Ideal protein concept in poultry and pigs

It can be concluded that, despite the existence of different methods for determining the ideal protein profile, the results obtained are relatively consistent. Any apparent variations could be attributed to the specific method used for determining the ideal protein, as well as interactions between amino acid requirements and other factors related to the animal and its environment. Important factors include age, climate, management practices, and genetic factors such as sex or strain.

INTRODUCTION
The pig’s ability to utilize dietary protein efficiently depends on the digestibility of protein and its constituent amino acids (AA), as well as the amount and balance of AA in relation to the animal’s needs. If there is an excess of amino acids, they will be broken down, and the resulting urea will be excreted in the urine. Achieving a proper balance between AA supply and AA requirement is crucial for several reasons. Protein is a relatively costly nutrient, and many countries depend on imported protein sources for animal feeding. Moreover, inefficient utilization of dietary protein leads to nitrogen excretion, which contributes to the environmental impact of animal production, particularly in pig-producing regions worldwide. Thankfully, the availability of crystalline amino acids such as L-Lys, DL-Met (or analogues of L-Met), L-Thr, L-Trp, and L-Val has increased, enabling the formulation of low-protein diets with well-balanced AA content. However, reducing protein content in the diet while maintaining optimal animal performance is only feasible if accurate knowledge exists regarding the requirements for all amino acids.

WHAT IS IDEAL PROTEIN
The concept of the “ideal protein,” which refers to protein with an optimal pattern of essential amino acids (EAA) precisely meeting animals’ physiological needs, was first introduced in the 1950s. However, this concept overlooked other amino acids, known as AASAs. In animal nutrition, relying solely on the ideal protein concept is not considered ideal. Instead, it is important to provide diets that include all amino acids that are both physiologically and nutritionally essential. Recent years have seen the proposal of improved amino acid patterns in diets for various animals such as swine, chickens, zoo animals, and companion animals. Animal-based feed sources offer a rich supply of essential amino acids and other important amino acids like glutamate, glutamine, glycine, proline, 4-hydroxyproline, and taurine. By incorporating these amino acids into the diets of swine, poultry, fish, and crustaceans, their growth, development, reproduction, and overall health can be improved, while also supporting global animal production. It’s worth noting that the specific requirements for amino acids such as Lysine, Methionine, Threonine, and Tryptophan in growing pigs have been extensively studied, taking into account factors like the study design, chosen response criteria, and statistical models used to estimate these requirements.

STRUCTURES AND ROLES OF AMINO ACIDS
Ensuring the availability of high-quality animal-derived proteins for human nutrition is a crucial aspect of animal production. Proteins are composed of amino acids, which act as the fundamental building blocks. Each amino acid consists of an amino group (-NH2), a carboxyl group (-COOH), and a unique side chain. Proteins are formed through the polymerization of amino acids, where the carboxyl group of one amino acid reacts with the amino group of another.

In the 1930s, William Rose from the University of Illinois proposed the idea that certain elements within proteins are essential components of the diet. He later identified Threonine as one of these essential elements. Amino acids such as Lysine, Methionine, Threonine, Tryptophan, Phenylalanine, Histidine, Valine, Isoleucine, and Leucine are considered dietary essential or indispensable amino acids for pigs. This means that pigs lack the metabolic capacity to synthesize the carbon chains of these amino acids. On the other hand, amino acids like Serine, Glycine, Arginine, Alanine, Proline, and Glutamine can be synthesized de novo, and therefore, they are referred to as dietary non-essential amino acids.

Although pigs have the ability to synthesize these non-essential amino acids, it doesn’t necessarily mean that their synthesis capacity is sufficient to meet the requirements. Arginine, for instance, is often considered a non-essential amino acid, but the synthesis capacity may be inadequate in young pigs. Hence, the overall dietary protein supply must be adequate to provide the necessary nitrogen required for synthesizing non-essential or semi-essential amino acids.

Due to their role in protein synthesis, the requirements for amino acids during growth are largely influenced by the quantitative aspects of protein deposition.

The concept of the ideal protein revolves around a scenario where all essential amino acids (AA) act as limiting factors for optimal performance, ensuring that the supply of AA precisely matches the AA requirement. Typically, AA requirements in ideal protein formulations are expressed relative to the requirement for Lysine, with Lysine being assigned a value of 100%. This relative expression is highly practical and beneficial from a practical standpoint. Lysine is commonly the most limiting amino acid in pig diets, attracting significant attention from nutritionists. Extensive research has been conducted to understand the changes in Lysine requirements throughout the growth, gestation, and lactation stages. If the requirements for other amino acids are predominantly driven by the need for protein synthesis, their requirements should remain relatively stable compared to Lysine. This simplifies practical pig nutrition because nutritionists only need to consider the changes in Lysine requirement over time and combine it with a consistent ideal protein profile. The application of the ideal protein concept for pigs was initially implemented by the Agricultural Research Council (ARC).

While the AA requirements for growing pigs, gestating sows, and lactating sows undergo rapid changes during the production period, most dose-response studies maintain constant diets to evaluate the response accurately.

AMINO ACID REQUIREMENTS FOR GROWING PIGS IN THE INRA PORC MODEL
The deposition of protein plays a significant role in determining the amino acid (AA) requirements in growing pigs. These requirements, along with the level of feed intake, dictate the necessary AA content in the diet. However, both protein deposition and feed intake undergo changes throughout the growth process, which means that the required AA content in the diet depends on the protein deposition curve and the feed intake curve.To obtain accurate estimates of AA requirements, users need to provide information about these curves. A strong relationship exists between body protein and body water, as well as between body protein and body weight. A procedure is available where users can input serial measurements of body weight and feed intake (with a minimum of three measurements during the growing-finishing period). Based on this data, the software can determine the feed intake and protein deposition curves.

The protein deposition curve is described by three model parameters: the initial protein mass (which is closely associated with the initial body weight), the average protein deposition over the growing period (related to the average daily gain), and a “precocity” parameter that indicates whether the animal matures early or late.

Inra Porc provides an estimation of the amino acid (AA) requirements for an individual animal, which is lower than the requirement of a population with the same average performance. Inra Porc also provides an indication of the requirement for a population of pigs, which is approximately 10% higher than the requirement of an average pig.

One significant advancement in animal nutrition, including broiler nutrition research, has been the development of the “Ideal Protein” concept. This concept is based on the idea that birds require amino acids in a specific balance to achieve optimal performance. When an absorbed amino acid is in excess compared to the first limiting amino acid, it is oxidized, and nitrogen is excreted. Therefore, adjusting the dietary amino acid supply based on the Ideal Protein Concept maximizes nitrogen utilization. A practical application of this concept is the formulation of low protein diets that can achieve the same level of animal performance compared to unbalanced high protein diets. Another fundamental idea of the Ideal Protein concept is that while the bird’s requirement for essential amino acids may vary depending on different practical situations, the ratios between these amino acids remain relatively stable. Therefore, it is necessary to evaluate the response of a single reference amino acid, usually lysine, to changes in production conditions, and then adjust the remaining amino acids through calculation. Only essential amino acids are considered in this concept. However, there is still ongoing scientific discussion regarding the appropriate ratio between essential and non-essential amino acids, particularly in low protein diets.

Lysine has been widely adopted as the reference amino acid in swine nutrition because it is typically the most limiting amino acid in swine diets. While methionine and cystine (Met+Cys) are often the first-limiting amino acids in common broiler diets, lysine is still chosen as the reference amino acid based on the following three arguments:
– Lysine is primarily utilized for the synthesis of body protein, and its requirement is minimally affected by other metabolic functions, such as maintenance requirements or feathering, which is the case for Met+Cys.
– There are no significant metabolic interactions between lysine and other amino acids. In contrast, birds have the ability to convert methionine to cystine, but not the reverse conversion.
– From an analytical standpoint, lysine is easier to measure compared to methionine and particularly cystine.

Therefore, despite the difference in the first-limiting amino acids between swine and broilers, lysine is still considered the reference amino acid due to its specific metabolic characteristics and practical advantages.

THERE ARE VARIOUS WAYS TO DETERMINE AN “IDEAL AMINO ACID PROFILE”
Various scientific approaches have been employed to determine ideal amino acid profiles. One method involves conducting a comprehensive review of existing literature, where data from experiments investigating individual amino acid requirements are compiled. The average optimum levels of amino acids obtained from these studies are then presented as the ideal amino acid profile. However, this approach has limitations due to variations in experimental conditions, such as environmental factors, housing systems, climate, stocking density, and diet composition. These factors can introduce inaccuracies and may not be adequately considered in determining amino acid ratios. Although not explicitly stated as ideal ratios relative to lysine, the amino acid recommendations for broilers by NRC (1994) serve as an example of this method.

More advanced methods, such as the factorial approach, involve conducting multiple dose-response experiments that simultaneously investigate the complete range of essential amino acids. Another method is the deletion method, where individual amino acids are selectively removed from the diet to assess their impact on performance. These approaches offer more sophisticated means of obtaining ideal amino acid profiles.

When comparing the outcomes of the various approaches to determine ideal amino acid ratios, it is noteworthy that the obtained amino acid profiles are surprisingly similar, despite their respective advantages and limitations. This suggests that we have gained a reasonably clear understanding of the ideal amino acid profile requirements for current broiler strains. However, certain circumstances can influence ideal ratios. One factor is the age or body weight of the birds, as their requirements may vary at different stages. Additionally, environmental conditions can also impact ideal amino acid ratios, with notable examples being the ratios of Arginine to Lysine and Threonine to Lysine.

CURRENT SUGGESTIONS FOR AN IDEAL PROTEIN
It can be concluded that, despite the existence of different methods for determining the ideal protein profile, the results obtained are relatively consistent. Any apparent variations could be attributed to the specific method used for determining the ideal protein, as well as interactions between amino acid requirements and other factors related to the animal and its environment. Important factors include age, climate, management practices, and genetic factors such as sex or strain. Applying the ideal protein concept, with minor variations, has proven to be effective.

The concept of ideal crude protein (ICP) aligns directly with the principles of precision nutrition for poultry. Here are some key points:
1. The ICP concept sets amino acid requirements based on ratios to lysine and emphasizes the use of digestible amino acid levels. This approach eliminates the over- or under-fortification of diets with critical amino acids, while also reducing nitrogen excretion.
2. The ICP concept allows for accurate estimation of amino acid requirements, apart from lysine, during later growth periods using the concept of “ideal individual crude protein” (IICP). However, it is recommended to validate these estimates with empirical data for older birds.
3. Lysine requirement for maintenance may be significantly higher than previously thought, which raises questions about the validity of increasing the ratio of amino acids to lysine during later growth stages.

The concept of ideal protein refers to a protein source that provides the precise balance of amino acids required for optimal performance and maximum growth, encompassing factors such as size, carcass weight, yield, and body composition. By formulating feeds based on ideal protein, it is possible to meet the amino acid requirements while using a reduced amount of protein in the diet. This approach not only maximizes the efficient utilization of protein but also helps lower production costs, increase farm profitability, and reduce the dependence on wild fish harvest for fishmeal.

Applying the concept of ideal protein to fish feed formulation can also contribute to the reduction of nitrogen pollution associated with fish production. Similar to its successful implementation in poultry and swine feeds, the concept of ideal protein focuses on meeting the fish’s specific requirement for essential amino acids that make up proteins. It is important to note that fish do not have a specific protein requirement; rather, they have a definite need for essential amino acids.During protein digestion, the amino acids comprising the protein are released and absorbed by the fish’s body as individual amino acids or shorter chains, such as di- and tri-peptides. There are approximately 20 known amino acids that serve as the building blocks for proteins in all living organisms. Out of these, ten amino acids are considered essential as fish cannot synthesize them or do so at an inadequate rate to meet cellular demands. The remaining amino acids are classified as non-essential or dispensable since fish can synthesize them at a rate sufficient for protein synthesis as per cellular requirements. Both essential and non-essential amino acids are crucial for body cells during protein synthesis.

IDEAL PROTEIN CONCEPT IN AQUACULTURE FEED
Excessive nitrogen, in the form of ammonia excreted by fish, can have detrimental effects on the environment as a major contributor to water pollution. Given that each fish species and its individual proteins have unique amino acid compositions, the ideal scenario would involve formulating low-protein feeds that minimize nitrogen excretion while meeting essential amino acid requirements. This practice is already routine in other species like poultry and swine, where commercially available synthetic essential amino acids (e.g., methionine, lysine, threonine) are efficiently utilized.

Reducing the overall protein content in fish feeds can enhance profitability and minimize nitrogen pollution. This can be achieved by selecting highly digestible proteins and formulating feeds with an appropriate amino acid balance. The utilization of nutrient-dense feeds is another effective strategy to decrease costs and reduce nitrogenous waste in aquaculture facilities. Nutrient-dense feeds provide a significant amount of specific nutrients relative to the energy content within a feed pellet. These approaches have been successfully implemented in poultry and swine, resulting in notable improvements in feed conversion ratios and reductions in nitrogenous waste.

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