Because of inheritance, some birds lay eggs with poor shells regardless of how well they are fed. Even so, no bird can be expected to form shells of the quality she is capable of unless the feed she eats furnishes the materials necessary for maximum shell formation. Environment, diseases and physiological changes in the birds themselves also affect the strength of shells produced.
Our present knowledge of feeding indicates that there are four nutrients of prime necessity in the ration in the proper amounts for maximum shell formation. These four nutrients are calcium, phosphorus, manganese, and vitamin D.
Nearly 95% of the egg shell is calcium carbonate. The hen depends on two different sources of calcium for the formation of her egg shells. These are which is available in her daily ration; and that which is present in her bones, which she is capable of drawing upon for use in shell formation. Both sources of calcium enter into the shell of each egg produced.
Normally, if enough calcium is provided in the ration, calcium will be deposited in the bones in quantities sufficient to balance that withdrawn from the bones for shell formation. If insufficient calcium is provided in the bird's ration for normal shell formulation, she will continue to withdraw calcium from her bones until she is depleted as much as about 50% of her entire skeletal reserve.
Even though birds will draw on bone calcium for shell formation, if the ration does not supply their needs they will not withdraw sufficient to maintain shell quality. Birds fed rations containing too little calcium will produce shells, which become thinner and thinner. However, the shells will not become thinner to the point of shell-less eggs being produced. A lack of calcium in the ration will cause production to stop entirely before shell-less eggs will be produced.
Shell-less eggs or so-called "soft-shelled eggs" are, as a rule, not the result of faulty feeds, but instead of physiological imperfections within the bird. Soft-shelled eggs are often seen in outbreaks of Newcastle disease.
The requirement of the laying bird for calcium has been set by the National Research Council at 2.25% of the total ration. The entire amount need not be included in the mash. Experiments have shown that laying birds given access to such calcium supplements as hen size particles of oyster shell, clam shell, or limestone grit will supplement the calcium present in the mash with enough of the shell or grit to meet her particular needs for shell-forming materials.
Generally, laying birds receiving a mash containing 2.25 to 2.50% calcium with a calcium supplement available will be supplied with sufficient calcium for shell formation.
Role of Phosphorus
The role of phosphorus in shell formation is a minor one. There are little or no data available that show that the level of phosphorus in the ration influences the quality of egg shell produced.
The shell itself contains only small amounts of phosphorus. Phosphorus, however, is required for egg production. It is an important factor in the complex method whereby the bird uses bone calcium for shell formation.
Phosphorus must be present in the diet in order for calcium to be deposited in the bone. The calcium is deposited in the bone as a calcium phosphate compound. When calcium is withdrawn from the bone, the phosphorus is also withdrawn, but instead of the phosphorus being utilized as the calcium is, it is eliminated from the body through the droppings.
The National Research Council has established the phosphorus requirement for laying birds at 0.75% of the ration. The common practice is to include the entire amount in the laying mash. To do this, it is necessary to include from 1.1 to 1.3% phosphorus in the mash.
Such materials as steamed bone meal, defluorinated phosphate, and dicalcium phosphate have been used as supplements to increase the phosphorus level of the mash to the desired amount.
Adequate vitamin D, secured either through irradiation from sunlight or from the feed, is necessary if the laying bird is to produce shells of maximum strength. Although the egg is one of the few natural foods containing vitamin D, it is not a component part of the shell. Nevertheless, a lack of vitamin D will cause egg shells to become progressively thinner in the same manner as a lack of calcium will.
Vitamin D Necessary
Vitamin D is necessary in the laying ration if the bird is to be able to utilize the calcium and phosphorus, which are provided to her. The actual amount of vitamin D necessary in the ration is somewhat dependent upon the level of calcium and phosphorus in the ration. Inadequate levels of calcium and phosphorus can be compensated for to some extent by increased levels of vitamin D. Higher levels of calcium and phosphorus also tend to decrease requirements for vitamin D.
At levels of 2.25% calcium and 0.75% phosphorus, it is recommended that the ration contain 450 A.O.A.C. units of vitamin D per pound of feed. Birds having access to sunlight will not require this much in their feed. In fact, it is generally felt that the level of vitamin D in the feed can be reduced to about 225 A.O.A.C. units during the summer months.
Experimental evidence has been brought forth in the past few years to show that small amounts of manganese are necessary in the diet of the laying hen for optimum shell formation. It has been determined that a deficiency of manganese will cause reduced breaking strength of shells and an abnormal appearance of the shells when observed before a candling machine.
The exact role of manganese in shell formation has not been definitely established. Recent reports from the Texas Experiment Station indicate that there may be a supplementary relationship between manganese and vitamin D, if not enough of the vitamin is present in the feed. The data of these investigators indicate that laying hens require more manganese than laying pullets.
It is generally accepted that laying rations should contain about 50 parts per million of manganese. The ration can, as a rule, be brought up to this level by including eight ounces of a commercial grade of manganese sulphate in each ton of mash.
Our knowledge of the role of calcium, phosphorus, manganese, and vitamin D in the formation of egg shells does not necessarily mean that we can write the final chapter on the effect of feeding on shell formation. Generally speaking, the quality of shells produced by our heavy laying strains of birds is poor, particularly during the spring and summer months. That additional nutritional factors may be responsible, in part at least, for the summer decline in shell quality is considered a definite possibility.
The role of many of the minor elements and of most of the recently discovered vitamins in shell formation has not been investigated. It is quite possible that, as more research work is completed, a way will be found to improve shell quality by means of better nutrition.