Calcium is toxic during growth at levels only slightly above the requirement in birds which have been tested. Levels of 1.2% (6), 1.35% (7), and 2.5% (6) have been shown to result in poorer performance than lower levels in growing chickens. Calcium at 2.5% (6) resulted in high morbidity and mortality in pullets between 8 and 20 weeks of age. From these data it appears that the calcium intake of growing cage and aviary birds should be limited to no more than 1.2% of the diet unless specifically indicated by unique conditions.
IRON STORAGE DISEASE
Iron storage disease is a frequently observed but poorly understood syndrome. One effective treatment, bleeding, is usually used in conjunction with low iron diets. The cause of excess iron storage and a less invasive treatment have not been elucidated.
Iron excess in the liver is called hemosiderosis if there is no alteration of tissue morphology and function and hemochromatosis if the iron accumulation alters either the appearance or function of the cells or tissues involved (12). Iron excess, usually as hemochromatosis, has been observed in a number of species (13, 14, 15, 16, 17) and remains a problem in many pet and zoo birds. Diets with less than 100 ppm iron are recommended, because most affected birds had been consuming diets in excess of 100 ppm iron. Since practical diets with less than 100 ppm iron are difficult to formulate, this observation may prove to relate to what diets are available rather than what is needed to prevent excess iron storage. Even 100 ppm iron is in excess of the 60 to 80 ppm (18) required for growth in poultry. Corn, a major constituent of poultry diets, contains 300 ppm iron. If this level of iron were the major cause of excess iron storage, major die offs of birds fed corn based diets should have been seen. This has not happened in flocks fed diets containing 250 to 300 ppm iron for ten years. Other primary causes of excess iron storage need to be considered. Some possibilities are stresses related to disease exposure (immunologic stress 19, 20, 21); crowding; and nutritional stress related to periodic starvation associated with diet changes (22), since the disease occurs with a high frequency in recently imported birds (23). Periodic starvation has been associated with excess iron storage in the livers of reindeer (22). Immunologic stress may prove to be a factor in iron metabolism since the sequestering of many trace elements occurs after infection with coccidia in poultry (19, 20, 21). Another stress which has been shown to be associated with increased iron stores is intoxication with heavy metals (24).
With our present level of understanding of iron storage disease it appears that nutrition is, at best, a minor player in the disease. Stress factors as mediators of iron metabolism need further investigation.
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