The laying diets shown in Table 1 contain all the calcium needed by the layer under most conditions. However, if egg shell quality is a problem during hot weather, or if the pullets have come into production at a fairly young age and have peaked very quickly, it may be advisable to increase the levels of calcium by at least 0.4%. Research has indicated that a marked improvement in shell quality can be obtained by feeding part of the dietary calcium as oyster shell or limestone chips. This is especially true if limestone flour rather than a granular source of limestone is used. The hen’s requirement for calcium is relatively low, except at the time of the day when egg shell formation is taking place. The greatest rate of shell deposition occurs in the dark phase, when birds are not actively eating feed. The source of calcium during this period then becomes residual feed in the digestive tract and the labile medullary bone reserve.
In the first six hours of the 24h laying 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 six to 12 hours, 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 six hours, for a total of around 1.7g shell calcium, depending upon egg size. The voluntary intake of oyster shell or large particle calcium at various times during a normal 16h day is shown in Figure 8.
With the control diet, the hen had no choice as to the time of day calcium was consumed. However, when given the simultaneous choice of diets providing energy, protein or calcium (E,P,Ca), the hen was able to select calcium at any time. Under these conditions, the hen consumed little or no calcium until the afternoon. This is the time of the day that the egg is usually in the shell gland, and the requirement for calcium should be higher at this time. When feeding limestone chips or oyster shell, it is recommended that the diet contain 1 to 1.5% calcium and that the remainder be supplied by the supplemental source. The ideal time to feed this calcium supplement would be in the afternoon, since this is when the hen normally has a high calcium requirement. Since separate feeding of calcium is not very practical, the only apparent solution is to have the calcium supplement mixed in the feed. The hen has the opportunity of leaving the oyster shell or limestone chips until the latter part of the day when it is required. This type of feeding method is being used by a number of producers with very good results. The feeding of limestone or oyster shell on a continuous free choice basis, or on top of a diet containing the full calcium requirement, is not recommended. It has been shown that egg shells with chalky deposits and rough ends are probably a direct result of feeding too much calcium to laying hens. Feeding birds oyster shell ad-libitum can also result in the production of soft shelled eggs. This unusual circumstance is due to a deficiency of phosphorus. If too much calcium is ingested, it must be excreted, usually as soluble calcium phosphate. This can lead to a deficiency of phosphorus, which results in no medullary bone being redeposited between successive periods of calcification.
Calcium is the nutrient most often considered when shell quality problems occur, although it is realized that deficiencies of vitamin D3 and phosphorus can also result in weaker shells. Vitamin D3 is required for normal calcium absorption, and so 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 9). The same situation is seen in Figure 10 when birds are fed deficient diets, and shell quality quickly deteriorates over two to three weeks. In this study, the basal diet was resupplemented with vitamin D3 after four weeks, and there was rapid normalization of shell quality (Figure 10).
However, a more serious problem occurs with sub-optimal levels of vitamin D3, where changes in shell quality are more subtle but nevertheless of economic significance (Figure 10).
A major problem with deficiency of vitamin D3 is that this nutrient is very difficult to assay in complete feeds. If it is only at concentrations normally found in vitamin premixes that meaningful assays can be carried out, and so if D3 problems are suspected, access to the vitamin premix is usually essential. In addition to uncomplicated deficiencies of D3, problems can arise due to the effect of certain mycotoxins. 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.
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/litre. If a laying hen consumes 100g feed and 200 ml water per day, then water at 250 mg salt/litre 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 often occur under these conditions. It appears that saline water results in limiting the supply of bicarbonate irons 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 greatly reduced by adding 1 g vitamin C/litre drinking water.