Protein and Amino Acid Sources for Swine Diets

Pigs of all ages and stages of the life cycle require amino acids to enable them for normal body function. Amino acids are the structural units of protein. During digestion, dietary protein is broken down into amino acids and peptides (more than one amino acid linked together). The amino acids and peptides are then absorbed into the body and are used to build new proteins, such as muscle, which is composed of about 21 different amino acids. Thus, pigs require amino acids, not crude protein. Diets must be balanced with respect to a desirable level and ratio of the 10 essential amino acids; they must also contain adequate amounts of 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. Two other amino acids (cysteine and tyrosine) are semi-essential since both can be synthesized if adequate amounts of methionine and phenylalanine are present for cysteine and tyrosine, respectively. The other nine amino acids (aspartic acid, asparagines, glutamic acid, glutamine, glycine, alanine, proline, hydroxyproline and serine) are considered nonessential because they can be synthesized at sufficient rates if an adequate amount of non-specific protein is present in the diet. [3, 9, 10, 11]





  • Provide general background and concepts about protein and amino acids as a nutrient source
  • Provide information about protein sources
  • Overview of amino acid formulation in swine diets


Amino Acid Sources


The proteins of corn and other cereal grains are deficient in certain essential amino acids for swine. Thus, protein supplements or sources are used in combination with cereal grains to correct the amino acid deficiencies. 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 throughout the United States. However, economic conditions can change making alternative plant-based amino acid sources (cottonseed meal, canola meal, sunflower meal, and peanut meal), animal co-products (meat and bone meal, fish meal, spray-dried egg, blood co-products, poultry meal), grain co-products (dried distillers, and corn gluten meal) or synthetic amino acids attractive for use in pig feed. Soybean meal is the only plant protein that compares with animal protein in terms of quality of amino acid content and can be used as the sole protein based ingredient in most swine diets. Therefore, there is generally no nutritional need to have both animal and plant protein sources in a swine diet, with the exception of early nursery diets. [3, 9] However, depending on commodity markets, there may be an economical advantage for using both animal and plant proteins in a swine diet.


Animal protein products (meat and bone meal, fish meal, spray-dried egg, blood co-products, poultry meal) may vary in composition more and quality more than plant protein sources. Meat and bone meal and meat meal are byproducts of the meat packing industry, and their composition depends on the animals slaughtered. Methods of processing also influence the quality of animal proteins. The rendering process (270 to 280°F) is sufficient to kill salmonella and other bacteria present in the raw material, but through improper handling, the rendered product can be recontaminated.


Strategies exist for dealing with variability of animal proteins. First, animal proteins from a single rendering plant have nutrient variability that is similar to soybean meal, because the raw material is relatively constant. The variability is when comparing a class of animal proteins (such as meat and bone meal) across the industry. One strategy then is to purchase from a single facility or monitor variability on each of a few select facilities to assist in formulation. A second strategy is to purchase from a protein blending plant. These plants take meat and bone meal from different sources and blend to reach constant endpoints.


Many plant proteins are more uniform because they are made from a single source. Also, methods of processing plant proteins have become standardized, and the same kind of product can be produced year round. However, improper processing can occur in the production of soybean meal and other plant proteins. Also, calcium carbonate (limestone) can be added to plant protein products (up to 0.5%) to prevent them from becoming lumpy and to maintain good flow characteristics. The additional calcium is not a problem as long as it is accounted for in diet formulation.


In order to determine relative feeding value of alternative protein sources, it is important to first compare the lysine content and digestibility of lysine of the alternative source to soybean meal. It is also important to consider the maximum inclusion rate of the alternative protein source. The software program included with the nutrition guide will help you establish the relative feeding value of alternative protein sources; thus a table is not included in this factsheet since relative feeding value is based on the current value of the protein source.


Concepts of Amino Acid Nutrition


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 be used to provide a more ideal balance of amino acids in pig feed. They are to use a combination of supplemental protein sources or to formulate the diet with crystalline amino acids. [6, 7, 8, 10, 11]


Table 1. Amino acid ratio recommendations for nursery pigsa

Type of diet Phase 1 to 4 (9-45 lb.)
Standardized ileal digestible % of lysine
Lysine 100
Threonine 62
Methionine 28
Methionine + cysteine 58
Tryptophan 17
Isoleucine 55
Valine 65
Arginine 42
Histidine 32
Leucine 100
Phenylalanine + tyrosine 94
Phenylalanine 60

aDerived from the 1998 National Research Council (NRC) [10] and research published since 1998.


The ideal amino acid pattern or ratios are presented in Table 1 (nursery), Table 2 (growing-finishing), Table 3 (gestating females and breeding boars) and Table 4 (lactating females) and in PIG Factsheet 07-02-03, Understanding the Nutrient Recommendations in the National Swine Nutrition Guide. Lysine is used as the base value with other amino acids expressed as the percentage of the lysine requirement. [10, 11] The threonine, methionine, methionine + cysteine and phenylalanine + tyrosine ratios increase as the growing and finishing pig matures due to increased in maintenance needs.


Table 2.  Amino acid ratio recommendations for growing and finishing swinea

Type of diet and body weight (lb) Phase 1 (45-90) Phase 2 (90-135) Phase 3 (135-180) Phase 4 (180-225) Phase 5 (225-270) Phase 5 + RACb (225-270) Phase 6 + RACb (270-315)
Standardized ileal digestible % of lysine
Lysine 100 100 100 100 100 100 100
Threonine 63 63 64 65 67 67 68
Methionine 29 29 29 29 30 30 31
Methionine + cysteine 58 58 60 60 62 62 63
Tryptophan 16 16 16 16 16 16 16
Isoleucine 55 55 55 55 55 55 55
Valine 65 65 65 65 65 65 65
Arginine 40 38 36 34 34 34 34
Histidine 32 32 32 32 32 32 32
Leucine 100 100 100 100 100 100 100
Phenylalanine + tyrosine 94 94 94 94 95 95 96
Phenylalanine 60 60 60 60 60 60 60

aDerived from the 1998 National Research Council (NRC) [10] and research published since 1998.
bRactopamine hydrochloride (Paylean®).

Use of Digestibility Values for Diet Formulation


The amino acid requirements of pigs are expressed as digestible amino acids; this reflects the fact that only a certain proportion of each amino acid in a feedstuff is digested and absorbed. Not all of the amino acids in feedstuffs are biologically available to the pig. Many attributes can limit digestion and absorption of the amino acid from the feedstuff. Therefore, the digestibility values of feedstuffs are being determined and utilized for diet formulation. There are two common methods to express digestibility of amino acids for feedstuffs: 1) Apparent and 2) Ileal. Apparent total tract digestibility is the amount of nutrient in the diet minus the amount in the feces divided by the amount of nutrient in the diet. However, research has shown that protein that disappears from the large intestine is not used effectively by the pig; thus digestibility at the end of the small intestine (terminal ileum) before the digesta enters the large intestine have been determined to be more accurate. [1, 2, 4, 5, 6] This type of digestibility is referred to as ileal digestibility.


Table 3. Amino acid ratio recommendations for gestating females and breeding boarsa

Standardized ileal digestible % of lysine for Gestating females % of lysine for Breeding boars
Lysine 100 100
Threonine 76 74
Methionine 27 27
Methionine + cysteine 70 70
Tryptophan 18 18
Isoleucine 57 57
Valine 68 68
Arginine 89 89
Histidine 30 30
Leucine 94 94
Phenylalanine + tyrosine 100 100
Phenylalanine 58 58

aDerived from the 1998 National Research Council (NRC) [10] and research published since 1998.

Due to the high amino acid digestibility of corn and soybean meal, ileal digestibility values can only improve the precision of formulating swine diets when any alternative grain source, high fiber or high protein product is utilized, then the diet should be formulated on a digestible amino acid basis. Otherwise, pigs may not perform as expected due to an overestimation of amino acid absorption.


Table 4. Amino acid ratio recommendations for lactating femalesa

Standardized ileal digestible Parity 1 Litter Weaning Weight (lb) Parity 2 or greater Litter Weaning Weight (lb)
% of lysine for 145 % of lysine for 115 % of lysine for 160 % of lysine for 125
Lysine 100 100 100 100
Threonine 59 63 62 66
Methionine 25 27 26 28
Methionine + cysteine 46 49 48 51
Tryptophan 18 19 18 20
Isoleucine 54 57 57 59
Valine 82 86 85 89
Arginine 53 53 57 59
Histidine 38 40 40 42
Leucine 110 114 114 121
Phenylalanine + tyrosine 96 101 101 108
Phenylalanine 53 55 55 59

aDerived from the 1998 National Research Council (NRC) [10] and research published since 1998.
When formulating diets on an ileal digestibility, the tryptophan, threonine, and methionine levels need to be checked to ensure requirements are met and that the amino acid balance (ratio) is maintained. In addition, isoleucine, valine, arginine, histidine, leucine, phenylalanine plus tyrosine and phenylalanine ratios may be checked as well. Formulating on a digestible amino acid basis is the most precise. Do not formulate diets on a crude protein basis because the diet could be deficient in lysine and (or) other amino acids, resulting in reduced pig performance. [1, 2, 4, 5, 6, 12]


Ileal digestibility values may be expressed as apparent (AID), standardized (SID), or true (TID). These various terms reflect how ileal endogenous amino acid losses are accounted for and separated. Ileal endogenous amino acid losses are separated into basal losses, which are not influenced by feed ingredient composition and specific losses, which are influenced by feed characteristics such as fiber and antinutritional factors. Apparent ileal digestibility (AID) values are calculated by deducting the total ileal outflow of amino acids (sum of endogenous losses and non-digested dietary amino acids) from dietary amino acid intake. However, when apparent ileal digestibility (AID) is corrected for the basal endogenous losses of amino acids then standardized ileal digestibility (SID) values are established. Furthermore, when AID is corrected for both basal and specific endogenous amino acid losses then true ileal digestibility (TID) values are calculated. Currently, SID values should be used for feed formulation until more information on TID values becomes available. [12]



1. Gaines, A. M., G. F. Yi, B. W. Ratliff, P. Srichana, D. C. Kendall, G. L. Allee, C. D. Knight, and K. R. Perryman. 2005. Estimation of the ideal ratio of true ileal digestible sulfur amino acids:lysine in 8- to 26-kg nursery pigs. J. Anim. Sci. 83: 2527-2534.
2. Gaines, A. M., R. D. Boyd, M. E. Johnston, J. L. Usry, K. J. Touchette, and G. L. Allee. 2006. The dietary valine requirement for prolific lactating sows does not exceed the National Research Council estimate. J. Anim. Sci. 84: 1415-1421.
3. Kansas State University Swine Nutrition Guide. General Nutrition Principles of Swine. MF2298 2007; http://www.oznet.ksu. edu/library/lvstk2/s99.pdf
4. Kendall, D. C., A. M. Gaines, G. L. Allee, and J. L. Usry. 2008. Commercial validation of the true ileal digestible lysine requirement for eleven- to twenty-seven-kilogram pigs. J. Anim. Sci. 86: 324-332.
5. Kendall, D. C., A. M. Gaines, B. J. Kerr, and G. L. Allee. 2007. True ileal digestible tryptophan to lysine ratios in ninety- to one hundred twenty-five-kilogram barrows J. Anim. Sci. 85: 3004-3012.
6. Kim, S. W., W. L. Hurley, G. Wu, and F. Ji. 2009. Ideal amino acid balance for sows during gestation and lactation. J. Anim. Sci. 87:E123-E132.
7. Knowles, T. A., L. L. Southern, and T. D. Bidner. 1998. Ratio of total sulfur amino acids to lysine for finishing pigs. J. Anim. Sci. 76: 1081-1090.
8. Lenis, N. P., H. T. van Diepen, P. Bikker, A. W. Jongbloed, and J. van der Meulen. 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.
9. Nebraska and South Dakota Nutrition Guide. Nebraska Cooperative Extension EC 95-273. 2000; http://ianr.unl.edu/PUBS/ swine/ec273.pdf
10. NRC. 1998. Nutrient Requirements of Swine. 10th ed. National Academy Press, Washington, DC.
11. PIC Nutrient Specifications. 2008.
12. Stein, H.H., B. Seve, M. F. Fuller, P.J. Moughan and C. F. M. de Lange. 2007. Invited review: Amino acid bioavailability and digestibility in pig feed ingredients: Terminology and application. J. Anim. Sci. 85:172-180.


Frequently asked questions


Will over feeding protein or amino acids affect animal 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, there is potential that concentrations of some amino acids in the diet will exceed recommended levels. It is encouraged for producers to feed diets using the ideal amino acid ratios in order to reduce nitrogen excretion through the urine and feces. 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 are the most economical.


How do you determine whether another source of amino acids is more economical?
The formulation software helps establish a utilization (shadow) price for determining the time to consider an alternative protein source other than soybean meal. Average daily gain and reproductive performance will not be altered by replacing soybean meal with an alternative amino acid source if the digestible amino acid ratios and diet energy level are maintained. Most amino acid sources are byproducts and subject to some variation in quality, because of the processing methods used. Also, be sure to consider factors such as storage costs, anti-nutritional factors, fiber content, spoilage, product variability, handling characteristics, and availability. 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 (or see Factsheet 27 on Feed Processing).


What is the maximum level that crystalline amino acids can be used in swine diets?
It depends on the price of the synthetic amino acids and the prices of grain and supplemental protein sources. The use of both dry and liquid sources of synthetic lysine is economically feasible. Synthetic methionine and threonine are commercially available and generally are economical to use in swine diets. Synthetic tryptophan, valine and isoleucine can be purchased as well, but are currently are rather expensive and uneconomical to use in most swine diets


Traditionally, 3.4 pounds of L lysine•HCl (containing 78% pure lysine) plus 96.6 lb of corn contribute the same amount of digestible lysine as 100 lb of 44% CP soybean meal. However, research has shown that larger amounts of synthetic lysine up to 6 or 8 lbs per ton are being used in growing and finishing diets with the addition of synthetic threonine, methionine and tryptophan. Finishing pig diets containing ractopamine may contain up to 8 pounds of synthetic lysine with other synthetic amino acids with no impact on animal performance. These methods could dramatically reduce feed costs when soybean meal prices are high.


If synthetic lysine is used, one must monitor dietary tryptophan, threonine, and methionine ratios 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 lysine and tryptophan. The level of amino acids supplemented will depend on the feeds utilized in the formulation and is usually dependent on the second limiting amino acid. The second limiting amino acid changes depending on the ingredients utilized. In most swine diets lysine is first limiting and threonine, methionine, or tryptophan is second limiting. However, starting pig diets containing large amounts of plasma proteins and blood meal need to be supplemented with synthetic methionine.


Should crystalline amino acids be used in gestation diets?
We recommend caution when considering synthetic amino acids as substitutes for intact protein in gestation diets. Gestating sows are usually fed once per day, and research in limit fed pigs indicates that synthetic amino acids are used less efficiently than they are when pigs consume feed several times per day.


What is the impact of using crystalline amino acids on swine manure content?
A factor not traditionally considered when evaluating the use of synthetic amino acids in swine diets is nitrogen content of the manure. Reducing excess amino acids in the diet will result in a decrease in the nitrogen content of the manure. When incorporated properly, the use of synthetic 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.


What does the “L” or “D” before the name of a synthetic amino acid mean?
The L and D refer to the isomeric form. Pigs utilize the L-isomer of all amino acids. Synthetic amino acids are a mixture of both L and D isomers. Pigs utilize the D isomer form of methionine and tryptophan; however, the D isomer of lysine and threonine are not utilized by the pig. [10, 11]


Can soybean meal serve as the sole source of supplemental protein in the diet?
Yes, but only for pigs heavier than about 25 lb. Younger, lighter pigs have a reduced ability to utilize the complex proteins found in soybean meal. In addition, starting pigs may develop an allergic reaction to certain proteins in soybean meal, causing difficulty in digesting and utilizing feed. It is desirable to include less allergenic, highly digestible amino acid sources in diets for starting pigs; for example, spray dried plasma proteins and blood meal, menhaden fish meal, dried whey, and(or) soy protein concentrate, although soybean meal would be a less expensive source of amino acids.