TABLE 5–2 Maximum Tolerable Concentrations of Mineral Elements Toxic to Cattle

Element

mg/kg

Aluminum

1,000.00

Arsenic

50.00 (100.00 for organic forms)

Bromine

200.00

Cadmium

00.5

Fluorine

40.00 to 100.00

Lead

30.00

Mercury

2.00

Strontium

2,000.00

Source: Adapted from Table 1 in National Research Council. 1980. Mineral Tolerance of Domestic Animals. Washington, D.C.: National Academy of Sciences.

a limited period, will not impair animal performance and should not produce unsafe residues in human food derived from the animal” (National Research Council, 1980: p. 3).

MACROMINERALS

Calcium

Calcium is the most abundant mineral in the body; approximately 98 percent functions as a structural component of bones and teeth. The remaining 2 percent is distributed in extracellular fluids and soft tissues, and is involved in such vital functions as blood clotting, membrane permeability, muscle contraction, transmission of nerve impulses, cardiac regulation, secretion of certain hormones, and activation and stabilization of certain enzymes.

CALCIUM REQUIREMENTS

Estimated requirements for calcium were calculated by adding the available calcium needed for maintenance, growth, pregnancy, and lactation and correcting for the percentage of dietary calcium absorbed. Calcium requirements are similar to those in the previous edition of this volume (National Research Council, 1984) because new information is not sufficient to justify a change. The maintenance requirement was calculated as 15.4 mg Ca/kg body weight (Hansard et al., 1954, 1957). Retained needs in excess of maintenance requirements were calculated as 7.1 g Ca/100 g protein gain. Calcium content of gain was calculated from slaughter data (Ellenberger et al., 1950). The calcium requirement for lactation in excess of maintenance needs was calculated as 1.23 g Ca/kg milk produced. Fetal calcium content was assumed to be 13.7 g Ca/kg fetal weight. This requirement was distributed over the last 3 months of pregnancy.

Absolute calcium requirements were converted to dietary calcium requirements assuming a true absorption for dietary calcium of 50 percent. Lower absorption values have been obtained in older cattle, but in many instances calcium intake may have exceeded dietary requirements in these animals (Hansard et al., 1954, 1957; Martz et al., 1990). Absorption of calcium is largely determined by requirement relative to intake. True calcium absorption is reduced when intake exceeds the animal’s need. The Agricultural and Food Research Council (AFRC) recently used a value of 68 percent absorption to calculate calcium requirements of cattle (TCORN, 1991).

FACTORS AFFECTING CALCIUM REQUIREMENTS

Calcium is absorbed primarily from the duodenum and jejunum by both active transport and passive diffusion (McDowell, 1992). It should be noted that diets high in fat may decrease calcium absorption through the formation of soaps (Oltjen, 1975). Vitamin D is required for active absorption of calcium (DeLuca, 1979). The amount of calcium absorbed is affected by the chemical form and source of the calcium, the interrelationships with other nutrients, and the animal’s requirement. Requirement is influenced by such factors as age, weight, and type and stage of production. In natural feedstuffs, calcium occurs in oxalate or phytate form. In alfalfa hay, 20 to 33 percent was present as insoluble calcium oxalate and apparently unavailable to the animal (Ward et al., 1979). True absorption of alfalfa calcium was much lower than absorption of corn silage calcium when fed to dairy cows (Martz et al., 1990). In cattle fed high-concentrate diets, dietary calcium in excess of requirements improved gain or feed efficiency in some studies (Huntington, 1983; Brink et al., 1984; Bock et al., 1991). Improvements in performance were likely the result of manipulation of digestive tract function and may not represent a specific calcium requirement. Increasing calcium from 0.25 to 0.40 or 1.11 percent reduced organic matter and starch digestion in the rumen but increased postruminal digestion of organic matter and starch (Goetsch and Owens, 1985). In finishing cattle fed a high-concentrate diet, increasing calcium more than 0.3 percent increased gain in one of two trials but did not affect calcium status based on bone calcium, bone ash, and plasma ionizable calcium concentrations (Huntington, 1983).

SIGNS OF CALCIUM DEFICIENCY

The skeleton stores a large reserve of calcium that can be utilized to maintain critical blood calcium concentrations. Depending on their age, cattle can be fed calcium-deficient diets for extended periods without developing deficiency signs if previous calcium intake was adequate. Calcium deficiency in young animals, however, prevents normal bone growth, thus causing rickets and retarding growth and development. Rickets can be caused by a deficiency



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