Blood calcium is tightly regulated through the absorption of dietary calcium and release or uptake of calcium from bone. Hypocalcemia prevention, added Amaral-Phillips, generally occurs though modifications to the prefresh or close-up diet. "These changes allow for the calcium mobilization system to be primed and ready for greater demand. Three precalving options include:

1. Low calcium diets: While this practice reduces hypocalcemia incidence, it is difficult to implement on-farm. For effectiveness, diets must provide less than 20 grams of available calcium. This strategy undesirably leads to low prefresh intakes as it often employs very low-quality forages. In some grazing situations, low calcium diets may be possible.


2. Low potassium forages/diets: Incorporating low potassium forages into prefresh diets may reduce the chance of clinical, but not subclinical, milk fever. Dietary cation-anion difference (DCAD) changes may not be large enough to cause metabolic acidosis and prevent a subclinical drop in blood calcium concentration when low potassium forages are fed without additional chlorine and sulfur modifications. DCAD influences blood pH and the cow's ability to reabsorb calcium from bone and absorb dietary calcium from the small intestine.


3. Feeding anionic salts for 21 days: Feeding a negative DCAD diet (-10 to -15 mEq/100 grams of dry matter) 21 days prior to calving has been shown to prevent clinical and subclinical milk fever. Before formulating, forage and feedstuff potassium and sodium levels should be kept as low as possible. Close-up diets should be formulated with 1.0 percent calcium and 0.35 percent magnesium to prevent hypocalcemia. Phosphorous concentrations should be between 0.25 and 0.3 percent as excess phosphorous elevates milk fever risk.

Urine pH is an indicator of whether DCAD management is effective, but does not indicate a reduction in milk fever risk. Urine should be sampled after cows are fed an anionic salt diet for at least 48 hours. Target a pH between 6.2 and 6.8 for Holsteins and 5.8 and 6.3 for Jerseys. If the average pH is between 5.0 and 5.5, excessive anions are being fed. Anionic salts are usually fed for 21 days prior to calving and are not recommended to be fed for the entire dry period.

---

The author is an associate editor and an animal science graduate of Cornell University. Smith covers feeding, milk quality and heads up the World Dairy Expo Supplement. She grew up on a Medina, N.Y., dairy, and interned at a 1,700-cow western New York dairy, a large New York calf and heifer farm, and studied in New Zealand for one semester.