**2. Essential amino acids**

Essential amino acids are ones that living organisms are unable to biosynthesize themselves and must obtain from their food source [7, 12, 16]. Therefore, in this term, "essential" refers to the amino acid requirements in dietary ingredients. The nine standard essential amino acids for humans present in soybean are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine [17]. Arginine is regularly considered an essential amino acid for fish, poultry and sometimes swine due to absent or deficient urea cycles [9, 16]. Poultry and reptiles also require dietary glycine because of differing waste excretion pathways [16]. While crude protein content is normally recognized as the driving nutritional factor for soybean meal, these essential amino acids provide true utility.

It has long been recommended that protein quality is based upon essential amino acid content. However, for many reasons, animal feed and human food markets have only recently begun assessing accordingly [5, 18, 19]. Equipment required for accurate amino acid measurement and the diversity of markets for amino acids makes it difficult for supply chain evaluators like elevator operators to appraise amino acid content on site. To some degree, the well-balanced soybean amino acid profile also devalues the need to measure individual amino acid levels. Since all essential amino acids are present, less attention is paid to deficient amino acids such as methionine and tryptophan [17, 20].

Deficiencies in soybean's essential amino acid profile has led to a large section of the livestock industry focusing on feed mixing and supplementation. Rationing with other feed sources such as cereal grain and synthetic amino acid augmentation can effectively resolve the issue. Although, this comes with economic and environmental problems. Supplementing amino acids adds costs to farmers. For example, the average cost for amino acids supplementation for dairy farmers is 20 cents per head per day [21]. Maximizing crude protein for a growth limiting factor also negatively impacts livestock nitrogen-use-efficiency and environmental nitrogen outputs [8, 22]. Synthetic amino acid production can produce hazardous environmental waste and synthetic methionine, the most limiting soybean amino acid for poultry, has also been banned for organic poultry production [20, 23]. Movement towards sustainable agriculture will pressure the feed industry to alter how soybean meal is enhanced for essential amino acid livestock maximization. Furthermore, the increasing popularity of meat-less diets in humans will create new markets for soybean's well-balanced amino acid profile.
