The impact that GMO crops have had on yields is dependent on several factors. The first wave of GMO 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, especially where conventional insect- and weed-management control technologies are limited in their effectiveness or availability. For example, conventional insecticides may become ineffective as a result of their overuse and the consequent evolution of insecticide-resistant populations. Also, access to insecticide and herbicide inputs may be limited in some areas or groups due to a lack of infrastructure or resources. These conditions may be more common in developing countries.
At this point, a large number of surveys have been done asking farmers about their yields and other differences in practices and outcomes for currently commercialized GMO vs. conventional crops. Farmer surveys are a valuable measure of the impact of GMO crops because they reflect actual farmer practices, given constraints on time, access to resources and information, differing levels of risk aversion and other factors. In an analysis of 49 peer-reviewed publications by a diverse group of researchers, 168 results compared yields of GMO and conventional crops, with 124 results indicating higher yields for GMO crop adopters, 32 results showing no difference and 13 negative results.
The results for yields show that farmers in developing countries are achieving greater yield increases than farmers in developed countries. 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 reported yield increases that range from no change for herbicide-tolerant cotton to a 7% increase for herbicide-tolerant soybean and insect-resistant cotton.
As the most frequently studied case, GM insect-resistant 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 the review were 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. The results from the first year of commercialization in India are believed to have been due to the limited number of Bt cotton varieties that were approved for commercialization in the first year, which had been in the regulatory pipeline for several years, during which time conventional breeding had continued to produce varieties with superior yields and disease resistance.
The accumulated evidence from farmer surveys on the yield performance of currently commercialized GMO crops helps to explain the widespread popularity of the technology in several regions of the world. Given the diversity of conditions across adopting areas, it is striking that the results are so consistently positive.