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Protein and Amino Acids

Birds do not require protein, but rather the amino acids that make up protein. Although minimum recommendations for dietary crude protein contents are shown in the accompanying tables, it is strongly recommended that diets be formulated on an amino acid basis with no crude protein minimums. However, when no minimum crude protein content is specified, it is important to consider the content of all amino acids to avoid deficiencies. With the use of synthetic (crystalline) amino acids (lysine, methionine, threonine), the limiting amino acids in most diets will likely be tryptophan, valine, or isoleucine.
 
Digestible amino acids
A portion (typically 10 to 15%) of the dietary amino acids is not digested and instead excreted in the feces. Because the indigestible portion varies greatly among feed ingredients, it is highly recommended that diets are formulated on a digestible amino acid basis. For instance, soybean meal, meat and bone meal, and cottonseed meal contain about the same amounts of total methionine, but their methionine digestibilities differ widely (see table below). Therefore, diets formulated on a total amino acid basis must contain large (and costly) safety margins to account for the differences in digestible amino acid content of different feed ingredients, especially when low-quality or coproduct ingredients are used.

Differences in digestibility and contents of digestible methionine among feed ingredients with similar contents of total methionine

1There may be legal and practical limitations in the use of meat and bone meal and/or cottonseed meal in laying-hen diets other than the amino acid digestibility.
 
By formulating diets on a digestible amino acid basis, safety margins can be reduced and feed ingredients can be more accurately valued based on their content of bioavailable amino acids. Formulation of diets on a digestible amino acid basis is more accurate, results in more economical diets, and can reduce the impact on the environment compared to formulation on total amino acid basis or on a crude protein basis. Note that the amino acid recommendations for Hy-Line birds are based on digestible amino acids. Values for amino acid digestibilities in feed ingredients are available through amino acid companies, such as Evonik and Ajinomoto. A table of amino acid digestibilities in selected feed ingredients is available here in pdf format (322 kb).
 
Ideal amino acid ratios
The hens' requirements for individual amino acids change due to genetic or environmental factors. However, the ratios among individual amino acids are only slightly affected. Thus, once the ideal amino acid ratios have been determined, the requirement for a single amino acid (i.e., lysine) can be determined experimentally for a given field situation and the requirements for all the other amino acids calculated from the ideal ratios. This approach has been adopted with success by the swine industry and is finding use in the broiler industry as well.

The amino acid recommendations for all Hy-Line varieties are based on digestible lysine and all other amino acids are calculated using ideal amino acid ratios. The ideal amino acid ratios used are based on a review of the literature (see table below) as well as analyses of practical commercial diets that result in excellent performance. The ideal amino acid ratios used to set amino acid recommendations may differ slightly among Hy-Line varieties due to differences in body weights and egg weights.
 
Ideal amino acid profiles1 for laying hens.

1Lysine requirement set at 100%
2Based on true digestible requirements for maximal egg mass (egg mass is defined as percent egg production × grams of egg weight)
3Based on total amino acid requirements
4Based on nitrogen balance
5Based on digestible amino acid requirements
6Based on total amino acid requirements for 32-to-45-wk-old laying hens
7The arginine-to-lysine ratio was estimated to be 107 or less

The differences in individual ideal amino acid ratios shown in the table above reflect differences in how they were determined (i.e., by calculation from averages of amino acid requirements from many or few experiments) and likely by differences in body weight and egg production of the hens used in the respective experiments (i.e., partitioning of amino acid needs among maintenance, body weight gain, and egg production). Currently, all estimates of the ideal amino acid ratios for laying hens combine ratios for maintenance, body weight gain, and egg production. It should therefore be expected that the ratios can vary depending on the specific flock and its egg-production performance.
 
Literature Cited
1. Bregendahl, K., S. A. Roberts, B. Kerr, and D. Hoehler. 2008. Ideal ratios of isoleucine, methionine, methionine plus cystine, threonine, tryptophan, and valine relative to lysine for white leghorn-type laying hens of twenty-eight to thirty-four weeks of age. Poultry Science 87:744-758.

2. Centraal Veevoederbureau (CVB). 1996. Aminozurenbehoefte van Leghennen en Vleeskuikens [Amino acid requirements for laying hens and broiler chickens]. Documentation Report nr. 18 (in Dutch). Lelystad, The Netherlands.
 
3. Coon, C., and B. Zhang. 1999. Ideal amino acid profile for layers examined. Feedstuffs 71(14):13-15, 31.
 
4. Jais, C., F. X. Roth, and M. Kirchgessner. 1995. The determination of the optimum ratio between the essential amino acids in laying hen diets. Archiv für Geflügelkunde 59:292-302.
 
5. Leeson, S., and J. D. Summers. 2005. Commercial Poultry Production. 3rd ed. University Books, Guelph, ON.
 
6. NRC. 1994. Nutrient Requirements of Poultry. 9th ed. National Academy Press, Washington, DC.
 
7. Rostagno, H. S. 2005. Brazilian tables for poultry and swine. Composition of feedstuffs and nutritional requirements. 2nd ed. Departamento de Zootecnia, Universidade Federal de Vicosa, Brazil.

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