The crude protein content of a diet has been the term historically used as an indicator that indirectly reflects the pig's requirement for amino acids. The term "crude" is used because feedstuffs not only contain amino acids (each amino acid contains nitrogen), but grains and other feeds also contain non-amino acid nitrogen. When the crude protein content of a grain is determined, it is the total nitrogen that is analytically determined, whereupon it is then calculated mathematically to a protein equivalent value, thus the term "crude protein." The nonprotein portion of the diet (e.g., urea, nitrates, etc.) is not effectively used by the pig but its nitrogen is analytically determined along with the nitrogen in the proteins.
Pigs in fact do not specifically need protein, but rather require amino acids for the formation of muscle and other body proteins. Ten of the 20 amino acids required are considered as dietary essentials because the pig is not able to synthesize them at all or at a rate that will adequately meet the pig's needs. Therefore, there are 10 essential amino acids (Table 1) that must be provided in the diet at a minimum level in order to meet the pig's requirement for the development of the body proteins.
Amino acids are also required in a precise proportion or ratio for each body protein. Because the development of the different body proteins occurs at differing rates, the amino acid requirements are constantly changing.
The balance of dietary amino acids can affect their overall utilization. Expressing the die-tary requirements for the 10 essential amino acids on the basis of an ideal amino-acid balance (ideal protein) is based on the principle that the amino-acid composition of the diet resembles the amino-acid composition of developing pig tissue with an additional amount needed for maintenance. An ideal protein mixture in the diet thus contains the optimum balance of all amino acids required for the maintenance, growth, and/or reproduction functions of the pig. The ratios of the different amino acids are, however, usually expressed in relation to the dietary lysine requirement. Although we currently use "total" amino-acid concentration, the use of "digestible" or "available" values will be more widely used in the near future, as they more accurately reflect what is absorbed and/or utilized from the various feed sources.
Lysine is generally the first-limiting amino acid (i.e., the first amino acid to be deficient) in a cereal grain-soybean meal mixture. Therefore, we usually formulate swine diets to a specific lysine, rather than protein, level. This results in the other amino acids being in excess. There is no scientific information that indicates that slight excesses of amino acids in corn-soybean meal diets are detrimental to the performance of pigs. Rather, excess amino acids are metabolized in the pig to urea, the end-product of nitrogen metabolism, and it is subsequently excreted in the urine. Practical methods to provide a more ideal amino acid balance include the use of various protein source combinations and/or "synthetic" amino acid additions. Supplementing a low-protein diet with amino acids such as lysineHCl (78.8% lysine) is oftentimes more cost effective than using an additional amount of a particular protein source. It also has the advantage of reducing nitrogen excretion in the manure. Other than synthetic lysine or methionine, it is generally not economical to consider adding other synthetic amino acids to swine diets because they generally are too expensive.
Lowered protein quality may occur during the processing of grain or animal products, especially when excessive heat is used. This heat modifies the chemical structure of the protein which may reduce the digestibility and bioavailability of certain amino acids to the animal.
Although protein or amino acids, when fed in excess of the pig's need, can be a source of energy, the use of protein as an energy source is not recommended because of the higher cost and higher nitrogen excretion in the animal's waste products.