Potassium carbonate as a cation source for early-lactation dairy cows fed high-concentrate diets

Previous studies reported that addition of K₂CO₃ to high-concentrate diets improved milk fat synthesis, although the mechanism is yet to be established. The objective of the current experiment was to investigate the effects of dietary cation-anion difference (DCAD), cation source, and buffering ability of the mineral supplement on rumen biohydrogenation of fatty acids and production performance in dairy cows fed a high-concentrate diet. Thirty-five early-lactation Holstein cows (25 multiparous ruminally fistulated and 10 primiparous nonfistulated) were used in a randomized complete block design (7 blocks) with 33-d periods, including a 5-d pre-treatment collection period used as a covariate. Diets were (1) control, a basal diet [47% nonfibrous carbohydrates, DCAD (Na + K - Cl - S) = 65 mEq/kg of dry matter (DM)] containing 40% forage (including 60% corn silage) and 60% concentrate, (2) K₂CO₃ (control + K₂CO₃, 1.8% of DM, DCAD = 326 mEq/kg of DM), (3) KHCO₃ (control + KHCO₃, 2.6% of DM, DCAD = 324 mEq/kg of DM), (4) KCl (control + KCl, 2.0% of DM, DCAD = 64 mEq/kg of DM), and (5) Na₂CO₃ (control + Na₂CO₃, 1.4% of DM, DCAD = 322 mEq/kg of DM). Pre-planned orthogonal contrasts were used to assess the effects of K₂CO₃ (control vs. K₂CO₃), buffering ability (K₂CO₃ vs. KHCO₃), DCAD (K₂CO₃ vs. KCl), and cation type (K₂CO₃ vs. Na₂CO₃). Supplementing K₂CO₃ in a high-concentrate diet did not improve milk fat yield or 4% fat-corrected milk yield. Milk fat concentration was greater in cows fed K₂CO₃ compared with control (4.03 vs. 3.26%). Milk yield tended to decrease (34.5 vs. 38.8 kg/d) and lactose yield decreased in cows fed K₂CO₃ as compared with KCl (1.64 vs. 1.87 kg/d). Milk fat concentration of trans-10 18:1 was increased when cows were fed Na₂CO₃ as compared with K₂CO₃. A positive relationship was observed between concentrations of anteiso 15:0 and trans-10,cis-12 18:2 in milk fat from cows receiving K₂CO₃. Milk Na concentration was increased, whereas milk Cl was decreased with K₂CO₃ as compared with KHCO₃ or KCl. A positive relationship was established between milk Cl concentration and milk yield (R² = 0.34) across all dietary treatments. Cation-anion difference (Na + K - Cl - S) in ruminal fluid was increased with K₂CO₃ as compared with control or KCl. Blood pH tended to decrease in cows fed KCl compared with K₂CO₃. Our results suggest that mineral supplementation tends to affect milk and milk fat synthesis and that factors other than DCAD, potassium ion, or buffer ability may be implicated. The variations observed in mineral composition of milk suggest an allostatic process to maintain an ionic equilibrium in mammary epithelial cells in response to mineral composition of the diet.