XV. Minerals

What Are Minerals?

Minerals are inorganic elements that are essential for the animal body’s physiological functions and metabolic processes. The mineral matter constitutes about 4% of the animal body’s weight, and their presence is essential for maintaining life and animal health. Minerals are more integrally a part of all biological functions in the body than any other single class of nutrient. The functions include expression and regulation of genes and enzyme systems that regulate cellular function, activity and functionality of vitamins, osmotic balance, detoxification, immunity, cell membrane function, acid-base balance and regulation, and structural support and growth (i.e., bone). Scientific literature lists 21 essential minerals. The current chapter will discuss only those definitely implicated in different animal nutrition problems in practical situations.

Minerals are classified into two groups—macro and micro (trace) minerals—based on the amounts needed in diet and not based on their importance for physiological functions. Macrominerals are those minerals that occur in appreciable amounts in the animal body and are required in large quantities in the diet (> 0.01%). Macrominerals include calcium, phosphorus, magnesium, sulfur, and electrolytes (sodium, potassium, chloride). The functions and deficiencies of macrominerals and electrolytes (Na, K, Cl) discussed in this section are shown in table 15.1.

Microminerals are required in trace amounts (< 0.01%), in milligrams, micrograms, or parts per million. Microminerals discussed include manganese, zinc, iron, copper, selenium, iodine, cobalt molybdenum, and chromium. Minerals cannot be added to a diet in their elemental forms but rather need to be added as salts that are combined with other minerals (NaCl, CaCO3, MnSO4, etc.).

Calcium and Phosphorus

Functions

Both calcium (Ca) and phosphorus (P) function as structural components in the animal body. Approximately 99% of the Ca and 80% of the P in the animal body occur in bones and teeth as a compound called hydroxyapatite. The other 1% of Ca is distributed in cellular fluids, where they are involved in different metabolic and physiologic activities such as blood coagulation, nerve impulse and cell permeability maintenance, activation of certain enzymes, muscle contraction, or serving as activators of ion channels.

The phosphorus that is found in the soft tissues of the body is involved in important phosphorylation reactions that are part of cellular oxidative pathways for energy metabolism. For example, phosphorus is a component of the central compound in energy metabolism, adenosine triphosphate (ATP), which is a phosphorylated compound. Similarly, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) contains phosphorylated pentose sugars. Phosphorus is also part of cell membrane phospholipids that are involved in maintaining cellular fluidity and transport of nutrients into cells. Thus calcium and phosphorus are crucial to different metabolic processes that sustain animal life. Cereal grains are rich in phosphorus. However, P in cereal grains are present in the bound form as phytate or phytic acid. The availability of P from such bound sources varies (20%–60%). Monogastric animals lack the enzyme phytase to release them from the bound form and the term available P is commonly used to designate unbound forms of P in the diet of monogastric animals. Ruminant animals produce microbial phytase enzyme that can split and liberate P.

Regulation of Blood Calcium Levels: The body has a strictly controlled physiological regulation called homeostasis—that is, maintenance of a steady state of circulating blood plasma calcium. This is brought about by the action of parathyroid hormone (PTH), calcitonin, and active forms of vitamin D as shown below. When blood Ca is low (hypocalcemia), PTH is released from the parathyroid gland, which leads to increased Ca and P resorption from bone, increased P excretion into urine, and increased synthesis of active forms of vitamin D in the kidneys; this in turn is due to an increase in absorption of dietary Ca from the gastrointestinal (GI) tract.

In conjunction with PTH, vitamin D also enhances the mobilization of Ca from bone by increasing the activity of osteoclasts. Overall, the net result is an increase in blood Ca level to normal levels. When blood Ca is high (hypercalcemia), another hormone called calcitonin is released by the parafollicular cells of the thyroid gland. Calcitonin reverses PTH functions to lower blood Ca level to normal by decreasing calcium mobilization from bones.

Calcium and phosphorus also have an important relationship to each other within the diet. The ratio of calcium to phosphorus is also important. Excess dietary Ca forms insoluble complexes with phosphorus, resulting in decreased P absorption. High P or phytate P in the diet can inhibit Ca absorption. Cereal grains are rich in P, but most of it is in the bound form as phytate P (> 30%–60%) and limits the absorption of other nutrients by forming complexes that are resistant to the action of digestive enzymes. The recommended ratio of Ca:P is 1:1 (small animals) to 2:1 (large animals). Feeding diets with improper ratio of Ca:P or supplementing feeds with high levels of one of these minerals can lead to calcium phosphorus imbalance. Such problems affect skeletal health, productivity, and animal welfare and may lead to economic loss.

Disorders Associated with Calcium Phosphorus Deficiency or Imbalance in Animals

Bone serves as the storehouse of minerals, especially calcium and phosphorus. Thus imbalance in calcium and phosphorus leads to structural deformities in animals as well as eggshell quality in egg-laying hens. Several bone growth disorders are associated with calcium phosphorus deficiency, imbalance, or excess in food-producing animals.

Rickets is a condition occurring in young growing animals due to normal growth in the organic matrix but insufficient mineralization. Osteomalacia occurs in adult animals with a Ca-deficient diet. Excessive loss of Ca from bone causes brittle, demineralized bones. Osteoporosis is the result of a loss of both mineralization and the organic matrix of bone. In both rickets and osteomalacia, bones become soft and often deformed due to improper calcification. In fast-growing animals, such as chickens and pigs, where skeletal mineral turnover is rapid, Ca deficiency may produce profound changes. In large animals, such as cows and sheep, it takes a longer time to show deficiency symptoms. Lameness, leg weakness, abnormal gait, and spontaneous fractures may accompany osteomalacia. A reduction in bone ash content occurs in all cases of Ca deficiency or Ca-P imbalance.

Severe Ca deficiency may produce hypocalcemia, which causes tetany and convulsions. Milk fever, or parturient paresis, in dairy cows is a classic example of hypocalcemia and Ca tetany. The animal’s body temperature drops, it shows signs of tetany, and it eventually collapses with head bent over the flank. This is attributed to the lowered blood Ca levels. Treatment is aimed at increasing blood Ca through an intravenous supply of Ca salts such as CaCl2, Ca-lactate, or Ca-gluconate. A high-dose vitamin D injection should be given five days before calving to enhance Ca absorption.

Milk fever always happens in high-producing dairy cows within the first 24 hours after calving because of the high Ca demand of lactation coupled with hormonal insufficiency. Under normal conditions, bone Ca minerals are utilized to meet the high demand for milk Ca. However, mobilization of bone minerals is under hormonal control, especially by PTH. A good management practice is providing a low Ca diet at least 14 days before calving to “prime” or stimulate endocrine activity so that when lactation begins, Ca mobilization from bones increases due to increased PTH secretion. Electrolyte balance is also important to prevent milk fever and is discussed under electrolytes. Cows with milk fever usually recover rapidly following intravenous administration of Ca.

Similar to milk fever, cage layer fatigue often happens to high-producing young hens during the peak egg production phase (>35 week of age). Egg laying demands a high supply of Ca for eggshell formation. Lack of enough Ca leads to increase mobilization from bones leading to leg weakness. Affected birds may show reluctance to move, may move to a corner of the cage, or may produce deformed or soft-shelled eggs. The Ca requirements of egg laying hens are much higher than other animals, and the hens should be provided a minimum of 3.3 g of Ca/day for egg production. Use of Ca sources and larger particle size that enhances retention in the gut are highly recommended in hen diets. Hen osteoporosis is also one of the major welfare issues in older hens after the laying cycle; this leads to broken bones and leg weakness.

The ratio of Ca:P is important in bone growth and development. Excess P and low Ca is the common situation in animals fed grain-based diets and low-quality hay or in pets fed homemade meat-based diets. Developmental bone-related disorders occur in young horses fed high-energy diets and in large breeds of dogs fed extra Ca–supplemented diets. Diets that provide Ca:P ratio of 1:1 to 2:1 with slow bone growth is recommended. Similar cases related to Ca-P imbalance has been reported in large cats (tigers, cheetahs) kept in a zoo when fed meat-only diets compared with the meat and bone diets they consume in the wild. A low ratio of Ca-P leads to high level of P and low Ca in the blood. Such a situation causes PTH to increase its secretion, stimulating urinary P excretion and mobilization of Ca from bone. In chronic cases, prolonged dietary imbalance leads to nutritional secondary hyperparathyroidism. Affected animals have demineralized bones, loss of bone mass, joint pain, swelling, lameness, and reluctance to move. In many tropical areas of the world, soil is deficient in P, and animals grazing in such places often develop a depraved appetite and abnormal chewing and eating behaviors, which is termed pica. High fluoride interferes with P digestion and absorption. Pastures contaminated with fluorine gas from industrial sources will precipitate P deficiency in animals.

Magnesium

Magnesium is the third most abundant element in the body, is present in the body as phosphates, and carbonates in bone and in liver and skeletal muscle cells. In the skeletal system, Mg is involved providing structural roles, while in the cells, Mg is required to activate several enzymes that split and transfer phosphatases. As a cation in the intracellular fluid, Mg is involved in the metabolism of carbohydrates and proteins. Along with Ca, sodium, and potassium, Mg plays an important role in muscle contraction and transmission of nerve impulses.

Dietary Mg is absorbed mostly from the ileum. No carrier is needed for Mg absorption. Vitamin D does not affect Mg absorption. Homeostatic control of blood and tissue Mg is not well understood, and PTH increases the release of Mg from bone. Nutritional secondary hyperparathyroidism is associated with increases in urinary excretion and reduced serum Mg.

Deficiency: Magnesium is widespread in food sources. A common problem of grazing livestock is called grass tetany. It is also known as “wheat grass poisoning.” It occurs most frequently in livestock that feeds on lush green pastures of cereal forages or native pastures in the spring season. It most frequently occurs on wheat grass pastures. It has been linked to increased trans aconitate in the green pasture in the spring. Trans aconitate binds Mg and leads to Mg deficiency. The symptoms include muscle tetany, head retraction, staggering, convulsion, and extreme sensitivity to noise or touch. Both nitrogen and potassium inhibit Mg absorption. High levels of N and K are usually present in lush fertilized pastures; livestock grazed on fertilized pastures are more susceptible to grass tetany. Treatments include intravenous injection of an Mg solution, feeding Mg from different sources, pasture rotation, and providing dry forages.

Sulfur

Sulfur (S) serves as a structural component of skin, hair, wool, feather, cartilage, and connective tissue. Sulfur is required by the body mainly as a component of S-containing organic compounds. These include chondroitin sulfate; mucopolysaccharide, found in cartilages; the hormone insulin; and the anticoagulant heparin. Sulfur is also an integral part of three amino acids: methionine, cysteine, and cystine. The largest portion of S in the body is found within S-containing amino acids. A high-S-containing amino acid is generally recommended in the diets of birds during rapid feather growth as well as in the diets of sheep for wool growth. Sulfur is also found in enzymes such as glutathione peroxidase, which functions as an antioxidant. In addition, S is a component of two B vitamins, thiamin and biotin, involved in carbohydrate and lipid metabolism. As a component of coenzyme A, S is important in energy metabolism too.

Deficiency: Reduced feather and wool growth and weight gain can occur due to S deficiency. Inorganic S is very poorly absorbed from a diet. S requirement can be met with organic S found in S-containing amino acids. In sheep, S supplementation may help in microbial protein synthesis and weight gain when nonprotein nitrogen is included in the diet.

Toxicity: Because intestinal absorption is very low, S toxicity is not a practical problem.

Electrolytes

Sodium, Potassium, and Chlorine

Electrolytes are electrically charged, dissolved substances; the animal body is kept electrically neutral.  Acid-base balance is determined by the difference between total anion and cation intake and excretion. In this section, sodium (Na), potassium (K), and chlorine (Cl) are discussed together because these three minerals are electrolytes and help in creating an ionic balance and in keeping cells alive. The electrolytes play a vital role in maintaining the acid-base balance (pH maintenance in the blood and tissue), cell membrane signal transductions, and osmotic pressure in intra- and extracellular fluids.

Normal ratios among electrolytes are remarkably constant among species. The animal body has regulatory systems to control the concentrations of these minerals. However, they cannot be stored and need to be supplied in the diet daily. Common salt (NaCl) is added to the diets of all animals and is given free choice to grazing animals. Salt is also used as a vehicle to deliver other trace elements such as iodized salt or trace-mineralized salt. In pigs and poultry diets, the addition of 0.3% to 0.5% salt is standard practice.

Sodium (Na+) is the main extracellular cation found outside the cells (extracellular) and blood. Sodium functions in conjunction with other ions to maintain cell permeability in the active transport of nutrients across membranes. The sodium pump (Na-pump) controls electrolyte balance and is a major part of the basal metabolic rate in the body. Sodium is also required for muscle contraction and nerve impulse transmission. Sodium is included in animal diets as sodium chloride (NaCl)

Potassium (K) is the major cation found in greater concentrations within the cells (intracellular fluid). Ionized K within the cells provides osmotic force, which maintains fluid volume. Cellular potassium is also involved in several enzymatic reactions. Maintaining potassium balance is important for the normal functioning of the heart muscle.

Chloride is the negatively charged anion that counterbalance the role of positively charged cations (K and Na). Chlorine (Cl) accounts for two-thirds of anion present in extracellular fluid involved in regulating osmotic pressure. Chlorine is also necessary for the formation of hydrochloric acid, which is needed for the activation of gastric enzymes and initiation of protein digestion in the stomach. Chloride is supplied through NaCl in the animal diet.

Deficiency: Usually, these three elements are fairly abundant in normal diets and deficiency is rare.

Toxicity: The kidneys normally regulate the excretion of electrolytes. Therefore, toxicity is very rare; unless, it is due to renal diseases, restricted water intake, or high salinity in water. High Na intake although associated with hypertension in humans is not reported in animals.

Dietary Electrolyte Balance in Food Animal Health and Production

There is an increased interest in the balance of electrolytes in food animal production to maintain animal health, welfare, and productivity. Alterations in acid-base balance can lead to acidosis or alkalosis in animals affecting animal health and productivity. Under most circumstances, dietary electrolyte balance is expressed as Na+K-Cl (meq/kg). For poultry, the optimal balance is 250 meq/kg, and for pigs, it should be in the range of 100–200 meq/kg dry matter (DM). Dietary electrolyte imbalance has been associated with leg abnormalities, such as tibial dyschondroplasia (slipped tendon); reduced growth, affecting appetite; and poor poultry growth, productivity, and welfare.

In ruminant animals, electrolyte balance is important in preventing acidosis and alkalosis. Dietary cation-anion difference is usually adopted in dairy cattle feeding to reduce the incidence of milk fever. Prepartum alkalosis may increase the incidence of milk fever in dairy cattle, whereas acidosis may prevent it. Prepartum diets high in forages are also rich in K and could reduce the ability of the cow to maintain Ca homeostasis and could cause milk fever. Diets that reduce blood pH can cause blood Ca to increase and reduce the milk fever.