Protein in swine nutrition

Protein in swine nutrition

Contents

Introduction

Importance of protein

Ideal protein

Crystalline amino acids

Low crude protein diets for swine

Conclusions

References

Introduction

Livestock production is growing rapidly as a result of the increasing demand for animal products. FAO projections suggest that global meat production and consumption will rise from 233 million tones (2000) to 300 million t (2020). This forecast shows a massive increase in animal protein demand, needed to satisfy the growth in the human population. Asia is experiencing the world's highest growth rates in production and consumption of livestock products (meat, milk and eggs) [1].

Swine production in the United States has changed dramatically over the last 50 years. Specifically, the industry has shifted toward a highly specialized systems approach, which has resulted in the concentration of production in certain areas of the country. The disposal of animal waste becomes a challenge in the intensive production of swine because the amount and nutrient composition of the manure may exceed the capacity of the land to accept it as a fertilizer. Therefore, alternative methods of animal manure disposal are needed urgently. Growing-finishing pigs are responsible for the bulk of nutrient excretion in manure. On average, each finishing pig produces approximately 1.2 gallons of manure (not including flush water or water wastage) every day. This adds up to 21 pounds of nitrogen and approximately 15 pounds of phosphorus per pig per year. To reduce the environmental impact of pig production, methods may be focused on finding different techniques and strategies for processing manure, reducing odor, or reducing the excretion of nutrients in manure. Nutrient excretion is a result of the inefficiencies associated with digestion and metabolism. Typically only 20 percent to 50 percent of the nitrogen and 20 percent to 60 percent of the phosphorus consumed are retained in the body; therefore, 50 percent to 80 percent of the nitrogen and 40 percent to 80 percent of the phosphorus consumed are excreted into the environment. This illustrates the potential for using dietary strategies to reduce nutrient excretion. In addition, by minimizing the amount of nutrients, particularly protein, that enters the large intestine without being digested, odor formation by microorganisms in the large intestine can be reduced. Thus, in general, methods that reduce nutrient excretion often reduce odor emission [2].

The feeds commonly used for swine may be roughly divided into two classes: Fattening feeds, such as corn, barley and rye that do not carry a sufficient proportion of protein to meet swine needs; and growing feeds, rich in protein. Those of the second group are usually higher in price than those of the first. Adding suitable protein feed to the fattening feeds reduces the amount of feed required per unit of gain and adds to the thrift and vigor of the animals. Even if the protein feed is much higher priced than the fattening feed, a small proportion of it usually results in cheaper gains. The problem is to feed the proportion which pays best. Young pigs require more protein than do the older ones. Growing shotes and nursing brood sows will pay for more than will fattening hogs or dry brood sows. The amounts of protein feeds given in the following examples of successful feed mixtures for hogs of different ages are such that increasing the proportion of protein feed would not materially reduce the total feed requirements for 100 pounds gain. As the average weight of the pigs approaches the heavier weight stated for each group, the proportion of protein supplement could profitably be cut down toward the proportion given in the next group [3].

The big increase in animal protein demand over the last few decades has been largely met by the world wide growth in intensive livestock production, particularly poultry and pigs. This is expected to continue as real income grows in the emerging economies.

Feed grains are thought to compete directly, or in the use of land, with grains for human consumption and because there is inefficient use of feed and energy in some livestock systems, it is often blamed for this occurrence. However, if efficiency is seen over the entire production chain, and expressed as input of edible human food/output in human edible food, the view of animal production takes on a more positive outlook. Note that 233 million t meat, 568 million t milk and 55 million t eggs produced globally contain more than 65 million t of protein. So while input is higher than output, if improved protein quality on the output side is considered, a reasonable balance emerges. A recent FAO study shows that the increasing use of feed grains has not had an adverse effect on the provision of cereals for human consumption. Indeed, many argue that the production of cereals for feed acts as a global buffer and therefore has a positive effect on global food security. Over the last 30 years, FAO has worked in the field to develop technologies for integrated farming systems appropriate to small producers, particularly in the tropics. For ruminant livestock, urea treatment of straw and the use of multi-nutrient blocks have been shown to greatly improve nutrition of animals fed on low quality roughage diets. Legumes and tree forages have also provided needed protein inputs into cattle, sheep and goat production systems, while benefiting the environment through nitrogen fixation and organic matter [1].

Importance of protein

Next to water, protein is the major component of body tissues. It is the essential nutrient for growth. The body is in a dynamic state, with protein and other nitrogenous compounds being degraded and resynthesised continuously. More protein is turned over daily in the body than is ordinarily consumed in the diet.

Proteins are large molecules made up of amino acids bonded together by peptide linkages. They provide the essential amino acids, which are the initial materials for tissue synthesis and constituent of tissue protein. Thus, it was often referred to as the “currency” of protein nutrition and metabolism. The maintenance of body tissue is essential because the body is constantly undergoing wear and tear, and proteins and amino acids provide continuous repairs. Some of the important physiological functions of proteins are summarised below.

Proteins are important for the formation of regulatory compounds. Some hormones, all enzymes, and most other regulatory materials in the body are proteins substances. Proteins defend the body against disease. When the body detects invading antigens, it manufactures antibodies, giant protein molecules designed specifically to combat them. The antibodies work so swiftly and efficiently that in normal, healthy individual, most diseases never have a chance to get started. The body's fluids are contained within the cells (intracellular) and outside the cells (extracellular). Extracellular fluids are found either in the spaces between cells (interstitial) or within blood vessels (intravascular). Wherever proteins are, they attract water and this helps to maintain the fluid balance in their various compartments. In addition, proteins help maintain the balance between acids and bases within the body fluids by accepting and releasing hydrogen ions. Even though proteins are needed for growth, maintenance and repair, they will be used to provide glucose when the need arises [4].

Protein is the basic structural material of all cells. Biologically active proteins include enzymes, immunoglobulins, hormones, neurotransmitters, nutrient transport and storage compounds, and cell membrane receptors. Plasma proteins (e.g., albumin) contribute to oncotic pressure that directs the flow of fluid and metabolic waste from the intracellular compartment into the capillary venules. These proteins (e.g., hemoglobin) also contribute to plasma buffering capacity and oxygen-carbon dioxide transport (e.g., hemoglobin, myoglobin).

The digestion of protein starts in the gastric lumen continuing in the small intestinal lumen and is completed at the brush-border membrane of the enterocytes. Hydrochloric acid and gastric proteases start the protein digestion action in the gastric lumen. Hydrochloric acid is secreted by the parietal cells. Hydrochloric acid activates gastric proteases and denatures the dietary protein. The gastric secretory capacity increased more rapidly after pigs were fed with creep diet rather than fed by sow. The low gastric secretory capacity at birth may relate to immaturity of the parietal cells in newborn pigs. The acidity of gastric contents is about pH 5.5 to 7 in neonatal pigs during early postnatal period due to low gastric secretory capacity and the high buffering capacity of sow's milk.

Gastric proteases are secreted by the chief cells in the gastric gland. Pepsin A, pepsin B, pepsin C, and chymosin are four proteases. Chymosin has strong milk-clotting ability but weak proteolytic activity. Clotting milk by chymosin occurs through a specific cleavage of k-casein. Milk-clotting may regulate gastric emptying and stimulate gastric development through gastric distention. Prochymosin has the highest concentration at the time of birth. The concentration of prochymosin in fetal pig gastric tissue is detected as early as at 80 d of gestation. Pepsinogen A replaces the prochymosin to become the dominant protease in the gastric tissue of pigs by the 5th wk of age. The proteolytic activity of neonatal piglets is relatively low in the stomach due to gastric acid secretory capacity and the small amount of pepsinogen A secreted. The bioactive compounds, such as immunoglobulin, hormone, growth factors, and bioactive peptides presented in the colostrum and milk are able to pass undegraded because of the low proteolytic activity. As a consequence, postnatal gastrointestinal development in neonatal pigs possibly could be regulated by those bioactive compounds [5].

The amino acid that we commonly hear the most about is lysine for two major reasons. First, the concentration of lysine in muscle and other tissues is relatively high (about 7%); and second, many of the feedstuffs that we feed to pigs are quite low in lysine. Cereal grains are notoriously low in lysine.

For example, corn and grain sorghum contain about .25% lysine - less than one-third to one-fourth the amount of lysine that is required by the growing pig. Wheat and barley are higher in lysine than corn (.35 to .40%), but they are still well below the pig's requirement. Consequently, these feed grains must be supplemented with a protein source that is high in lysine (such as soybean meal) in order to meet the lysine requirement of pigs.

We generally hear much less about the other nine amino acids because they are normally provided in adequate amounts by the protein supplements that are used to supply lysine. In other words, if one adds sufficient amounts of a good quality protein supplement to meet the lysine requirement, the requirements for the other amino acids will be met [6].

There are several parameters which can be used to indicate indirectly the nutritional values of proteins in different feeds, but the most important feature of a protein is its capacity to supply amino acids in forms which are available for absorption from the digestive tract [7].

Biological value of a dietary protein is determined by the amount and proportion of essential amino acids it provides. If any one of the essential amino acids is not available in sufficient amounts or is present in excessive amounts relative to other essential amino acids, protein synthesis will not be supported. Under these circumstances, labile body proteins such as plasma albumin will be catabolized to provide the limiting amino acid so that protein synthesis may continue.

Although there are 20 primary amino acids that occur in proteins, not all of them are essential dietary components. Some amino acids can be synthesized by using carbon skeletons (derived primarily from glucose and other amino acids) and amino groups derived from other amino acids present in excess of the requirement. Amino acids synthesized in this manner are termed nonessential (or dispensable). Amino acids that cannot be synthesized, or cannot be synthesized at a sufficient rate to permit optimal growth or reproduction, are termed essential (or indispensable). Although amino acids in both categories are needed at the physiologic or metabolic level, normal swine diets contain adequate amounts of nonessential amino acids or of amino groups for their synthesis. This seems to be true even for low-protein diets that are supplemented with crystalline amino acids (Brudevoid and Southern, 1994). Thus, most of the emphasis in swine nutrition is on the essential amino acids [8].

Protein from animal sources (meat, fish, dairy products, egg white) is considered high biological value protein or a "complete" protein because all nine essential amino acids are present in these proteins. An exception to this rule is collagen-derived gelatin which is lacking in tryptophan [9].

Sources of protein for animal feeds are many and varied, with considerable opportunities for further diversification and substitutions. More research is required on alternative sources before many of the opportunities can be exploited in practice [1].

Plant sources of protein (grains, legumes, nuts, and seeds) generally do not contain sufficient amounts of one or more of the essential amino acids. Thus protein synthesis can occur only to the extent that the limiting amino acids are available. These proteins are considered to have intermediate biological value or to be partially complete because, although consumed alone they do not meet the requirements for essential amino acids, they can be combined to provide amounts and proportions of essential amino acids equivalent to high biological proteins from animal sources.

Plants that are entirely lacking in essential amino acids are considered incomplete proteins or sources of low biological value protein. These sources include most fruits and vegetables. A low biological value means that it is difficult or impossible to compensate for insufficient amounts of essential amino acids by combining different sources as with partially complete proteins [9].

Cereal grains, such as corn, sorghum, barley, or wheat, are the primary ingredients of most swine diets and usually provide 30 to 60 percent of the total amino acid requirements. But other sources of protein, such as soybean meal, must be provided to ensure adequate amounts of, and a proper balance among, the essential amino acids. Supplements of crystalline amino acids may also be used to increase intakes of specific amino acids. The protein levels necessary to provide adequate intakes of essential amino acids will depend on the feedstuffs used. Feedstuffs that contain ``high quality'' proteins (i.e., they have an amino acid pattern relatively similar to the pig's needs) or mixtures of feedstuffs in which the amino acid pattern in one complements the pattern in another will meet the essential amino acid requirements at lower dietary protein levels than feedstuffs with a less desirable amino acid pattern. This is important if one of the goals is to minimize nitrogen excretion. Another method of reducing dietary protein levels, and thereby reducing nitrogen excretion, is the judicious use of supplements of crystalline amino acids [8].

Ideal protein

The concept of an ideal protein or ideal amino acid balance is to provide a perfect pattern of essential and nonessential amino acids in the diet without any excesses or deficiencies. This pattern is supposed to reflect the exact amino acid requirements of the pig for maintenance and growth. Therefore, an ideal protein provides exactly 100% of the recommended level of each amino acid. Although standard diets are usually formulated to meet the pig's requirement for lysine (the most limiting amino acid), excesses of many other amino acids exist. Two practical methods can provide a more ideal balance of amino acids in pig feed: Use a combination of supplemental protein sources or formulate the diet with crystalline amino acids. Questions often are asked about whether the excess amino acids hurt pig performance and whether reduction or elimination of the excesses would improve pig performance. There is little evidence to indicate that the performance of pigs fed diets containing a more ideal balance of amino acids is better or worse than that of pigs fed practical corn- or milo-soybean meal-based diets. However , if excess amino acids are reduced, nitrogen excreted through the urine and feces will be reduced, meaning that less nitrogen is in the manure. This will reduce the amount of land required to properly manage the nitrogen in the manure. Unless there is a strong incentive to reduce nitrogen in the manure, choose sources of amino acids that will produce the lowest cost gain [10].

Some feed manufacturers formulate swine feeds on an ideal protein basis. An ideal protein is one in which the amino acids closely match the animal's requirements for lean tissue protein synthesis and maintenance. One way of doing this is to reduce the crude protein level in the diet and supplement with synthetic amino acids. Although nutritionists cannot prepare perfect amino acid balances from natural feed ingredients, using computers and an array of many different ingredients and synthetic amino acids allows them to produce feeds that have reduced amino acid excesses. Reducing the crude protein in the diet by 3 to 4 percent and supplementing with synthetic amino acids (generally, lysine, methionine, threonine, and tryptophan) have shown a 20 to 40 percent reduction in N excretion [11].

In determining amino acid requirements, a fundamental concept of this publication is that there is an optimal dietary pattern among essential amino acids that corresponds to the needs of the animal. This optimal dietary pattern is often called ``ideal protein'' [8].

The use of synthetic amino acids in swine nursery and growing diets is a common practice implemented throughout the world. The majority of the research to date has focused on the use of synthetic lysine. Researchers have spent considerable time and money determining the lysine requirement and the maximum amount of synthetic lysine that can be fed in a practical pig diet.

With the present knowledge of lysine requirements, drastic variations in ingredient costs, and environmental pressures to lower crude protein, it has become increasingly more important to know the requirements for the next limiting amino acids. Methionine, threonine, and tryptophan are available in synthetic forms. These amino acids are also considered to be the next limiting amino acids in swine nursery and grower diets. A better understanding of the pigs' requirements for each of these amino acids will result in lower feed costs, improved diet formulations, and a reduction in nitrogen excretion [12].

In swine diet formulation, the concept of ideal protein is being applied worldwide. Due to the relative importance of maintenance requirements when compared to those of production, it seems appropriate to think that composition of ideal protein will vary with age of the pig. Several recent studies appear to show that the ratio 18% Trp/Lys should be incremented to 22 % for weaning pigs in order to optimise their production performance [13].

An ideal amino acid pattern has been developed for young pigs (Wang and Fuller, 1989; Chung and Baker, 1992; Baker et al., 1993). However, limited research has been conducted in finishing pigs to determine an ideal amino acid ratio. A projection of the ideal amino acid ratio has been made by Baker (1994). This projected ideal amino acid ratio is based on the pattern for 10-kg pigs with increases in the ratios of threonine, tryptophan, and total sulfur amino acids (TSAA) to lysine. The amino acid ratios are increased due to the increased maintenance requirement of these amino acids for finishing pigs (Hahn, 1994; Hahn and Baker, 1995). The proposed ideal amino acid pattern for finishing pigs may not be correct for pigs fed practical corn-soybean meal or sorghum-soybean meal diets. For example, sorghum-soybean meal and corn-soybean meal diets (depending on the level of lysine and on the use of crystalline L-lysine·HCl) calculate to be first- or second-limiting in TSAA when the proposed ideal amino acid ratio is used. However, research indicates that lysine and threonine are the first- and second-limiting amino acids in sorghum-soybean meal diets, respectively (Cervantes-Ramirez et al., 1991; Hansen et al., 1993; Page et al., 1993; Brudevold and Southern, 1994). In addition, tryptophan has been reported to be second-limiting or equally limiting with threonine in corn-soybean meal diets (Russell et al., 1983, 1986, 1987). Consequently, the proposed ideal amino acid ratio for finishing pigs may require additional refinement. Therefore, the research reported herein was conducted to determine the optimum ratio of TSAA to lysine for finishing pigs [14].

Crystalline amino acids

Pigs of all ages and stages of the life cycle require amino acids to enable them to grow and reproduce. Amino acids are the structural units of protein. During digestion, proteins are broken down into amino acids and peptides. The amino acids and peptides are absorbed into the body and are used to build new proteins, such as muscle. Thus, pigs require amino acids, not protein. Diets that are “balanced” with respect to amino acids contain a desirable level and ratio of the 10 essential amino acids required by pigs for maintenance, growth, reproduction and lactation. Those 10 essential amino acids for swine are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The proteins of corn and other cereal grains are deficient in certain essential amino acids. Protein supplements are used to correct the amino acid deficiencies in grains. For example, the correct combination of grain and soybean meal provides a good balance of amino acids. Soybean meal is often the most economical source of amino acids for pigs in Nebraska and South Dakota. However , economic conditions can change, making alternative amino acid sources attractive for use in pig feed [10].

The use of industrially produced amino acids in animal feeds is not new. Synthetic amino acid incorporation in feed has at least a 40 year history. DL methionine was produced by chemical synthesis in the 1950s and 1960s for inclusion in poultry feeds. L-lysine production by fermentation began in the 1960s in Japan, followed by L-threonine and L-tryptophan in the late 1980s.

The adoption of modern biotechnology has revolutionized the synthesis process, and has significantly reduced the costs of amino acid production. The exploitation of genetically modified microbial strains has substantially improved competitiveness. The economics of production has dramatically changed, providing much greater opportunities for synthetic amino acid use. It is persuasively argued that improvements in protein use from animal feeds are required to meet the substantial growth in global demand for animal protein products. The increased substitution of synthetic amino acids for plant protein could provide greater efficiency and effectiveness of protein utilization, but the cost effectiveness of their use needs to be continually assessed.

It is suggested that the incorporation of one tonne of L-lysine hydrochloride could save the use of 33 t of soybean meal. Or, if 550,000 t of L-lysine hydrochloride is used globally, it could replace 18 million t of soybean meal, representing about half of the USA soybean meal production. There is potential for considerable impact on current protein supply channels and the types of protein which are now used.

It is also argued that greater synthetic amino acid use could reduce nitrogen pollution from animal wastes, as a result of better and more efficient nutrient utilization.

Future developments of synthetic amino acid production could apparently include synthetic isoleucine, valine and arginine, thus extending the range of amino acids available for use in feeds. The degree of use would be mainly determined by the economics [1].

In chemistry, an amino acid is a molecule that contains both amine and carboxylic acid functional groups. Amino acids are the basic structural building units of proteins. They form short polymer chains, peptides or polypeptides, which in turn form structures called proteins. There are 20 primary amino acids that occur in proteins. The biochemical and molecular actions of amino acids are of interest for designing new means to improve health and growth.

Arginine, histidine, isolecine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine are indispensiable or essential amino acids for piglets. The pig body cannot synthesize all of these amino acids except arginine, and they, therefore, must be obtained from the diet. Conversely, the amino acids can be synthesized in the body are termed dispensable or nonessential, including alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine. Essential amino acids have been emphasized for swine nutrition.

The main function of dietary amino acids is to stimulate muscle protein synthesis. Individual amino acid has been proposed to act as signalling molecule that serves to regulate mRNA translation. Leucine has been suggested to have a signalling role in the simulation of muscle protein synthesis by enhancing availability of specific eukaryotic initiation factors. Some amino acids have been implicated in immune function, and some are important precursors of neurotransmitters and certain hormones [5].

The use of crystalline amino acids has become an important part of diet formulation within the swine and poultry industries. Crystalline amino acids are relatively purified sources of amino acids that can be added to diets to meet an amino acids requirement. Their use not only allows for producers to lower feed costs per unit of gain but also may help producers in their efforts to manage nutrients more effectively and prevent damage to the environment caused by excess nitrogen (N) in manure.

There are four commercially available crystalline amino acids (AA) (Lys, Met, Thr, and Trp) that are commonly used in diets for swine and poultry. Their use is largely dependant upon feed ingredient prices such as corn and soybean meal. Specifically, as the price of soybean meal increases the use of crystalline AA becomes more economical. Conversely, when corn prices are high the economics of using crystalline AA are decreased. In most diets for swine, Lys is the first limiting AA. Therefore, the use of other crystalline AA may be dependent upon the price of Lys. Furthermore, the price of other crystalline AA directly affect how much crystalline Lys will be used. In most poultry diets, Met is the first limiting AA and Lys is the second limiting AA [15].

Lysine is most commonly sold commercially as L-Lys monohydrochloride (L-lysine*HCl). There are no known mammals that are able to utilize D-Lys and therefore only L-Lys is considered to have bioavailability in swine and poultry [16].

Methionine has three basic functions: 1) utilization for protein synthesis, 2) conversion to S-adenosylmethionine, or 3) conversion to cystathionine, cysteine, or derivatives of cysteine. With the latter two functions in mind, methionine is converted to S-adenosylmethionine, which is converted to S-adenosylhomocysteine, and then to homocysteine. Homocysteine then has one of two fates: 1) reconversion tomethionine by either the betaine or tetrahydrofolate pathway, or 2) irreversible conversion to cysteine [17].

Methionine seems to be equally available for use in swine and poultry in the D- or L-form. Feed grade sources of Met are available as DL-Met (99% pure) and as Met hydroxy analog (a liquid that contains 88% Met hydroxy analog). There is considerable controversy over the relative bioactivity of Met hydroxy analog; however, previous research has indicated that it is equivalent to DL-Met on an equal molar basis [18].

The bioactivity of D-Trp varies between species and may vary from 60 to 100% in the pig [16].

As a limiting essential amino acid, Trp is typically supplied in the diet at levels required for maximum animal growth. When excess Trp is supplied to the diet and not used for the purpose of protein synthesis, it may be used as a therapeutic supplement. The rationale for the therapeutic use of Trp is based on the fact that alterations in brain Trp levels can influence the synthesis of serotonin, an inhibitory neurotransmitter in the central nervous system (CNS). Tryptophan is the primary precursor of serotonin. Serotonin has a sedative effect, such as suppressing sleep-wake mechanisms, temperature regulation, pain sensitivity, and aggressive behaviors [19].

Threonine has four chemical isomers: D- and L-Thr and D- and L-allothreonine. Poultry can only utilize L-Thr. It is also assumed that pigs can only utilize L-Thr, because of an inability for transamination to occur. Thus, commercially available Thr is in the L-form (98.5% pure) [15].

Threonine is the third-limiting amino acid in corn-soybean meal diets for growing pigs and is second-limiting in sorghum-soybean meal diets, but there have been few experiments to determine the bioavailability of threonine in feed ingredients [20].

The use of other crystalline AA in diets for swine and poultry will be dependant upon the extent to which CP can be lowered without affecting growth and carcass traits.

It seems likely that in the future, crystalline forms of other essential AA will be produced commercially and priced for inclusion into swine and poultry feeds[15].

It is difficult to use intact protein diets effectively in the determination of individual amino acid requirements. A diet containing individual crystalline L-amino acids as the sole source of dietary nitrogen and capable of supporting optimal growth and performance is necessary for the determination of most amino acid requirements [21].

The production of feed-grade, crystalline amino acids (lysine, tryptophan, threonine, and methionine) has enabled the formulation of diets that provide amino acids in proportions similar to the pattern needed for optimum growth. To make maximum use of crystalline amino acids, however, diets must be formulated on a bioavailable amino acid basis, and therefore the bioavailability of amino acids in other feed ingredients must be known [20].

In most swine diets, a portion of each amino acid that is present is not biologically available to the animal. This is because most proteins are not fully digested and the amino acids are not fully absorbed, and also because not all absorbed amino acids are metabolically available. Diets vary considerably in the proportions of their amino acids that are biologically available. The amino acids in some proteins such as milk products are almost fully bioavailable, whereas those in other proteins such as certain plant seeds are much less so (Southern, 1991; Lewis and Bayley, 1995).

Expressing amino acid requirements in terms of bioavailable requirements is, therefore, desirable. However, it means that to formulate swine diets, the bioavailable amino acid content of the ingredients being considered must be known.

The bioavailability of amino acids in the protein of dietary ingredients has been determined for a wide range of protein sources fed to swine (Tanksley and Knabe, 1984; Sauer and Ozimek, 1986; Southern, 1991; Lewis and Bayley, 1995). The primary method to determine bioavailability has been to measure the proportion of a dietary amino acid that has disappeared from the gut when digesta reach the terminal ileum. Values determined in this manner are termed ``ileal digestibilities'' rather than bioavailabilities because amino acids are sometimes absorbed in a form that cannot be fully used in metabolism. Furthermore, unless a correction is made for endogenous amino acid losses, the complete terminology is ``apparent ileal digestibilities.'' In this publication, minimum endogenous losses are accounted for, and both requirements and ingredient contents are expressed in terms of ``true'' (or standardized) ileal digestible amino acids. When apparent digestibilities are determined, feedstuffs with low protein content are undervalued relative to feedstuffs with high protein content because of the relatively greater contribution of endogenous amino acids. True digestibilities correct for this. In addition, because of the way in which ideal protein patterns were determined, these patterns reflect true ileal digestibility rather than apparent ileal digestibility [8].

When is it economical to use crystalline amino acids in swine diets and how can they be used? It depends on the price of the crystalline amino acids and the prices of grain and supplemental protein sources. The use of L-lysine*HCl as a source of crystalline lysine is often economically sound. Crystalline methionine is commercially available and inexpensive. Crystalline tryptophan and threonine can be purchased in feed-grade forms, but currently they are rather expensive. Crystalline lysine and tryptophan together in the same source is now commercially available. Other sources combining these crystalline amino acids as well as others may be developed in the future.

Three pounds of L-lysine*HCl (containing 78% pure lysine) plus 97 lb of corn contribute the same amount of digestible lysine as 100 lb of 44% CP soybean meal. If L-lysine*HCl is used, one must monitor dietary tryptophan, threonine and methionine levels and maintain sufficient intact protein (e.g., soybean meal) in the diet to meet the requirements for these amino acids. Greater reductions of intact protein may be possible when using products containing both crystalline lysine and tryptophan. As when adding L-lysine*HCl, monitor dietary threonine and methionine levels when using these products. The level of crystalline amino acids supplemented will depend on the feeds used in the formulation and is usually dependent on the second limiting amino acid. That amino acid changes depending on the ingredients used. In most swine diets lysine is first limiting and either tryptophan or threonine is second limiting. However , starting pig diets containing large amounts of plasma proteins and blood meal need to be supplemented with crystalline methionine. Use caution when considering crystalline amino acids as substitutes for intact protein in gestation or lactation diets. Gestating sows are usually fed once per day, and research in limit-fed pigs indicates that crystalline amino acids are used less efficiently than they are when pigs consume feed several times per day . There is evidence that in some circumstances lactation diets can be co-limiting in lysine and another amino acid(s). In these circumstances, replacement of intact protein with lysine alone could lead to a deficiency of other amino acids. An amino acid deficiency causes reduced litter weight gain and sow lactation feed intake.

A factor not traditionally considered when evaluating the use of crystalline amino acids in swine diets is nitrogen content of the manure. As stated previously, reducing excess amino acids will result in a decrease in the nitrogen content of the manure. When incorporated properly , the use of crystalline amino acids will accomplish that without affecting growth performance. This means the producer needs fewer acres to spread the manure on and potentially less odor. To ensure proper distribution in the complete feed, amino acids must be combined with a carrier to achieve a minimum volume before they are added to the mixer [10].

Low crude protein diets for swine

Following digestion of proteins, amino acids are absorbed and used for muscle protein synthesis and other physiological functions. The ideal protein concept, introduced by the Agricultural Research Council (ARC, 1981), proposes that ideal protein consists of amino acids in exactly the right proportions for maintenance and lean tissue growth. According to this concept, each amino acid is equally limiting and, per definition, excretion of nitrogen is minimized. Requirements of all essential amino acids are expressed as a percentage of lysine (the first amino acid that would limit pig growth in practical swine diets). From a practical standpoint, the ideal protein concept allows for a quick calculation of amino acid requirements as long as the lysine requirement of the pig is defined. However, the exact ideal protein may be variable, depending on the physiological state of the animal. This may explain the different ideal protein or amino acid patterns that have been proposed. Baker (1996) calculated ideal amino acid patterns for pigs in different weight categories. Others (Fuller et al. 1989) studied optimum amino acid patterns for maintenance and growth. Because the maintenance component increases relative to protein deposition, the combined ideal pattern of amino acids will change with increasing body weight. Similarly, the amino acid requirements for swine as calculated in the new NRC publication (1998) are based on different patterns for maintenance and growth [2].

Protein generally refers to crude protein, which is defined for mixed feedstuffs as the nitrogen content 6.25. This definition is based on the assumption that, on average, the nitrogen content is 16 g of nitrogen/100 g of protein. Proteins are composed of amino acids, and it is actually the amino acids that are the essential nutrients. Therefore, the dietary provision of amino acids in correct amounts and proportions determines the adequacy of a dietary protein concentrate. Supplemental nonprotein nitrogen, such as urea, has not produced beneficial responses in swine that were fed practical diets [8].

Increasing environmental concerns related to N concentrations of swine manure has generated interest in the use of crystalline AA to lower the CP of swine diets [15].

Nitrogen losses from pig production can be reduced by feeding diets with a lower crude protein content but supplemented with essential (indispensable) amino acids ( EAA) according to the animal's requirements. However, for optimal performance, the amount by feeding diets with a lower crude protein content but and composition of nonessential (dispensable) amino acids ( NEAA) should also be taken into account [22].

Kerr and Easter (1995) estimated that each one percentage unit reduction in dietary CP results in 8% less N excreted in manure. However, there are some inconsistencies in the literature as to the extent CP can be lowered without affecting growth performance and carcass traits. Most reports suggest reducing CP by more than 3 to 4% leads to a reduction in rate and efficiency of growth, even when all known nutrient requirements are met (Tuitoek et al., 1997a,b; Shelton et al., 2001; Gomez et al., 2002a,b). Furthermore, reductions in CP with the use of crystalline AA often lead to increased fat deposition (Tuitoek et al., 1997a,b; Knowles et al., 1998; Shelton et al., 2001; Figueroa et al., 2002; Gomez et al., 2002a,b) [15].

Reduction of dietary CP in growing pig diets by four percentage units without amino acid (AA) supplementation dramatically reduces pig growth performance and carcass leanness [23].

Conclusions

Further reductions in dietary protein, even when additional lysine is supplemented, results in depressed pig performance. The reason is that other amino acids become deficient when more than 100 pounds of soybean meal is deleted from the diet. The depression in performance can be prevented by adding threonine, tryptophan, and methionine, along with additional lysine*HCl to the diet.

One of the major advantages in using lower protein, amino acid supplemented diets is the positive impact that these programs have on the environment. Recent studies at the University of Kentucky have shown that nitrogen excretion is reduced by 15-20% when the dietary protein is decreased by 2 points and lysine added; and is reduced by 30-35% when the dietary protein is decreased by 4 points and the four amino acids added. Ammonia and other odorous emissions from manure also are substantially reduced with low protein, amino acid supplemented diets. Studies at UK have shown that ammonia emissions can be cut in half by feeding these types of diets.

Several recent studies at our university indicate that carcass leanness is decreased slightly when low protein, amino acid supplemented diets are fed to finishing pigs. This is probably due to the higher net energy in corn, as compared with soybean meal, and the greater amount of corn (and lesser amount of soybean meal) in a reduced protein, amino acid supplemented diet. We are currently investigating ways of preventing this from happening.

So what about the bottom line? Is the use of supplemental amino acids cost effective? In situations where only lysine is supplemented, the answer is yes. Lysine*HCl is competitively priced and the cost closely follows the price of soybean meal. Presently, lysine*HCl can be purchased in ton quantities at $1.20 to $1.25 per pound. When used according to the guidelines discussed previously and at present costs of corn ($4.40 per bushel) and soybean meal ($270 per ton), lysine usage reduces the feed cost by about $1.25 per ton.

However, if further reductions in dietary protein are implemented and lysine, threonine, tryptophan, and methionine are supplemented, diet cost is increased. The cost of a diet in which the protein is reduced by 4 percentage points and the diet then supplemented with adequate amounts of lysine, threonine, tryptophan, and methionine is from $7.00 to $11.00 per ton more than a conventional diet at today's prices of corn, soybean meal, and amino acids. This additional cost would have to be weighed against the benefits to the environment from reducing nitrogen in the manure and reducing ammonia and other odor emissions from the manure.

References

1. FAO. 2004. Protein sources for the animal feed industry. Animal Production and Health. Expert Consultation and Workshop, Bangkok.

2. Heugten, E. and Kempen, T. 1999. Understanding and Applying Nutrition Concepts to Reduce Nutrient Excretion in Swine. NC State University

3. Baldwin, R. J. 1925. Swine Feeding. Michigan Agricultural College Extension Division

4. Young, V.R. (2001). Protein and amino acids. In: Present Knowledge in Nutrition. 8^th Edition. Bowman BA and Russel RM (eds). International Life Sciences Institute, Washington DC. Chapter 5, pp. 43-58.

5. Carol Lin, B.S. 2007. A Thesis In Animal Science. Texas Tech University.

6. Cervantes, M.R., G.L. Cromwell, and T.S. Stahly.1996. Synthetic amino acid supplementation in the low protein grain sorghum-soybean meal diet for pigs. Cuban Journal of Agricultural Science, 29:197-208.

7. Kathleen, H. Carlson 2007. Nitrogen and Amino Acids in the Feces of Young Pigs Receiving a Protein-free Diet and Diets Containing Graded Levels of Soybean Oil Meal or Casein. J. NUTRITION, 100; 1353-1362.

8. NRC. 1998. Nutrient Requirements of Swine. 10th rev. ed. Natl. Acad. Press, Washington, DC.

9. Protein // Feinber School of Medicine, Northwestern University: http://www.feinberg.northwestern.edu/nutrition/factsheets/protein.html. - Дата доступа: 25.04.2008

10. Duane, E. Reese. 2000. Swine Nutrition Guide, Nebraska and South Dakota.

11. NRCS. 2003. Feed and Animal Management for Swine. United States Department of Agriculture.

12. Peak, S. 2005. TSAA Requirements for Nursery and Growing Pigs. Advances in Pork Production Volume 16, pg. 101

13. Daza, A. 2000. Optimum Trp/Lys ratio in nursery pig diets (6-20 kg). Departament Production animal, ETSIA.

14. Knowles, T. A. 1998. Ratio of Total Sulfur Amino Acids to Lysine for Finishing Pigs. J. Anim. Sci. 1998. 76:1081-1090.

15. Dean, W.D. 2005. Amino acid requirements and low crude protein. Amino acid supplemented diets for swine and poultry. M.S., Kansas State University.

16. Lewis, A. J., and L. L. Southern. 2001. Swine Nutrition. 2nd ed. CRC Press, Boca Raton, FL.

17. Matthews, J. O. 2001. Estimation of the total sulfur amino acid requirement and the effect of betaine in diets deficient in total sulfur amino acids for the weanling pig. J. Anim. Sci. 79:1557-1565.

18. Waldroup, P. W., C. J. Mabray, J. R. Blackman, P. J. Slaugter, and R. J. Short,

and Z. B. Johnson. 1981. Effectiveness of the free acid of methionine hydroxy analogue as a methionine supplement in broiler diets. Poult. Sci. 60:438-443.

19. Li, Y. Z. 2006. Use of supplementary tryptophan to modify the behavior of pigs. J. Anim. Sci. 84:212-220.

20. Kovar, Joy L. 1993. Bioavailability of Threonine in Soybean Meal for Young Pigs. J. him. Sci. 71:2133-2139.

21. Jeffery, M. 1982. Crystalline Amino Acid Diet for Determining Amino Acid Requirements of Growing Guinea Pigs. J. Nutr. 112: 1118-1125.

22. Lenis, P. 1999. Effect of the Ratio Between Essential and Nonessential Amino Acids in the Diet on Utilization of Nitrogen and Amino Acids by Growing Pigs. J. Anim. Sci. 77:1777-1787.

23. Kerr, B. J., F. K. McKeith, and R. A. Easter. 1995. Effect onperformance and carcass characteristics of nursery to finisher pigs fed reduced crude protein amino acid-supplemented diets. J. Anim. Sci. 73:433.