What specific nutritional strategy, involving dietary anion-cation difference (DCAD), is implemented in the close-up dry period to reduce the incidence of subclinical hypocalcemia?

The specific nutritional strategy implemented in the close-up dry period to reduce the incidence of subclinical hypocalcemia is the feeding of a negative Dietary Anion-Cation Difference (DCAD) diet. Subclinical hypocalcemia is a condition characterized by lower-than-normal blood calcium levels without overt clinical signs of milk fever, often occurring around calving when calcium demand dramatically increases for colostrum and milk production. The DCAD represents the balance of positively charged minerals (cations) and negatively charged minerals (anions) in the diet, typically calculated as (Sodium + Potassium) - (Chlorine + Sulfur), with all minerals expressed in milliequivalents per kilogram of dry matter. The close-up dry period typically refers to the final two to three weeks before expected calving. The goal of a negative DCAD diet is to induce a mild, compensated metabolic acidosis in the cow. Metabolic acidosis refers to a slight decrease in the pH of the cow's blood, which is a key physiological trigger. This mild acidosis, achieved by increasing the intake of anionic minerals like chlorine and sulfur and/or decreasing the intake of cationic minerals like potassium and sodium, enhances the cow's ability to mobilize calcium from her bones and increase calcium reabsorption from the kidneys. Specifically, the acidosis increases the sensitivity of target tissues, such as bone and kidney cells, to parathyroid hormone (PTH). PTH is a crucial hormone that regulates blood calcium levels by stimulating calcium release from bones and increasing calcium reabsorption in the kidneys. By making the cow more responsive to PTH before calving, her body is primed to efficiently draw calcium from her skeletal reserves and conserve it through kidney function when the massive calcium demand for lactation begins. Common anionic salts used to achieve this negative DCAD include calcium chloride, magnesium sulfate, ammonium chloride, and calcium sulfate. The target DCAD value for close-up dry cows typically ranges from -100 to -250 mEq/kg of dry matter. This strategic dietary intervention effectively prepares the cow's calcium homeostatic mechanisms, allowing her to better maintain blood calcium levels at or after calving, thereby significantly reducing the risk of subclinical hypocalcemia.