DCAD calculations from Dairyland Laboratories, Inc. are based on work presented by Shire and Beede at the 2013 Southwest Dairy Nutrition and Management Conference. Shire and Beede concluded: “Based on all of the available information the four-element equation seems most acceptable.”
Per Shire and Beede:
DCAD = (Na+ + K+ ) - (Cl-+ S2-)
Target range for close up groups is -10mEq/100g
+30 mEg/100g appears adequate for early to mid-lactation cows
Many other DCAD calculations are available which include as few as three or as many as seven elements: Na+ , K+ , Mg+ , Ca2+ , Cl-, S2-, P3-.
For a more thorough discussion on their conclusions please see “DCAD Revisited, SWNMC 2013” and "Influence of DCAD on Lactation Performance PSDCNW 2013."