Nutrition and shell quality

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Nutrition can have a major impact on eggshell quality, and is often the first parameter considered when problems arise. After peak egg production, the layer pro duces a fairly consistent quantity of shell material for each egg, regardless of its size. As the egg gets larger, therefore, the shell necessarily gets thinner, and this becomes more prone to breakage.

Even with ideal conditions,4-5% of eggs leaving the farm will be graded as 'cracks', and together with cracked and broken eggs on-farm, means that 7-8% of eggshells break for various reasons. The composition of the shell is very consistent since the major constituent is calcium carbonate. When considering eggshell quality, the nutritional factors most often investigated are diet levels of calcium, phosphorus, and vitamin D3. Since larger eggs have thinner shells, then levels of protein, methionine, and TSAA may also come under scrutiny.

A shell contains around 2 g of calcium the origin of which is the feed, with a portion of this cycling through the medullary bone. The most active period for shell formation usually coincides with the dark phase of the photoperiod, and so birds are not eating at this time (Figure 4.10). In the first 6 hours of the 24 h ovulatory cycle, there is virtually no shell deposition. This is the time of albumen and shell membrane secretion, and the time of redeposition of medullary bone. From 6 - 12 hr about 400 mg calcium are deposited, while the most active period is the 12 - 18 hr period when around 800 mg shell calcium accumulates. This is followed by a slower deposition of about 500 mg in the last 6 hr, for a total of around 1.7 g shell calcium, depending upon egg size.

During the evening, when shell calcification is greatest, a portion of the required calcium will come from the medullary bone reserves. The total medullary calcium reserves are probably less than 1 g. This reserve normally contributes no more than 0.1 g to a shell containing 2g calcium, yet are essential for the almost daily shell formation process of the modern layer. The medullary bone is composed of calcium phosphate, and so the quantity of calcium liberated for shell synthesis, is associated with a similar release of phosphorus.

Fig. 4.10 Shell mineral deposition over a 24h ovulation cycle

Medullary Bone Birds
  1. 4.10 Shell mineral deposition over a 24h ovulation cycle
  2. 4.11 Schematic of daily calcium balance in a laying hen.

Since there is little immediate need for this phosphorus, it is excreted and there is need for both calcium and phosphorus to replenish this medullary reserve during periods between successive ovulations. Figure 4.11 shows the calcium and phosphorus balance of a bird at around 35 weeks of age.

Figure 4.11 shows zero net accretion of calcium and phosphorus in medullary bone. It is likely that the quantity of medullary calcium and phosphorus reserves are maximum when the bird is around 30 weeks of age, and a slight negative balance over time contributes to reduced shell quality in older birds.

There is often discussion about the physical form and source of calcium supplied to layers. Calcium is usually supplied as limestone, or as oystershell which is much more expensive. Oystershell and large particle limestone are expected to be less soluble than is fine particle limestone, and so remain in the gizzard for longer and will hopefully be there in the period of darkness when the bird is not eating. Table 4.23

Table 4.23 Limestone types and solubility


Particle size (mm)

Relative1 solubility







- 0.5






Extra coarse


- 2.0


Large (hen size)


- 5.0




- 8.0


1 Reduced solubility results in longer retention within the digestive tract

1 Reduced solubility results in longer retention within the digestive tract gives an example of descriptions used for limestone and associated relative solubility.

Twelve hours after feeding, there will likely be twice as much calcium in the gizzard from large vs. fine particle limestone. Oystershell is expected to have solubility characteristics similar to those of large particle limestone. The large particles are more important for older birds and seem to help maintain the quantity and activity of medullary bone. The only problem with large particle limestone is its abrasive characteristic with mechanical equipment.

Using particulate limestone or oystershell does allow the bird a degree of nutrient selection. The peak in calcium requirements coincides with shell calcification, and this starts each day in the late afternoon. If given a choice situation, layers will voluntarily consume more calcium at this time of day. In fact, a specific appetite for calcium is the likely reason for the late afternoon peak in feed intake seen when layers do not have the opportunity at nutrient/ingredient selection.

If birds do not receive adequate quantities of calcium there will be almost immediate loss in shell integrity. If the deficit is large, ovulation often ceases and so there is no excessive bone resorption. With marginal deficiencies of calcium, ovulation often continues, and so the birds rely more heavily on bone resorption. Total medullary bone calcium reserves are limited and so after production of 3 - 4 eggs on a marginally calci um deficient diet, cortical bone may be eroded with associated loss in locomotion. As calcium content of the diet decreases, there is a transient (1 - 2 d) increase in feed intake, followed by a decline associated with reduced protein and energy needs for egg synthesis. Calcium deficiency is exacerbated by high levels of dietary chloride (0.4 - 0.5%). In such dietary situations, there is greater benefit to feeding sodium bicarbonate. If birds are fed a calcium deficient diet, egg production and eggshell calcium return to normal in 6 to 8 days after the hens receive a diet adequate in calcium. After three weeks, the leg bones will be completely recalcified. The finding that the adrenal gland is enlarged in calcium deficiency indicates that this is a stress in the classical sense.

Calcium is the nutrient most often considered when shell quality problems occur,although

Fig. 4.12 Decline in shell weight for hens fed a diet devoid of Vitamin D3 supplementation.

Fig. 4.12 Decline in shell weight for hens fed a diet devoid of Vitamin D3 supplementation.

deficiencies of vitamin D3 and phosphorus can also result in weaker shells. Vitamin D3 is required for normal calcium absorption, and if inadequate levels are fed, induced calcium deficiency quickly occurs. Results from our laboratory suggest that diets devoid of synthetic vitamin D3 are quickly diagnosed, because there is a dramatic loss in shell weight (Figure 4.12).

A more serious situation occurs when a marginal, rather than absolute deficiency of vitamin D3 occurs. For example, birds fed a diet with 500 IU D3/kg showed only an 8% decline in shell quality, yet this persisted for the entire laying cycle and would be difficult to detect in terms of cracked and reject eggs etc. A major problem with such a marginal deficiency of vitamin D3 is that this nutrient is very difficult to assay in complete feeds. It is only at concentrations normally found in vitamin premixes, that meaningful assays can be carried out, and so if vitamin D3 problems are suspected, access to the vitamin premix is usually essential. In addition to uncomplicated deficiencies of vitamin D3, problems can arise due to the effect of certain myco-toxins. Compounds such as zearalenone, that are produced by Fusarium molds, have been shown to effectively tie up vitamin D3, resulting in poor egg shell quality. Under these circumstances dosing birds with 300 IU D3 per day, for three consecutive days, with water soluble D3 may be advantageous.

Vitamin D3 is effectively 'activated' by processes occurring first in the liver and then in the kidney. This first activation in the liver yields 25(OH)D3 while the second product is the result of further hydroxylation to yield 1,25(OH)2D3. This latter compound is a very potent activator of calcium metabolism, although is not likely to be available as a feed ingredient. The first hydroxylation product, 25(OH)D3, is however, now available to the feed industry, and seems to promote increased calcium retention in layers (Table 4.24).

Table 4.24 Effect of Hy-D®25(OH)D3 on daily calcium rentention




retained (mg)











Adapted from Coelho (2001)

Adapted from Coelho (2001)

Minimizing phosphorus levels is also advantageous in maintaining shell quality, especially under heat stress conditions. Because phosphorus is a very expensive nutrient, high inclusion levels are not usually encountered, yet limiting these within the range of 0.3% to 0.4%, depending upon flock conditions, seems ideal in terms of shell quality. Periodically, unaccountable reductions in shell quality occur and it is possible that some of these may be related to nutrition. As an example, vanadium contamination of phosphates causes an unusual shell structure, and certain weed seeds such as those of the lathyrus species, cause major disruptions of the shell gland.

Up to 10% reduction in eggshell thickness has been reported for layers fed saline drinking water, and a doubling in incidence of total shell defects seen with water containing 250 mg salt/liter. If a laying hen consumes 100 g of feed and 200 ml of water per day, then water at 250 mg salt/liter provides only 50 mg salt compared to intake from the feed of around 400 mg salt. The salt intake from saline water therefore, seems minimal in relation to total intake, but nevertheless, shell quality problems are reported to occur under these conditions. It appears that saline water results in limiting the supply of bicarbonate ions to the shell gland, and that this is mediated via reduced activity of carbonic anhydrase enzyme in the mucosa of the shell gland. However, it is still unclear why saline water has this effect, in the presence of overwhelmingly more salt as provided by the feed. There seems to be no effective method of correcting this loss of shell quality in established flocks, although for new flocks the adverse effect can be minimized by adding 1 g vitamin C/liter drinking water.

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    What is the annual production cycle for a laying hen diagram?
    8 years ago

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