Domestic animals in the elucidation of zinc's role in nutrition.

Parakeratosis in swine, a disease that caused severe economic losses in many commercial herds during the late 1940's and early 1950's, was caused by an inadequate intake of zinc. In addition to poor growth, the disease involves primarily the epidermal layer of the skin, which in severe cases becomes thickened and heavily encrusted. Finally, deep fissures develop in the encrusted areas. The low availability of zinc in plant protein sources and the increasing use of soybean meal in practical-type diets during the late 1940's contributed to the increasing incidence of the disease. Excessive levels of calcium also increased the incidence and severity of the disease. Prominent features of zinc deficiency in poultry were failure of normal development of long bones and the occurrence of severe lesions on the feet and legs. As with the pig, the low bioavailability of zinc from plant protein sources led to the observation that practical-type diets were limiting in this metal. Although parakeratosis can be produced in cattle fed diets low in zinc, it does not seem to have been a major nutritional problem in ruminants.