Coinciding with a manifesto from Country Life launched today, which urges people to 'learn to love GM crops', the Soil Association has published a report on the latest available research on GM crop yields over the last ten years. The yields of all major GM crop varieties in cultivation are lower than, or at best, equivalent to, yields from non-GM varieties.
Peter Melchett, Soil Association policy director, said: "GM chemical companies constantly claim they have the answer to world hunger while selling products which have never led to overall increases in production, and which have sometimes decreased yields or even led to crop failures. As oil becomes scarcer and more expensive, we need to move away from oil dependent GM crops to producing food sustainably, using renewable energy, as is the case with organic farming."
Latest Research on GM Crop Yields
GM crops as a whole
First generation genetic modifications address production conditions (insect and weed control), and are in no way intended to increase the intrinsic yield capacity of the plant.
An April 2006 report from the United States Department of Agriculture (USDA) states that "currently available GM crops do not increase the yield potential of a hybrid variety. [.] In fact, yield may even decrease if the varieties used to carry the herbicide tolerant or insect-resistant genes are not the highest yielding cultivars". (Fernandez-Cornejo, J. and Caswell, 2006)
The United Nations Food and Agriculture Organization's 2004 report on agricultural biotechnology acknowledges that GM crops can have reduced yields (FAO, 2004). This is not surprising given that first-generation genetic modifications address production conditions (insect and weed control), and are not intended to increase the intrinsic yield capacity of the plant.
A 2003 report published in Science stated that "in the United States and Argentina, average yield effects [of GM crops] are negligible and in some cases even slightly negative". (Qaim and Zilberman, 2003). This was despite the authors being strong supporters of GM crops.
Yields of both GM and conventional varieties vary - sometimes greatly - depending on growing conditions, such as degree of infestation with insects or weeds, weather, region of production, etc. (European Commission, 2000)
Roundup Ready (RR) GM soya
Studies from 1999 - 2007 consistently show RR GM soya to yield 4 12% lower than conventional varieties.
A 2007 study by Kansas State University agronomist Dr. Barney Gordon suggests that Roundup Ready soya continues to suffer from a yield drag: RR soya yielded 9% less than a close conventional relative.
A carefully controlled study by University of Nebraska agronomists found that RR soya varieties yielded 6% less than their closest conventional relatives, and 11% less than high yielding conventional lines (Elmore et al, 2001). This 6% 'yield drag' was attributed to genetic modification, and corresponds to a substantial loss in production of 202 kg/ha.
In 1998 several universities carried out a study demonstrating that, on average, RR soy varieties were 4% lower in yield than conventional varieties (Oplinger et al., 1999). These results clearly refuted Monsanto's claim to the contrary (Gianessi, 2000).
Yields of GM soybeans are especially low under drought conditions. Due to pleiotropic effects (stems splitting under high temperatures and water stress), GM soybeans suffer 25% higher losses than conventional soybeans( Altieri and Pengue, 2005)
5 studies between 2001-2007 show that glyphosate applied to Roundup Ready soybeans inhibits the uptake of important nutrients essential to plant health and performance. The resultant mineral deficiencies have been implicated in various problems, from increased disease susceptibility to inhibition of photosynthesis. Thus, the same factors implicated in the GM soya yield drag may also be responsible for increased susceptibility to disease. (Motavalli, et al., 2004; Neumann et al., 2006; King, et al.,2001; Bernards,M.L, 2005; Gordon, B., 2007). The yield drag of RR soya is reflected in flat overall soybean yields from 1995 to 2003, the very years in which GM soya adoption went from nil to 81% of U.S. soybean acreage. By one estimate, stagnating soybean yields in the U.S. cost soybean farmers $1.28 billion in lost revenues from1995 to 2003 (Ron Eliason, 2004). More recent evidence shows that the kilogram per hectare ratio of soybean has been in decline since 2002, leading to the conclusion that RR soy does not have an impact on yield (ABIOVE, 2006a).
Only maize shows a persistent trend of yield increase into the biotech era, but even here the rate of increase is no greater after than before biotech varieties were introduced.
A rigorous, independent study conducted in the U.S. under controlled conditions demonstrated that Bt maize yields anywhere from 12% less to the same as near-isoline (highly similar) conventional varieties (Ma & Subedi, 2005).
Despite claims of increased yield, Bt cotton has had no significant impact in real terms.
Average cotton yields have increased 5-fold since 1930, and staged an impressive surge from1980 to the early 1990s. Cotton yields then went flat, and continued to stagnate during the seven years of GM cotton's rise to dominance. The steep yield and production increases in 2004 and 2005 were chiefly attributable to excellent weather conditions (Meyer et al., 2007).
Bt cotton, introduced to Australia in 1996, has not offered a boost to the cotton sector, and since its adoption has not provided improvements in either yield, or quality (ISAAA, 2006b).
Cotton South Africa show constant yield levels before and after adoption of Bt cotton (Witt et al 2005, cited in FoEI Who Benefits 2007), in contradiction to ISAAA claims that Bt has brought about a 24% yield increase in the region. Outbreaks of the secondary pests that are not killed by the Bt insecticide have rendered Bt cotton ineffective in China (Connor, S., July 27, 2006), and are also becoming a problem in North Carolina (Caldwell, D. 2002) and Georgia (Hollis, P.L., 2006).
An article in Nature Biotechnology notes that the poor performance of Bt cotton varieties used in India (which were developed for the short U.S. growing season) is linked to the loss of their insecticidal properties late in India's longer growing season, and because Bt cotton insecticide is not expressed in 25% of the cotton bolls of India's preferred hybrid cotton varieties (Jayaraman, K.S., 2005)
During the Government's 2003 'national debate' on whether or not to allow commercial planting of GM crops, the Royal Institute of Chartered Surveyors, which represents land agents amongst others, predicted 'long-term chaos' and possible declines in land values if GM crops were planted.  Recent research in Sweden has confirmed that GM seeds can remain active in farmland for at least 10-years, adding scientific support to the RICS's concern about the impact on land values of growing GM crops.
For media enquiries please contact Clio Turton, Soil Association senior press officer, 0117 914 2448 / firstname.lastname@example.org
Notes to editor:
 Extract from an article published in Daily Telegraph: GM crop trials 'pose threat to property prices' By Charles Clover, Environment Editor (4 June 2003) Property prices could be undermined if land is polluted with traces of genetically modified crops, the Royal Institution of Chartered Surveyors said yesterday at the start of a Government-sponsored debate on whether Britain should approve commercial GM varieties. Surveyors and land agents warned of "long-term chaos" in the property market unless buyers were provided with information on the farms, allotments and gardens where GM crops were or had been grown. The RICS said accurate information on where GM crops were planted was essential to buyers wishing to purchase or rent land for non-GM or organic production and to financial institutions lending against land and property.
ABIOVE, 2006a. Sustainaibility in the Legal Amazon. Presentation by Carlo Lovatelli at the Second Roundtable on Responsible Soy. Paraguay, 1 September 2006. http://www.abiove.com.br/english/palestras/abiove_pal_sustent_amazonialegal_ us.pdf
Altieri, M., Pengue, W., 2005. GM Soya Disaster in Latin America: Hunger, Deforestation and Socio-ecological Devastation.
Bernards, M.L. et al, 2005. Glyphosate interaction with manganese in tank mixtures and its effect on glyphosate absorption and translocation. Weed Science 53: 787-794.
Caldwell, D. 2002. A Cotton Conundrum. Perspectives OnLine: The Magazine of the College of Agriculture and Life Sciences, North Carolina State University,Winter 2002. http://www.cals.ncsu.edu/agcomm/magazine/winter02/cotton.htm
Connor, S., July 27, 2006. Farmers use as much pesticide with GM crops, US study finds. The Independent. http://news.independent.co.uk/environment/article1199339.ece
Elmore et al, 2001. Glyphosate-Resistant Soybean Cultivar Yields Compared with Sister Lines, Agron J 2001 93: 408-412, quote from the University of Nebraska press release online at http://ianrnews.unl.edu/static/0005161.shtml
European Commission, 2000. Economic Impacts of Genetically Modified Crops on theAgri-food Sector. http://europa.eu.int/comm/agriculture/publi/gmo/cover.htm
FAO, 2004. The State of World Food and Agriculture 2004. Biotechnology: Meeting the Needs of the Poor? http://www.fao.org/newsroom/en/focus/2004/41655/
Fernandez-Cornejo, J. & Caswell. April 2006. Genetically Engineered Crops in the UnitedStates. USDA/ERS Economic Information Bulletin n. 11. http://www.ers.usda.gov/publications/eib11/eib11.pdf
FoEI, January 2007. Who Benefits from GM crops? An analysis of the global performance of GM crops (1996-2006)
Gianessi, L.P., April 2000. Agriculture Biotechnology: Benefits of Transgenic Soybeans. National Center for Food and Agricultural Policy, p. 63. http://www.ncfap.org/reports/biotech/rrsoybeanbenefits.pdf
Gordon, B., 2007. Manganese nutrition of glyphosate-resistant and conventional soybeans. Better Crops, Vol. 91, No. 4: 12-13
Hollis, P.L., February 15 2006. Why plant cotton's new genetics? Southeast Farm Press. http://southeastfarmpress.com/mag/farming_why_plant_cottons/
ISAAA, 2006b. GM crops: the first ten years- Global Socio-Economic and Environmental impacts. http://www.isaaa.org/resources/publications/briefs/36/download/isaaa-brief- 36-2006.pdf
Jayaraman, K.S., November 2005. Monsanto's Bollgard potentially compromised in India. Nature Biotechnology.
King, A.C., L.C. Purcell and E.D. Vories, 2001. Plant growth and nitrogenase activity of glyphosate-tolerant soybean in response to foliar glyphosate applications. Agronomy Journal 93:179-186.
Ma & Subedi, 2005. "Development, yield, grain moisture and nitrogen uptake of Bt corn hybrids and their conventional near-isolines," Field Crops Research 93 (2-3): 199-211, at
Meyer, L., S.MacDonald& L. Foreman,March 2007. Cotton Backgrounder. USDA Economic Research Service Outlook Report.
Motavalli, P.P. et al., 2004. "Impact of genetically modified crops and their management on soil microbially mediated plant nutrient transformations," J. Environ. Qual. 33:816-824;
Neumann, G. et al., 2006. "Relevance of glyphosate transfer to non-target plants via the rhizosphere," Journal of Plant Diseases and Protection 20:963-969.
Oplinger, E.S et al., 1999. Performance of Transgenetic Soyabeans, Northern US. http://www.biotech-info.net/soybean_performance.pdf
Qaim, M. and Zilberman, D., 7 February 2003. "Yield Effects of Genetically Modified Crops in Developing Countries" in Science, vol. 299, p. 900.
Ron Eliason, 2004. Stagnating National Bean Yields. 2004 Midwest Soybean Conference, cited by Dan Sullivan, "Is Monsanto's patented Roundup Ready gene responsible for a flattening of U.S. soybean yields," NewFarm.org, September 28, 2004, online at http://www.newfarm.org/features/0904/soybeans/index.shtml