Understanding Calcium and Phosphorus Calculations in Poultry Diets: Available, Retainable, Digestible & Other Forms

In poultry nutrition, calcium (Ca) and phosphorus (P) are two critical minerals that need precise calculation in feed formulations. Different regions or feed formulations may use distinct terms like available, retainable, and digestible to describe how these minerals are utilized by poultry. Here's the difference between these terms:

1. Available Phosphorus (Avail-P or Available-P):

• Definition: This refers to the portion of phosphorus that can be absorbed by the bird and is physiologically usable for metabolism. It is a measure of how much phosphorus can be utilized by the bird after it passes through the digestive system.
• Common use: This term is often used in North America and other regions.
• Example: Inorganic phosphorus sources like dicalcium phosphate are mostly available, but plant-based phosphorus (like that from grains) may not be fully available without the use of phytase enzymes.

2. Retainable Phosphorus (Retain-P or Retained-P):

• Definition: Retainable phosphorus is the portion of phosphorus that is not only absorbed but also retained in the bird’s body for various physiological functions, such as bone mineralization or egg production. It excludes the phosphorus that is absorbed but later excreted.
• Common use: This term is sometimes used in more precise or research-driven contexts where phosphorus retention and utilization need to be measured.
• Example: Phosphorus that is retained in bones, eggs, and tissues would count towards retainable phosphorus, while any that is absorbed but excreted in urine or feces would not.

3. Digestible Phosphorus (Dig-P or Digestible-P):

• Definition: Digestible phosphorus represents the portion of phosphorus that can be broken down and absorbed through the digestive tract, but it may not necessarily be retained by the body. This is a direct measure of phosphorus that has passed through digestion but doesn’t indicate if it is used or lost through excretion.
• Common use: Digestible phosphorus is commonly used in European feeding systems, and focuses on absorption rather than retention.
• Example: A certain percentage of phosphorus in a feed ingredient might be digestible, but not all of that phosphorus will be retained in the body.

Calcium (Ca) Calculation:

• Similar concepts can apply to calcium in terms of availability and digestibility, but calcium is usually calculated as total calcium in most regions.
• Available Calcium: Like phosphorus, this refers to the portion of calcium that can be utilized by the bird after digestion.
• Retainable Calcium: The calcium that remains in the bird’s body (e.g., in bones, eggs).
• Digestible Calcium: The amount of calcium that is absorbed from the digestive tract, regardless of how much is retained.

Key Differences:

• Available vs. Digestible: Available refers to the phosphorus that the bird can actually use for its physiological needs, while digestible just refers to what gets absorbed through the gut, which might not always be fully used by the bird.
• Retainable vs. Digestible: Retainable focuses on what remains in the body for long-term use, whereas digestible focuses only on what gets absorbed during digestion.
• Practical Impact: The use of phytase enzyme in feed increases phosphorus availability and digestibility, but may not always affect phosphorus retention equally, especially if phosphorus is excreted post-absorption.

Understanding these differences is crucial for optimizing feed efficiency and minimizing excess mineral excretion, especially in modern poultry production systems that seek to reduce environmental impacts.

There are a few additional terms and concepts related to calcium (Ca) and phosphorus (P) in poultry diets, which are important for understanding their bioavailability and utilization. These terms are often used in more advanced nutrition discussions:

1. Phytate Phosphorus (Phytate-P):

• Definition: This refers to phosphorus that is bound in the form of phytate (or phytic acid) in plant-based ingredients such as grains and oilseeds. Poultry have limited ability to digest phytate phosphorus without the help of the enzyme phytase, making it largely unavailable unless phytase is added to the feed.
• Importance: Many feed formulations now include phytase to increase the availability of phytate-bound phosphorus.

2. Non-Phytate Phosphorus (NPP):

• Definition: This is the portion of phosphorus that is not bound to phytate and is naturally available to the bird for absorption and use. Non-phytate phosphorus is usually more bioavailable than phytate-bound phosphorus.
• Common use: In feed formulations, non-phytate phosphorus is calculated separately to ensure adequate phosphorus levels are provided after accounting for phytate-bound phosphorus.

3. Total Phosphorus (Total-P):

• Definition: This is the total amount of phosphorus present in the feed, including both the phytate-bound phosphorus and the non-phytate phosphorus. It does not account for bioavailability or how much is usable by the bird.
• Importance: Total phosphorus gives a broader picture of phosphorus content but doesn’t give insight into how much can actually be utilized by the bird unless further analysis (like available, retainable, or digestible phosphorus) is done.

4. Net Phosphorus (Net-P):

• Definition: This refers to the amount of phosphorus that is retained in the bird after accounting for what is absorbed and what is lost through excretion. It reflects the true efficiency of phosphorus utilization in the bird.
• Common use: Net phosphorus is used in advanced feed formulation and research contexts, and can vary depending on factors like phosphorus source, age of the bird, and enzyme supplementation (e.g., phytase).

5. Bioavailable Phosphorus (Bioavailable-P):

• Definition: Similar to available phosphorus, but with a focus on how much phosphorus can be absorbed, utilized, and retained by the bird based on its physiological needs. It considers the efficiency of the bird's body to use the phosphorus for growth, egg production, bone health, etc.
• Common use: Bioavailable phosphorus may be used in some contexts to describe the effectiveness of phosphorus in meeting the nutritional needs of poultry, taking into account digestion, absorption, and retention.

6. Apparent vs. True Digestibility (Phosphorus or Calcium):

• Apparent Digestibility: The apparent digestibility of phosphorus or calcium measures how much is absorbed from the diet based on fecal output, without accounting for endogenous losses (such as those from intestinal secretions).
• True Digestibility: True digestibility takes into account the endogenous losses of minerals, providing a more accurate estimate of how much of the ingested phosphorus or calcium is actually utilized by the bird.
• Importance: True digestibility provides a more precise picture of nutrient absorption, especially when determining the effectiveness of various feed additives like enzymes.

7. Endogenous Phosphorus Losses:

• Definition: These are phosphorus losses that occur naturally within the bird’s digestive system, such as phosphorus secreted by the bird’s own tissues during digestion. This amount is not part of the dietary phosphorus but still contributes to overall phosphorus excretion.
• Importance: Endogenous losses need to be accounted for in certain contexts, particularly when calculating true digestibility or net phosphorus retention.

8. Phosphorus Retention Efficiency:

• Definition: This is a ratio or percentage that indicates how efficiently phosphorus is retained in the bird’s body after ingestion and absorption. It’s used to assess the effectiveness of phosphorus sources and feeding strategies in maximizing the use of phosphorus for productive functions, like bone mineralization and egg production.

9. Soluble Calcium:

• Definition: Soluble calcium is the portion of calcium in the diet that can dissolve in the gut and be absorbed by the bird. It is influenced by factors such as the form of calcium in the feed (e.g., calcium carbonate vs. calcium citrate) and the pH of the gut.
• Importance: Soluble calcium is essential for bone formation, eggshell quality, and other physiological functions, and can be influenced by factors like vitamin D levels.

Summary of Additional Terms:

• Phytate Phosphorus (Phytate-P):?Phosphorus bound to phytate is less available unless phytase is used.
• Non-Phytate Phosphorus (NPP): The portion of phosphorus not bound to phytate and readily available.
• Total Phosphorus (Total-P): Total phosphorus content, regardless of availability.
• Net Phosphorus (Net-P): Retained phosphorus after accounting for losses.
• Bioavailable Phosphorus (Bioavailable-P): Phosphorus efficiently used by the bird.
• Apparent vs. True Digestibility: Measures of how much phosphorus or calcium is absorbed, with true digestibility accounting for endogenous losses.
• Endogenous Phosphorus Losses: Phosphorus lost from the bird’s own tissues during digestion.
• Phosphorus Retention Efficiency: How well phosphorus is retained in the body.
• Soluble Calcium: The portion of calcium that dissolves in the gut and is absorbed.

Each of these terms helps refine the understanding of mineral utilization in poultry diets, ensuring optimal nutrient absorption and efficiency in feed formulations.