reported that not only are there less farm jobs available but many people are fleeing the farm industry. Many children of farmers see how hard it can be for a little reward and choose to pursue other career options (Olmstead, 2016).

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Small farmers face regulatory burdens, negative stereotypes, and children fleeing the family business.

Lucy and Joe Lourenco emigrated from Portugal in 1984 as a newly married couple; they were only 19 and 24 years old. They spent the first few years of their life in America working in California, saving all the money they could. Idaho seemed like the perfect place to start a farm, so they moved to Emmett in 1992. "We started with 40 cows and barely made enough money to eat," Lucy says. "But it's a beautiful business if you love the animals."

Lucy has always had a gift with the cows. Around the time their first son was born, the dairy's calves began to get sick, and Joe couldn't help them. They hired a nanny so that Lucy could help nurse the calves back to health. Many times, a cow would begin birthing in the middle of the night, prompting Lucy to pull her boots on over her pajamas and head outside. That's why she keeps their bedroom window open a crack.

Joe prefers working in their corn and wheat fields: He's the dreamer, Lucy says, the one with vision. "Reading and writing is not his thing, but he can see and do what he has a vision for." Lucy handles the bookkeeping and finances and makes sure they have money saved for the bad years. "When you start with nothing and build up from nothing, you don't want to lose it or give it away."

But now, Lucy and Joe are facing an unknown future, as neither of their two sons has expressed any interest in taking over the dairy farm someday. "Younger people don't want it because the money isn't there," she says. "Younger people want fancy, higher -paying jobs."

In the 1930s, the United States was home to 6.3 million farms; today, there are approximately 2.2 million, and fewer every day. The average age of today's farmer or rancher is 59 years old, and many are retiring without a successor, as their children don't want - or can't afford - to take over the family business. Thus, as farms' inheritors increasingly abandon the farm, a vacuum of stewardship opens up, leaving many wondering who, or what, will take their place.

Toward Freer Markets and More Local Control

Small family farms were traditionally tied to the good of their communities and their environment: Their smallness and generational longevity helped foster communal rapport, accountability, and stewardship. But today, formerly vibrant rural communities are slowly turning into ghost towns as industrialized farms corrode the traditional relation of commerce and community that surrounded their smaller counterparts. Farmers are motivated to "get big or get out," as President Nixon's secretary of agriculture, Earl Butz, said in 1973, urging farmers to buy up their neighbors' lands, and replace laborers with tractors. Without the traditional cycles of employment and production that small farms fostered, rural communities are falling apart.

Formerly vibrant rural communities are slowly turning into ghost towns as industrialized farms corrode the traditional relation of commerce and community.

Many writers who cover food production - such as Michael Pollan, author of the bestselling book The Omnivore's Dilemma - blame the perpetuation of such circumstances on capitalism itself, arguing that the U.S. economic system enables big businesses to triumph through corruption and cruelty. But this fails to address the huge surge in consumer demand for sustainable, human-scale agricultural practices. If the market were truly a "free" one, wouldn't it respond to this demand? Trends such as "locavorism" and the increasing number of farmers' markets nationwide indicate some change, but the system is still rewarding practices that more and more consumers dislike.

To find an advocate for both sustainable agriculture and the free market, look no further than famous farmer and author Joel Salatin. Salatin's parents settled in southern Virginia in 1961, buying up severely eroded farmland that they and their son have worked steadily to restore. He staunchly advocates organic, diversified agriculture, free of pesticides and chemicals. Salatin's book Everything I Want to Do Is Illegal details the many ways that the federal government saddles farmers with excess paperwork, regulations, and bureaucracy. He thinks this burden is, at least in part, facilitated by big agricultural producers who have the staff and funds to meet heavy regulatory demands. Big farms set high barriers of entry for small ones, making it difficult for operations such as Salatin's to compete.

Whether it's subsidies, crop insurance, federal regulations, or even conservation programs, says Salatin, there is often a "prejudice against scale." In a centralized system of regulation and control, diversity makes standards more difficult to enforce. But to dispense with variation is not just detrimental to land, animals, and community; Salatin also believes it hampers new farmers, because "innovation starts embryonically, creation starts small."

Federal insurance programs traditionally come in two forms: yield and revenue insurance. The USDA's Risk Management Agency (RMA) formulates insurance policies, sets premium rates, and subsidizes the cost for farmers and insurance providers. But big businesses often receive larger subsidies, rewarding powerful and politically connected farming operations over small family farms. "There have been a series of policy decisions at the federal level that have greatly influenced where we are today - where farms are larger, there are fewer family farms, and fewer vibrant rural communities," Paul Wolfe of the National Sustainable Agriculture Coalition told me.

One particularly bad culprit is the harvest price option (HPO) crop-insurance policy. While traditional crop insurance protects farmers from catastrophic losses, those who can afford higher premiums buy an HPO policy. Traditional crop insurance pays farmers when the price at harvest is less than projected prices at planting. But HPO policies guarantee that farmers will be paid either the projected price at planting time, or the market price at harvest - whichever is higher. "This product goes above and beyond the definition of a safety net," wrote the R Street Institute, a libertarian think tank, in a letter to Congress last year. "It is the crop insurance equivalent of your auto insurer surprising you with a new Cadillac Escalade after you've totaled your Toyota Corolla."

Craig Cox, the senior vice president for agriculture resources at the Environmental Working Group (EWG), says the HPO is less an insurance policy than a price support. "The odds are very high they're going to make money," says Cox. "The rate of return in some instances was 300 or 400 percent. With crop insurance, we're subsidizing so much normal business risk, you can make decisions to expand, buy, or rent more land, without having to worry about risk that would normally make you think twice, or make you diversify your crops."

Those who oppose the Farm Bill (of which the HPO is one component) cross party lines, forming unexpected partnerships. The EWG joins libertarian-leaning groups such as The R Street Institute and Taxpayers for Common Sense in opposing current agricultural policies. Their reasons are ecological, economic, and political: Crop insurance is bad for the land and also bad for the taxpayer. In 2012, the harvest price option boosted payouts to corn and soybean farmers by $6 billion - money that comes from the federal government and thus from the pockets of average citizens. "The overwhelming cost is borne by us," says Joshua Sewell, a policy analyst for Taxpayers for Common Sense. "We also bear the cost of the crop-insurance companies, as well as the cost of loss when it occurs. It's a triangle of waste."

Catherine Cochran, press secretary for the U.S. Department of Agriculture, acknowledged that because of the way USDA programs function, "more funds go to larger farms," and perceptions of a disparity in coverage "may not be totally false." However, she also pointed to efforts the department is making to reach out to smaller growers, such as their microloan initiative and "Whole Farm Revenue Protection" (WFRP), a measure introduced in the 2014 Farm Bill. In contrast to more conventional crop-insurance policies, WFRP offers greater security to smaller, diversified farm producers.

This year, President Obama proposed $9 billion in cuts to the USDA budget - cuts that focus in on crop-insurance programs such as the HPO policy. It's impossible for the budget to entirely eliminate the Harvest Price Option plan, but Obama's proposed cuts would drastically trim the federal funds that undergird it. In a joint press release, the R Street Institute and National Taxpayers Union joined Friends of the Earth U.S. and the EWG in applauding these budget cuts.

Many libertarian-leaning Americans are also advocating for what they call "local food freedom" as a way to free up the connection between farmer and consumer. The Wyoming Food Freedom Act is one example of this. Signed into law last March, it fights for the deregulation of locally produced foods. It exempts local sales from government licensure, permitting, certification, inspection, packaging, and labeling requirements, and applies to farmers' markets as well as on-site stores at farms.

Salatin and other "food freedom" advocates argue that, while federal oversight is an expensive means of accountability, local policing - in which consumers visit, interact with, and personally inspect the farms where they procure their food - is a more efficient system, policed without government cost and reinforced by self-interest. While it cannot replace interstate commerce and regulation, it can provide an alternative to them and open up opportunities for small growers to succeed.

Passing the Family Farm to the Next Generation

But financial and regulatory challenges aren't the only things driving farmers' children off the land. While farming was once considered important and even prestigious work, today "there's a cultural condescension toward farmers," Salatin says. Many believe farming is blue-collar and unintellectual. "When my high-school counselor found out I wanted to be a farmer, I thought she'd have an apoplectic seizure," Salatin recalls. "Farming is seen as what you do if you can't do anything else."

'When my high-school counselor found out I wanted to be a farmer, I thought she'd have an apoplectic seizure.'

Despite the negative stereotypes, Salatin's interns and apprentices are stepping out onto the farm. One of his current interns worked ten years for a Fortune 500 company, until he "got tired of computers." Farming is something the intern always wanted to do, Salatin says, but society, his family, and academia all conveyed the message that he shouldn't, or couldn't. "These are the stewards of our air, soil, water, food - isn't it important that we have our best and brightest out here?" Salatin asks.

Marie Bowers is one such person: a bright college graduate who decided to return to her family farm and work with her father. She was raised on a six-generation grass farm in Oregon's Willamette Valley, where her family grows ryegrass and fescue. "I do a lot of political activities," she says. "When I told someone who does similar stuff that I was heading back to the farm, they said it was a waste of my potential."

But Bowers loves everything about her family farm: driving tractors, puzzling over the crops, putting in the hours and the physical labor. "If I don't continue [the farm], I don't know who would, and I would hate to see something my family has worked so hard be taken over," she says.

When land was a highly coveted commodity and land ownership conferred a degree of prestige, most farmers' children could see the value in carrying on their parents' work. But today, the prestige of farming has diminished, and children are no longer expected or encouraged to follow in their parents' footsteps.

This is the dilemma the Lucy Lourenco and her husband, Joe, now confront: Their sons are proud of their parents, she says, but "they aren't driven." She wonders whether working for another farmer might have helped her sons see the value and dignity in farm work, but she also believes the culture has offered her sons different incentives. They aspire to an ease of life that never appealed to her or Joe. "I walk to the barn; they take the four-wheeler," she says. "They'll help out on the farm but only because they have to - not because they care for it." There are still young people like Marie Bowers who are interested in farming despite its difficulties. But there are fewer such people than there were 20 or 50 years ago, thanks to the physical strains of the job, a lack of financial incentives, and widespread cultural prejudices.

Yet as farmers' children vacate the family farm, other young people across the U.S. are stepping in to fill the void: Eric Hansen, a policy analyst for the National Young Farmers Coalition, has seen their passion firsthand. His organization started in 2010, with a vision to help these young people start their careers in agriculture.

For many of the farmers with whom Hansen works, procuring land is the biggest challenge: A March 2016 study by Iowa State University economics professor Mike Duffy demonstrates that crop insurance has hiked up the price of farmland substantially, with farmland appraisal values rising 8.5 percent per year between 2006 and 2015, compared with 7.2 percent per year between 2001 and 2015. This can hurt both small farm owners and farmers renting their land: "When growers have to pay more for land because crop insurance raises the rental rates they pay, their overall costs of doing business increase," Anne Weir, a senior analyst at EWG, wrote this spring. "Some growers are approaching the point where they will no longer break even."

But a lot of these young farmers need a helping hand: Unable to inherit land, they need to find acreage at a good price, from landowners who share their vision and are willing to support them. This is where agricultural partnership or "matchmaking" services are stepping in. Some are as simple as a "land link" - connecting property owners with new farmers. Other matchmaking efforts, however, are more complicated, bringing together an elderly, retiring farmer, and an aspiring, young farmer in a sort of geographical and vocational marriage.

Salatin talked with me about a couple such services: Eager Farmer is an advertising site for farmers and landowners, as well as for potential farm apprentices, interns, and managers. The grassroots organization Greenhorns offers a similar linking service, as does the National Young Farmers Coalition. Hansen notes that some of the Coalition's land links have been more successful than others. "You aren't just passing on an asset," he says. "It's a business, a big piece of your identity. People want to make sure the places they're passing on will be valued and taken care of and managed properly."

People like the Lourencos are well aware of the difficulties, unknowns, and grueling work farming requires. They don't have visions of grandeur about it. But can they work with the younger generation in passing along this legacy? This is what a family farm traditionally represents: the preservation of tradition, the handing down of the land and its stewardship from one generation to another. Though the generational chain has loosened and even broken, perhaps land links can make the bonds strong again.

Hansen notes that many of the young farmers with whom he works aspire to hand down their farms to their children. They want to continue the tradition of family farming, if they're able, and this is what he hopes to help them do: "A farm should be a place where you can raise a family, where you have the income you need, where you can pass down to the next generation."

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Small farmers face regulatory burdens, negative stereotypes, and children fleeing the family business. Lucy and Joe Lourenco emigrated from Portugal in 1984 as a newly married couple; they were only 19 and 24 years old. They spent the first few years of their life in America working in California, saving all the money they could.