Here’s a program note from ABC indicating that yield of GMO crops is no greater than conventional crops. Unfortunately there is no formal citation of the source article, nor is it available on line as far as I can tell, and ABC tends to remove their program notes after a few weeks. However, the article, from the International Journal of Agricultural Sustainability, was written by Professor Jack Heinemann of the University of Canterbury in New Zealand, which might be enough information to locate it. Heinemann’s web page links to an article which may even be the one in question.