The Role of Calcium in Phosphorus Digestibility

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Article | 08.09.2021
In the past there was little interest in the assessment of the calcium (Ca) digestibility of the main ingredients contributing to dietary Ca in poultry diets. This was due to abundant supplies of low-cost limestone being used to supply 80-85% of the total Ca in feed.

However, driven by the numerous studies on phosphorus (P) digestibility and phytase efficacy, Ca requirements have gained increasing interest. In particular, the question is raised whether we should also formulate feed using a digestible Ca value for feedstuffs.

Calcium as an anti-nutrient

Research from recent years has indicated that oversupplying Ca can negatively affect animal performance:

  • Limestone is a buffering substance and hinders sufficient acidification of the feed in the gizzard
  • An excess of dietary limestone reduces feed intake
  • Ca exerts a negative effect on the tight junctions, reducing gut integrity
  • Excess Ca reduces P digestibility of feeds

Ca inhibits the phytate-degrading ability of a phytase

Dietary Ca forms complexes with phytate that can remain insoluble at high pH levels. These precipitated phytate-Ca complexes (Figure 1) are not available to the phytase for hydrolysis, thereby reducing the P digestibility of the feed.

Figure 1. Calcium chelates phytate making it less accessible for phytase

The speed of limestone solubilization will dictate the concentration of Ca available to chelate with the phytate. Fine limestone is more soluble than coarse limestone, meaning it will bind faster to phytate, and therefore has a stronger negative impact on P digestibility. Using coarser limestone sources with lower Ca solubility gives the phytase more time to reach the phytate molecule before it is complexed by Ca (Figure 2). 

Figure 2. Effect of limestone granulometry (pulverized (PUV), average diameter < 75 μm; particulate (PAR), average diameter 402 μm) on P digestibility with or without phytase (Source: Kim et al., 2018)

Ca and P need to be in balance

Absorbed P can only be retained in the bones when there is enough Ca present in the blood. At low Ca levels, P digestibility can be improved, but P retention will remain low due to insufficient Ca in the blood. A deficiency in Ca can therefore cause poor bone mineralisation, even when P digestibility is high. 

In contrast, an excess of Ca will further decrease P digestibility (Figure 3). Therefore, dosing a phytase at higher levels (1,000 FTU/kg or more) is the best way to counterbalance excessively high dietary levels of Ca without causing any obvious adverse effects on broiler performance or bone mineralisation.

Figure 3. Impact of Ca (5 to 9 g/kg feed) on performance and the beneficial effect of adding phytase (0 or 500 FTU/kg; Source: Rousseau et al., 2011)

Formulating with digestible Ca for a limestone is not that straightforward

Published data on the digestibility of Ca has only appeared in recent years and are sometimes contradictory. There is a high variability in compiled data on the Ca digestibility of limestone, from as high as 84% standardized ileal digestibility of Ca, to below 40%. Calcium digestibility in broilers can vary dramatically depending on such factors as particle size, acid solubility of limestone, geology, hardness, and mine source. It is therefore not always straightforward to attach a digestible Ca value to limestone.

Formulating with analyzable Ca

Based on the above, it might still be advisable to formulate on analyzable Ca, which is Ca that can be measured by lab analysis and does not contain the theoretical Ca matrix value of a phytase.

The necessary levels of analyzable Ca in feeds for different species is difficult to establish due to the highly variable digestibility of the Ca sources. Therefore, it is better to require a minimum analyzable Ca level in the feed during formulation, where Ca requirements can be adjusted by the nutritionist depending on the Ca source:

  • Increasing requirements when using fine limestone (due to its high solubility)
  • Decreasing requirements when using coarse limestone (which is less soluble)
  • Making adjustments depending on the minimum P requirements imposed in the formula

Please consult with your local Huvepharma contact or the local Huvepharma distributor for more information on this subject.


References are available on request.
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