The GM 'higher yields' mantra

May 2011
Image from Wikimedia Commons
Throughout the short history of GM plants, the crops have been promoted for their higher yields. British academics are repeatedly quoted as saying that GM crops allow farmers to attain “high yields” while using less chemicals.

However, at least one Canadian Professor of Plant Agriculture, Ann Clark, has suggested “let's stop with the 'higher yield' mantra and stick with the scientifically defensible evidence”.

Take GM cotton ...

Cotton has been subject to extensive commercial genetic transformation. The first GM cotton, in 1995, was resistant to bromoxynil herbicide, followed quickly by Bt-generating insecticidal and glyphosate herbicide-resistant varieties. After that, the artificial traits began to stack up with other Bt-toxin variants, and different kinds of herbicide-tolerance.

In Monsanto's 1997 Report, readers were informed that, the previous year, “The company's insect-protected cotton provided a 7% yield improvement”. In 2011, Monsanto was complaining about the efforts of NGOs seeking to protect farmers from Monsanto's infamous lawsuits when their fields suffered genetic pollution. The company stated that “These (NGO) efforts seek to reduce private and public investment in the development of new and higher-yielding seed technologies.” The link between 'GM' and 'higher yields' has thus been firmly established in the public, agricultural, political and academic perception.

But is it true?

A report from the US Department of Agriculture (USDA) in 2006 stated 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.”

A year-on-year comparison of cotton yields based on USDA data, reveals that these have increased an impressive 5-fold since 1930. This included a distinctive surge from 1980 to the early 1990s before GM cotton was introduced. During the seven years when GM cotton was rising to dominance, 1996-2003, yields went flat and stagnated until excellent weather conditions produced higher yields in 2004-5.

Reports coming in from all over the world are telling a similar story: Bt cotton introduced to Australia in 1996 has not boosted the cotton sector with improvements in either yield or quality; South Africa has shown a constant yield level before and after the adoption of Bt cotton; secondary pests on Bt cotton have crippled yields in China; Indian farmers chasing the “white gold” promise of Bt cotton have ended “knee-deep in debt” from the need to buy expensive GM seed, pesticides and fertilizers on credit (shored up apparently by bribes and threats if they try to complain). (For more on India, see INDIA JUDGES Bt COTTON – April 2011).

A US study published in 2008 made a direct comparison between several GM and non-GM cotton varieties over a four-years period. In contrast to the standard scientific procedures for conducting official cultivar trials (OCTs) which prescribe a common pest management system for all tests, each crop variety was instead managed to maximise profit as recommended by the University of Georgia Extension Service.

The conclusions reached were that:
“When considered as a whole, no transgenic technology system provided greater returns than a nontransgenic system in any year or location.” and that “profitability was most closely associated with yields and not the transgenic technologies.”
In one instance, the return from the GM test crop was 40 percent lower than its non-transgenic counterpart. Also, there was “no difference in returns (profit)” between the GM and non-GM cultivars once the technology fees for the biotech seeds were factored in. Growers do, however, associate GM crops with convenience in terms of reduced labour time and costs: a benefit often attributed to transgenic production systems is the ability to farm more hectares or to reduce personnel.

Interestingly, the authors identified a number of biasing factors in previous data on GM crop yields. Farmers have only industry advertising and OCT information to base their choice of seed on. These are not necessarily based on best management practice for the crops. Besides this, the rate of development of GM crop varieties, and especially the newer multiple-trait types, has outpaced the capability of OCTs to evaluate their agronomic merits.

Another factor which might become increasingly important in future trials of GM crops is that the supply of appropriate non-GM varieties to compare them with is dwindling, not just because the biotech industry isn't producing them but because seed stocks are becoming contaminated.

Note also the yield claims made for GM soya and maize:
  • With regard to GM soya, which Monsanto claimed provided a 5 percent yield improvement in 1997, university studies from 1999 to 2007 consistently show Roundup Ready soya to yield 4-12 percent less than conventional varieties.
  • With regard to GM corn, which Monsanto claimed provided an 8 percent yield improvement in 1997, a university study of several Bt maize hybrids compared with their closest non-GM relatives found “no yield advantage of Bt hybrids” when pest levels were low to moderate, which is the case for most farms and for most years.


Not much hard evidence there that GM cotton crops yield more or are more profitable than correctly-chosen and correctly managed non-transgenic varieties. The biggest danger suggested here is that farmers are being made dependent on biased industry information when choosing seed, and are being deskilled. As long as the biotech industry can churn out new GM seeds faster than any one can evaluate them, it's likely to be successful in bamboozling US farmers.

Make sure EU regulators are not tempted to be similarly bamboozled. Make it clear you don't want untested, industry-dependent, GM food. Also, consider challenging any claims in the press that GM crops yield more: ask for “scientifically defensible evidence”.

  • Monsanto 1997 Report, March 1998
  • P. Jost et al., 2008, Economic Comparison of Transgenic and Nontransgenic Cotton Production Systems in Georgia, Agronomy Journal 100:1
  • Transgenic Cotton Offers No Advantage, Institute of Science in Society Press Release, 27.03.08
  • Mitch Lies, Researcher finds GMO traits in non-GMO canola, Capital Press, 22.04.11
  • B. L. Ma and K. D. Subedi, 2005, Development, yield, grain moisture and nitrogen uptake of Bt corn hybrids and their conventional near-isolines, Field Crops research 93:2-3
  • New Soil Association report shows gm corps do not yield more – sometimes less, Food Democracy, 18.04.08
  • Carey Gillam, Organic farmers sue, seek protection from Monsanto, Reuters, 29.03.11
  • E. Ann Clarke, Let the Roeld Learn From Our Experience with GMOs,

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