Phosphorus is a key nutrient for bone development and biochemical reactions.
This is an inorganic element that is essential for growth performance and development and maintenance of the skeletal system. Approximately two-thirds of the body concentration of P is found in the pig skeleton, while the remaining P is found in muscle tissues where it is involved in different biological function.
Pigs should be provided with adequate P to meet their physiological needs, and the accurate estimation of P bioavailability and requirements is essential to achieve this. Diets formulated with excess P can lead to an increase in P excretion, negatively effecting the environment. In addition, this mineral is the third most expensive component in swine diets after energy and protein. Thus, diets are typically formulated to avoid excess P, with low margins of safety.
In the past, swine diets had been formulated based on the relative bioavailability of P calculated using the slope/ratio technique.
As an alternative to the relative bioavailability of P, digestible total tract P has been commonly used when formulating swine diets. In contrast to a total P basis, digestible total tract P accounts for the digestibility of P in feed ingredients and can be expressed on an apparent (ATTD), true (TTTD), or standardized total tract digestible (STTD) basis. However, ATTD values are not corrected for endogenous losses of P. Consequently, ATTD values would not be expected to be additive in mixed diets, particularly when the ingredients used in the mixed diets have very low P concentrations.
In simple terms, STTD P, is the true usable phosphorus in pig feed after correcting for natural phosphorus losses from the pig itself.
It is well known that muscle tissue constitutes the second most abundant P reserve in the body after skeletal tissue, with minimum P found in the adipose tissue.
Phosphorus : a driver of lean deposition and swine efficiency
Pig genotype can influence the extent of bone and muscle tissue deposition, which can lead to different dietary P requirements. In today’s pig production, pigs are not only marketed at heavier weights but are also highly selected for high lean tissue deposition.
The link between STTD P (Standardized Total Tract Digestible Phosphorus) and swine performance is quite direct because phosphorus is essential for growth, bone development, and metabolic functions.
STTD P directly influences pig growth, feed efficiency, bone integrity, and reproductive performance. Using STTD P in diet formulation improves performance while minimizing waste and environmental impact.
Observations from a study conducted in 2019 (Carine M. Vier and coll, Journal of Animal Science, 2019, 4023–4031) have determined the STTD Phosphorus requirements to optimize growth rate and feed efficiency at 116% to 122% of the NRC estimates (2012), corroborating results from the earlier studies. Moreover, low dietary P concentrations can negatively affect ADG (Growth) and G:F (Feed efficiency), which is in agreement with the quadratic improvements in growth performance.
So, this is critical to have the best phosphorus digestibility and availability, to secure performances and avoid any zootechnical issue.
Calcium: the hidden threat
Calcium (Ca) has a direct and important effect on STTD P (Standardized Total Tract Digestible Phosphorus) in swine because Ca and P interact closely in the gut and in bone metabolism.
Excess dietary Ca can bind phosphorus in the intestine, forming insoluble Ca–P complexes. These complexes are poorly absorbed, reducing STTD P availability.
When Calcium levels are high in a diet, Calcium reacts with soluble phosphate (from feed) and forms insoluble calcium phosphate. Insoluble Phosphorus cannot be digested and consequently, standardized digestibility decreases.
This can lead to suboptimal growth and weaker bones, even if total Phosphorus seems adequate. Too little Calcium also affects bone development and Ca:P balance, but STTD P may appear higher because less Calcium is binding it.
Even if feed has enough digestible phosphorus (theoretically), excess calcium can “lock” Phosphorus in the gut, reducing STTD P and impairing pig performance and lean deposition.
Driving the Ca:P ratio depends on the production objectives.
he graph below shows the relation between Calcium and phosphorus ratio : to optimise lean deposition, swine require more Phosphorus related to calcium.
If Ca–P nutrition is misbalanced, lean growth efficiency (ADG, feed efficiency, carcass lean) declines mainly because excess Ca reduces P digestibility and limits metabolic processes related to protein deposition. There are some indicators that can alert if there is a Ca–P balance that is limiting lean growth.
Signs in production :
- Reduced feed efficiency
- Lower ADG with adequate energy & amino acids
- Higher fecal P excretion
- Weaker bones in finishing pigs
- Poorer carcass lean %
In modern lean-growth swine nutrition, calcium (Ca) and standardized total tract digestible phosphorus (STTD P) interact with lysine deposition models because protein deposition (PD) can only occur when skeletal growth and cellular energy metabolism are not mineral-limited.
If Ca–P supply is suboptimal or imbalanced, the pig cannot fully utilize dietary lysine for lean tissue growth. Phosphorus is required for several metabolic steps in muscle growth: ATP production. And lean and protein synthesis requires large amounts of ATP.
Many growth models now link STTD P requirement directly to protein deposition:
Typical relationship:
STTD P requirement ≈ 5.0–6.0 g per kg protein deposition
To support high lean deposition that is allowed with AXIOM sire line, to express full genetic potential and optimize FCR, excess of Calcium is one of the most common hidden limitations in high-lean genetic lines.
We recommend to control limestone inclusion and to verify feed quality by analysing regularly the total analytical calcium in fattening swine diet. Higher phytase doses can offset Ca-induced reductions in P availability.
To optimise bones deposition, swine need more calcium related to phosphorus (see the graph below): that is especially important when farmers are producing breeders and gilts which require specific feed recommendation.
Optimizing bone health requires precise management of calcium (Ca) and phosphorus (P) because the skeleton is the largest mineral reservoir in the pig.
Around 80–85% of body P and 98–99% of Ca are stored in bone, mainly as hydroxyapatite crystals. Poor mineral balance leads to reduced bone strength, lameness, and structural problems that can limit growth and welfare.
Conclusion
Depending on the production objective (Lean for meat or bones for breeders), Ca:P nutrition must be precisely managed and controlled.
Correctly optimizing STTD P allows pigs to express their genetic potential for lean growth while maintaining skeletal integrity and minimizing environmental phosphorus losses.
Key takeaways
- Genetic selection increased lean growth and mineral demand.
- Lean growth drives phosphorus requirement.
- Environmental pressure reduced dietary P.
- Excess Ca reduce effective P availability and can impair lean deposition potential.
Writer
- Article writed by Laurent Roger, Axiom Technical Director
- Article adapted for a general audience by Axiom’s Communications Department
Sources and Bibliographies
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- F. Wu and coll, Animal (2019), 13:11, pp 2473–2482 – Standardized total tract digestible phosphorus requirement of 6 to 13 kg pigs fed diets without or with phytase.
- Woong Bi Kwon 1, Beob Gyun Kim , Anim Sci J2017 Jun;88(6):918-924 – Standardized total tract digestibility of phosphorus in various inorganic phosphates fed to growing pigs
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- NRC (2012) Nutrient Requirements of Swine. – 11th Edition, The National Academies Press, Washington DC.