Dietary Ca concentration to minimize the risk of hypocalcaemia in dairy cows is affected by the dietary cation–anion difference

The objective of this study was to determine whether dietary Ca concentration affects the ability to maintain Ca homeostasis in non-lactating non-pregnant dairy cows fed diets differing in dietary cation–anion difference (DCAD). Eight non-lactating non-pregnant multiparous Holstein cows (594 ± 80.3 kg body weight; 34.5 ± 11.4 month old) were fed diets Low or High in DCAD (−64 vs. 82 mequiv./kg dry matter, respectively) in combinations with Low or High dietary Ca concentration (3.0 vs. 9.1 g/kg of dry matter, respectively) in a duplicated 4 × 4 Latin square design with 14-d periods. On d 14 of each period, cows were subjected to an EDTA challenge that consists of an intra-jugular infusion of EDTA solution to decrease blood Ca concentration. In this protocol, the time required to recover to 90% of the pre-challenge blood Ca concentration was determined as recovery time. During the EDTA challenge, mean blood bicarbonate concentration was lower for cows fed Low-DCAD diets although mean blood pH was not affected by treatment, indicating that cows fed Low-DCAD diets had mild compensated metabolic acidosis. Feeding High-Ca diet shortened the recovery time (106 vs. 134 min; P=0.04) when DCAD was low, while Low-Ca diet shortened the recovery time (125 vs. 159 min; P=0.02) when DCAD was high. These results suggest that the optimum dietary Ca concentration to minimize the risk of hypocalcaemia in dairy cows is likely different depending on the DCAD value.