EGG Yield of Chickens Is Affected by Content of Vitamin D in Diet
To obtain good egg production it is not enough to give chickens all the feed they will eat. Unless their diet is carefully compounded, so that it contains an adequate quantity of all the necessary nutrients and accessory food factors, they will not lay all the eggs they are capable of producing.
A deficiency of vitamin D in the diet has a detrimental effect on the production of eggs and also decreases the strength and thickness of shells and the vitamin D content of yolks. If, in the case of pullets, the feed contains an inadequate supply of this accessory food factor, skeletal development is delayed. The net result is that the time required to reach full production is increased and an unnecessarily large number of small eggs is obtained. In the case of both pullets and hens, the ability of their eggs to hatch is materially decreased, if a diet containing an inadequate supply of vitamin D is fed.
It is fortunate, therefore, that vitamin D is very easily supplied to laying chickens. Among the practical means of supplying this vitamin are sunshine, cod-liver oil, sardine oil, some of the other fish oils, and cod-liver meal. Ultraviolet irradiation of the chickens, or the use of irradiated yeast and solutions of irradiated ergosterol may be resorted to, but at present these methods of supplying vitamin D are either unsatisfactory or not economical. It is probable, however, that, in the near future, satisfactory irradiated products will be produced cheaply enough to make their use economical.
The cheapest source of this highly important vitamin is sunshine; but in many parts of the country during late fall, winter, and early spring, it is not possible for the chickens to get enough sunlight to supply all the vitamin D required. At such times it is necessary to have a more dependable source, such as cod-liver oil or sardine oil.
Inasmuch as not all cod-liver oils, sardine oils, and other fish oils containing vitamin D have the same potency, it is necessary that only products of guaranteed vitamin D content be used. A good cod-liver oil will contain 2,400 or more international vitamin D units per ounce, or 85 or more international vitamin D units per gram. Dependence should not be placed on cod-liver meal, unless its potency is definitely known.
Experiments on the vitamin D requirements of laying chickens in full production indicate that each bird should receive between 70 and 80 international vitamin D units per day. In other words, to meet this requirement, each ounce of feed consumed would have to supply at least 20 of these units.
If the all-mash system of feeding is used, and the chickens are kept in strict confinement without access to sunlight, 1 pound of good cod-liver oil per 100 pounds of feed mixture will ordinarily supply enough vitamin D. If the mash-and-scratch system of feeding is employed, from 1.5 to 2 pounds of good cod-liver oil should be added to each 100 pounds of mash, depending on the proportions of mash and scratch which are fed.
Laying chickens are not usually kept in strict confinement without access to sunlight; and when they are not so kept, it is unnecessary to supply the full quantity of cod-liver oil indicated above. The quantity to use will depend on the amount of sunshine the birds receive. During November, December, January, February, and March, from 75 to 80 percent of the quantity of cod-liver oil recommended for strictly confined birds should be used; and during the other months of the year, between 25 and 50 percent as much. In my case, the amount of cloudy weather should be the determining factor.If cod-liver oil that has been fortified, sardine oil, or other fish oils are used, the quantity to be added to each 100 pounds of feed will depend on the guaranteed potency of the oil in question. A fortified cod-liver oil is one to which additional vitamin D has been added.
A word of warning should be added about using too much cod-liver oil. Although 1 or even 2 percent of cod-liver oil ordinarily gives excellent results, it does not follow that 4, 6, or 8 percent will give still better results. Experiments conducted at the United States Animal Husbandry Experiment Station, Beltsville, Md., indicated that, in general, no advantage is to be gained by feeding a diet containing 3 percent of cod-liver oil, as compared with 2 percent. Also, it was found that when the diet contained as much as 4 percent of cod-liver oil, the hatchability of the resulting eggs was decreased, and that 6 to 8 percent of cod-liver oil materially decreased egg production, as well as hatchability.