defluorinated phosphate, and bone meal (Peeler, 1972). More recent studies with calves have indicated that defluorinated phosphate (Miller et al., 1987) and monoammonium phosphate (Jackson et al., 1988) are equal in availability to dicalcium phosphate. Phytate phosphorus is not well utilized by nonruminants, but seems to be utilized by ruminants as readily as phosphorus from inorganic sources (McGillivray, 1974).

Potassium is the third most abundant mineral in the body and the major cation in intracellular fluid. Potassium is important in acid-base balance, regulation of osmotic pressure, water balance, muscle contractions, nerve impulse transmission, and certain enzymatic reactions.

Feedlot cattle require approximately 0.6 percent potassium. Studies conducted with potassium in cattle receiving no ionophore have been inconsistent. Roberts and St. Omer (1965) observed a response in gain with potassium supplementation of steer diets containing 0.50 to 0.56 percent potassium in only one of three trials. Devlin et al. (1969) noted improvements in steers’ gain and feed intake when potassium was added to diets already containing 0.5 percent potassium. More recently, however, Kelley and Preston (1984) observed no improvement in steer performance when potassium was supplemented to a basal diet containing 0.4 percent potassium. Studies with feedlot cattle fed lasalocid (Ferrell et al., 1983; Spears and Harvey, 1987) or monensin (Brink et al., 1984) indicate that potassium requirement does not exceed 0.55 percent. Potassium requirements in young dairy calves not fed an ionophore also do not exceed 0.55 percent (Weil et al., 1988; Tucker et al., 1991). Because of the lower rates of gain observed in growing cattle in range conditions, potassium requirements for range cattle may be lower than those for feedlot cattle. Clanton (1980) concluded that growing cattle in range conditions require 0.3 to 0.4 percent potassium.

Potassium requirements of beef cows are not well defined. Clanton (1980) suggested that gestating beef cows require 0.5 to 0.7 percent potassium. Because of the relatively high secretion of potassium in milk (1.5 g/kg), requirements for potassium may be slightly higher in beef cows during lactation—for example, for cows producing 9 kg milk/day, approximately 13.5 g K/day or 0.13 percent of dry matter intake would be needed for milk production.

A deficiency of potassium results in reduced feed intake and weight gain, pica, rough hair coat, and muscular weakness (Devlin et al., 1969). In beef cattle, a severe deficiency of potassium is unlikely. A marginal potassium deficiency results in decreased feed intake and retarded weight gain. Dietary potassium concentration is the best indicator of potassium status. Serum or plasma potassium is not a reliable indicator of potassium status. Reduced feed consumption appears to be an early indicator of marginal potassium deficiency, but the depression in feed intake is usually of relatively small magnitude, making it difficult to detect in field conditions.

Potassium is absorbed from the rumen and omasum as well as the intestine, and absorption is very high. The major route of potassium excretion is the urine. Body stores of potassium are small; therefore, a deficiency can occur rapidly (Ward, 1966).

Forages are excellent sources of potassium, usually containing between 1 and 4 percent potassium. In fact, high potassium content in lush spring pastures seems to be a major factor associated with the occurrence of grass tetany in beef cows (Mayland, 1988).

As forages mature, the potassium content decreases, and low concentrations of potassium have been observed in range forage and in accumulated tall fescue during the winter (Clanton, 1980). Cereal grains are often deficient (<0.5 percent) in potassium, and high-concentrate diets may require potassium supplementation unless a high-potassium forage or protein supplement is included in the diet. Oilseed meals are good sources of potassium. Potassium can be supplemented to cattle diets as potassium chloride, potassium bicarbonate, potassium sulfate, or potassium carbonate. All forms are readily available.

Increasing the potassium content of a liquid diet from 1.2 to 5.8 percent on a dry matter basis resulted in the deaths of 3 of 8 calves as a result of cardiac insufficiency (Blaxter et al., 1960). In calves, increasing dietary potassium from 2.77 to 6.77 percent reduced feed intake and retarded weight gain (Neathery et al., 1980). The maximum tolerable concentration of potassium has been set at 3 percent for cattle (National Research Council, 1980). Cattle grazing lush, spring pastures often consume more than 3 percent potassium, and other than reduced absorption of magnesium, no adverse effects have been reported.

Sodium is the major cation, while chlorine is the major anion, in extracellular fluid. Both sodium and chlorine are involved in maintaining osmotic pressure, controlling water