CONCEPT NOTES ON ROLE OF CACIUM FEEDING IN COMMERCIAL LAYING HENS
Post no-1344Dt 31/07/2019
(Compiled & shared by-DR RAJESH KUMAR SINGH, JAMSHEDPUR, JHARKHAND, INDIA, 9431309542, rajeshsinghvet@gmail.com)..
Calcium is one of the most abundant minerals in the body and is often the major cation in the diet. Ninety nine percent ofthe body’s calcium is located in the skeleton and the remaining one percent is distributed throughout the extracelluallar and intracellular fluids. When there are bone or shell problems, calcium is the nutrient that is mostly considered, although in actual fact many hormones and minerals can influence bone and shell quality. Blood is the transport medium by which calcium is moved from the gastrointestinal tract to other tissues for utilization. Because the shell gland does not store enough calcium for shell formation, the ion must be extracted continuously from the blood. All blood calcium is in the plasma and can be separated into two components: an ultrafiltrable fraction , which is primary ionic calcium, and a non-ultrafiltrable fraction, which is protein bound. The most obvious functions of calcium are to provide structural integrity to the skeleton and to contribute to the maintenance of blood calcium levels through on-going resorption and deposition. Calcium in bone tissue is not in a steady state, it is constantly being mobilized and deposited as bone growth. During the last 15 hours of shell formation, calcium moves across the shell gland of the hen at a rate of 100-150 mg/h. Because shell formation involves the removal of 100-150 mg of calcium Ih, the concentration of calcium in blood would be zero within 8-18 minutes if it were not replenished constantly through intestinal absorption and mobilization from the bone (Sturkie, 1976). Calcium is also important for controlling the excitability of nerves and muscles. Low calcium (Ca2 +) concentrations result in increased excitability of pre- and postganglionic nerve fibers and muscle. Calcium is also essential for normal blood-coagulation and for muscle relaxation because it stimulates adenosine triphosphatase. Calcium regulates the level of phosphorylation of endogenous protein in the nervous system॰
MEDULLARY BONE IN BIRDS
Just prior to the onset of sexual maturity, pullets develop unique labile and dynamic reserves of calcium in the form of non-structural bone, and because it is easily observed in the marrow cavity of the long bones, it is called medullary bone. It is believed that most of the increase in the pullet skeletal weight before sexual maturity is caused by the formation of this bone material (Miller, 1992). Hurwitz and Bar (1987) reported that medullary bone formation is a primary function of hormones rather than a result of the increased accumulation of calcium in the body due to increased absorption. The formation of medullary bone starts from the endosteal surface ofthe cortex in the form of interlacing spicules, which eventually fill the entire marrow cavity. Blood supply in the marrow cavity increases, which facilitate mobilization of the bone mineral during shell formation as well as its subsequent replacement, so the supply of blood plays a significant part in medullary bone formation. During calcium reduction, medullary bone calcium is maintained despite the exhaustion of calcium from structural bone, indicating
that medullary bone is not a simple reservoir of calcium: it also serves to buffer acute interruptions in calcium absorption, preventing hypo-or hypercalcaemia situations through hormonal control. Dietary calcium content had a significant (P < 0.05) effect on total medullary bone calcium, as measured (Table 1.1) in an experiment conducted by Clunies et al. (1992). However, dietary calcium level appeared to have no significant (P > 0.05) effect on the individual medullary bones. The largest reserves of medullary bone calcium were found in the femur, followed by the tibiotarsus, regardless oflevel of calcium fed. Also, all other long bones (the carpus, radius, ulna, tarsus, and clavicle) when combined did not contain as much medullary bone calcium as the femur and the tibiotarsus separately. Birds fed diets containing 45 g calcium/kg had intermediate levels of medullary bone calcium that were not significantly different from those fed diets with 25 and 35 g calcium/kg. In an experiment conducted by Parks et al. (1992) an increase in dietary calcium to 45 g calcium/kg increased shell weight, but in the trial by Clunies et al. (1992) it was found that 45 g calcium/kg did not increase medullary calcium deposition. Therefore, high dietary calcium content might increase shell weight but do not have the same effect on medullary bone. Birds fed diets containing 45 g calcium/kg had the lowest amount of medullary bone calcium, while the birds fed 35 g calcium/kg had the highest. So increasing the dietary concentration to 45 g calcium/kg did not increase the deposition of calcium into medullary bone
How medullary bone metabolism is affected by differences in dietary calcium levels remains controversial. Clunies et al. (1992) found that the relationship between medullary bone and dietary calcium was not linear, and an optimal level of dietary calcium was required to maximize medullary bone calcium reserves in active laying hens. However, Hurwitz and Bar (1966) found that low medullary bone calcium in laying hens was a result of low dietary calcium, not optimal levels of dietary calcium. Sohail and Roland (2000) reported that increasing dietary calcium from 31 to 37 g calcium/kg increased bone mineral content from 0.165 to 0.174 g/crrr’ and bone density from 0.252 to 0.274 g/cm”. For each increase of 3.5 g calcium/ kg diet there is approximately 5% increase in bone mineral content and bone density. Dietary calcium had an effect on bone breaking strength: increasing calcium from 31 g calcium/ kg to 37 g calcium/ kg increased bone breaking strength from 16.5 to 21.5 kg. For every 3 g calcium/kg increase there was approximately 10% increase in bone breaking strength. Frost and Roland (1991) found that feeding three levels of dietary calcium (27.5, 37.5 and 42.5 g calcium/kg) to layers at 31 weeks of age had a significant effect on tibia breaking strength. Tibia ash and bone mineral content increased significantly with increasing dietary calcium. Birds that were fed 27.5, 37.5, and 42.5 g calcium/kg had 6.74,7.14 and 7.48 kg bone strength respectively. They concluded that hens at peak production require at least 45.3 g calcium/kg per day for maximum bone strength. The size of the medullary calcium pool also plays a critical role in the metabolism of calcium. Hurwitz and Bar (1969) reported that the overall ability of the hen to direct calcium from medullary bone to the eggshell depends on the amount of calcium stored in the medullary bone. An inadequate intake of calcium during the prelay period results in lower calcium stores at the onset of production. Hurwitz and Bar (1971) believed that during the early stage of egg production the calcium absorption mechanism is not fully developed. In conclusion, dietary calcium deficiency or over- supply during the prelaying period will adversely affect bone development, and will therefore influence skeletal integrity. A deficiency or inappropriate levels of diertary calcium will lead to rickets and other disorders as well as impaired laying performance.
METABOLISMS AND HORMONAL CONTROL OF CALCIUM
Hormones involved in calcium regulation
There are three major hormones that control calcium in the body, these being parathyroid hormone (PTH), calcitonin and 1,25-dihydroxycholecalciferol (1,25(OH2)D3, the active metabolite of vitamin D3), but prostaglandins, reproductive steroids and other hormones also play a role. Increased secretion of oestrogen and androgen at the beginning of follicle maturation stimulates the development of medullary bone. Changing levels of oestrogen may also regulate some of the actions of PTH on bone (Miller, 1992). Only the major hormones that are involved in calcium homeostasis will be discussed in this section.
Parathyroid hormone —————
The parathyroid hormone is a protein or a polypeptide with a molecular weight of 9500, which contains 84 amino acid residues (Georgievskii, 1982). Parathyroid hormone regulates .calcium metabolism, which is essential for eggshell formation, muscular contraction, blood clotting, enzyme systems, calcification of tissue, neuromuscular regulation, and the maintenance of a constant level of calcium in the blood (Georgievskii, 1982). The principal targets of parathyroid hormone are bone cells for mobilization of calcium and the renal tubules for tubular excretion of phosphate. Thus, parathyroid hormone fulfils its function by intensifying the excretion of phosphate in urine , as a result of increased secretion of phosphate anions in the distal segments of the renal ducts. This results to an increased level of calcium ions in the blood serum, so the greatest effect of parathyroid hormone is when the concentration of phosphate changes. The parathyroid hormone maintains the correct Ca:P ratio, is implicated in the absorption of calcium from the intestine, and stimulates the reabsorption of calcium in the convoluted tubules when calcium in the serum is low
Calcitonin —————–
Calcitonin is synthesised by the ultimobranchial glands, which are just distal to the parathyroid glands. It is a polypeptide hormone with a molecular weight of 3000-4500 consisting of 32 amino acid residues with certain specificity (Georgievskii, 1982). The main function of calcitonin is the protection of the bones or skeleton from excessive resorption of calcium during the egg-laying period.
Vitamin D —————–
Vitamin D is the collective name for a family of compounds having antirachitic properties. The most important of these compounds is ergocalciferol (Vitamin D2) and cholecalciferol (Vitamin D3) . The form that affects calcium metabolism in birds is cholecalciferol. Consequently more emphasis will be given to this form throughout the review. The general function of vitamin D3 is to raise plasma calcium and to maintain the level of phosphorous to the ratio that maximises bone mineralization. Parathyroid hormone, together with vitamin D3, prevents tetany by increasing plasma calcium concentration and by stimulating the active transport of calcium and phosphorous across the intestinal epithelium.
Calcium metabolism in birds——————
The maintenance of relatively constant levels of plasma ionic calcium is called calcium homeostasis. In the body, calcium homeostasis takes priority over other processes involving calcium,such as bone and eggshell calcification. Calcium homeostasis is essential in the regulation of important biological processes, such as cellular information transfer, hormonal biosynthesis and release, and cellular replication (Miller, 1992). The body makes use of many homeostatic mechanisms, which are designed to maintain a constant level of circulating calcium plasma. The mechanisms act directly on the intestine, bone and kidney. The kidney serves as a safety valve, operating on a time-totime basis to excrete excess calcium and phosphate. The quantities of calcium and phosphorous excreted in urine are determined by the cumulative rates of three processes: glomerular filtration, tubular reabsorption and tubular secretion with the help of hormones in order to maintain the right amount of calcium in the body
When a low calcium diet is fed the condition that results is called hypocalcaemia. A decline in plasma calcium concentration triggers the release of parathyroid hormone, which stimulates the biosynthesis ofthe metabolically active form of vitamin D3 [1,25- (OH)2D3] by the kidney. This in turns increases calcium absorption from the gastrointestinal tract and re-absorption of calcium from the bone and the renal tubules (Figure 1.1). When the kidney increases calcium reabsorption it also decreases phosphate reabsorption, resulting in increased retention of calcium in the body (Shafey,1993). When a high calcium diet is fed the resultant condition is called hypercalcaemia. An increase in plasma calcium concentration triggers the parafollicular cells in the thyroid to release calcitonin, which depresses plasma calcium by inhibiting bone resorption. Thus the amount absorbed from the gastrointestinal tract depends on the amount ingested and the proportion absorbed. As the percentage of calcium in the diet increases the proportion absorbed tends to decline. This is related to the fact that calcium absorption is an active process under the control of a calcium binding protein, which is vitamin D3 dependent. In a vitamin D3 ddeficiency, calcium absorption is reduced because of impaired calcium binding protein formation, so skeletal and shell abnormalities can occur even in the presence of enough dietary calcium.
FACTORS AFFECTING THE ABSORPTION OF CALCIUM
Source and form of calcium Bioavailability of calcium is important in explaining the difference between the total amount of calcium in a feed ingredient and the amount that is used by the bird consuming the feed ingredient. The bioavailability of calcium is determined by measuring growth rate, ash percentage of the bone or the composition of the bone ash (Shafey, 1993). There are several types of calcium supplement on the market, but not all of them are created equal. There are inorganic and organic sources, which are also different in calcium bioavailabi1ity, since both the solubility and the concentration of calcium in these sources differ.
Inorganic Sources ofcalcium————-
Inorganic sources of calcium are found in limestone, gypsum, rock phosphate, calcium gluconate, mono- and dicalcium phosphate, meal and dolomite. All these supplements have different solubilities and concentrations of calcium.
Organic Sources ofcalcium————-
Organic sources of calcium such as that found in fishrneal are more readily available to birds than calcium from dicalcium phosphate and limestone.
Research has indicated that improvements in shell quality can be obtained by feeding part of the dietary calcium as oyster shell or limestone chips. The hen’s requirement for calcium is relatively low except at the time ofthe day when eggshell formation is taking place . Calcium is the major nutrient involved in shell calcification but researchers have questioned whether to use limestone or oyster shell. These calcium sources can vary widely in price. Oyster shell and limestone were probably the first, and are still the most common, concentrated sources of calcium fed to laying hens, and most experiments have been based on them A good quality limestone or oystershell should contain 380 to 390 g calcium! kg. To ensure maximum shell quality it is recommended that hens consume a minimum of 3.75g calcium per day, and that in older hens or hens with shell quality problem, the calcium intake should be increased by as much as 1 gld depending on the severity and type of shell quality problem (Roland, 1986). How limestone or oystershell should be fed to poultry is still not clear: some researchers believe that feeding limestone or oystershell continuously on a free choice basis, or on top of a diet which contains the full calcium requirement is not recommended, because egg shells show some chalky deposits and rough ends, and some soft shelled eggs are produced as a result. These unusual conditions are due to a deficiency of phosphate, because much ofthe phosphate ingested is excreted as soluble calcium phosphate
Particle size ————
The particle size ofthe calcium source plays a role in ensuring whether or not calcium is available in the gut when needed. It was reported by Coon et al. (2001) that larger particle size limestone results in increased weight gain, feed efficiency and calcium availability to the poultry at low dietary calcium levels. He suggested that the increased availability to laying hens of larger particles is the result of a slower rate of passage through the digestive tract of the hen, allowing absorption when needed for eggshell formation. Roland (1986) reported that larger particles are retained in the gizzard during the day. This means that large particles of calcium are released more slowly and this may be important for continuity of shell formation, especially in the dark period when birds are reluctant to eat or when there is no feed. The rate and percentage of calcium absorption may also depend on the rate of passage of food, which is quite variable in the various intestinal segments
Intestinal pH ————-
A major effect of high calcium concentration in poultry may be a reduction in the bioavailability of other minerals. Phosphorous and calcium absorption is optimal at pH 6. When the pH is higher than 6.5 absorption of phosphorous is markedly decreased and calcium tends to precipitate from the solution (Shafey, 1993). Excess free fatty acids in the diet can cause the pH to decrease and therefore interfere with calcium and phosphorous absorption. Shafey et al. (1991) found that high dietary calcium concentration increased the pH of the intestinal crop contents, but did not influence the pH of the contents in the rest of the gastrointestinal tract. They also reported that increased intestinal pH reduced the soluble fraction of minerals, and consequently their availability for absorption also decreases.
Calcium and phosphorous ratio ————
High calcium levels in the intestine reduce the absorption of phosphorous, zinc and manganese, and increase the pH in the gut. High plasma phosphorous decreases both calcium absorption from the gut and mobilization from the bone. When the concentration of phosphate in the serum increases, the hormones encourage an increase of calcium by excreting phosphate through the kidney in order to maintain the correct concentration of calcium and phosphorous (Hurwitz, 1976). Hartel (1987) reported that the amounts of calcium and phosphorous required by laying hens at a given stage of production vary not only from day to day but also through the day. Shell calcification improves as the supply of dietary calcium increases, whereas it declines with increasing dietary phosphorous, therefore good shell quality is assumed to result from feeding diets high in calcium and low in phosphorous.
Amount fed————
The amount of dietary calcium given to birds is one of the essential factors that affect calcium absorption. Usually the calcium requirement for birds is calculated on the basis of calcium retention. Morris (1972) used the data of MacIntyer et al.(1963) to calculate the calcium rentention of hens fed diets ranging in calcium from 1 to 6 % over a period of 280 days. The formula for calculating retention is: (calcium output in the shell/calcium intake in the diet) x 100. This formula ignores changes in skeletal calcium; but the data from which calcium rentention was derived covers a period of 280 days, during which the output of the shell is about 450 glbird and the skeletal calcium is unlikely to have exceeded 4 glbird. It is commonly believed that laying hens retain 50 percent of dietary calcium and it makes use of only this amount to maintain all the calcium requiring processes.
EFFECTS OF FEEDING CALCIUM IN EXCESS OR BELOW THE REQUIREMENT ON PERFORMANCE OF PULLETS
Many attempts have been made to reduce the incidence of poor shell and bone problems in laying hens through supplementation of the diet with additional calcium. In attempting to solve these problems in this way the added calcium has been shown occasionally to interfere with some of the body mechanisms, which result in different outcomes. If the diet fed to birds is deficient in calcium or contains excess calcium, it does not only affect the kidney and the hormones concerned. It also affects egg production, bone strength, egg quality, appetite, growth and sexual maturity. Hartel (1987) reported that an excess of dietary calcium interferes with the availability of other minerals, such as phosphorous, magnesium, manganese and zinc, and diets of high calcium content are detrimental to egg weight and sometimes to rates of lay. In terms of calcium metabolism in laying hens, early introduction of high calcium diets to pullets prior to the onset of lay would appear to be beneficial in order to optimise the calcium balance in the body. But it can be argued that feeding very high levels of calcium prior to lay imposes unnecessary stress on the bird’s kidneys. The stress is because birds try to maintain the right amount of calcium by excreting excess calcium. In a study by Shane et al. (1969), it was found that feeding diets containing 30 g calcium/kg to chickens between 8 and 12 weeks of age resulted in higher levels of visceral urate deposits, mortality, nephrosis (kidney degeneration) and a smaller parathyroid gland. High levels of calcium during the growth period will interfere with the proper development of the parathyroid gland by increasing gut pH, which will decrease absorption of calcium (Shafey, 1993). The parathyroid gland is central in the hormonal control of calcium and is activated when pullets mature.
In conclusion, the amount of calcium included in the feed and the rate of metabolism of calcium in the body’ are closely interconnected. Calcium homeostasis is disturbed to a larger extent by the presence of high dietary calcium levels, and the adaptation of birds to such levels depends primarily on the ability of the bird to reduce the efficiency of calcium absorption. So before deciding how much calcium to feed the criteria for adequacy of calcium should be considered, since excess calcium intake in the late rearing period has been shown to reduce both the subsequent performance and egg quality.
