* Benbrook publication now available online
* excerpt from exec summary
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http://www.biotech-info.net/first_generation_GMC.pdf
The above is a 57 pp Benbrook publication entitled Economic and Environmental Impacts of First Generation Genetically Modified Crops. published in 2002, just uploaded this week at his brilliant website.  He covers Argentina as well as the US, and comments on the Fernandez-Conejo-McBride 2002 report.
Ann
E. Ann Clark, Ph.D.
Associate Professor
Department of Plant Agriculture
University of Guelph
Guelph, Ontario, Canada N1G 2W1
(519) 824-4120 Ext. 52508
http://www.plant.uoguelph.ca/research/homepages/eclark/
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Excerpt from the executive summary:

2.3. Yield data
There is clear and consistent evidence in the U.S. that since introduction in 1996 most RR soybean cultivars produce 5 percent to 10 percent fewer bushels per hectare/acre in contrast to otherwise identical varieties grown under comparable field conditions. There is evidence that this "yield drag" has been reduced somewhat in recent years, as the Roundup tolerant trait has been moved into a broader diversity of varieties, offering farmers a better match to their soil type and maturity zone.

A team at the University of Nebraska estimated that the yield drag between RR varieties and otherwise similar varieties, when grown under comparable conditions, is about 6 percent. In a January 2001 story on corn and soybean seed selection, Farm Journal magazine published the results of independent soybean yield trials in three states conducted under conditions designed to match those on commercial farms. In Indiana, the top RR variety offered by three seed companies yielded, on average, 15.5 percent fewer bushels than the top conventional variety from the same company. In Illinois plots, however, the top RR to top conventional yield drag across eight companies was less than 1 percent. In Iowa trials, the RR yield drag was just under 19 percent across 17 companies.