It’s not total protein that counts; it’s amino acid balance and availability.
That is one conclusion from a research study a few years ago at Kansas State University, in which cows fed an 18-percent-protein diet milked less than cows fed a 16-percent-protein diet. (See chart below.)
Heat-treated screw press (expeller) soybean meal is more “bypass” in nature than solvent-extracted soybean meal. That allows many of the essential amino acids to escape rumen degradation and get to the small intestine for absorption. In particular, more methionine is delivered to the small intestine, which aids milk production. Methionine is often limiting in the diets of high-producing cows (i.e., those producing more than 90 pounds per day).