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Trace minerals Six trace minerals have been shown to be needed as supplements in poultry diets: iron, copper, zinc, manganese, iodine and selenium. Subclinical trace mineral deficiencies probably occur more frequently than are recognized by poultry producers. Some soils are naturally deficient in trace minerals. In addition, crops and plants vary in their uptake of minerals. Consequently, feedstuffs grown in certain geographic areas may be marginal or deficient in specific elements. Certain areas in North America experience a high rainfall, which results in leaching of the soil and selenium deficiency. As a result, selenium deficiencies have been observed in livestock in Asia when fed US-produced maize and soybean meal but not when fed locally grown feed. Feed suppliers are usually aware of deficient (and adequate) levels of the trace minerals present in feedstuffs and will provide trace minerals mixes formulated appropriately.Several studies have shown that omitting trace minerals from poultry diets depresses productivity and tissue mineral concentrations. Patel et al. (1997) found that removal of supplemental trace minerals and vitamins from the diet during the period 35–42 days post hatching decreased daily weight gain in three different broiler strains. In addition, removal of supplemental riboflavin from the finisher diet 7 days prior to slaughter resulted in a 43% decrease in the content of riboflavin in breast muscle. Shelton and Southern (2006) reported that omission of a trace mineral premix from broiler diets had no effect on productivity during the early stage of growth but had progressively deleterious effects on productivity with increasing age of the birds. In addition, removal of trace minerals had a negative effect on bone strength and on tissue trace mineral concentrations. A study conducted in Turkey by Ïnal et al. (2001) with laying hens showed that omission of a trace mineral and vitamin supplement resulted in reduced egg production, feed intake, egg size and zinc content of eggs. These findings are of importance to organic producers, in view of their relevance to production efficiency and product quality.CobaltCobalt is a component of the vitamin B12 molecule but a deficiency of cobalt has not been demonstrated in poultry fed a diet adequate in vitamin B12. Therefore, supplementation with this element is not normally necessary. Diets containing no ingredients of animal origin (which contain vitamin B12) contain no vitamin B12. Therefore, poultry fed on all-plant diets may require dietary cobalt, unless the diet is supplemented with vitamin B12. In practice, many feed manufacturers use a cobalt-iodized salt for all species since cobalt is needed in ruminant diets. This avoids the need to stock separate salt types for ruminant and non-ruminant diets and the inclusion of cobalt provides some insurance in case the poultry diet is lacking sufficient vitamin B12.CopperCopper is required for the activity of enzymes associated with iron metabolism, elastin and collagen formation, melanin production and the integrity of the central nervous system. It is required with iron for normal red blood cell formation. Copper is also required for bone formation, brain cell and spinal cord structure, the immune response and feather development and pigmentation. A deficiency of copper leads to poor iron mobilization, abnormal blood formation and decreased synthesis of elastin, myelin and collagen. Leg weakness, various types and degrees of leg crookedness and incoordination of muscular action also result. Tibial dyschondroplasia is an example of a leg disorder in poultry that can be caused by a copper deficiency. Poor collagen and/or elastin formation can also lead to cardiovascular lesions and aortic rupture, particularly in turkeys.IodineIt has been known for over 100 years that iodine is required for the proper functioning of the thyroid gland and that an iodine deficiency causes goitre. As a result, iodized salt is now used to prevent this disease in humans and animals. Iodine metabolism is greatly influenced by selenium nutrition, thus influencing basal metabolic rate and several physiological processes. Some dietary factors are goitrogenic. Cruciferous plants contain potential goitrogens of the thiouracil type, while brassicas and white clover contain cyanogenetic glycosides that are goitrogenic (Underwood and Suttle, 1999). Canola meal has resulted from the selection of rapeseed that is low in glucosinolate, a common goitrogen. There are also goitrogenic substances in other feeds such as carrots, linseed, cassava, sweet potatoes, lima beans, millet, groundnuts, cottonseed and soybeans which impair hormone release from the thyroid gland. Goitre can then occur even though the iodine level in the diet may appear to be adequate.A high calcium level in drinking water is also known to reduce iodine absorption and result in goitre, particularly if the dietary iodine level is borderline. Signs of iodine deficiency include an enlargement of the thyroid gland (which might not be noticed because of the feathers on the neck), poor growth and reduced hatchability of the eggs. At necropsy, the thyroid is enlarged and haemorrhagic.Most feedstuffs contain only low levels of iodine. The exception is seaweed, which can contain 4000mg/kg Most of the iron in the body is in the form of haemoglobin in red blood cells and myoglobin in muscle. The remainder is in the liver, spleen and other tissues. Haemoglobin is essential for the proper functioning of every organ and tissue of the body. Iron has a rapid turnover rate in the chicken; therefore, it must be provided in a highly available form in the diet on a daily basis. Iron deficiency can result in microcytic, hypochromic anaemia in poultry. Any internal infection such as coccidiosis can also interfere with iron absorption and lead to a deficiency.Soil contains iron and may provide sufficient for poultry raised outdoors on pasture. It is important, however, that the soil be free of disease organisms and parasites. Manganese Manganese is essential for the synthesis of chondroitin sulfate, a mucopolysaccharide which is an important component of bone cartilage. Manganese is also required to activate enzymes involved in the synthesis of polysaccharides and