The anatomy and physiology of birds show specific adaptations for the egg laying. Laying hens have a unique calcium and bone metabolism, adapted for egg production and eggshell formation.

Because of productive, economic and animal welfare reasons, it is essential to understand the structure and function of the bone and the calcium metabolism in laying hens. This knowledge will help us to apply the correct management and feeding strategies in order to achieve appropriate mineral reserves in the skeletal system, an eggshell of good quality, and the prevention of problems or imbalances derived from an alteration of these factors.

It is worth mentioning that, in addition to the skeletal system, the digestive system, including the liver and the intestine, is key for laying hens.

1. SOURCES OF CALCIUM: FEED AND SKELETON

The high levels of calcium required for the eggshell formation are obtained from different sources: the diet (through intestinal absorption) and the body (through bone resorption and reduced renal excretion of calcium). The control of the calcium levels in the plasma is done through hormonal regulation, which maintains the balance between these three mechanisms of calcium obtention.

The calcium absorption capacity at the intestinal level is an important quality in high-production layers, since this factor should guarantee the calcium supply for the formation of the eggshell and its thickness, as well as its capacity to withstand handling without breaking.

When the calcium from the diet is insufficient, or it is not absorbed correctly, the bird obtains the calcium from the bone reserves through a process called bone resorption, defined in the following sections.

2. ROLE OF THE BONE IN THE LAYING PERIOD

• ANATOMY AND PHYISIOLOGY OF THE BONE

The skeletal system of birds is lightweight to facilitate the flight, thanks to the fusion of several bones into one and the large presence of pneumatic bones, which contain air instead of bone marrow. The bones with bone marrow are the long bones distal to the humerus and pelvis (Image 1).

Bones are made up of a collagen matrix that surrounds the cellular component, made up of osteoclasts, osteocytes, and osteoblasts. The different types of bone cells and tissues, as well as the bone metabolism, are described below.

Types of bone cells

The bone-creating action of the osteoblasts, together with the bone destruction of the osteoclasts, forms the process of bone remodeling, intended to preserve the mechanical integrity of the skeleton.Osteoblasts are bone-forming cells. When these cells are active, they secrete proteins and alkaline phosphatase for the synthesis of new bone matrix and its mineralization. During bone formation, some osteoblasts remain in the matrix they deposit, and differentiate into osteocytes, with structural function. Osteoclasts are responsible for bone resorption by releasing hydrogen ions, that dissolve the mineral phase of bone and proteolytic enzymes that degrade collagen.

Types of bone tissue

There are three different types of bone tissue: cortical, trabecular, and medullary bone.

The cortical bone is tubular in shape to enclose the medullary cavity, and makes up the outer structure of round bones, providing great strength to the bone.

The trabecular bone, located in the inner part of the cortical bone, is less dense than the first one and has a faster and more efficient osteogenic surface. Thus, it experiences greater remodeling. These two types of bones are present in all birds and have a structural function to support the skeleton.

The medullary bone is characteristic only of laying hens. It is developed during the sexual maturity under the effect of estrogens, and it is easily mobilized. Because of that, it acts as a reserve of calcium for the eggshell formation when calcium from the diet is insufficient.

Bone metabolism

Bone formation and resorption are regulated by several cells, the osteoblasts and osteoclasts, mentioned above, and different hormones, which are named with their main activity in the bone in the following table :

Estrogens decrease bone resorption. Therefore, bone remodeling occurs when the estrogen levels are low, like in the moulting or during daily rest in posture.

This article highlights the process of calcium release from the bone and its hormonal regulation by calcitonin and parathyroid hormone (Image 2):

• Calcitonin is a hypocalcemic hormone that inhibits bone resorption. In addition, it acts on the kidney, increasing the urinary excretion of calcium to reduce serum calcium levels.
• Parathyroid hormone (PTH) has the opposite action to calcitonin. It is hypercalcemic and it is released during periods of low serum calcium. PTH has acts directly on the bone and the kidneys, and indirectly on the intestine. It binds to osteoblasts, decreasing their activity, and activates osteoclasts, promoting bone resorption. At the same time, it stimulates the reabsorption of calcium in the urine, which is exchanged with the elimination of phosphorus. At an intestinal level, it acts on the mucosa of the small intestine, increasing the absorption of calcium from the diet.

• DEVELOPMENT OF THE SKELETON IN PULLETS AND SEXUAL MATURITY

The development of the intestinal tract occurs, mainly, during the first weeks of life of the pullets and is vital for the absorption of nutrients and the productive efficiency of the future layer. During this time the skeleton is also under development.

The highest growth rate of the structural bone occurs between weeks 6 and 12. At the end of this period, the size of the bird is established, since 95% of the skeleton is developed, although it has only 75% of its mature weight. Any delay in growth will affect the size of the adult bird and delay the start of production.

Weight gain in the later 6 weeks after this phase will correspond to the development of the muscle, the reproductive tract, and the medullary bone, which will be completed by week 32 of age.

When the animal is close to the sexual maturity, there is an increase in estrogen that closes the growth plates and stops the development of structural bone. In turn, this hormone stimulates the formation of medullary bone on the inner surface of the structural bone.

• BONE REMODELING PROCESS LINKED TO THE LAYING PERIOD

Bone content and mineral density, as well as the proportion of the different types of bone (cortical, trabecular, or medullary) can change dramatically during the laying period.

This is because both osteoclastic activity (bone resorption) and osteoblastic activity (bone formation) are present simultaneously during the ovulatory cycle and will modify the structure of the bone. Depending on the calcium intake rate and the hormonal activity, one will predominate over the other.

Eggshell formation takes place, usually, during darkness and after several hours since the last feed intake. By that time, the serum calcium levels are very basic, and the bone is responsible for the calcium supply thanks to the osteoclastic activity, mainly from the medullary bone. After oviposition, there is a short period of rest until a new eggshell begins to form, which is the time when the hen redeposits the medullary bone.

• PROBLEMS DERIVED FROM THE REDEPOSITION OF THE MEDULLARY BONE

Immature birds have a thick cortical bone and a good trabecular structure on the inside, but they lack medullary bone. As sexual maturity arrives, the medullary bone is deposited on the inner surfaces of the trabecular and cortical bone, which protects the structural tissues from bone resorption.

Over time, hypocalcemic periods when the medullary bone is mobilized cause a gradual diffusion of the medullary bone through the spaces of the structural bone, when it is redeposited. This diffuse deposition does not provide the same level of protection over structural bone to prevent its resorption and, therefore, mobilization of both trabecular and cortical bone occurs.

At the end of the production cycle, the cortical bone layer is very thin, few trabecular structures remain, and the medullary bone is widespread in the medullary cavity.

Because of this, the problems of fractures, crooked keels, osteoporosis, or cage fatigue syndrome that may appear towards the end of the laying period can be related to the stability of the structural bone, rather than with the reserve of medullary bone.

3. ORIGIN OF THE LOSS OF INTEGRITY OF THE SKELETON OF LAYING HENS

Nutritional deficiencies are usually the first cause of the decrease in the integrity of the hen’s skeleton that may lead to problems in the eggshell quality.

The low mineral intake, especially of calcium, due to insufficient supply or availability; intestinal dysbiosis affecting intestinal integrity and absorption; as well as feed restriction during the dark period, which is the moment when eggshell calcification is greater, lead to a predominant use of calcium of bone origin that can end up destabilizing the structural bone and generate problems of prostration, fractures, cage fatigue syndrome and an increase in the rate of broken eggs, consequences which have already been described in the previous section.

Heat stress is another important factor that causes the loss of the reserves of bone calcium. When animals hyperventilate, respiratory acidosis occurs. This is compensated by the mobilization of carbonates from the bone. These carbonates are bound to calcium, so there is a loss of this mineral, and, indirectly, the quality of the eggshell is affected.

4. EGG SHELL FORMATION

The eggshell is made up of approximately 95% calcium carbonate, mainly in the form of calcite crystals, and 5% of organic material, in the form of membranes and an organic matrix.

The eggshell formation process consists of different stages. Initially, the deposition of water, salts and glucose occurs, which increases the volume of the egg and works as a stimulus to initiate the rapid calcification of the eggshell.

The maximum rate of calcium deposition occurs between 12 and 18 hours after ovulation, when the egg is in the uterus. The calcium transfer from the blood to the eggshell at that time is very high (rates of 100-200 mg/h).

5. STRATEGIES TO IMPROVE BONE QUALITY AND AVOID EGGSHELL PROBLEMS

Biovet S.A. has developed Alquerfeed Layers to reduce the use of calcium of bone origin, avoid the weakness of structural bone and the related problems in the eggshell quality, and maximize intestinal absorption of calcium for egg production. Alquerfeed Layers is an oral solution based on the combination of:

• Minerals as an additional calcium source to that of the diet, crucial in periods of high demand.
• Carbonates to prevent the loss of calcium reserves in situations of heat stress.
• Pronutrients, compounds of plant origin that maximize calcium absorption capacity at intestinal level.

In this way, the product aims to maintain the internal balance of the birds and optimize the laying process in order to extend the laying peak, slow down the post-peak decrease in production, and prevent egg breakage and fractures or the cage fatigue syndrome.

Thanks to the benefits of the use of Alquerfeed Layers after the peak of production (<80% of the laying rate), the post-peak decrease in the production was slowed down, obtaining a 2.6% greater persistence compared to the use of similar products.

In another trial, in hens of 43 to 53 weeks of age, Alquerfeed Layers improved bone density by more than 3%, a factor related to bone strength (Table 1), compared to the control group, while thickness of the eggshell was 1% higher in the group with Alquerfeed Layers, considering that it started with lower values.

Additionally, the number of osteoclasts per medullary bone surface was 13.7% lower with Alquerfeed Layers, indicating that this solution prevents bone resorption and the use of bone as a source of calcium, thanks to providing this mineral and enhancing its intestinal absorption.

6. CONCLUSIONS

It is unlikely that there is another animal that can consume, absorb, transport and metabolize more calcium per unit of weight than birds.

Bone tissue is physiologically active and plays an important role in the homeostasis of serum calcium levels to compensate the temporary lack of intestinal calcium using the bone for the eggshell formation.

Consequently, the continuous bone remodeling that occurs due to the use of calcium from the bone, as well as the loss of calcium derived from heat stress situations, can weaken the skeleton of the bird and lead to bone disorders, such as fractures or cage fatigue syndrome, and decrease the eggshell quality, increasing the rate of broken eggs.

The use of Alquerfeed Layers in the drinking water offers a calcium source which is available continuously and, in turn, maximizes the intestinal absorption of this mineral in order to counteract the use of bone calcium for the eggshell formation and avoid locomotor problems, while optimizing the laying rate and the eggshell quality.

The effectiveness of Alquerfeed Layers has been demonstrated in commercial farming evaluations, where it showed a positive impact on the economy of the farms. The use of Alquerfeed Layers slowed down the post-peak decrease in the laying rate, obtaining a 2.6% greater persistence compared to the use of similar products, and improved the eggshell thickness and the bone density and reduced the osteoclastic activity.

BIBLIOGRAPHY:

• Kim, W. K.; Bloomfield, S.A.; Sugiyama, T. and Ricke, S.C. (2012). Concepts and methods for understandingbone metabolism in laying hens. World’s Poultry Science Journal, Vol. 68, March 2012. doi:10.1017/S0043933912000086
• Whitehead, C. C. (2004). Overview of bone biology in the egg-laying hen. Poultry Science, 83(2), 193–199.doi:10.1093/ps/83.2.193 
• Hy-Line, Boletín Técnico – Entendiendo la función del esqueleto en la producción de huevo. Disponible en: https://www.hyline.com/ViewFile?id=d4876f9e-a263-49e3-9d23-af09f6d4df25
• Núria Martín Gairal (2019). Formación de la cáscara del huevo. Disponible en: https://www.veterinariadigital.com/articulos/formacion-de-la-cascara-de-huevo/
• David Díez (2020). La importancia de la fisiología intestinal en la puesta de huevos. Disponible en: https://www.veterinariadigital.com/articulos/la-importancia-de-la-fisiologia-intestinal-en-la-puesta-de-huevos/
• Júlia Pié Orpí (2020). El papel del hígado en la producción de huevos. Disponible en: https://www.veterinariadigital.com/articulos/el-papel-del-higado-en-la-produccion-de-huevos/
• Veterinaria digital – Atlas de patología – Imágenes:
  • https://www.veterinariadigital.com/post_blog/quilla-sigmoidea-aviar/
  • https://www.veterinariadigital.com/post_blog/fracturas-oseas-por-estres-termico-en-avicultura/