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Peer-reviewed surveys indicate positive impact of commercialized GM crops

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- Janet E Carpenter, Nature Biotechnology, April 2010, Vol. 28, p 319–321. Excerpt below. Full paper at

The benefits of genetically modified (GM) crops continue to be disputed, despite rapid and widespread adoption since their commercial introduction in the United States and Canada in 1995. Last year, 14 million farmers in 25 countries grew GM crops commercially, over 90% of them small farmers in developing countries1. Farmer surveys are a valuable measure of the impact of GM crops. These surveys estimate the technology's performance as it is incorporated into farmer practices, given constraints on time, access to information, differing levels of risk aversion and other factors. This analysis summarizes results from 49 peer-reviewed publications reporting on farmer surveys that compare yields and other indicators of economic performance for adopters and non-adopters of currently commercialized GM crops. The surveys cover GM insect-resistant and herbicide-tolerant crops, which account for >99% of global GM crop area1. Results from 12 countries indicate, with few exceptions, that GM crops have benefitted farmers. The benefits, especially in terms of increased yields, are greatest for the mostly small farmers in developing countries, who have benefitted from the spillover of technologies originally targeted at farmers in industrialized countries.

Of 168 results comparing yields of GM and conventional crops, 124 show positive results for adopters compared to non-adopters, 32 indicate no difference and 13 are negative. By far the largest numbers of results comparing yields of adopters and non-adopters come from India and the United States, which account for 26% and 23% of the results, respectively (Table 1).

The results for yields indicate that farmers in developing countries are achieving greater yield increases than farmers in developed countries (Table 2). The average yield increases for developing countries range from 16% for insect-resistant corn to 30% for insect-resistant cotton, with an 85% yield increase observed in a single study on herbicide-tolerant corn. On average, developed-country farmers report yield increases that range from no change for herbicide-tolerant cotton to a 7% increase for herbicide-tolerant soybean and insect-resistant cotton. The first wave of GM crops to be commercialized has embodied traits intended to improve pest management and therefore reduce or eliminate losses from insect damage or weed competition. These technologies do not raise yield potential, but they can improve yields substantially owing to improved pest management. Where conventional weed- and insect-control technologies were lacking because of inherent limits to the effectiveness of available conventional pest-management options or limited access to conventional control methods, yields would be expected to increase. These conditions may be more common in developing countries.
As the most frequently studied case, GM insect-resistant cotton (Bacillus thuringiensis (Bt) cotton) in India provides examples of both the highest yield increases observed as well as several of the negative results. The largest yield increases found in this review are reported for Bt cotton in India, where surveys show yield increases of up to 150%. Of the negative results, six are for the first year of commercialization of Bt cotton in India, and the rest of the negative results are from developed countries in the first few years of commercialization. <cut>

The accumulated evidence from farmer surveys on the performance of GM crops helps to explain the widespread popularity of the technology in several regions of the world. The surveys reviewed here reflect a wide variety of conditions in terms of environment, pest pressure, farmer practices, social context, intellectual property rights and institutional arrangements. Given this diversity of conditions, it is striking that the results are so consistently positive. Even so, these results cover less than half of the countries currently growing GM crops and are sparse for some already widely adopted technologies, such as GM herbicide-tolerant corn and canola. Furthermore, GM crops have been grown for only 14 years—fewer for those countries that were not among the first adopters—a relatively short period for assessing the long-term impact of any technology. In some cases, results reflect a single growing season, which may not be an adequate basis for judging the sustainability of the technology's impact. Nevertheless, the window of opportunity for directly comparing the outcomes of adopters and non-adopters has closed where adoption rates are very high, and different methods of impact assessment will now be required.

Of interest in the future will be the assessment of the impacts of stacked traits, incorporating a combination of traits, which already represent over 28% of total global GM crop acreage but have been studied by only two surveys. Also of interest will be the assessment of farmers' experiences with GM crop technologies created specifically to address the most pressing constraints of developing-country farmers, such as technologies being developed in cassava, cowpea and rice, as those reach the commercialization stage.