of calcium, phosphorus, or vitamin D. It is characterized by improper calcification of the organic matrix of bone, which results in weak, soft bones that may be easily fractured. Signs include swollen, tender joints, enlargement of the ends of bones, an arched back, stiffness of the legs, and development of beads on the ribs.
Osteomalacia is the result of demineralization of the bones of adult animals. Because calcium and phosphorus in bone are in a dynamic state, high demands on calcium and phosphorus stores, such as occur during pregnancy and lactation, may result in osteomalacia. This condition is characterized by weak, brittle bones that may break when stressed.
Blood calcium concentration is not a good indicator of calcium status because plasma calcium is maintained at between 9 and 11 mg/dL by homeostatic mechanisms. Parathyroid hormone is released in response to a lowering of plasma calcium. It stimulates the production of 1,25-dihydroxy cholecalciferol (vitamin D3). The 1,25-dihydroxy cholecalciferol increases calcium absorption from the intestine and, in conjunction with parathyroid hormone, increases calcium resorption from bone. If plasma calcium concentrations become elevated, calcitonin is produced and parathyroid hormone production is inhibited. Thus, calcium absorption and bone resorption are decreased.
The calcium content in forage is affected by species, portion of plant consumed, maturity, quantity of exchangeable calcium in the soil, and climate (Minson, 1990). Forages are generally good sources of calcium, and legumes are higher in calcium content than grasses. Cereal grains are low in calcium, so high-grain diets require supplementation. Oilseed meals are much higher in calcium than grains. Sources of supplemental calcium include calcium carbonate, ground limestone, bone meal, dicalcium phosphate, defluorinated phosphate, monocalcium phosphate, and calcium sulfate. True absorption in young steers of calcium from different sources ranged from 45 percent for ground limestone to 64 percent for dibasic calcium phosphate (Hansard et al., 1957).
High concentrations of dietary calcium are tolerated well by cattle. Protein and energy digestibility were reduced when cattle were fed a diet containing 4.4 percent calcium (calcium carbonate) (Ammerman et al., 1963). High concentrations of dietary calcium may affect metabolism of phosphorus, magnesium, and certain trace elements, but the changes are relatively small (National Research Council, 1980; Alfaro et al., 1988).
More than 300 enzymes are known to be activated by magnesium (Wacker, 1980). Magnesium is essential, as the complex Mg-ATP, for all biosynthetic processes including glycolysis, energy-dependent membrane transport, formation of cyclic-AMP, and transmission of the genetic code. Magnesium also is involved in the maintenance of electrical potentials across nerve and muscle membranes and for nerve impulse transmission. Of the total percentage of magnesium in the body, 65 to 70 percent is in bone, 15 percent in muscle, 15 percent in other soft tissues, and 1 percent in extracellular fluid (Mayland, 1988).
Dietary requirements for magnesium vary depending on age, physiological state, and bioavailability from the diet. As a percentage of dry matter, recommended magnesium requirements are as follows:
growing and finishing cattle, 0.10 percent;
gestating cows, 0.12 percent; and
lactating cows, 0.20 percent.
Absolute requirements for magnesium have been estimated as follows:
replenishment of endogenous loss, 3 mg Mg/kg liveweight;
growth, 0.45 g Mg/kg gain;
lactation, 0.12 g Mg/kg milk; and
pregnancy, 0.12, 0.21, and 0.33 g Mg/day for early, mid, and late pregnancy, respectively (Grace, 1983).
O’Kelly and Fontenot (1969, 1973) found that beef cows required 7 to 9 g Mg/day during gestation and 18 to 21 g Mg/day during lactation to maintain serum magnesium concentrations of 2.0 mg/dL. These daily quantities corresponded to 0.10 to 0.13 percent during gestation and 0.17 to 0.20 percent during lactation. In young calves fed milk, 12 to 16 mg Mg/kg body weight was adequate to maintain blood magnesium concentrations (Huffman et al., 1941; Blaxter and McGill, 1956).
Magnesium deficiency in calves results in excitability, anorexia, hyperemia, convulsions, frothing at the mouth, profuse salivation, and calcification of soft tissue (Moore et al., 1938; Blaxter et al., 1954). Grass tetany or hypomagnesemic tetany is characterized by low magnesium concentrations in plasma and cerebrospinal fluid and is a problem in lactating beef cows. Initial signs of grass tetany are nervousness, reduced feed intake, and muscular twitching around the face and ears. Animals are uncoordinated and