Amino acids (AA) are the organic compounds made from amine (-NH2) and carboxylic acid (-COOH) functional groups, along with a side-chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, though other elements are found in the side-chains of certain amino acids (Fig. 1).
Fig. 1: General structure of amino acids.
Regulatory roles for AA in nutrition and metabolism have long been ignored until recent important findings that dietary glutamine is necessary for intestinal mucosal integrity and dietary arginine is required for maximum neonatal growth and embryonic survival (Wu, 2010).
Nutritionally essential AA (EAA) are those whose carbon skeletons are not synthesized by animal cells and, therefore, must be provided from the diet. Twenty-two amino acids are needed for body protein synthesis. Out of these twenty-two amino acids EAA for pigs are Arginine, Methionine, Histidine, Phenylalanine, Isoleucine, Threonine, Leucine, Tryptophan, Lysine, valine. In contrast, nonessential AA (NEAA) are those AA that are synthesized de novo within the body and not need them in diet. However, growing evidence from cell culture and animal studies shows that some of the traditionally classified NEAA (e.g. glutamate, glutamine and arginine) play important roles in multiple signaling pathways, regulation of gene expression, nutrient metabolism and oxidative defense (Brasse-Lagnel et al, 2009; Yao et al, 2008). A deficiency of a FAA (either EAA or NEAA) impairs not only protein synthesis but also whole-body homeostasis (Fig.2). Notably, supplementing a specific FAA (e.g. glutamine or arginine) to a conventional diet that was traditionally thought to provide adequate AA can maximize growth potential in young animals (Wu, 2009; Wu et al,2004 ) and prevent diseases (e.g. obesity, diabetes, necrotizing enterocolitis, and intrauterine growth retardation) in both animals and humans (Wu et al,2009).
Fig.2: Roles of AA in whole-body homeostasis. Besides serving as building blocks for proteins, AA has multiple regulatory functions in cells. (Adopted from Guoyao Wu, 2010. Functional amino Acids in growth, reproduction, and health. Advances in Nutrition. 1: 31-37)
Limiting amino acids in pig:
The amino acid that is present in a feed in the least amount relative to pig requirements is said to be the ‘limiting amino acid’. The requirements of amino acids in animals are well defined in various sets of recommendations such as those of NRC (National Research Council), USA. Requirements vary depending on the species and age of animals. Amino acids should be supplied either in the form of protein or crystalline amino acids in feed to meet requirements. High quality protein contains all of the essential amino acids at acceptable levels, poor quality protein is deficient in one or more. Since the proteins used in pig diets are of variable quality some of the essential amino acids may be deficient. These are called the limiting ones and in most cases lysine is the most likely, followed by methionine and both are often added to diets routinely. If the diet is deficient in one or more of these essential amino acids then protein synthesis will only continue to the level associated with the first limiting amino acid. By comparing requirements and the actual amino acids present in feed, the order of ‘limiting amino acids’ is estimated. The orders of limiting amino acids in pig and broiler feeds, composed of corn (or wheat) and soybean meal, are-
Crystalline amino acids should be added to feed in the order of limiting amino acids when the protein content of the feed is reduced. Now, with a more economic supply of L-Threonine and L-Tryptophan available, use of amino acids has entered a new era, in which the use of second and third limiting amino acids is taking off.
Table: Limiting amino acids in cereal grains used for pig diet. (Adopted from http://informedfarmers.com/amino-acids/)
Metabolism of AA-
Swine feed contains vegetable and animal sources of AA; soyabean is the best-known of the vegetable sources AA and fishmeal is an animal source of AA. Amino acids in these feeding stuffs are found mainly in complete protein chains. The proteins are broken down by the enzymatic digestion in the gastro intestinal tract to release individual AA (or short chains of them). These are then absorbed into the blood stream and used to make new proteins that will be useful to the animal. The major metabolic fate of amino acids in the gut is considered to be their incorporation into cellular proteins.
Fig. 3: Metabolism of protein.
General effect of AA deficiency in livestocks:
Insufficient AA intake in young animals results in lowered body weights. In young animals, there is reduced appetite, lowered feed intake, reduced nitrogen retention, poor feed efficiency, inferior growth rate, lack of muscle development and prolonged time to reach maturity. In mature animals there is loss of weight, and reduced milk or egg production. In both young and mature animals there is a drop in haemoglobin concentration, packed cell volume (PCV), total serum protein and serum albumin. In the late stages, there is oedema associated with hypoproteinaemia.
Dietary AA Requirements (%) of Growing Pigs Allowed Ad Lib Feed (90% dry matter)
Dietary Nutrient Requirements (%) of Gestating and Lactating Sows (90% dry matter)
Alternative AA sources can be fed to pigs-
Substitutes for soybean meal in swine diets as a protein source should appear to be economical compared to soybean meal, comparable chemical composition to ensure optimum pig performance.
1. Cottonseed Meal.
Cottonseed meal ranks second in production compared to soybean meal. However, its use in swine diets is limited because of the deleterious effects produced by the residual free gossypol found in the pigment glands of the seed. Although fairly high in protein, cottonseed meal is low in lysine and tryptophan. It is recommended that cottonseed meal replace no more than 50 percent of the soybean meal or protein supplement in the diet so that this inclusion rate does not exceeds 0.01 percent free gossypol concentration in diet as pig performance begins to be reduced at gossypol concentrations of 0.04 percent of the diet. However, gossypol-free (olvent-extracted) cottonseed meal can be used to replace 75 percent of the protein source in growing-finishing diets when balanced on a lysine basis.
2. Sunflower Meal.
Sunflower meal is produced by extraction of the oil from sunflower seeds. Because of its high fiber content (22-24%), it should be used in limited quantities in swine diets. Sunflower meal is relatively low in lysine yet high in sulfur-containing amino acids in comparison to soybean meal. Sunflower meal containing high levels of oil will produce soft pork because of the oil's unsaturated fatty acid content. It appears that sunflower meal may replace up to 25 percent of the protein in the diet for growing-finishing pigs.
3. Meat and Bone Meal.
Meat by-products are often economically feasible to add to swine diets. In general, meat and bone meal is an excellent source of calcium and phosphorus. However, it is often very low in tryptophan and methionine. Since there is considerable variation in the type and quality of the raw materials used, there is potential for greater variation in the quality of meat and bone meal. However, excessive heating during the processing of meat and bone meal may decrease its digestibility and value as a protein source. Therefore, it is recommended that meat and bone meal should not exceed 25 percent of the protein supplement.
4. Raw Soybeans.
Raw soybeans, especially weather damaged or low test-weight beans, are often attractive alternatives to add to swine diets. However, raw soybeans contain high quantities of trypsin inhibitors, which block normal protein digestion in pigs. As the pig becomes older, its susceptibility to trypsin inhibitor decreases. Therefore, raw soybeans may be used only in gestation diets without adversely affecting performance. If raw soybeans are to be sued in diets for young pigs, it is important to heat the beans to inactivate the trypsin inhibitor.
5. Rapeseed Meal.
It is the by-product of vegetable oil processing from rapeseed. Rapeseed meal contain averages between 35 and 40 percent crude protein and has less lysine but more sulfur-containing amino acids than soybean meal. Some varieties of rapeseed contain high levels of a toxic compound, glucosinolate, which effects thyroid functioning. Rapeseed meal can be used to replace up to 50 percent of the protein from soybean meal in growing-finishing and sow diets without adversely affecting performance.
Environmental nitrogen contamination
Environmental nitrogen contamination attributed to the livestock industry arises from the fact that animal efficiency for converting feed into animal protein is less than 100%. Though swine are among the most efficient, they convert only 35 to 45% of ingested dietary nitrogen to meat. It has been reported that reducing dietary protein by four percentage units with addition of lysine, methionine, threonine, and tryptophan results in a marked reduction in urinary nitrogen excretion from growing-finishing pigs (Carter et al., 1996). Effective reduction of nitrogen excretion in swine production requires an examination of the origin of the problem, namely nitrogen (protein) intake.
Nitrogen excretion in pigs may be reduced by:
1. Phase feeding to provide a series of diets closely matching the pig's requirements.
2. Utilization of commercially available amino acids combined with reduced protein levels to provide better balanced diets. Formulating diets based on ideal protein concept would be useful.
3. Reducing nitrogen excretion via nutritional means may provide a less expensive solution on a long term basis. (Adopted from Reducing Nitrogen Output in Pig Production; SWINE RESEARCH REPORT 13; http://www.lysine.com/pdf/swine/srr13.pdf)
USE OF SYNTHETIC AMINO ACIDS IN PIG DIET:
Synthetic amino acids can be used to replace a portion of the protein in the diet to meet the amino acid needs of the pig. The increasing availability of synthetic amino acids continues to make their use a more economically viable option for swine diets.
Advantages use of synthetic amino acids in pig diet:
Use of synthetic amino acids lowers the crude protein level of the diet. Synthetic lysine is almost always an economical addition to swine diets, but prices vary between synthetics and one may be economical while another may not at a given price. Nitrogen excretion and ammonia emissions are reduced as greater levels of synthetic amino acids are used in the diet and relative crude protein levels decrease. The net energy level of the diet also increases as grain and synthetic amino acids replace protein sources in the diet.
Disadvantages use of synthetic amino acids in pig diet:
Most synthetic amino acids are available in dry or liquid forms. In order to use liquid forms, pumps and metering devices are required. Te dry forms are free ?owing and do not pose great handling concerns and are relatively stable during storage. Because of the precision required with low inclusion rates of some amino acids, accurate scales and mixing equipment are required for their use.
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Wu G, 2010. Functional amino Acids in growth, reproduction, and health. Advances in Nutrition. 1: 31-37
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Yao K, Yin YL, Chu WY, Liu ZQ, Deng D, Li TJ, Huang RL, Zhang JS, Tan BE,2008. Dietary arginine supplementation increases mTOR signaling activity in skeletal muscle of neonatal pigs. J Nutr. 138:867–72.