A paper written 35 years ago summarized soybean meal use in the 1980s.  This paper still has direct application to the role soybean meal has in current livestock and poultry operations.  A few paragraphs from the paper seem appropriate for citing:

“The usefulness of any feed ingredient is dependent on several factors.  The three most important are nutrient composition, availability and price.  Since the mid-1950s nutritionists have been programing the computer with information on these three factors to calculate the relative value of various feed ingredients.

The complementary relationship between soybean products and cereal grains in meeting animal nutritional requirements has rapidly expanded soybean product utilization.  Soybean meal is the dominant supplemental protein in the U.S. livestock and poultry rations.  Heat processed full-fat soybeans and soybean hulls are valuable feed ingredients often used when their price allows inclusion in rations.”