USDA: GM seeds do not increase yields, may even lower them
“Over the first 15 years of commercial use, GE seeds have not been shown to increase yield potentials of the varieties. In fact, the yields of herbicide-tolerant or insect-resistant seeds may be occasionally lower than the yields of conventional varieties if the varieties used to carry the HT or Bt genes are not the highest yielding cultivars, as in the earlier years of adoption” (page 12).
US Department of Agriculture, Economic Resource Service, Jorge Fernandez-Cornejo, Seth Wechsler, Mike Livingston, and Lorraine Mitchell, “Genetically Engineered Crops in the United States,” ERR-162, February 2014, accessed July 29, 2014, http://www.ers.usda.gov/publications/err-economic-research-report/err162.aspx#.U9fa9yjyC8A