Calcium Nutrition of Broilers: A review: History
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Subjects: Agriculture, Dairy & Animal Science
Contributor:
Laura Shiromi David
,
M. Naveed Anwar
,
M. Reza Abdollahi
,
Michael R. Bedford
,
Velmurugu Ravindran

Calcium (Ca) is essential for the skeletal growth and a plethora of other functions in broilers. Over 80% of Ca in broiler diets is supplied by inorganic Ca sources among which limestone is the predominant Ca supplement. Currently, considerable attention is being directed towards the use of digestible Ca in poultry feed formulations.The specific aim of the present overview was to highlight the recent advances in the measurement of ileal Ca digestibility of Ca sources and digestible Ca requirement of broilers.

 

  • calcium
  • ileal digestibility
  • limestone
  • phosphorous
  • requirement

1. Introduction

Humphry Davy, a British chemist, was the first to isolate and investigate the chemistry of calcium (Ca) and to recognise it as an essential component of bone in 1808. Ever since, it has been the subject of a voluminous amount of research in both human and animal nutrition. Today, the biological significance and economic importance of Ca in animals are intuitive. Calcium is the most abundant mineral in the body and is essential for the skeletal health and animal welfare. Over 99% of Ca in the animal body is located in the skeleton [1] where it exists in the form of hydroxyl–apatite in a ratio of 2:1 with phosphorus (P). Calcium is also important for a wide range of functions in the body, for example, blood clotting, muscle contraction, nerve impulse transmission, enzyme activation, metabolic reactions, protein synthesis and maintenance of osmotic and acid-base balance [1].
The absorption and utilisation of Ca and P are mutually dependent, and these two minerals should be considered together in animal feeding. For maximum efficiency of utilisation, the supply of Ca and P must be matched as closely as possible to the requirements of the animal. Studies have demonstrated that increasing concentration of one mineral (Ca or P) decreases the absorption of the other [2][3].
In recent decades, considerable attention has been directed towards the P nutrition of poultry for the following reasons: (i) P is the third most expensive component in poultry diets, (ii) its supply is nonrenewable and (iii) P excretion is a major contributor, after nitrogen, to environmental pollution [4]. On the other hand, Ca nutrition and metabolism have been relatively neglected, because of the abundance of Ca in the earth’s crust. The supply of Ca is inexpensive and its excretion into the environment does not represent a threat to the environment. To improve the utilisation of P and conserve it for the future, the poultry industry is currently moving to a digestible P system from the historically used available (nonphytate) P system. Thus, there is a need to develop a digestible Ca system in order to derive more precise requirements and ultimately provide accurate digestible Ca to digestible P ratios for each growth or production phase of the bird. Some progress has been made during the last decade in defining P and Ca availability in terms of ileal digestibility [4][5][6].

2. Calcium Sources

Over 80% of Ca in broiler diets is supplied by inorganic Ca sources because the Ca content of feed ingredients of plant origin are very low [7]. Limestone, oyster shell, monocalcium phosphate (MCP), dicalcium phosphate (DCP) and monodicalcium phosphate (MDCP) are the commonly used Ca sources, but limestone is the predominant supplement. Animal-based feed ingredients (meat and bone meal [MBM], bone meal, poultry by-product meal) and some plant-based ingredients (canola meal, soybean meal [SBM]) can also contribute reasonable amounts of Ca to poultry diets.

2.1. Limestone

Limestone is the common Ca source used in poultry feed formulations because of its high Ca content. The primary component of limestone is calcite, but it may also be contaminated with Ca oxide (CaO), aragonite and dolomite (CaMg(CO3)2). Calcite and aragonite differ in the crystal arrangements of Ca carbonate (CaCO3). Different types of limestone are formed through a variety of processes such as precipitation, secretion by marine organisms, shells of dead sea creatures and cementation of sand or mud by calcite. Limestone can be classified into three categories based on its depositional environment such as platform, basin and geosynclinals [8]. An array of names are used for limestone based upon how the rock is formed, its appearance or its composition, along with other factors. Different colours of limestone (tan, grey, etc.) are due to impurities such as sand, clay, iron oxides and organic materials.
The Ca content of limestone is usually assumed to be 380 g/kg in most feed formulation matrices, but analysed values have been reported to vary from 304 to as high as 420 g/kg [9][10][11][12]. In a survey of 641 samples from 40 countries, Gilani et al. [13] found a range of 333 to 400 g/kg Ca, with an average of 379 g/kg. Based on chemical formula (CaCO3) and molecular weights, the maximum possible Ca content is 400.4 g/kg. Values higher than this may indicate analytical errors or contamination with CaO which has a higher Ca content. On the other hand, lower values are indicative of contaminants.
Limestone supplies up to 70% of the Ca in typical broiler diets. The availability of Ca in limestone was historically assumed to be 100% and therefore the limestone or purified forms of Ca carbonate has been used as the standard in the measurement of Ca bioavailability of other Ca sources. The measurement of Ca digestibility, however, is now the preferred method to express Ca availability and recent findings clearly demonstrate that Ca in limestone is not 100% digestible. Ileal Ca digestibility in limestone for broilers was determined to vary widely from 27 to 77%, depending on the source of limestone [12][14][15], particle size [15][16][17], in vitro solubility [15], dietary P content [16] and bird age [18]. The digestible Ca determined for a particular limestone sample, therefore, will not be applicable to other samples with different intrinsic factors, challenging the use of an average value in feed formulations.

2.1.1. Limestone as a Source of Ca

Different sources of limestone contain differing Ca and mineral composition [12][13][14][19]. For example, dolomitic limestone contains high concentrations of magnesium (12%; [20]) when compared to calcitic limestone. Dolomitic limestone is derived from deposits of Ca carbonate combined with magnesium carbonate, whereas calcitic limestone is derived from deposits of primarily Ca carbonate. Magnesium competes with Ca for absorption sites and also excess magnesium binds with Ca in the intestinal tract, lowering availability of Ca for the animal. For these reasons, dolomitic limestone should be avoided in broiler diets.
Equally important, limestone samples of different origins also vary in their particle size and in vitro solubility. Furthermore, different limestones with similar particle size may have different in vitro solubilities [15][21]. Zhang and Coon [15] reported that the amount of limestone retained in the gizzard (5.90 vs. 3.81 g) was affected by limestone source which was related to the difference in in vitro solubility between the limestones. Anwar et al. [12] and David et al. [14] reported an influence of limestone sources on their ileal Ca digestibility. The study by Anwar et al. [12] found that the ileal Ca digestibility of three limestone samples with the same particle size in broilers were 0.54, 0.58 and 0.61. Such variation in Ca digestibility across limestone samples are inevitable due to a myriad of interacting factors affecting solubilisation and digestion.
 

2.1.2. Limestone and Particle Size

Limestone particle size plays a major role in the Ca utilisation by poultry. Because coarser limestone particles are retained longer in the gizzard of poultry [15][17][22], they result in increased in vivo solubility and digestibility of Ca. Anwar et al. [16] reported a substantially higher true ileal Ca digestibility (0.71 vs. 0.43) for coarse limestone (1–2 mm) than fine limestone (<0.5 mm). Similarly, Kim et al. [23] reported a difference in the apparent ileal Ca digestibility between particulate (0.633 mm) and pulverised (0.063 mm) limestone samples of the same origin, with the digestibility in particulate samples being higher. These findings are in agreement with other studies [24][25][26].

2.1.3. Limestone and Solubility

Solubility of limestone is determined as the percentage weight loss of limestone samples during an in vitro assay. Cheng and Coon [27] developed a technique to measure limestone solubility in vitro where the limestone samples (2 g) were allowed to solubilise for 10 min in 100 mL of 0.1 N hydrochloric acid (HCl), which was warmed for 15 min in a 42 °C water bath oscillating at 60 hertz. After 10 min, the contents were filtered gravimetrically through a Whatman 42 filter paper, dried at 70 °C for 10 h, cooled and weighed to determine the percent weight loss. Zhang and Coon [15] proposed the use of 200 mL of 0.2 N HCl instead of the 100 mL of 0.1 N HCl to avoid the excess buffering of the acid when a highly soluble limestone is tested. This method involves a very acidic solution (pH 0.76) and a one-time point (10 min) solubility determination, which is the widely used method for the measurement of in vitro Ca solubility. Recently, a dynamic model [23] with more than one solubility time point (5, 15 and 30 min) and of a solution that closely represents gizzard conditions (pH 3 and buffered) has been reported to correlate better with in vivo Ca digestibility when compared to the digestibility results obtained at an assay at one time point. These researchers developed prediction models for ileal Ca digestibility that included the solubility at 15 and 30 min time points as these time points were more relevant compared to that at a 5 min time point. Regardless, a robust correlation utilising multiple samples of limestone of varying particle sizes with in vivo Ca digestibility is yet to be established.
However, in vitro solubility of limestone has been reported to be inversely related with its in vivo solubility in laying hens [15][28]. The in vitro solubility of limestone samples greatly depends on the particle size and source of origin. For instance, coarse particles are known to less soluble in vitro when compared to fine particles [15][17] but as indicated above, such coarse particles are retained longer in the gizzard where the Ca is able to dissolve, which explains this inverse relationship on the other hand, different limestone sources of similar particle size were found to have different in vitro solubilities [21]. Gilani et al. [13] reported that the average in vitro solubility of 566 fine limestone samples at 5 min incubation (pH 3.0) ranged from 19 to 99%, whereas that of 75 coarse limestone samples at 30 min incubation (pH 3.0) ranged from 23 to 96%. The in vivo relevance of these assay results has not been ascertained thus far.

2.2. Other Inorganic Ca Sources

Oyster shell is another natural source of Ca carbonate. The Ca content of oyster shells is similar to that of limestone [7]. According to Reid and Weber [9], the Ca content of oyster shell varies from 344 to 390 g/kg. Augspurger and Baker [29] determined that the relative bioavailability of Ca from oyster shell is comparable to that of limestone and, as is the case for limestone, particle size and other factors influence Ca utilisation of oyster shells. Anwar et al. [17] reported ileal Ca digestibility coefficients of 0.33 and 0.56, respectively, for oyster shell with fine (<0.5 mm) and coarse (1.0–2.0 mm) particles. Scott et al. [30] reported that feeding Ca in the form of oyster shells is more effective than feeding the same amount of finely ground limestone because the larger particles that solubilised slowly resulting in higher Ca absorption. Overall, most studies report that feeding oyster shells has benefits on eggshell quality, similar to that of feeding limestone [31].
Dicalcium phosphate (CaHPO4) is Ca phosphate with its dihydrate, an odourless white-coloured powder. Dicalcium phosphate contains about 220 g/kg Ca and 190 g/kg P [7]. There are three forms, namely dihydrate (CaHPO4.2H2O, the mineral brushite), hemihydrate (CaHPO4.0.5H2O) and anhydrous (CaHPO4, the mineral monetite). Depending on the form of DCP, the P and Ca contents vary [32]. Dicalcium phosphate is produced by the neutralisation of Ca hydroxide with phosphoric acid, which precipitates the dihydrate as a solid or by reacting phosphoric acid with limestone. The purity of the DCP depends on the origin of the raw material and procedures employed in its industrial production. Dicalcium phosphate could also be produced through precipitation from bones and it is a coproduct from gelatine manufacture [33]. The relative biological value of bone-precipitated DCP was reported to be higher than commercial feed phosphates [33].
Monocalcium phosphate is an inorganic compound (Ca(H2PO4)2) and is commonly found as the monohydrate (Ca(H2PO4)2.H2O). Monocalcium phosphate is produced by treating Ca hydroxide with phosphoric acid and contains around 160 g/kg Ca and 220 g/kg P [7] MCP may contain some DCP, but more than 80% of the P should be derived from the MCP fraction to be classified as an MCP. Monodicalcium phosphate contains less than 80% P from MCP [32].
Tricalcium phosphate (TCP; Ca3(PO4)2) is a calcium salt of phosphoric acid, which is a white solid of low solubility. It exists as three crystalline polymorphs such as α, α’ and β. The α and α’ states are stable at high temperatures [34]. Tricalcium phosphate is produced commercially by treating hydroxyapatite with phosphoric acid and slaked lime. Tricalcium phosphate is also produced by heating a mixture of Ca pyrophosphate (Ca2P2O7) and Ca carbonate (CaCO3). Tricalcium phosphate occurs naturally in several forms such as rock, skeletons and teeth of vertebrate animals and milk. In some countries, TCP is used as the main inorganic phosphate supplement in poultry diets [35]. The production process of TCP from bones involves the degreasing of the bones in counter-flow with hot water (bone chips less than 14 mm). Then it undergoes continuous cooking with steam at 145 °C for 30 min at 4 bars and the separation of the protein broth from the hydroxyapatite (TCP) by centrifugation. The granulation of TCP is carried out after drying in a fluid bed with air at 200 °C. This TCP is not pure and, on average, composed of 750 g/kg hydroxyapatite, 170 g/kg gelatine, 4 g/kg fat and 4 g/kg moisture [36]. Feed grade TCP may be available in different trade names depending on the production process. For example, Hamdi et al. [37] used a TCP called lipocal, which is a TCP powder that has been treated with lecithin to reduce its interactions with other minerals and feed ingredients, especially in aqueous media. Rao et al. [38] used a TCP called Multifos, which is a deflourinated TCP derived from phosphate rock. Rao et al. [38] reported that the in vivo solubility of TCP (Multifos) was lower than that of DCP (Dynafos) or MCP (Biofos). Kwon and Kim [39] found that Ca digestibility was lower for TCP than for DCP and MCP.
Defluorinated phosphate (DFP; Ca4Na(PO4)3) is another mineral supplement used in poultry. Defluorinated feed phosphate is produced by removing the fluorine, which is toxic, from the phosphate rock. Natural phosphate rocks vary in fluorine content from less than 10 g/kg to above 40 g/kg [40]. Phosphate rock and sand are ground, mixed in a definite ratio and fed as a slurry to a rotary kiln. The material is treated at temperatures of 2700° to 2900° F. Water vapour is introduced at the hot end of the kiln and quick cooling of the product is undertaken. The discharged, compacted residue which is not fused, is ground and bagged. The product contains about 20% phosphorus pentoxide and 0.1% or less fluorine. The phosphate constituent is α-tricalcium phosphate [41]. Defluorinated feed phosphate is a good source of available P. Defluorinated feed phosphate is nonhydroscopic and noncaking powder or granules, light-brown to dark-brown in color and odorless. Compared to other mineral P sources, the DFP supplies Ca, P and sodium to animals at minimum concentrations of heavy metals and harmful components, and is highly soluble in the gut.
The use of lesser-known Ca sources has also been reported. Calcium citrate and Ca citrate-malate are reported to have similar relative bioavailability to that of limestone [29][42]. Although some other inorganic sources like agricultural grade phosphates and raw rock phosphates are cheaper than DCP, these contain high concentrations of heavy metals and can be toxic to animals [43].

2.3. Animal-Based Ca Sources

Animal protein sources, namely, MBM, fish mea, and poultry by-product meal also supply significant amounts of Ca in poultry diets. Meat and bone meal is a product of the rendering industry and an important organic Ca source, which contains an average of 103 g/kg Ca [7]. However, the Ca concentration of MBM varies widely depending on the source [44][45][46][47]. This variation in Ca concentration is due to the nature of the raw materials as well as the processing methods. Meat and bone meal is made from animal offal that is not suited for human consumption. Offal is cooked, defatted, sterilised and ground to obtain MBM. Depending on the proportions of bone and soft tissues used in the manufacture, the finished product is categorised as meat meal (containing > 55% crude protein and < 4.4% P) or meat and bone meal (containing < 55% crude protein and > 4.4% P). Similarly, fish meal is manufactured by cooking, pressing, drying and grinding the fish or fish waste that are not intended for human consumption. In the same way, poultry by-product meal is made by a similar rendering process.

2.4. Plant-Based Ca Sources

Plant-based feed ingredients are mostly used in poultry diets as an energy or protein supplements. They can, however, contribute only minimal amounts of Ca to the diet. Among the plant-derived feed ingredients, canola meal (6.8 g/kg) and rice bran (7.0 g/kg) contribute are the richest sources of Ca for poultry diets [7]. The Ca content of SBM, the major protein source in poultry diets, is 2.8 g/kg [7] whereas those of cereals is negligible.

This entry is adapted from the peer-reviewed paper 10.3390/ani13101590

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