Calcium to Phosphorus Ratios in Broilers

I. INTRODUCTION

Historically, there has been a move from the use of a total Ca (tCa) to total P (tP) ratio system (NRC, 1950) to a tCa to inorganic P (iP) ratio (NRC, 1954), to the use of tCa to available P (aP) that appeared in the 1984 NRC. In 1950, the requirements were 1.0% tCa and 0.6% tP (1.66 tCa:tP) (NRC, 1950) and, in the 1954 NRC, the qualification was made giving importance to the availability of P by specifying that, of the 0.6% tP requirement, 0.56% needed to be from an inorganic source. An allowance of 0.3% availability of plant P was also specified. The 1977 NRC still gave requirements in terms of tCa and tP with the proviso that part of the 0.7% tP requirement for P from 0 to 8 weeks be supplied from inorganic sources of P. In the 1984 NRC, requirements for P were given as aP and aP values were also given for ingredients but no change was made to Ca. The Ca:aP ratios recommended were 2.22 to 2.28 throughout growth (hatch to 8 weeks of age) in broilers. By 1994 (NRC, 1994), the term non phytin P (nPP) instead of aP was used but there were minimal if any changes to the values. The tCa to nPP ratios recommended were 2.22 to 2.67 Ca depending on growth stage.

The low digestibility of P in plant sources (Van Der Klis and Versteegh, 1996; Coon and Leske, 1998; Angel et al., 2002; Tamim and Angel, 2003; Tamim et al., 2004) and the variable digestibility of P in animal and inorganic sources (Van Der Klis and Versteegh, 1996; Coon and Leske, 1998) prompted the change in the use of P, from total P to aP, nPP, dP or retainable P that better reflected availability of P in dietary sources. But there has been no move toward a digestible Ca system (dCa). Because of the extensive use of phytases in poultry diets worldwide, the negative effect of Ca on phytate P and on phytase efficacy digestibility (Tamim and Angel, 2003), there has been a push downwards in the use of Ca in poultry diets. Because there are very few data on actual availability or digestibility, as opposed to relative availability, of calcium in feed ingredient sources, we assume in general a 100% digestibility for Ca. Clearly this is not correct. Based on reported Ca digestibilities in corn soy diets with no added inorganic Ca or P sources, and the same diet with added limestone, one can calculate an availability of Ca in corn and SBM portion of the diet of 20 to 33% (Tamim and Angel, 2003; Tamim et al., 2004) and for Ca from limestone of between 60 and 70%. The Ca from corn and soy in a corn soy diet represents between 0.17 and 0.21%. We have, up to now, disregarded this low digestibility of this "organic" Ca because it represents a small (20%) percentage in a diet containing 1% Ca, typical of a broiler starter diet. But when we start seeing commercial broiler diets with 0.6% Ca, or even 0.5% Ca in the withdrawal phase, then Ca from the corn and SBM becomes a greater proportion of total Ca.

It is important that we start developing a dCa to dP system if we are to really feed broilers at a requirement concentration; that is, provide diets with concentrations of dCa and dP that meet the needs of the birds. For this, we will need to develop methodologies to determine dCa in ingredients and we will need to review the methodologies used to determine dP, and expected contributions of P and Ca when phytases are used.

II. VARIABILITY IN REPORTED CALCIUM TO PHOSPHORUS RATIOS

Standard implementation of tCa to dP ratios of 2:1 has occurred commercially for more than 30 years, with the P being called available or digestible. In practical diets, nutritionists try to maintain a 1.8 to a 2.2 tCa to dP ratio. The question is why, and does this constraint lead to diets that do not contain the correct dCa and dP needed by broilers?

Reports abound in the literature where the tCa to dP ratios at "optimal" performance or bone mineralisation are not 2:1. For example, Driver et al., (2005a) reported that a 1:1 tCa to tP ratio maximised body weight gain (BWG) and feed to gain ratios from 0 to 16 d of age in broilers. They fed a corn-SBM diet with graded increases in Ca and P from limestone, monocalcium phosphate and dicalcium phosphate. But they also reported similar BWG with tCa:tP ratios of 0.94 to 1.25. Tibia ash was maximised at ratios of 1.07 to 1.35. Incidence of tibial dyschondroplasia was lowest when diets with a 1.29 to 1.89 tCa to tP ratio were fed but when Ca concentration was between 0.89 and 0.98.

In another study where Ca requirements between 1 and 16 d of age were determined (Driver et al., 2005b), using broken line analysis and 1 concentration of tP (0.63%) and calculated nPP of 0.45%, the requirements reported were at tCa to tP ratios of 0.77, 0.99, 1.14 for BWG, feed to gain ratio and tibia ash, respectively. If one puts these values in terms of tCa to nPP ratios, the ratios would be 1.08, 1.39, and 1.60, respectively. Clearly, the ratios reported by Driver et al., (2005a, b) differ from our standard industry use of 2:1 for tCa to aP ratios and open the door for diets with broader or tighter ratios.

In recent Ca and P requirement work done at the University of Maryland (JimenezMoreno et al., unpublished), tCa to nPP ratio for maximal bone ash was found at 1.05% Ca and 0.65% nPP or 1.61 tCa to nPP ratio from hatch to 10 days of age. When retainable Ca and retainable P were measured for this diet, the ratio was 1.64.

Intestinal absorption, or what we are defining as apparent or true (if corrected for endogenous losses) digestibilities or disappearance of Ca or P up to the distal ileum, does not necessarily reflect the bioavailability of these minerals to the whole animal. This is because both Ca and P must be retained and used for metabolism and/or in bone formation and mineralisation. In the case of bone where the bulk of the Ca and P in the body are found, both Ca and P must also be available to the animal and in the body for the production of hydroxyapatite (a complex tricalciumphosphate).

III. THE CASE FOR THE NEED FOR A DIGESTIBLE CALCIUM SYSTEM IN THE CONTEXT OF CALCIUM TO PHOSPHORUS RATIOS

We have known for some time that the digestibility of P is low in seed based ingredients and variable and not 100% in animal or inorganic based sources (Table1).

Table 1 - Phosphorus availability from plant and animal sources and feed phosphates (From a Van Der Klis and Versteegh, 1996; b Coon and Leske, 1998; c National Research Council, 1994)

Calcium to Phosphorus Ratios in Broilers - Image 1

In a study where the digestibility of Ca and P were determined in a corn-SBM starter diet devoid of inorganic Ca or P sources, the digestibility of Ca in the corn and SBM was 33.6% and of P was 67.9% (Tamim et al., 2004). These authors demonstrated the important and large negative impact of Ca on phytate P digestibility when 0.5% Ca from CaCO3 was added to the corn-SBM diet (analysed Ca in the corn-SBM diet as 0.17% and after adding 0.5% Ca from CaCO3 was 0.65%). As the Ca in the diet went from 0.17 to 0.65%, the digestibility of phytate P went from 69.2 to 25.4%; this when the only dietary change was the addition of 0.5% Ca from CaCO3. Key also from this work was the low digestibility of Ca reported from the corn-soy diet, with no added inorganic sources of Ca or P of 33.6% and by calculation the digestibility of the Ca from the inorganic source, CaCO3 was 62.7%.

More recent work at the University of Maryland, where true digestibilities of Ca and P from SBM, CaCO3 and mono calcium phosphate were determined in trials lasting from 8 to 96hr (Proszkowiec-Weglarz et al., unpublished), where purified diets were fed and the specific ingredients were the only source of the P or Ca, true digestibilities of Ca for limestone and mono calcium phosphate were 34.1 and 67.9%, respectively when determined at 40hr post feeding of the ingredient to 25 d old broilers. Of importance is the low Ca digestibility of a feed grade limestone as compared to that in mono calcium phosphate when the absolute Ca concentration was similar.

If, indeed, the digestibility of Ca from mono calcium phosphate is 2 times higher than that of Ca from a limestone, then what are the impacts on Ca to P ratios when phytases are used? When we use phytases in broiler diets, usually inorganic phosphates are decreased or removed and Ca from limestone is used. If we use an example of a corn-SBM diet with and without phytase, where we give phytase similar matrixes for Ca and P, the amount of mono calcium phosphate in the diet would be reduced by 60% and of limestone by 12%. What are the implications from a digestible ratio perspective? If we assume a dCa to dP ratio in the diet without phytase of 1.6 to 1, then the ratio in the diet with phytase would be close to a 1 to 1 ratio. This change is brought about by the large decrease in the amount of mono calcium phosphate with the much higher dCa.

What Ca to P ratios to use become even more difficult to access in diets with animal Ca and P sources where sometimes, under commercial conditions, we find ourselves adding inorganic P sources to maintain Ca to P ratios. Having ingredient dCa as well as dP values is essential and, to date, little if any dCa ingredient data are available. Requirement work, in the future, should provide data as dCa and dP requirements for optimisation of performance and bone measures. We also must strive to define ranges of dCa to dP that optimise performance and bone mineralisation and minimise welfare issues.

IV. CONCLUSION

It is important to move to a dCa and a dP system that allows nutritionists to formulate diets that meet the needs of the animal. We must accept that dCa in inorganic sources are widely different. Furthermore, when we use phytase, we change Ca source from a calcium phosphate that usually has higher dCa to a limestone that has usually lower dCa and that has a dCa that varies with source.

The current system of tCa to a P value that partially reflects digestibility no longer serves our needs for formulating diets where we minimise P concentrations, reduce costs, use alternate ingredients and feed additives that change both the digestibilities of Ca and P.

REFERENCES

Angel R, Tamin NM, Applegate TJ, Dhandu AS, Ellestad LE (2002) Journal of Applied Poultry Research 11, 471-480.

Coon C, Leske K (1998) Maryland Nutr Conf 18-31.

Driver JP, Pesti GM, Bakalli RI, Edwards HM Jr (2005a) Poulty Science 84, 1406-1417.

Driver JP, Pesti GM, Bakalli RI, Edwards HM Jr (2005b) Poulty Science 84, 1629-1639.

National Research Council (NRC) (1950) Nutrient Requirements for Poultry National Academy Press, Washington DC.

National Research Council (NRC) (1954) Nutrient Requirements for Poultry National Academy Press, Washington DC.

National Research Council (NRC) (1977) Nutrient Requirements for Poultry National Academy Press, Washington DC.

National Research Council (NRC) (1984) Nutrient Requirements for Poultry National Academy Press, Washington DC.

National Research Council (NRC) (1995) Nutrient Requirements for Poultry National Academy Press, Washington DC.

Tamim NM, Angel R (2003) Journal of Agricultural and Food Chemistry 8, 4687-4693.

Tamim N M, Angel R, Christman M (2004) Poulty Science 83,1358-1367.

Van Der Klis JD, Versteegh HAJ (1996) In: Recent Advances in Animal Nutrition. (p 71-83) Ed.: P. C. Garnsworty, J. Wiseman, and W. Haresign. Nottingham University press, Nottingham, UK.