Trace Mineral Function (cont.)

Copper. Copper is a necessary component of a number of enzymes including superoxide dismutase, lysyl oxidase and thiol oxidase (15, 51). These enzymes function to eliminate free radicals that increase tissue susceptibility to bacterial infections, increase structural strength and elasticity of connective tissues and blood vessels and increase strength of horn such as in the claw (hoof), minimizing lameness (15, 51).

Reproductive problems that relate to copper deficiency manifest themselves in inhibited conception even though estrus may be normal. Symptoms of a copper deficiency include early embryonic deaths, resorption of the embryo, increased retained placentas and necrosis of the placenta (41, 52). Weak and silent heats have been also reported. Kappel et al. (32) reported dairy cows with higher serum copper levels had significantly less days to first service, fewer services per conception and fewer days open. Proper copper supplementation of the sire is needed for production of quality semen (52).

The copper requirement of dairy cattle typically ranges between 9 and 16 ppm, depending upon stage of the life cycle and dry matter intake (46). However, copper's availability is greatly diminished by sulfur and molybdenum as they form insoluble complexes in the rumen that render copper unavailable to the animal (51). Zinc and iron also reduce the availability of copper to the animal (52). Montana State research (66) indicates that supplementing zinc and copper in a ratio between 3 and 5:1 results in the best utilization of dietary copper. Producers utilizing by-products (i.e. corn gluten products, etc.) need to be aware of possible antagonists and adjust their copper levels appropriately.

Cobalt. Cobalt is needed for proper vitamin B12 synthesis. Maintaining adequate vitamin B12 status benefits both the dam and offspring. When adequate, sufficient amounts of vitamin B12 cross the placenta and are present in colostrum (41). Milk, and colostrum in particular, contain high levels of vitamin B12. Consequently, lactation depletes cobalt and vitamin B12. Vitamin B12 is required for the conversion of propionate to glucose and for folic acid metabolism (30).

Depletion of cobalt and vitamin B12 at parturition causes depressed milk production and colostrum yield and quality (52). Reduced fertility and sub-optimal conditioning of the offspring are noted in a cobalt deficiency. Inadequate cobalt levels in the diet have been correlated with increased early calf mortality (52). A cobalt deficiency ultimately results in a vitamin B12 deficiency.

Research has also shown that ketosis may be partially alleviated with cobalt. Dairy cows will respond to proper additions of cobalt to the diet with decreased occurrence of ketosis. In general, ruminants will tend to respond with better appetites and improved fiber digestion. These improvements appear to be enhanced during periods of reduced feed intake due to heat stress, poor fiber quality and by-product feeding.

The required dietary content of cobalt for dairy is 0.11 ppm (46). Manganese, zinc, iodine and monensin may reduce cobalt availability (52).

Iodine. Iodine is required for the synthesis of the thyroid hormone, thyroxin, which regulates the rate of metabolism (46). Prior to regulation of the feeding rate of ethylenediamine dihydriodide (EDDI), many producers fed iodine compounds to cattle in excess of the nutritional requirement to prevent foot rot (40).

Signs of a subclinical iodine deficiency in breeding females include suppressed estrus, abortions, stillbirths, increased frequency of retained placentas and extended gestation periods (28, 52). Calves born to cows that are marginally deficient in iodine are weak and may be hairless (52). Furthermore, animals that have a subclinical iodine deficiency will also have increased incidence of foot rot and respiratory disease due to suppressed immune responses (52). One notable characteristic of a clinical iodine deficiency is an enlargement of the thyroid gland, often termed a goiter (28).

The iodine requirement for dairy cattle typically ranges between 0.27 and 0.88 ppm (46). Soybean, rapeseed and canola increase the iodine requirement of the animal as they contain goitrogenic compounds that reduce the availability of iodine (52). High dietary nitrate also inhibits uptake of iodine by the animal (52).

Iron. Iron is a necessary component of hemoglobin and myoglobin for oxygen transport and cellular use (30). An iron deficiency is rare in adult cattle, but calves are frequently iron deficient, especially if fed milk replacer containing no supplemental iron or whole milk for extended periods of time (46).

Iron supplementation is usually not needed in ruminant diets due to the high iron content of many feedstuffs and soil contamination of many feedstuffs that are ingested by cattle. Cadmium, cobalt, copper, manganese, phosphorus and zinc all reduce absorption and utilization of iron by cattle (52). Excess iron increases the risk of infection because it enhances bacterial growth (19). The iron requirement for weaned dairy cattle typically range between 13 and 43 ppm (46).

Chromium. Chromium potentiates insulin action, resulting in increased uptake of glucose and amino acids by cells in the body (57). A chromium deficiency in lactating cows may result in increased incidence of ketosis and decreased milk production (52). Improved energy balance in early lactation may improve reproduction.

Selenium. Prior to 1957, the nutritional significance of selenium was related to its toxicity (41). Today, selenium is recognized as an essential element that defends the body against oxidative stress.

Marginally selenium deficient animals will abort, or calves will be weak and unable to stand or suckle (52). Research indicates that selenium supplementation reduces the incidence of retained placentas, cystic ovaries, mastitis and metritis (52). In addition, cattle that maintain adequate blood selenium levels have reduced incidence of abortions, stillbirths and periparturient recumbency. (41, 52). Compromised selenium status has also been associated with poor uterine involution, and weak or silent heats. In males, selenium supplementation has been shown to increase semen quality (52).

Symptoms of a chronic selenium toxicity include lameness, sore feet, deformed claws and loss of hair from the tail (52). In pregnant animals, selenium toxicity will produce abortions, stillborns and weak and lethargic calves as selenium accumulates in the fetus at the expense of the cow (52).

Cadmium, copper, mercury, lead, zinc and sulfur can induce a selenium deficiency (52). Dietary calcium levels greater than 0.8% reduce selenium absorption (52). The selenium requirement of dairy cattle is 0.3 ppm (46).