Postwar hybrid corn

I'm investigating the breeding of fertilizer-intensive hybrid corn in the 1950s in the US.  Two questions:

One source (Steele 1978, "The hybrid corn industry in the United States") says by 1955 some hybrids were being marketed as especially bred for high plant densities.  I need to track down this marketing material -- adverts, speeches, brochures, anything. 

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