Effects of reducing dietary [(Na + + K +) − (Cl − + SO 4=] on the rate of calcium mobilisation by dairy cows at parturition

The effects of feeding diets with different milliequivalents (meq) of dietary [(Na + + K +) − (Cl − SO 4 =)] to dairy cows during the last seven weeks of pregnancy on their acid-base status and calcium mobilisation rate around parturition were studied. Ten monozygotic twin pairs of pregnant cows (five pairs of parity 1 or 2, and five pairs of parity 3 or more) were allocated to two diets which were formulated to provide either - 4 meq (anion diet) or +572·5 meq (cation diet) of [(Na + + K+) − (Cl − + SO 4 =)] kg −1 dietary dry matter ( dm). The daily rations consisted of 4 kg grass hay and 7 kg concentrates. Changes in meq of dietary [(Na + + K +) − (Cl − + SO 4 =)] were achieved by adding KCl, K 2SO 4 and (NH 4) 2SO 4 (anion diet) or K 2CO 3 (cation diet) to basal concentrates. Plasma calcium concentration and blood acid-base parameters were not affected by dietary treatment. However, urinary calcium excretion was markedly higher and urinary pH and bicarbonate excretion significantly lower in cows fed the anion diet than in cows fed the cation diet. The responses to hypocalcaemia induced by an intravenous infusion of edta solution were similar in the cows fed either diet.