XVI. Microminerals

The difference between macro- and microminerals is based on their requirements in the diet. Microminerals are required in trace amounts (< 0.01%, milligrams or micrograms) and function as activators of enzymes or as components of organic compounds. The following microminerals will be discussed in this chapter: manganese, zinc, iron, copper, selenium, molybdenum, iodine, and cobalt.

Manganese (Mn)

Functions

Manganese (Mn) is a trace mineral that is a dietary essential for animals. In the animal body, Mn is widespread but is concentrated in bone and liver. Manganese is essential for the maintenance and production of the mucopolysaccharide of the organic matrix of the bone. Thus Mn is essential for bone formation and health. Consequently, Mn-deficient animals have normal tendon growth but slow or abnormal bone growth. This leads to symptoms such as perosis (slipped tendon) in chicks and crooked calf in young ruminants. Manganese also serves as an important cofactor for many enzymes that catalyze carbohydrate, fat, and protein metabolism. A large portion of Mn is located within the mitochondria, where it activates a number of metal-enzyme complexes, such as pyruvate carboxylase, that regulate carbohydrate metabolism. Manganese also functions as a cofactor in lipid metabolism through its role in cholesterol and fatty acid synthesis. The absorption of manganese from the diet is very poor and is less than 10% of intake. Excessive dietary Ca or P inhibits Mn absorption. Manganese is absorbed from the gastrointestinal tract as Mn 2+, oxidized to form Mn 3+, and transported to tissues using transferrin as a carrier. Excessive Mn in diet can induce iron deficiency.

Deficiency: Many skeletal abnormalities are associated with manganese deficiency and are related to default in mucopolysaccharide synthesis. Lameness, shortening and bowing of legs, and enlarged joints in pigs, sheep, goats, and cattle are reported. In poultry, perosis (slipped tendon) occurs with Mn deficiency. Affected birds will have a malformation of the tibiotarsal joint, bending of long bones, and gastrocnemius tendon slipping from its condyle. In cattle eating range lupine, bone-related disorders are reported. This is because lupine contains substances that interfere with Mn absorption causing deficiency. Reproductive problems such as delayed estrus, poor conception, decrease in litter size and livability in large animals, and reduction in hatchability in birds are reported due to Mn deficiency. Toxicity: Manganese toxicity is very rare.

Zinc (Zn)

Zinc (Zn) is widely distributed in the animal body. High concentrations of Zn can be found in the liver, bones, and animal body coverings, such as hair, wool, skin, and feathers.

Zinc is a cofactor or constituent (metalloenzyme) for more than 100 enzyme systems in the animal body. These include nucleic acid and protein synthesis and metabolizing enzymes (e.g., as DNA and RNA polymerases). Zinc concentration in tissue is highly related to the tissue distribution of enzymes to which it is related. Zn is a component of insulin and in this way functions with carbohydrate metabolism. Zn is also required for retinol-binding protein synthesis and is important for T-cell function in immunity and reproductive functioning. Absorption of Zn is about 5% to 40% of the intake and is affected by several factors.

Metallothionein, a low molecular weight binding protein, has a high affinity for binding to Zn and is involved in the transfer of Zn from intestinal mucosa cells to plasma and metabolism of Zn. High levels of Zn stimulates synthesis of metallothionein, which binds and traps Zn inside the mucosal cells. The absorption of Zn is affected adversely by high dietary Ca, and the presence of phytate aggravates it. Dietary phytate chelates with Zn, limiting its availability (and the availability of other minerals such as P too) to the animals. Zn absorption requires a common carrier shared by iron, copper, and zinc. Therefore, excessive iron impairs zinc absorption. When mucosal cells are sloughed off, Zn is lost in feces.

Cell differentiation and replication are impaired with Zn deficiency. Therefore, rapidly growing tissues such as the skin, gastrointestinal tract, and reproductive tract are most affected. As Zn is mainly distributed through body coverings such as skin, hair, wool, skin, and feathers, deficiency is associated with skin- or feather-related conditions. Zinc deficiency causes a condition called parakeratosis, or severe dermatitis, with dry, scaly, and cracked skin and poor feathering in poultry. Due to the role of Zn in immunity and T-cell functions, impaired or delayed wound healing occurs with Zn deficiency. Both high Ca and phytate decrease Zn absorption and thus precipitate Zn deficiency. Animal diets containing cereal grains and soybean meal increase Zn requirement due to the high content of phytic acid in these products.

Iron (Fe)

Iron is present in all cells of the animal body, but the largest proportion of the body’s iron is present as a component of the protein molecule hemoglobin (> 65%) and myoglobin (> 4%). Hemoglobin is a complex protein present in red blood cells consisting of a haem group (porphyrin) containing ferrous (Fe2+) iron and a protein (globin). The metabolic requirement for iron is for the synthesis of respiratory pigments (hemoglobin) that is needed for transporting oxygen from lungs to tissues.

Iron is also a cofactor for several metalloenzymes such as cytochromes, respiratory pigments (hemoglobin, myoglobin), peroxidases, and catalases. Dietary iron is supplied either as inorganic ions (ferric or ferrous iron) or as organically bound iron as a part of the hemoglobin molecule. Nonhaem iron is absorbed primarily in the ferrous (Fe2+) state. Ferric iron is reduced to ferrous iron in the intestine. Fe++ (ferrous iron) is the form that is being absorbed.

Absorption of Fe in the duodenum is poor and is regulated according to the body’s need for the mineral, type of food consumed, and intestinal environment. Acidic conditions in the intestine enhance iron absorption because inorganic iron in the ferrous form is more readily absorbed than iron in the ferric state. Organic haem iron originating from hemoglobin and myoglobin animal tissue, such as meat, is better absorbed than nonhaem iron from plant sources. Low body stores and an increase in metabolic need during periods of active growth and gestation lead to increased absorption. Dietary factors like phytates and tannins and other divalent elements, such as Zn, Mn, and Cu, can inhibit Fe absorption due to their competition for the same binding protein.

The ferrous iron must convert into ferric iron (Fe+++) before they can be transported. This requires a Cu-containing enzyme, ceruloplasmin. Transferrin is a ferric iron-containing protein, which is the major iron transporting protein found in blood. Once inside the enterocyte, iron can be stored as ferritin (an iron-containing protein) or transferred into the plasma, where it binds to transport protein transferrin, the form of which is transported through the plasma. Iron can be stored in tissues bound to two other proteins, a soluble form (ferritin) or an insoluble form (hemosiderin). Chief storage sites in the body are bone marrow, the liver, and the spleen. Most animals are efficient in conserving iron, so loss is minimal, unless it is due to blood loss such as in parasitic infections, injury, parturition, or surgery.
Iron deficiency leads to hypochromic (less hemoglobin) and microcytic (smaller cell) anemia and reduced growth. These can be attributed to simple iron deficiency and are common among baby pigs (piglet anemia) or to induced iron deficiency, such as cotton pelt in mink. Sow milk is low in iron, and competition among littermates, rapid growth of piglets, and low placental maternal transfer aggravate iron deficiency and cause anemia in baby pigs. Cotton pelt in mink is caused by formaldehyde in the pacific hake binding to iron makes it unavailable for absorption. These deficiencies can be treated by injecting animals with organic iron—that is, iron dextran.

Copper (Cu)

Copper (Cu) is required for hematopoiesis (red blood cell formation). As such, the metabolism of Cu and iron are very much related. Copper serves as a component of different enzyme systems. This includes lysyl oxidase needed for collagen and elastin crosslinking. Inadequate crosslinking can lead to rupture of major vessels and defective bone matrices. Copper is a component of cytochrome C oxidase, which is involved in electron transport and ATP generation. Most of the Cu found in the blood is bound to the plasma protein ceruloplasmin. This Cu-dependent protein functions as a carrier of Cu and is necessary for plasma iron for binding to transferrin. Copper is also a component in the antioxidant enzyme superoxide dismutase, responsible for destroying free radicals and preventing membrane damage and cell death. Copper is needed for the enzyme (tyrosinase) conversion of amino acid tyrosine to the pigment melanin. Lack of Cu can lead to inefficient melanin formation and lack of pigmentation causing changes in coat color and loss of crimp in wool (steely wool). Supplementing Cu has been shown to enhance immunity in ruminant animals.

Absorption: Like iron, Cu is absorbed according to the need in the animal. Metallothionein, a cysteine-rich protein, is involved in the absorption. After absorption, mainly in duodenum, Cu is complexed with plasma protein albumin and mainly stored in the liver, where it is used for ceruloplasmin and other proteins needed by the body. Zinc inhibits copper absorption, whereas phytate increases copper absorption by binding to zinc. In ruminants, there is an interaction between Cu and molybdenum. Excess molybdenum causes Cu deficiency by binding to Cu and forming an insoluble complex in blood. Ascorbic acid inhibits the absorption of Cu. Sheep are sensitive to Cu toxicity due to their low ability to excrete Cu in bile. Copper toxicity causes red blood cell hemolysis. Copper accumulates in the liver cells until they are saturated causing oxidative damage. The breakdown of liver cells releases large amount of Cu into the blood causing RBC damage. Hemolysis causes metallic-green-colored kidneys, chocolate-colored blood, and reddish urine. An inherited disorder of Cu metabolism causing Cu toxicosis occurs in certain breeds of dogs.

Selenium (Se)

Selenium is a component of glutathione peroxidase, an enzyme that deactivates lipid peroxides that are formed during lipid oxidation. Se shares this property with vitamin E in preventing peroxidation of polyunsaturated fatty acids in cell membranes and thus protecting cell integrity. Thus Se and vitamin E have a sparing effect on the requirements of each other micronutrients. Se is also a component of other selenoproteins in blood and muscle. The midpiece of sperm requires selenoprotein. Se is also involved in thyroid gland functions as deiodinase that converts the thyroid hormone thyroxine to its metabolically active form, triiodothyronine. Sulfur-containing amino acids are important in the metabolism of Se. Microbes in the rumen replace Se with S in their S-containing amino acid synthesis and are absorbed in the duodenum as amino acids. Se is stored as selenomethionine and selenocystine.

Se-deficient or toxic soils occur in different parts of the US and the world, affecting Se content of forages and grains produced from such places. Se deficiency causes nutritional muscular dystrophy in all species (white muscle disease, exudative diathesis). Affected animals have white streaks on both skeletal and heart muscles, stiffness, and difficulty in locomotion The white color of skeletal and heart muscles are due to the deposition of Ca salts in the degenerating muscle tissue. Exudative diathesis occurs in chickens due to cell membrane damage; cellular fluid exudes into body cavities under the skin, causing ascitic-like conditions. Se deficiency symptoms can be treated with both vitamin E and Se. Subclinical deficiency of Se may cause the incidence of retained placenta in dairy cattle. The addition of Se to animal diets has been shown to enhance antioxidant status and lipid stability through reduced lipid peroxidation.

Se is required in very small quantities. The range between deficiency and toxicity is very narrow. A 0.02 ppm of Se is required and 5 ppm is considered toxic. Se toxicity causes alkali disease. Animals affected by alkali disease show abnormal hoof and hair growth, loss of hair, and cracking and breaking of hooves. In addition, Se toxicity causes acute blind staggers, which are caused by central nervous system damage. The mode of action of Se toxicity is not known at this time. Toxicity can be prevented by providing animals with a high-protein diet or inorganic sulfate in their diets. Se can be provided as pellets placed in rumen. Recently, Se fertilization of forages has been attempted to enhance organic Se in the diet of animals foraging in Se deficient soils.

Cobalt (Co)

Cobalt (Co) is a constituent of vitamin B12. Cobalt is widely distributed in tissues such as in the liver, kidneys, and bones. The forms in which it appears in tissues other than as a part of vitamin B12 are not clearly known. Due to its close association as a chelated mineral with B12, the deficiency symptoms of cobalt align with vitamin B12 deficiency symptoms. Lack of cobalt in a diet leads to reduced ruminal synthesis of vitamin B12. Ruminant animals have high cobalt requirements. This is due to their inefficient vitamin B12 synthesis and low ability to absorb vitamin B12. Cobalt-deficient (and vitamin-B12-deficient) ruminant animals are unable to metabolize volatile fatty acids (propionic acid) for energy production, and thus affected animals will have high propionate in their blood and reduced appetite leading to emaciation. Because propionate is the precursor of blood glucose, affected animals will have hypoglycemia. Cobalt deficiency occurs in the soil in different parts of the world, thus leading to low levels of Co in the forages consumed by grazing ruminants. Dense pellets of cobalt are given orally to cobalt-deficient ruminants. These pellets lodge in the rumen and supply cobalt for rumen microbes for vitamin B12 synthesis. Inorganic Co is absorbed very poorly from the gastrointestinal (GI) tract, and due to the low absorption rate, toxicity is unlikely.

Iodine (I)

The only known function of Iodine is as a constituent of thyroxin (tetra iodothyronine) and triiodothyronine, thyroid gland hormones. Tetra iodothyronine is synthesized by the thyroid gland and is released into the tissues and is converted to the active form, triiodothyronine. An iodine-containing protein, thyroglobulin, is the precursor of thyroxine. Thyroxine stimulates cellular oxidative processes and regulates the basal metabolic rate. The thyroid gland contains the highest concentration of I and is followed by other organs such as the stomach, intestine, mammary glands, and skin. The key organ for I metabolism is the thyroid gland. More than 80% of total body iodine can be found in the thyroid gland. The uptake of I by the thyroid is enhanced by thyroid-stimulating hormone (TSH) secreted by the anterior pituitary gland. I is stored in the thyroid gland mainly as a glycoprotein called thyroglobulin.

Deficiency of I leads to reduced regulation of the basal metabolic rate (BMR). Tissues of I-deficient animals consume less oxygen, and a reduction in the basal metabolic rate is associated with reduced growth rates and gonadal activity. In these animals, the skin becomes dry and the hair becomes brittle. Reproductive problems are associated with abortion, stillbirths, or irregular estrus in females and deterioration of semen quality in males. I deficiency in young animals is called cretinism, a syndrome characterized by failure to grow, multiple skeletal deformities, and skin lesions. Thyroid enlargement leads to a condition called goiter. The enlargement is due to an attempt of the thyroid gland to secrete more thyroxin in response to TSH stimulation. TSH is released in response to reduced thyroxine production. In the absence of adequate thyroxine for inhibiting TSH release, the thyroid gland becomes hyperactive and increases in size (hypertrophy).

Goiter could occur in animals eating I-deficient forages or those feeds containing goitrogens (substances that interfere with the iodination process in thyroxin synthesis). Such feeds can cause induced I deficiency in animals. Numerous plants contain thyroid inhibitors or goitrogens. Plants in the cabbage family (Brassica forages, kale, turnip, rapeseed) are noteworthy for their goitrogenic activity. The requirement of I is about 0.2 to 0.3 ppm.
Long-term chronic intake of large amounts of I reduces thyroid uptake of I and leads to toxic symptoms called hyperthyroidism. Excess I disturbs all thyroid functions leading to increased BMR, increased pulse rate, and increased nervousness and excitability.

Molybdenum (Mo)

In addition to the microminerals discussed, there are several other elements that have been shown to have positive effects on animal growth, immunity, and health. These include molybdenum and chromium.

Molybdenum (Mo) is a cofactor of the enzyme xanthine oxidase and nitrogenase. Mo is used as fertilizer on pasture. It is rare to see Mo deficiency; however, it is common to see Mo toxicity. Excessive Mo inhibits Cu absorption and binds Cu in blood to form an insoluble complex and thus cause Cu deficiency.

Chromium (Cr) has been identified as an essential nutrient in animals. The role of Cr in glucose metabolism and in the ability of cells to take glucose has been identified. In swine nutrition, the addition of Cr is used as a feed additive to reduce carcass fat, and supplementation of Cr has been shown to enhance immunity and reduce respiratory disease in cattle.