Murgen, it would be wrong to dispute that a particular study does or does not show some pressure either positive or negative.  The problem with studies that have been conducted is that they only look for negative correlation at the producer price but fail to consider the positive impacts that can more then offset any detectable negative impact.  I believe the best study conducted was by Dr Azzam from the University of Nebraska.  His data did show a minor negative correlation.  However, he very clearly states that he did not look for or measure the positive impacts from marketing agreements that could lead to higher prices.   He was at least honest and made full discloser of the strengths and weakness of his findings.