Two interesting articles examining the contribution of GM to crop yields in the USA.
From the article The Contribution of Genetic Modification to Changes in Corn Yield in the United States by Elizabeth Nolan and Paulo Santos in the American Journal of Agricultural Economics2012,94 (5): 1171-1188. doi: 10.1093/ajae/aas069 First published online: July 18, 2012.
In relative terms, this suggests that the presence of GM technology may have led to corn yield gains 1.4–1.5 times higher than would have been achieved with traditional crop breeding techniques alone.
In the correspondence section the article “Commercialized transgenic traits, maize productivity and yield risk” by Guanming Shi, Jean-Paul Chavas and Joseph Lauer was published in the journal Nature Biotechnology, 31,111–114,(2013) doi:10.1038/nbt.2496. this article shows that
GM varieties have had an important risk-reducing effect, even if they sometimes led to slightly lower yields.