Protein is an essential component of feed, but animals require the amino acids within that protein. When the amino acids in a protein are in balance, the animal's muscles, skin, feathers, internal organs, enzymes, antibodies, and other physiological activities are at their optimal state. This article systematically describes the relationship between feed protein and amino acids, following the pathway from protein → amino acids → digestion → absorption → balance → protein synthesis → animal production performance.
1. What is Protein?
Proteins are composed of more than 20 amino acids. The main functions of protein in animals are: to form body tissues, participate in metabolic regulation, maintain immune function, and provide energy. It is important to note that crude protein in feed is not the same as complete protein; it is merely an estimated protein level, not true protein, and certainly not usable protein.
2. What are Amino Acids?
Amino acids are the basic building blocks of proteins. Based on whether the body can synthesize them, they can be classified into essential amino acids, non-essential amino acids, and conditionally essential amino acids. If you would like to learn more about feed-grade amino acids, please read this article: Basic elements of feed: Amino Acids.
3. The Relationship between Proteins and Amino Acids
In the animal's digestive tract, proteins in feed are gradually hydrolyzed by proteases into polypeptides, small peptides, and free amino acids. Protein is the main source of amino acids; adding crude protein to animal feed essentially provides amino acids. Therefore, feed formulation is fundamentally an amino acid formulation, not a protein formulation.
The amount of protein synthesized is determined by the most deficient amino acid: if the amount of the most limiting amino acid is not increased, no amount of other amino acids can compensate, and the excess can only be oxidized and deaminated, increasing the metabolic burden and nitrogen emissions. The core of formulation work is to "find the deficiency and fill it one by one".
4. Amino Acid Characteristics of Common Feed Ingredients
As clearly seen in the image above:
Corn contains 7.5–8.5 % crude protein and 0.24–0.28 % lysine . It is severely deficient in lysine and tryptophan , resulting in generally poor protein quality.
Soybean meal contains 43–48 % crude protein and 2.5–3.2% lysine. The amino acid profile of plant protein is relatively balanced, but the methionine content is low.
Fishmeal contains 60–67 % crude protein and 4.5–5.0 % lysine . The amino acid profile of fishmeal is well-balanced and highly digestible.
Rapeseed meal contains 33-40% crude protein and 1.8-2.1 % lysine . The lysine content is low and the digestibility is slightly low , and it contains anti-nutritional factors.
DDGS has a crude protein content of approximately 26% and a lysine content of 0.9 %. The lysine and tryptophan content is insufficient, and excessive heating during the drying process can lead to the Maillard reaction, reducing lysine digestibility. It also has a high phosphorus content.
5. What is an Ideal Protein?
Ideal protein refers to a feed composition that perfectly matches the animal's growth needs, neither lacking nor excessive. This concept was proposed by Mitchell around 1950 and later developed into a modern, workable amino acid model by the ARC, NRC, and industry organizations .
Animals at different growth stages have different amino acid requirements, and these requirements also vary depending on the farmer's production goals, such as weight gain, milk production, egg production, and feather growth. For example, piglets require higher levels of tryptophan and lysine than finishing pigs, while laying hens need higher levels of methionine and cysteine. Formulas should be dynamically adjusted according to the animal's growth stage.
6. Why Do Modern Feeds Emphasize the Addition of Amino Acids?
The introduction of the modern ideal protein concept and changes in feed formulation mean that past crude protein indicators can no longer meet the balanced nutritional needs of animals. What animals truly need is: specific amino acids + appropriate ratios + sufficient digestibility and absorption.
According to K-State's current data, feed-grade lysine, methionine, threonine, tryptophan, valine, and isoleucine are widely used as feed additives, continuously reducing nitrogen emissions while truly achieving low protein + amino acid fortification + amino acid balance.
Of course, emphasizing amino acid supplementation does not mean continuously reducing the proportion of crude protein. Crude protein also contains some non-essential amino acids, nitrogen, and other protein-related nutrients. These nutrients can help synthesize other nutrients in the body. Therefore, the goal of a low-protein diet is not to reduce protein to a minimum, but rather to maintain a reasonable level of other protein intake while meeting the animal's balanced nutritional needs.
7. What is the Difference between Total Amino Acids and Digestible Amino Acids ?
The total amount of protein in feed is not all the protein that animals can absorb. Therefore, protein quality depends not only on the amino acid content but also on digestibility. Modern animal nutrition requires consideration of both amino acid digestibility and bioavailability.
Total amino acids ( Total AA ): The total amount of amino acids in the raw materials of feed.
Apparent ileal digestibility ( AID ): (Ingested amino acids - Amino acids flowing out of the terminal ileum) / Ingested amino acids. This calculation method includes amino acids from digestive juices and sloughed intestinal cells in the undigested portion, thus underestimating the true digestibility of the raw material . Furthermore, this value is also affected by feed intake.
Standard ileal digestibility ( SID ): (Ingested amino acids − (Ileal outflow amino acids − Basal endogenous amino acids)) / Ingested amino acids. This calculates the loss of amino acids secreted within the animal's body and is closer to the true digestibility.
Currently, most mainstream animal feed formulations on the market use the SID method, which is convenient for accurate calculation based on the combination of raw materials and eliminates interference from feed intake levels and endogenous losses.
8. Why does Protein Processing Affect Amino Acid Utilization ?
During processing, some feed ingredients or additives may undergo changes due to high temperatures, prolonged heating, improper drying processes, or substandard storage conditions. Changes in lysine content are particularly important to monitor.
FAO data indicates that even if chemical analysis shows the presence of a certain amino acid, it does not necessarily mean that it has the same bioavailability . Therefore, protein content , amino acid content , digestible amino acids , and usable amino acids are four different concepts.
Therefore, many ingredients are used in combination in modern feed formulations, such as corn and soybean meal. As we learned from the amino acid characteristics of common feed ingredients above , corn is relatively low in lysine, while soybean meal is high in lysine. Combining these two, along with the addition of crystalline amino acids , can better provide animals with sufficient balanced amino acids, which is the protein complementarity function mentioned in the formulation.
9. The Role of Adding Amino Acids to Feed
9.1 Supplementing Limiting Amino Acids
Lysine, methionine, threonine, and tryptophan to mainstream corn and soybean meal diets can prevent amino acid deficiencies in the diet and allow it to fully play its role in balancing animal nutrition.
9.2 Reduce Crude Protein
Experiments show that for every 1% decrease in crude protein, nitrogen excretion decreases by approximately 8% to 10% (this percentage varies depending on the animal species and the types of amino acids). Furthermore, provided essential amino acids are sufficient, replacing some high-protein raw materials with crystalline amino acids can effectively avoid protein waste without compromising production performance.
9.3 Optimize the Breeding Environment
The effective reduction of nitrogen emissions is significantly related to low-protein diets . Reduced nitrogen in animal excrement leads to lower ammonia concentrations , resulting in better air quality and reduced manure treatment load , which is of certain significance for environmental protection.
9.4 Optimize Feed Costs
The price of amino acids is lower than that of soybean meal or fishmeal of the same amino acid equivalent . Currently, feed formulations that combine low-protein raw materials with complete sets of crystalline amino acids can effectively reduce the production costs of feed companies.
To improve protein utilization by balancing amino acids, it's crucial to ensure energy balance. Targeting ideal protein levels and formulating based on SID (Suitable Intake Levels) minimizes imbalances in limiting amino acids. Low-protein diets can reduce nitrogen excretion and feed costs without sacrificing or even improving production performance. Get the best results for less money! For more information on feed-grade amino acids, consider consulting industry experts at Polifar; they will provide you with the best advice!
FAQ
Q1: Is higher protein content always better?
A1: Incorrect. Excessive crude protein does not equal sufficient amino acids. Excessive protein means that excess amino acids are deaminated and excreted, which wastes nitrogen, increases the burden on the liver and kidneys and ammonia emissions, and drives up feed costs. The key is the quality and quantity of amino acids, not that the higher the crude protein, the better.
Q2: Is a higher amino acid content always better?
A2: Incorrect. Excessive amino acids are also harmful: they increase metabolic burden (e.g., excessive Met toxicity), disrupt the balance between amino acids (excessive Leu antagonizes Ile/Val), and reduce feed intake and feed conversion ratio. Precision means "just enough," not "the more the better."
Q3: Is the total amino acid count equal to the available amino acid count?
A3: Incorrect. The digestibility of different raw materials varies greatly (and even different batches of the same raw material may differ), so total amino acids cannot reflect the true amount obtained by animals. SID / digestible amino acids is the correct benchmark for formulation and evaluation.
Q4: Is lower protein always better?
A4: Incorrect. A low-protein diet requires a complete and sufficient supply of essential amino acids (especially SID amino acids). Without adequate amino acid supplementation to suppress CP (protein nutrient deficiency), performance will inevitably decline. The boundary of a low-protein diet is "amino acid balance."
Q5: Can supplementing with just one amino acid solve all protein problems?
A5: After lysine supplementation, the following may occur: lysine is no longer a limiting amino acid, threonine becomes the limiting amino acid, and once threonine is replenished, other amino acids may become the next limiting factors. Therefore, amino acid supplementation is a dynamic equilibrium process.
Conclusion
In summary, the economic relationship between protein and amino acids is the core concern for feed companies and farmers. A reasonable protein source and precise amino acid supplementation meet the growth needs of animals, resulting in lower feed costs, stronger animal production performance, lower nitrogen emissions, and higher overall profits. So why not use this more economical approach?