Changing dietary cation-anion difference for dairy cows fed with two contrasting levels of concentrate in diets

High-producing dairy cows are commonly fed diets containing a high proportion of rapidly degradable starch, which can cause subacute acidosis and reduce dry matter (DM) intake. Because of the properties of nonmetabolizable cations and anions, increasing the dietary cation-anion difference (DCAD = Na + K - Cl - S in mEq/kg of DM) may prevent a drop in DM intake. To test this hypothesis, 48 Holstein cows were blocked into 2 groups of 24 and assigned to two 3 x 3 Latin squares in a split-plot design. Each group received one level of concentrate at either 20% or 40% on a dry matter (DM) basis. The diet containing 20% concentrate was formulated to supply 4% rapidly degradable starch, whereas the diet containing 40% concentrate supplied 22% rapidly degradable starch. Diets in each square were formulated to provide a DCAD of 0, 150, or 300 mEq/kg of DM. The 3 values were obtained by manipulating Na and Cl contents. Intake, 4% fat-corrected milk yield, and milk fat percentage, as well as blood nonesterified fatty acids and beta-hydroxybutyrate increased with DCAD, but only on the diet providing 40% concentrate. The yield of trans-10 C(18:1) and odd-chain fatty acids decreased with increasing DCAD, whereas trans-11 C(18:1) increased. Again, this occurred only with the diet providing 40% concentrate. Blood pH and HCO(3) concentration increased along with DCAD, irrespective of the concentrate level. A positive DCAD led to increasing DM intake and fat-corrected milk yield in dairy cows fed highly degradable diets. The mechanism involved may be a localized rumen buffering effect, together with the ability of positive DCAD to maintain blood acid-base status in cows faced with a massive acid input.