Onset of lactation results in a large increase in demand for calcium. Although mechanisms in the body exist for maintenance of calcium homeostasis, some degree of hypocalcemia is common around parturition.1-9-1 If plasma calcium becomes too low to support nerve and muscle function, the condition of parturient paresis or milk fever results. This disease is most common in high-producing dairy cows. It is associated with the rapid loss of calcium due to formation of colostrum. For example, a cow producing 10 L of colostrum loses about 23 g of calcium in a single milking. This is about nine times as much calcium as is present in the plasma pool. Replacement calcium must come from intestinal absorption or bone resorption. Calcium treatments, such as intravenous infusion of a solution of calcium borogluconate, are used immediately postcalving to keep the cow alive until the homeostatic system has time to adapt.
In response to hypocalcemia, the body produces parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3. The effect of PTH is to increase calcium reabsorption at the kidneys and also to increase resorption of bone. In addition, PTH increases the production of 1,25-(OH)2 D3 by the kidney. The effects of 1,25-(OH)2 D3 are to synergize with PTH to promote resorption of calcium from bone and to increase the active transport of calcium in the intestinal epithelium.
Jersey cows are more susceptible than Holstein cows, which may be due to fewer intestinal receptors for 1,25-(OH)2 D3. Incidence also increases with age, which may be due to both increased milk production in older cows and decreased effectiveness of homeostatic regulatory mechanisms.
The current focus of prevention is on the dietary cation anion difference in conjunction with knowledge of the homeostatic system for Ca. Feeding inorganic acids reduced the incidence of milk fever. The cation anion difference (CAD) of diets has an impact on acid base status. Addition of anionic salts to the diet reduces blood and urine pH and is associated with decreased incidence of milk fever. This is likely due to increased responsiveness of tissues to PTH during metabolic acidosis. Reducing cations in the diet can also have the same effect. Most emphasis has been placed on decreasing dietary potassium.
Oral administration of calcium salts enhances the passive absorption of calcium from the gastrointestinal tract. A CaCl2 paste given at calving and shortly thereafter reduced the incidence of milk fever. In addition to increasing calcium absorption, it also reduces blood pH. An alternative is calcium propionate, which does not have an acidifying effect.
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