Factors affecting eggshell quality
During this time, there has been a significant shortening of the ahemeral cycle of the bird, reducing from 26–27 hours to closer to 24. This has reduced the time taken to form the egg, mainly due to a reduction in the time required for the eggshell to form. This means that greater care needs to be taken of today’s bird to ensure that its requirements for eggshell production are met; otherwise, problems with eggshell quality can cause economic losses from increased numbers of thin-shelled and cracked eggs.
This article will not discuss the disease-related causes of reductions in eggshell quality, as these should be managed through an effective targeted vaccination program during the rearing period. Instead, it will concentrate on nutritional and management aspects. First, we have to understand the process of egg formation, which requires knowledge of the bird’s calcium requirement throughout the 24-hour period.
The egg production cycle
Understanding egg formation helps us to understand the best way to feed calcium to our laying hens. Calcium demand will depend on the time of day and the time of egg laying. Many brown-egg layers lay early in the morning, and the next yolk is released from the ovary within 10 minutes of oviposition. This is followed by the secretion of albumen around the yolk in the magnum, followed by development of the internal egg membranes in the isthmus. This takes approximately 5–6 hours, then the egg enters the uterus where the albumen is hydrated over the next 4–5 hours. During the next 12 hours, the shell is formed through deposition of an even layer of calcium carbonate. Pigmentation and production of the final cuticle occurs during the last stages in the uterus, just prior to oviposition, and the cycle starts once again.
1 - Egg laying and release of the next yolk from the ovary
2 - Start of eggshell formation
3 - End of eggshell formation
4 - Pigmentation and final cuticle secretion
Figure 1. Stages in egg formation.
Figure 1 shows the stages in egg formation throughout the 24-hour period, and the likely lighting pattern for the bird. This illustrates how most eggshell formation occurs during the hours of darkness, when the bird has no access to feed.
Eggshell is formed from calcium carbonate (CaCO₃), and the layer will metabolize and transport several times the total weight of her skeleton to produce eggshells during the course of the laying cycle. The hen has a limited calcium storage capacity, mainly in the medullary bones and in particulate form in the gizzard, once the in-feed calcium has passed out of the crop. This means that the source, quality and particle size of the calcium in the diet all play an important role in availability, along with the calcium to phosphorus ratio and the effect of certain mycotoxins on vitamin D metabolism, a key element in the production of calcium-binding proteins in the blood.
Sources of calcium
The main source of calcium for chickens is limestone, a sedimentary rock formed from calcite and aragonite from sea creatures that lived many millions of years ago. Limestone is predominantly calcium carbonate, but not all limestones are equal, and quality and purity can vary considerably: it can be contaminated with magnesium salts, silica, clay and heavy metals. Dolomitic limestone is a cheap source of calcium, but should be avoided as it contains high levels of magnesium, which can bind calcium in the gut, making it unavailable to the bird.
In some geographical regions where marble deposits are found, off-cuts from the marble industry are ground as a source of calcium. Marble is a metamorphic rock, so it has been subjected to high temperatures and pressures, resulting in a harder, crystalline structure. It can be formed from calcitic or dolomitic limestone and is often made up of calcium magnesium carbonate (CaMg(CO₃)₂) rather than calcium carbonate. The high levels of magnesium, along with the increased hardness, make it less suitable for eggshell production.
Fossilized oyster shell is also sometimes used, but it is mainly used as a supplement rather than a main source due to its cost. It can be a good quality and readily available source of calcium for the bird.
Quality of the calcium source
As previously mentioned, there can be quite a variation in limestone quality due to differing contaminant levels. Many nutritional guides suggest that limestone contains 38% calcium but in reality, calcium content ranges from 32–38%, so calcium levels can be overestimated, with disastrous results. A simple rule would be the higher the calcium content, the lower the other contaminants. The price differential is rarely significant, so it is well worth spending a little more to ensure good, reliable quality.
Hardness and solubility can also vary between sources, depending on how the deposits were formed. Solubility tests can quite easily be carried out in a lab by adding a fixed amount of the limestone to hydrochloric acid and measuring how long it takes to dissolve. However, care must be taken to ensure that particle size is the same in all samples, as smaller particles have a larger surface area and will therefore dissolve faster.
Most eggshell development occurs during the night, and the bird has limited calcium storage options. At the start of the day, chickens will naturally ingest about 30% of their daily ratio, and about 60% during the last 3 hours of daylight. The remainder of the time, the bird will select whatever feed it finds attractive. Feed intake increases at the end of the day to ensure that energy levels are sufficient to maintain the bird through the night. By choice, birds will consume more energy in the afternoon, and in-lay layers will also select calcium particles at this time to meet the main requirement for calcium during the eggshell formation period. Chen and Coon (1990) demonstrated that eggshell characteristics improved as calcium particle size increased.
With limited storage capacity, particle size is of utmost importance, as particles of less than 1 mm in diameter can pass out of the gizzard into the duodenum with the chyme, which is buffered by bile salts, raising the pH (Rao and Roland, 1989). Once this occurs, no further calcium will be available from the particle. This is why most breeding companies recommend a mixture of particle sizes, with a third provided in powder form to replenish calcium in the medullary bones during the morning and two-thirds in particles of 2–4 mm in diameter to be stored in the gizzard and used to grind food particles and be used for eggshell production.
Calcium is stored in the bones in the form of calcium phosphate. So if only powdered limestone is used, phosphorus levels in the diet must be increased to offset the phosphorus lost by excretion when the calcium is transported from the medullary bones.
Calcium to phosphorus ratio
When a layer comes into production at approximately 18 weeks of age, it weighs only two-thirds of its expected adult body weight, so it still needs to grow during the early laying period. However, excess phosphorus is known to have a negative effect on eggshell quality by binding calcium in the intestine, rendering it unavailable. This is why the calcium to phosphorus ratio in the diet is adjusted during the laying cycle, starting with a ratio of approximately 7:1 while the bird is growing, and reducing to 10:1 once final body weight has been reached. This is achieved partly by reducing the level of phosphorus in the diet, but also by increasing the level of calcium from 3.7–4.2% as egg size and shell weight increase.
Mycotoxins and eggshell quality
The importance of vitamin D to eggshell quality is quite well known, however less is understood about the way mycotoxins can affect vitamin D. Vitamin D, or cholecalciferol, is converted to 25-hydroxycholecalciferol in the liver microsomes before being further converted to 1,25-dihydroxycholecalciferol in the kidney mitochondria (DeLuca, 2008). This compound circulates in the intestine and bones, and it is important in the transportation of calcium from the gut to the bones and shell gland. 25-hydroxycholecalciferol is selectively bioactive and promotes the uptake of calcium ions in the small intestine (Phandis & Nemere, 2003).
Aflatoxin and ochratoxin A attack the liver and kidneys respectively, reducing vitamin D metabolism. This reduces bird's ability to absorb calcium from the diet and transport it to the shell gland and medullary bones (Verma, 2006). Zearalenone has been also implicated in reducing eggshell quality by binding calcium to form a complex in the intestines, reducing calcium availability.
Figure 2. Vitamin D metabolism in the bird (adapted from Verma, 2006).
Experimental work carried out in broilers by the Free University of Berlin has demonstrated that digestion of calcium, phosphorus and other nutrients increases with the inclusion of phytogenic additives in the feed. The reasons are not fully understood, but it may be because better gut structure and function are maintained, with an increased villus to crypt ratio improving absorption. Field trials have also demonstrated that eggshell quality can be maintained or improved by the inclusion of Biomin phytogenic products, reducing the percentage of second quality eggs, in both breeders and commercial layers (BIOMIN data).
The continuing increase in commercial layer and parent stock productivity increases pressure on the birds to metabolize and mobilize ever-more calcium from the intestine to the bones and uterus for eggshell production. Producers must therefore pay greater attention to bird management to maximize the genetic potential provided by the breeding companies.