Effects of Biotechnology on Corn Yields

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.

That's 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 most recent issue of the American Journal of Agricultural Economics.