NOTE: As the abstract for this important new article notes, in general the scientific evidence indicates that "genetic engineering has been at best neutral with respect to yield and in many cotton growing countries the average cotton yields have stagnated since the adoption of Bt cotton." 

Despite this, in India there's been a massive campaign of hype in the media which attributes remarkable increases in yield and other benefits to genetically engineered Bt cotton. 

This hype's only maintained by attributing all good years in Indian cotton production to the "magic" of Bt cotton, while those years in which production or yield falls are explained away as due to a variety of other factors influencing production. 

The complex factors that account for any improvements in cotton production are studiously ignored. These include factors like good monsoons, low pest incidence, a large-scale shift in seed sources, the increased use of chemical fertilisers, a shift from unirrigated to irrigated cotton, etc., which have
all contributed to cotton yield increases
in some years in some states in India.
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Bt Cotton and the Myth of Enhanced Yields
Kavitha Kuruganti 
Economic and Political Weekly, VOL 44 No. 22 May - June 5 2009 
http://epw.in/epw/user/viewAbstract.jsp

ABSTRACT: It is presumed that remarkable increases in cotton productivity in India have come about through bacillus thuringiensis cotton and that this approach therefore must be replicated in other crops. This article explores the myth of rising yields of genetically modified crops and points out that genetic engineering has been at best neutral with respect to yield and in many cotton growing countries the average cotton yields have stagnated since the adoption of Bt cotton. 

View Full Article 
http://epw.in/epw/uploads/articles/13563.pdf