http://www.indexmundi.com/agriculture/?country=in&commodity=cotton&graph=yield
Our results show that Bt cotton contributed significantly to cotton yield growth ... total increase contribution of 19 percent over time between 1975 and 2010. ... Second, our findings suggest that Bt did contribute to the second increase in cotton productivity (after 2005) but remain inconsistent regarding the possible impact of unofficial Bt cotton adoption in the early years.
www.ifpri.org/sites/default/files/publications/ifpridp01170.pdf