It is standard practice for any modern feed and fodder manufacturer to optimise fodder composition to obtain the maximum nutritional value at the lowest possible cost. Nutritionists use sophisticated formulation software to reach these goals, but due to the inevitable theoretical nature of this formulation software (just consider the various available matrixes) and practical considerations in the factory (are raw materials accurately applied as specified in the formula?), discretion and a true understanding of the characteristics of the raw materials also play an important part. In this context this article discusses the role of calcium phosphate in animal fodder.
All fodder phosphate occurring in the form of calcium-phosphate products (MAP is also used) is produced by neutralising a lime source with phosphoric acid to a pH of 3 to 4. For all practical purposes all South African suppliers use the same source of phosphoric acid, but they have different sources of lime and employ different neutralisation methods. These differences can give way to variations in the molecular composition of calcium phosphate that is produced. Chemical composition and quality of Biofos21
The quality of fodder phosphates, usually calcium phosphate, is described in terms of the solubility, digestibility and/or bio-availability. Technically, these terms aren't interchangeable, but they are commonly used in this way to refer to the same concept.
The quantification of the digestibility can be expressed as 'relative' or 'real':
- It is 'relative' in cases where a product's digestibility is given relative (as %) to a reference source such as sodium phosphate. Sodium phosphate is considered 100% digestible.
- It is 'real' in cases where the product's real digestibility is given as % of the total P contained in the product.
Different molecular forms of calcium phosphate are generated during the process of neutralisation; these forms differ depending on the process. The following molecular forms occur in fodder phosphates:
- Monocalcium phosphate (MCP), also known as water-soluble P fraction (WSP) due to its solubility in water.
- Dicalcium phosphate (DCP), which is not soluble in water but in 2% citric acid or 0.1% hydrochloric acid. The acid-soluble P fraction (ASP) thus consists of both the MCP and the DCP. DCP thus is determined by ASP minus WSP.
- Tricalcium phosphate (TCP), which is determined by the ammonia-citrate solubility (ACSP) of the fodder-phosphate product. Biofos21 contains negligible levels of tricalcium phosphate or none at all, so the test is not deemed necessary.
- In cases where metals occur a fourth fraction, such as Al phosphate, can bind. It is insoluble and thus inaccessible for animals.
DCP's crystallisation water also influences digestibility. The anhydrite form (waterless) is less digestible. The process used to manufacture calcium phosphates in South Africa results in an end product that is a mix of both MCP and DCP; it is referred to as MDCP. The water solubility is an indication of the fraction size in a particular sample.
The Pas serves as a practical method to determine the relative digestibility of a calcium phosphate. The Pas includes both the MCP and DCP and must be >90%. In other words >192g of the 213g P per kilogram P21 must be soluble in 2% citric acid. Theoretically speaking, the Pas indicates which P is soluble, dissociable and thus available or digestible in the digestive tract. The Pas also indicates which P in the product has TCP or another insoluble molecular form.
This figure differs from the method determined in vivo (in the animal) that determines the real digestibility. The following table shows the latest authoritative distinction between the in vivo digestibility of calcium phosphate's various molecular forms:
| Phosphate || Total g P/kg || Poultry || Pigs |
| % digestibility || g digestible P || % digestibility || g digestible P |
| MCP || 229 || 85 || 195 || 90 || 206 |
|MDCP ||219 ||83 ||182 ||80 ||175 |
|DCP2H2O ||182 ||80 ||146 ||72 ||131 |
| DCP0H2O || 202 || 70 || 141 || 65 || 131 |
|DCP0 H2O ||180 ||70 ||126 ||65 ||117 |
|DFP(CaNaP) ||180 ||55 ||99 ||60 ||108 |
Sources: CLO (Gent, B), CVB (Nl), ID-TNO (Lelystad, Nl), Tessenderlo Group
Compared to similar P21 products, Biofos21's chemical composition differs mainly in terms of the water-soluble P content.
The influence of the above table on the theoretical digestibility % of Biofos21 compared to a competitor product is as follows:
| || Biofos21 || Competitor |
|MCP content (PWS),% ||55 ||70 |
|DCP content (PAS-PWS),% ||40 ||26 |
|Other P (PTOT-PAS),% ||5 ||4 |
|PAS of MDCP (%) ||95 ||96 |
|Total P ||12.12+8.02+1.2 = 21.34 ||16.03+5.25+.88 = 22.16 |
|Digestibility (%) ||30+42 = 72 ||60+18 = 78 |
The table is interpreted as follows:
- The MCP fraction (85% digestible) of Biofos21 is approximately 80% of that of the competitor.
- The Pas is effectively the same at 95 to 96%.
- The DCP of both products occurs in anhydrite form since both processes employ chemical or other external heat sources to dry the end product.
- Based on the aforementioned expected chemical composition the total P of Biofos21 should be 21.34% and that of the competitor 22.16%. The former corresponds but the latter does not. The reason for this difference is unclear.
- The theoretical digestibility of the two products, according to the ID-TNO (Lelystadt) model are 72 and 78% respectively.
The inclusion of inorganic phosphate constitutes only 25 to 30% of the total P in chicken (broiler chicks and layer hens) rations. This inclusion standard has even diminished in the last couple of years with the launch of the phytase enzyme that makes the organic form of P (phytic acid) available to the monogastric animal and makes a larger contribution to the total P-pool in the body.
The difference in between Biofos21 and its competitor is thus limited to indeterminable amounts; it remains a theoretical exercise that is not determinable through chemical analysis. Phytase use
At a recent symposium on phosphate consumption, the various phosphate-evaluation approaches were discussed by Dr Markus Rodehutscord. He pointed out that knowing and using the correct P-values have a direct impact not only on animal production but also on increasing P-supplies in the environment. There is no standard for expressing P-values, according to Dr Rodehutscord, and until such a standard has been set there will be confusion, and it will be impossible to draw up a formal list of animal P-requirements.
The differences in the various systems are summarised by Plumstead and Blake in the following table:
| Term || Definition || Value in a typical breeder diet (g/kg) and relative (%) |
|Total phosphorus (P) ||Analysed total P in the diet ||6.3 (100) |
|Non phytate P (NPP) ||Analysed total P less the P from phytate ||3.8 (60.0) |
|Available P (AvP) ||Bio-available P determined using a slope ratio assay and expressed relative to monocalcium P ||4.0 (63.5) |
|Assimilable P (OPL) ||Retained P system for layers developed by the CVB (1997) ||3.6 (57.0) |
The availability of P in the body is influenced by a variety of factors:
- Raw-material composition of the ration that determines the total organic P.
- The inclusion of phytase to make this organic form of P available to and digestible for the animal.
- Sufficient inorganic P-lock to compensate for a deficiency in the above.
- Sufficient inclusion of Vit D, which regulates the body's ability to metabolise P.
- Correct P:Ca ratio in the total fodder. An excess Ca gives way to the rebinding of P in the digestive tract, thus weaker P utilisation/digestibility.
According to a 2009 article by Drs Michael Bradford and Aaron Cowieson published in World's Poultry Science Journal, the phytate molecule should be considered as an anti-nutrient for its effect on P-digestibility and a number of other reasons. It is also a highly active chelator of minerals and also lowers pepsinogen discharge in the stomach. It is important to accommodate all these effects in the formulation and make provision for other effects as we increase our understanding of them.
The correct utilisation of phytase is important to ensure maximisation of the digestibility/utilisation of total P and avoid negative influences thereof in the body:
- The inclusion of phytase is not directly proportionate to the release of organic P. In other words, a double dose will not give way to double the P being released.
- The use of phytase does not only release organic P but also a number of other minerals and amino acids. An oversupply of phytase in this way can release an oversupply of Ca, which gives way to a skewed Ca:P ratio and inhibits the digestibility of P. Phytase efficiency in the diet is influenced by the Ca: available P ratio. The greatest efficiency is obtained when the ratio is smaller than 2.2:1. When the Ca level of broiler-chick starter feed is increased by adding Ca to the feed or water, the ratio changes as follows:
- The reactivity of phytase is negatively influenced in cases where the Ca:P ratio exceeds certain critical levels; this situation can easily lead to weak P-utilisation, especially in cases where rations are used that are dependent on plant-based protein sources and in effect on phytase.
- In 2007/08, the world experienced drastic price hikes, and phytase was wrongly applied as a replacement for the inorganic P-sources.
- It recently came to light that phytase releases Na for avian use, and subsequently the formulation specification should be increased to 0.3g. Bedding problems experienced subsequent to the start of phytase use can perhaps be explained by the shortage.