Diminished hypercalcemic response to parathyroid extract in prepartum cows

Adapting to the transition between gestation and lactation: differences between rat, human and dairy cow

Adequate blood calcium concentrations are vital for the normal function of mammals. Mechanisms for maintaining normal blood calcium function adequately most of the time; however, occasionally they fail and calcium homeostasis is compromised. Milk fever or periparturient hypocalcemia in dairy cattle is a well-documented example of a breakdown in the mechanisms of calcium homeostasis. This disease occurs at the time of parturition and is unique to adult dairy animals. The disease results from the inability of animals to cope with the sudden demand for calcium in support of colostrum formation. Animals developing the disease become hypocalcemic and require intravenous calcium to survive. The precise metabolic disorder(s) responsible for the onset of milk fever is still being debated. This report will highlight some of the current concepts related to the causes and prevention of milk fever in dairy cattle, as well as contrasting differences in calcium demands that exist between dairy cattle, humans and rats at the onset of lactation.

Strategies for preventing milk fever in dairy cattle

Milk fever is a complex metabolic disorder that occurs at the onset of lactation. Clinical symptoms of this disease include inappetence, tetany, inhibition of urination and defecation, lateral recumbency, and eventual coma and death if left untreated. The hallmark of this disease is severe hypocalcemia, which probably accounts for most of the clinical signs associated with a milk fever episode. Several factors have been consistently associated with increased incidence of milk fever, including parturition and initiation of lactation, advancing age, breed, and diet. Of the various methods used in attempts to control the disease, the most progress has been made in dietary management. Until recently, most attention has focused on manipulating the levels of dietary calcium to control milk fever incidence; results, however, have been inconsistent, except for those diets containing very low (8 to 10 g/d) concentrations of Ca. During the past decade, there has been renewed interest and research in the use of dietary anions (Cl- and SO4(2-) in controlling milk fever. An outgrowth of this research has been the surprising realization that dietary K is significant (perhaps more significant than Ca) in determining the susceptibility of dairy cows to milk fever. This knowledge has expanded the understanding of the pathogenesis of milk fever and has focused attention on research designed to study methods for neutralizing the detrimental effects of dietary K excess on periparturient animal health. This report discusses various practical strategies and potential research areas for managing the dietary forage components to minimize the effects of K on milk fever incidence.

Effects of the addition of potassium or sodium, but not calcium, to prepartum ratios on milk fever in dairy cows

The effects of prepartum dietary concentrations of K, Na, and Ca on the incidence of periparturient hypocalcemia or milk fever was determined for older (> or = 4th lactation) Jersey cows. Cows were fed one of six diets differing in K and Ca contents. In addition, the effect of dietary Na (tested only at the high concentration of dietary Ca, and low concentration of dietary K) was examined. Treatments were arranged in an incomplete 2 x 4 factorial design; dietary Ca (0.5 or 1.5%) and dietary strong cations (1.1, 2.1, and 3.1% K or 1.3% Na) were the main effects. Dietary Ca did not significantly affect the incidence of milk fever of the degree of hypocalcemia experienced by the cows. Milk fever occurred in 2 of 20 cows that were fed the prepartum diet containing 1.1% K and 0.12% Na. Increasing dietary K to 2.1 or 3.1% increased the incidence of milk fever to 10 of 20 cows and 11 of 23 cows, respectively. Increasing dietary Na to 1.3% in the diet containing 1.5% Ca induced milk fever in 5 of 8 cows. Addition of strong cations to the prepartum diet increased blood and urine pH and reduced plasma hydroxyproline concentrations, suggesting that bone resorption of Ca is inhibited in cows fed high K or high Na diets as a result of metabolic alkalosis. These data demonstrated that dietary Ca concentration is not a major risk factor for milk fever and that dietary strong cations, especially K, induce metabolic alkalosis in the prepartum dairy cow, which reduces the ability of the cow to maintain Ca homeostasis.

Effect of synthetic bovine parathyroid hormone in dairy cows: prevention of hypocalcemic parturient paresis

Intravenous infusion of synthetic bovine parathyroid hormone for 96 h increased 1,25-dihydroxyvitamin D, Mg, Ca, and hydroxyproline in plasma of pregnant cows within 16, 24, 48, and 72 h, respectively. Plasma Ca concentration was maximal at the end of the 96-h infusion (15.1 mg/100 ml). Plasma concentration of 1,25-dihydroxyvitamin D at 96 h was twice that before parathyroid hormone infusion, indicating that parathyroid hormone stimulated 1,25-dihydroxyvitamin D production in the presence of hypercalcemia. The urinary excretion of hydroxyproline indicated that at least 48 h of parathyroid hormone stimulation was required to stimulate bone resorption in the pregnant cow. Eight periparturient cows were on a high Ca diet prepartum. Four cows were treated with intravenous parathyroid hormone prior to parturition. Four cows were untreated. All four untreated cows developed parturient paresis. None of the cows treated with parathyroid hormone developed parturient paresis. However, two cows that received parathyroid hormone for less than 24 h prior to parturition became hypocalcemic, but not recumbent. Plasma Ca concentrations remained within normal limits in the two other cows that received greater than 60 h parathyroid hormone infusion prior to parturition. We conclude that exogenous parathyroid hormone (1-34) can prevent parturient paresis if administered at least 60 h prior to parturition.