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Unintended Effects of Genetic Manipulation
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Posted: January 2015

Unintended Effects of Herbicide-Resistance Genes on Crop Yield
Craig Holdrege

This review of unintended effects of herbicide-resistant genes on crop yields deals with crops developed by traditional breeding or through mutagenesis, and genetically modified (GM) crops (Darmency 2013). I refer here only to GM crops and the comparison to their isogenic lines. The author reviews only those studies that compare the two crop varieties when the GM crops are not sprayed with the herbicide to which they are resistant. As he remarks, “this comparison represents the true basis for evaluation under ‘natural’ conditions. In addition, relative yield and fitness of HRCVs [GM herbicide-resistant crop varieties] are important data to anticipate their potential to invade field and uncultivated habitats.”

By far the most widely planted GM herbicide-resistant crops are those that are resistant to the herbicide glyphosate. Darmency reviews studies of glyphosate-resistant soybeans (for which there are the most comparative studies), cotton, canola, and maize:

Crop Yield Impact
Glyphosate-resistant Soybeans – 5 %
Glyphosate-resistant Cotton no difference
Glyphosate-resistant Canola – small difference
Glyphosate-resistant Maize (Corn) + and –

Also for the other GM herbicide-resistant crop varieties (resistance to bromoxynil, chlorosulforon, and glufosinate) the reviews reported mainly no yield differences between the GM varieties and their isogenic counterparts. In some cases herbicide-resistant varieties had significantly lower yields.

Based on this review, GM herbicide-resistant varieties have negligible impact on yield and when the impact is substantial, then the GM varieties have lower, not higher yields.


Darmency, H. (2013). “Pleiotropic Effects of Herbicide-Resistance Genes on Crop Yield: A Review,” Pest Management Science vol. 69, pp. 897-904. doi:10.1002/ps.3522

Note: The author is a scientist at the French National Institute for Agricultural Research (INRA).

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