Iron is an essential component of a number of proteins involved in oxygen transport or utilization. These proteins include hemoglobin, myoglobin, and a number of cytochromes and iron-sulfur proteins involved in the electron transport chain. Several mammalian enzymes also either contain iron or are activated by iron (McDowell, 1992). More than 50 percent of the iron in the body is present in hemoglobin, with smaller amounts present in other iron-requiring proteins and enzymes, and in protein-bound stored iron.
The iron requirement is approximately 50 mg/kg diet in beef cattle. Studies with young calves fed milk diets have indicated that 40 to 50 mg Fe/kg is adequate to support growth and prevent anemia (Bremner and Dalgarno, 1973; Bernier et al., 1984). Iron requirements of older cattle are not well defined. Requirements in older cattle are probably lower than in young calves because considerable recycling of iron occurs when red blood cells turn over (Underwood, 1977), and in older animals blood volume is not increasing, or at least not to the extent that it is in young animals.
A deficiency of iron results in anemia (hypochromic microcytic), listlessness, reduced feed intake and weight gain, pale mucus membranes and atrophy of the papillae of the tongue (Blaxter et al., 1957; Bremner and Dalgarno, 1973). Iron deficiency can occur in young calves fed exclusively milk, especially if they are housed in confinement. Most practical feedstuffs are more than adequate in iron, and iron deficiency is unlikely in other classes of cattle unless parasite infestations or diseases exist that cause chronic blood loss. In the absence of blood loss, only small amounts of iron are lost in the urine and feces (McDowell, 1992).
Cereal grains normally contain 30 to 60 mg Fe/kg; oilseed meals contain 100 to 200 mg Fe/kg (Underwood, 1981). With the exception of milk and milk products, feeds of animal origin are high in iron, with meat and fish meal containing 400 to 500 mg Fe/kg; blood meal usually has more than 3,000 mg Fe/kg. The iron content of forages is highly variable but most forages contain from 70 to 500 mg Fe/kg. Much of the variation in forage iron is probably caused by soil contamination. Water and soil ingestion also can be significant sources of iron for beef cattle. Availability of iron from forages appears to be lower than from most supplemental iron sources (Thompson and Raven, 1959; Raven and Thompson, 1959). Iron from soil is probably of low availability; however, research by Healy (1972) indicated that a significant amount of iron from various soil types was soluble in ruminal fluid.
Iron is generally supplemented in diets as ferrous sulfate, ferrous carbonate, or ferric oxide. Availability of iron is highest for ferrous sulfate with ferrous carbonate being intermediate (Ammerman et al., 1967; McGuire et al., 1985). Ferric oxide is basically unavailable (Ammerman et al., 1967).
Iron toxicity causes diarrhea, metabolic acidosis, hypothermia, and reduced gain and feed intake (National Research Council, 1980). The maximum tolerable concentration of iron for cattle has been estimated at 1,000 mg Fe/kg (National Research Council, 1980). Dietary iron concentrations as low as 250 to 500 mg/kg have caused copper depletion in cattle (Bremner et al., 1987; Phillippo et al., 1987a). In areas where drinking water or forages are high in iron, dietary copper may need to be increased to prevent copper deficiency.
Manganese functions as a component of the enzymes pyruvate carboxylase, arginase, and superoxide dismutase and as an activator for a number of enzymes (Hurley and Keen, 1987). Enzymes activated by manganese include a number of hydrolases, kinases, transferases, and decarboxylases. Of the many enzymes that can be activated by manganese, only the glycosyltransferases are known to specifically require manganese.
The manganese requirement for growing and finishing cattle is approximately 20 mg Mn/kg diet. Skeletal abnormalities were noted in calves from cows fed diets containing 15.8 mg Mn/kg but were not present when diets were supplemented to contain 25 mg Mn/kg (Rojas et al., 1965). The quantity of manganese needed for maximum growth is less than that required for normal skeletal development. Manganese requirements for reproduction are higher than for growth and skeletal development, and the recommended concentration for breeding cattle is 40 mg/kg. Cows fed a diet containing 15.8 mg Mn/kg had lower conception rates than cows fed 25 mg Mn/kg (Rojas et al., 1965). Heifers fed 10 mg Mn/kg exhibited impaired reproduction (delayed cycling and reduced conception rate) compared to those fed 30 mg Mn/kg, but growth was similar for the two groups (Bentley and Phillips, 1951).