Calcium Digestibility: A Breakthrough Beyond Total Calcium

The Calcium digestibility parameter is an old concept, whereas the total Ca concept used so far allows excess Ca intake, which can have negative implications. Calcium is an essential macromineral that is not only involved in bone and eggshell formation but also plays a crucial role in various metabolic processes in the body, maintaining health and supporting poultry growth. Providing excessive calcium can impair the absorption of other nutrients, leading to health problems and reduced livestock performance. 

Ca intake is closely related to phosphorus intake because both have an optimum ratio that must be considered in feed formulation. An imbalance in the Ca/P ratio will interfere with absorption. For example, excess Ca interferes with P absorption, and vice versa. The ideal Ca/P ratio will also vary with age and production phase, as needs differ. 

In broiler chickens, the ideal Ca/P ratio ranges from 1.2 to 2:1. During the early stages of growth, chickens require relatively higher phosphorus (P) to support bone growth and skeletal formation. Meanwhile, laying hens entering the production phase requires a wider Ca/P ratio, ranging from 4:1 to 6:1. Ca requirements increase significantly because they are required for eggshell formation, which is largely composed of calcium carbonate. 

P is the third-most-expensive feed nutrient after metabolic energy and protein. It is also the most abundant mineral in the body after Ca, with 80% of it found in bones. P supply is important to consider formulas to meet poultry’s daily requirements. The challenge is that P, which is primarily found in plants such as grains, nuts, and other plant-based raw materials, is stored bound to phytate, which is difficult for poultry to digest.  

Phytate functions as a phosphorus storage site, especially in seeds. When seeds germinate, this phytate is broken down by the enzyme phytase to provide P for young plants. In addition to binding phosphorus, phytate can also bind to calcium and other minerals. The term phytate-P refers to the phosphorus embedded within the phytate molecule, which requires the enzyme phytase to make it available to birds. The phytate content and proportion of phytate-P in raw materials vary widely. 

Table 1. Ca content, total P, and proportion of phytate-P in raw materials

Raw materials	Ca (g/kg)	Total P (g/kg)	Phytat (g/kg)	Phytat-P (g/kg)	Phytat-P/Tot P(%)
Barley	0,3	3,2	7,0	1,9	61
Corn	0,2	2,6	6,7	1,9	72
Wheat	0,5	3,1	7,8	2,2	72
Sorghum	0,4	3,0	7,7	2,2	73
Soybean Meal	2,7	6,5	13,8	3,9	60
Canola meal	6,8	9,7	22,9	6,5	66
Wheat bran	1,4	11,0	29,6	8,4	76
Cottonseed	1,5	10,0	27,4	7,7	77
Rice bran	0,5	17,8	50,3	14,2	80
Source : Stuart Wilkinson et al., 2014

Phosphorus Availability and Retention 

In practice, available P (av. P) is more common and has been used longer than retained P (ret. P), and the two also have different meanings. Available P indicates the amount of P that can be absorbed by poultry and subsequently metabolized. In plant-based feed, most P is present in the form of phytic acid, which is indigestible and therefore unavailable to poultry. The total P content in grains, such as corn, is mostly in the form of phytate, and av. P may account for only 20-40% of the total P. 

P digestibility and P retention are distinct concepts. P digestibility is the percentage of P digested and absorbed by the digestive system, i.e., the ratio of the amount of P absorbed from feed to the total P consumed. Indigestible P is excreted in the feces. Conversely, P retention describes the percentage of P absorbed by the body and used or stored for bone formation, energy metabolism, egg production, and other processes. 

Phosphorus that has been digested and absorbed but not utilized by the body is excreted in the urine, so P retention calculations will always be lower than the average P or the digestible P. The P digestibility value is strongly influenced by the type of raw material, the addition of the enzyme phytase (which increases digestibility), and the ratio of phytase to Ca. The P retention value is determined by metabolic efficiency and physiological needs. The P content in grain raw materials has low digestibility due to the binding with phytic acid, compared to P from animal sources (MBM, PMM, fish meal), which are more easily digested. 

Environmentally friendly feed can be approached by minimizing the loss of excess P that is undigested and unused in poultry metabolism. Excessive phosphorus released into the environment will cause pollution as a side effect of using poultry manure as organic fertilizer. Soil pollution from increased phosphorus levels resulting from unbalanced use of organic fertilizers prevents P from being absorbed by plant roots. 

Calcium Digestibility 

Total dietary calcium is the sum of the calcium contributions from all feed ingredients. Like phosphorus, the calcium in grains and other plant-based sources (SBM, bran, and PKM) is bound to phytate, resulting in low digestibility. Meanwhile, primary sources of calcium, such as limestone (powder and coarse), contain CaCO3, which is not bound to phytate, resulting in much higher digestibility. 

Animal-derived raw materials contain calcium in the form of calcium phosphate, which has relatively high digestibility but varies widely depending on the manufacturing process. If a higher bone content is present in the mixture, the Ca (and P) percentage will be higher than if a higher meat content is present. Feed formulations will rely more on limestone as a cheaper, more consistent source of calcium. 

Digestion is the process of breaking down feed ingredients using enzymatic hydrolysis and microbial fermentation into smaller molecules. Next, absorption transfers nutrients across the intestinal wall into the blood. Calcium limestone is not 100% digestible but varies from 27 to 77% (ileal digestibility) based on measurements in broilers. 

This variation is determined by many factors, including: (1) limestone source (calcium carbonate is more digestible than calcium phosphate), (2) particle size (fine particles are more digestible than coarse particles), (3) pH (gastric pH ranges from 2-3 to dissolve calcium carbonate. If the feed contains a lot of fiber or sodium bicarbonate, the pH will increase, decreasing calcium digestibility), (4) the P content of the feed, which is due to the close interaction between Ca and P, where an ideal Ca/P ratio is required to prevent calcium phosphate precipitation, which can reduce Ca availability), and (5) the age of the chicken, as Ca absorption is lower in young chickens due to their underdeveloped digestive systems. 

Two methods are known for measuring calcium digestibility by collecting ileal digesta (the final part of the chicken’s small intestine) namely as AID (apparent ileal digestibility) and SID (standardized ileal digestibility). AID and SID measure calcium digestibility by comparing the amount of Ca consumed with the amount of Ca remaining in the digesta in the ileum. Unlike SID, the AID value does not account for endogenous calcium sources, i.e. calcium derived from bodily secretions, such as digestive fluids, or from cells released from the intestinal wall. The AID value is cruder, while the SID is more accurate. Carrie Walk (2023) compared AID and SID Ca measurements for several types of raw materials in broiler chickens. 

Table 2. Comparison of AID and SID values ​​in several raw materials

Ingredient	n	Average SID Ca (%) ± sd	n	Average AID Ca(%) ± sd
Limestone	10	55 ± 4	45	53 ± 12
Oyster shell (<500 µm)	1	33	1	32
Oyster shell (>1000 µm)	1	56	1	55
DCP	9	39 ± 12	12	44 ± 16
MCP	6	36 ± 7	6	34 ± 6
Meat and bone meal	6	46 ± 7	14	45 ± 6
Fish meal	1	24	1	23
Poultry by product meal	1	29	1	28
Corn	1	70	1	46
Wheat	1	71	1	73
Soybean meal	4	55 ± 8	1	47
Canola meal	1	31	2	34 ± 7
Sunflower meal			1	61
Sorghum			1	54
Full fat Soybean meal			1	70
Source : Carrie Walk (2023) DSM-firmenich

The calcium digestibility value reflects the percentage of Ca that can be digested and absorbed by poultry from the total Ca consumed. By comparing the amount of Ca consumed with that excreted in the feces, it is expressed as apparent digestibility. To formulate more precise Ca nutritional intake for poultry, it is helpful to consider Ca digestibility. Inorganic Ca is more digestible than organic Ca derived from animal sources such as MBM, PMM, and fish meals. 

Organic Ca is usually found in complex bonds with protein, fat, and other organic components. The rendering process also changes the structure of Ca-containing compounds, making them more difficult to digest. Inorganic Ca, on the other hand, has a simpler form and is more easily released during digestion. Inorganic Ca is more soluble in the acidic environment of the poultry stomach, enhancing release and absorption. 

Main Source of Calcium 

Limestone is the most efficient and economical source of Ca in formulations. In broiler feed, limestone supplies more than 70% of the Ca requirement. Quality control of the limestone used remains essential. Limestone can come from different mining locations and may be contaminated with heavy metals and magnesium, leading to variations in quality. The ideal color is white to light gray. A color that is too dark indicates excessive contamination with other minerals, such as Mg. A slightly reddish color indicates iron oxide (Fe2O3) contamination or clay content specific to the mining site. 

The calcium content in limestone ranges from >38%, but various contaminants can reduce it and its digestibility. Too high a moisture content can reduce calcium content, typically 0.3-0.5%. Mining during the rainy season at mining sites usually increases the moisture content. If the moisture content is too high, especially in limestone powder, it tends to clump, complicating the conveying or weighing process during production. 

Limestone Quality 

Limestone quality can be calculated using its calcium solubility approach. Solubility is the ability of limestone to dissolve in liquids, especially in the acidic environment of the digestive tract. In poultry, the solubility of calcium in the digestive tract is highly dependent on pH, and the dissolution rate is also influenced by particle size. The lower the pH, the higher the solubility of calcium. The primary dissolution process occurs in the proventriculus and gizzard (pH 2-4). 

Limestone particle size determines its solubility. Limestone is available in several sizes on the market: limestone powder (<1 mm), limestone grit (2-3 mm), and limestone coarse (>4 mm). Fine/small sizes have a larger surface area, resulting in higher solubility than coarser sizes. However, particles that are too fine will dissolve too quickly, potentially leading to more waste than absorbed in the digestive tract. 

In practice, limestone powder is used in pelleted feed, while mash feed, specifically for layers in production, uses limestone grit. Coarse limestone, with its slow-release Ca, helps meet Ca requirements in egg production, making it ideal for use in mash feed. Also aids in the physical grinding of feed as it passes through the gizzard.  

Ca Homeostasis 

Ca is the most abundant mineral in the body, and 99% of it is stored in the skeleton in the form of hydroxyapatite with a 2:1 ratio to phosphorus. The remaining 1% is distributed between intracellular and extracellular compartments. Ca is a major component in bone formation and supports the skeletal structure of rapidly growing broiler chickens. It is an essential component in eggshell formation, as 94-97% of the mineral is composed of calcium carbonate. 

In metabolism, calcium (Ca) plays a crucial role in regulating muscle contraction, transmitting nerve impulses, activating enzymes essential to cellular life, participating in blood clotting, and maintaining osmotic and acid-base balance. Enzyme secretion controlled by Ca includes vitamin D3 and parathyroid hormones. Ca deficiency will inevitably lead to abnormalities in skeletal development. 

The Ca balance in poultry is controlled by several key hormones, and it is important to understand how body Ca levels are also determined by the amount of Ca obtained from feed intake. When blood Ca decreases, parathyroid hormone (PTH) is secreted by the parathyroid glands, which increases blood Ca levels through several mechanisms: (1) increasing Ca release from bone, (2) increasing Ca reabsorption in the kidneys, and (3) stimulating the formation of active vitamin D (calcitriol) in the kidneys, which increases Ca absorption from the intestines. Calcitriol is a hormone produced in the kidneys and released into the blood. 

Another hormone is calcitonin, secreted by the thyroid gland to lower blood Ca levels. It works by inhibiting osteoclast activity (bone-degrading cells) and increasing Ca excretion in the urine. Ca excretion is one route of Ca homeostasis. 

Facing variable Ca supplies in the diet, Ca absorption becomes a primary adaptive mechanism to maintain Ca homeostasis in the body. When Ca supply from the diet is low, the hormonal system increases active absorption. Anti-nutritional factors in feed, such as phytic acid and oxalate, bind Ca to form insoluble complexes, inhibiting Ca absorption. 

Conversely, when Ca intake is excessive, passive absorption will dominate, with the body decreasing calcitriol production to reduce active absorption in the intestine. Ca excess or deficiency can occur due to physiological needs at certain stages. For example, during the egg-laying phase, significantly more Ca is required to support normal eggshell formation, requiring sufficient calcitriol production to enhance active Ca absorption from the intestine. 

Ca Absorption 

Ca absorption occurs in the duodenum, jejunum, and ileum, where Ca is absorbed as Ca ions (Ca2+). After undergoing a dissolution process in the proventriculus and gizzard, Ca is then transported to the small intestine. The increased pH will precipitate it along with P anions and phytic acid before absorption. Ca may be present in the form of Ca complexes still bound to organic acids (phytic acid and oxalate), reducing the availability of Ca for absorption as ions. 

There are two mechanisms for calcium absorption: active absorption, which occurs predominantly in the duodenum, and passive absorption, which occurs in the jejunum and ileum. In active absorption, calcium ions are taken from the intestinal cell membrane, transported across the cell, bound to the protein calbindin, transferred to the basolateral membrane, and then released into the bloodstream. Passive absorption occurs primarily via diffusion across intestinal cells, especially when the calcium concentration in the intestinal lumen is high. 

When plasma calcium levels are within the normal range, half of the absorbed calcium ions are stored in the bones. This serves to replace calcium previously released, ensuring a constant exchange between the blood and bones. The remaining 50% circulate and is filtered in the renal tubules, supporting physiological functions and is retained in the blood as free calcium ions or as protein-bound reserves to maintain calcium balance. 

Calcium Requirements in Feed 

In commercial broiler chickens, meeting Calcium requirements is more precise and specific with age. Broiler chickens are divided into three stages: starter (0-10 days), grower (11-24 days), and finisher (25 days until harvest). In practice, two phases are more commonly used: starter (0-21 days) and finisher (22 days until harvest). It is recommended to use pre-starter feed for the first 7 days, followed by starter feed from 8 days of age. Calcium nutritional requirements are always linked to phosphorus in an optimal ratio, as shown in the following table. 

Table 3. Ca requirements and Ca/P ratio in various growth phases of broiler chickens

Phase	Age	Ca requirement (total) %	Ratio Ca/P	Function
Starter	0-10 days	0.9 - 1,0	1.5:1	Bone and muscle growth
Grower	11-24 days	0.85 - 0.95	1.5-1.8 : 1	Skeletal growth
Finisher	25 days - harvest	0.8 - 0.9	1.5:1 - 1.7:1	Muscle mass and bone strength

Calcium requirements for layer in production and its allocation differ from those of broilers, given the high Ca requirement during the egg-laying phase. The life cycle of laying hens can be divided into several stages: pre-starter (0-5 weeks), starter (6-10 weeks), grower (11-16 weeks), pre-lay (17 weeks – 2% of production), first-stage production (2% of production – 55 weeks), and second-stage production (56 weeks – culled). While it is still recommended to use the third-stage production guide, it is rarely implemented in practice. In fact, most farmers use only one type of feed from the start of laying until the hen is culled. Ca requirements, in relation to the Ca/P ratio at various stages of laying hens, are outlined in the following table. 

Table 4. Ca and P requirements in ISA Brown laying hens

Phase	Age	Total Ca requirement (%)	Available P	Retention P
Starter	0-5 weeks	1.05-1.1%	0.45-0.50%	0.38-0.42%
Grower	6-10 weeks	0.9-1.1%	0.45-0.50%	0.38-0.42%
Developer	11-16 weeks	1.0-1.2%	0.42-0.47%	0.36-0.40%
Pre-lay	17 weeks - 2% production	2.1-2.5%	0.45-0.50%	0.38-0.42%
Layer 1 (105 g/day)	2% production -55 weeks	3.71-3.90%	0.43-0.46%	0.36-0.39%
Layer 2 (105 g/day)	56 weeks- culled	4.1-4.48%	0.34-0.36%	0.29-0.31%
Source : ISA Brown, Nutrition Guide, 2020

Ca Digestibility Perspective 

Current Ca requirements in formulations rely on total Ca, not on Ca digestibility (digestible Ca). Therefore, it is highly likely that current recommendations exceed the actual Ca requirements of chickens. Numerous studies have shown that excess Ca in feed, especially from limestone, significantly reduces feed digestibility. Among other things, excess Ca reduces P digestibility and leads to the formation of insoluble phytate mineral complexes. 

Many studies have shown that providing a total Ca of 6.5 g/kg and 6.0 g/kg is sufficient to meet the needs of broilers aged 1-14 days and 15-49 days, respectively. A study by Applegate et al. found that providing a total Ca of 9 g/kg of feed decreased intestinal phytase activity and intestinal phytate-P hydrolysis. A total Ca level of 10.0 g/kg of feed decreased P digestibility in broilers. 

Although it is widely recognized that calculations using total Ca carry the risk of Ca overload, the transition to the Ca digestibility parameter has been slow. This is due to several factors, including: (1) limited data on Ca digestibility in raw materials, not as extensive as ret. P data due to the complexity of measurement methods, both AIP and SID. (2) Ca digestibility values ​​in raw materials containing Ca vary widely and are strongly influenced by source, processing, storage, particle size, and other factors. (3) Recommendations for Ca requirements for broilers/layers by breeder producers still use total Ca.  

Research on Broiler Dig. Ca and Total Ca 

Research on Ross 308 broiler chickens in the starter phase (1-10 days), grower (11-24 days), and finisher (25-35 days) sought the value of Ca digestibility (SID) and dig. P (SID) requirements for maximum results in terms of body weight gain and tibia bone ash content. The dig. P (SID) requirement for maximum body weight gain and tibia ash is 5.0 g/kg, 3.5 g/kg and 3.5 g/kg for the starter, grower, and finisher phases, respectively. 

The Ca digestibility (SID) requirement for maximum weight gain is 3.32 g/kg, 3.05 g/kg, and 3.5 g/kg for the starter, grower, and finisher phases, respectively. Based on calculations, this is approximately equivalent to 7.0 g/kg, 6.1 g/kg, and 6.4 g/kg of total Ca, respectively. Meanwhile, to obtain maximum tibia ash, the Ca digestibility (SID) requirement is 4.51 g, 3.69 g, and 3.0-3.5 g/kg for the starter, grower, and finisher phases, respectively. This is equivalent to a total Ca of 9.2 g, 7.3 g, and 5.5-6.4 g/kg, respectively. The results of this study show lower requirement values ​​than the 2019 Ross 308 recommendations, namely total Ca at 9.6, 8.7, and 7.8 g/kg for starter, grower, and finisher, respectively. 

Table 5. Summary of SID Ca and SID P requirements for maximum body weight gain and tibia ash in Ross 308 broilers in the starter, grower, and finisher phases

Broiler phase	Starter (1-10 days)	Grower (11-24 days)	Finisher (25-35 days)
Body weight gain
SID Ca (g/kg)	3.32	3.05	3.50
SID P (g/kg)	5.00	3.50	3.50
SID Ca : SID P	0.66	0.87	1.00
Tibia bone ash
SID Ca (g/kg)	4.51	3.69	3.00-3.50
SID P (g/kg)	5.00	3.50	3.50
SID Ca : SID P	0.87	1.05	0.86-1.00
Ross 308 (2019) Recommendation
SID Ca (g/kg)	4.40 (9.6)	4.03 (8.7)	4.25 (7.8)
SID P (g/kg)	5.40	4.83	3.91
SID Ca : SID P	0.81	0.83	1.09
Source: Laura Shiromi David et al., 2023

Bibliography 

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