The Dietary Cation Anion Difference (DCAD) evaluates the anion-cation status of a diet.

Calculating a diet's DCAD is very important in preventing milk fever (hypocalcemia) in dairy cattle. Milk fever occurs when the demand for calcium is greater than the supply in the blood. It can be offset if the cow can mobilize calcium stores in their bones.

Anions are negatively charged and promote a more acidic metabolic state, resulting in a lower blood pH. Cations are positively charged and promote a more alkaline metabolic state, resulting in a higher blood pH. Major anions are sulfur (S^-) and chloride (Cl^-) and major cations are potassium (K⁺) and sodium (Na⁺).

In order for a cow to pull positively charged calcium ions from the bone to the blood stream the environment needs to be negative. A negative environment in the blood stream is the result of a lower blood pH which can be achieved through the feeding of anions.

The cation-anion status of a diet can be evaluated using the DCAD equation. The cations and anions of a diet must be converted into milliequivalents (mEq). The mEq of anions is then subtracted from the mEq of cations and the resulting value is the DCAD. The equation is ([Na⁺]+[K⁺]) - ([Cl^-]+[S^-]), and is measured in mEq/kg. An alkaline diet will yield a positive value and an acidic diet will yield a negative value.