production and eggshell thickness in laying hens (Ben-Dor, 1941; Gillis, 1948; Leach, 1974). It is not usually necessary to add potassium to practical feed formulations, since such formulas generally contain about 0.7 to 1.0 percent potassium.
A deficiency of sodium in chicken diets results in poor growth, increased adrenal weight, and decreased egg production (Burns et al., 1952, 1953; Nott and Combs, 1969). Frequently, sodium supplementation is minimized to reduce the moisture level in the excreta.
Signs of chlorine deficiency in chicks include poor growth, mortality, hemoconcentration, and reduced blood chlorine level (Leach and Nesheim, 1963). Chlorine-deficient chicks show a nervous condition resembling tetany and fall forward with legs extended backward when stimulated by a sharp noise.
Iodine is necessary for the synthesis of thyroid hormones. Iodine deficiency results in goiter, which is the enlargement of the thyroid glands (Wilgus et al., 1953; Rogler et al., 1959a). The glands may increase to many times their usual size. If the deficiency is not too severe, the increased efficiency of the enlarged gland in "trapping" iodine from the bloodstream may compensate for the low dietary concentration. When this is the case, the production of thyroid hormones is normal, although the thyroid glands are enlarged.
Inadequate production of thyroid hormones results in poor growth, egg production, and egg size. Iodine deficiency in breeders results in low iodine content of the egg and, consequently, decreased hatchability and thyroid enlargement in the embryos.
Copper deficiency in poultry causes an anemia in which the red blood cells are small and low in hemoglobin (Elvehjem and Hart, 1929). Bone deformities can occur (O'Dell et al., 1961). Pigmentation of feathers in New Hampshire and Rhode Island Red chickens is reduced (Hill and Matrone, 1961). Copper is required for the activity of the enzyme needed for the cross-linking of lysine in the protein elastin (O'Dell et al., 1961; Starcher et al., 1964). Dissecting aneurism of the aorta occurs in birds deficient in copper because of the defect in elastin formation. Copper deficiency also results in marked cardiac hypertrophy (Carlton and Henderson, 1963).
Iron deficiency in chickens and turkeys causes an anemia in which the red blood cells are reduced in size and low in hemoglobin (Elvehjem and Hart, 1929). In red-feathered chickens, pigmentation does not occur when the diet is deficient in iron (Hill and Matrone, 1961; Davis et al., 1962).
Selenium is closely associated with vitamin E and other antioxidants in practical feed formulation. The principal sign of deficiency in chicks is exudative diathesis (Creech et al., 1957; Patterson et al., 1957; Nesheim and Scott, 1958). A requirement for selenium supplementation, even in the presence of vitamin E, is demonstrated by the poor growth, muscular dystrophy, and mortality of chicks fed purified diets or diets based on grains produced on low-selenium soils (Nesheim and Scott, 1958). Selenium is required for prevention of myopathies of the gizzard and heart in turkeys (Walter and Jensen, 1963; Scott et al., 1967). Pancreatic fibrosis, with resultant reductions in the pancreatic output of lipase, trypsinogen, and chymotrypsinogen, has also been associated with selenium deficiency (Thompson and Scott, 1970; Gries and Scott, 1972c). Selenium is a structural component of glutathione peroxidase, an enzyme needed to quench peroxides generated during metabolism (Rotruck et al., 1973).
There is wide variability in the amount and availability of selenium in the soils of different geographic areas (Scott and Thompson, 1971; Scott, 1973). Consequently, cereals and plant-derived feedstuffs are variable sources of selenium. Grains from some areas contain sufficient selenium to render them toxic to chicks. The effects of toxic levels of selenium are listed in Table 8-1. The amount of supplementary selenium permissible in diets is regulated in the United States and Canada.
Zinc has many biochemical functions. Deficiency causes retarded growth and frayed feathers (O'Dell et al., 1958; Sullivan, 1961). The extent of fraying varies from almost no feathers on the wings and tail to only slight defects in the development of some of the barbules and barbicels. The long bones of the legs and wings are shorter and thicker than normal (Kratzer et al., 1958; Morrison and Sarett, 1958; O'Dell et al., 1958). The hock joint may be enlarged. Layer and breeder diets deficient in zinc reduce egg production and hatchability (Kienholz et al., 1961).