This talk will challenge the idea of defining the dietary protein requirements for lactating cows based solely on the limiting amino acid concept. One form of the ‘limiting amino acid model’ is that milk protein yield potential is set, but it can be reduced below this potential by a deficiency in a single essential amino acid. A corollary of this is that the total dietary crude or metabolizable protein (MP) can be minimized to the point that the requirement for this first limiting amino acid is met. This model certainly includes the possibility of sequential second and third limiting amino acids, which become limiting as the original first limiting amino acid reaches its requirement. One problem with this model in the ruminant animal is that a large portion of the MP is derived from rumen microbial reproduction. Unlike animal protein, a community of microbial species has a malleable genetic blueprint and may respond to the amino acid content of the rumen-degraded protein in a complex way. Another limitation of the limiting amino acid model is that potential milk protein yield may be influenced by many dietary (and other) factors. Insulin is known to increase milk protein yield in cases where milk protein yield is limited by MP or where milk protein secretion has been increased due to additional MP. Removing crude protein from the diet and replacing it with other dietary constituents could affect circulating insulin and modulate milk protein secretion independently of the limiting amino acid. One identified mechanism for insulin is through activation of mTOR, a regulatory protein in mammary secretory cells. This same mTOR is simultaneously regulated by essential amino acids, but the essential amino acids that have the largest effect (e.g., leucine, isoleucine), are different than the those normally suggested as most limiting (methionine, lysine and histidine). Optimizing milk protein secretion and nitrogen efficiency must consider the amino acid composition of MP and animal needs, but careful experimentation and quantitative model development must consider alternatives beyond only the concept of one or a few limiting amino acids.