This thesis is an investigation of the effect of a prepartum negative dietary cation-anion difference (DCAD) diet on postpartum health, immune function, production, and reproductive performance, as well as culling risk in dairy herds. The main study was a randomized clinical trial of a negative DCAD diet, including more than 1000 cows from 4 commercial farms. Close-up pens with cows 3 wk before calving were randomly assigned to a negative DCAD (TRT; -108 mEq/kg dry matter [DM]) or a control diet (+105 mEq/kg DM with a placebo supplement). The outcomes include blood Ca concentrations, neutrophil function, postpartum disease, milk production, reproductive performance, and culling. Additionally, the diagnostic accuracy of a point-of-care blood calcium meter was evaluated. We observed a significant reduction in milk fever incidence in over-conditioned multiparous cows fed TRT. In multiparous cows, the prevalence of subclinical hypocalcemia (tCa ≤ 2.14 mmol/L) in the first 2 days in milk (DIM) was lower when fed TRT, with no differences in primiparous cows. Multiparous cows fed TRT were less likely to have more than one disease in the postpartum period, which might have contributed to the tendency of reduced risk of leaving the herd by 30 DIM, and a reduced culling risk over the whole lactation. There was a significant reduction in displaced abomasum and a tendency for lower odds of clinical mastitis within 30 DIM in TRT. Contrary to our hypothesis, peripartum neutrophil killing activity was not enhanced in healthy multiparous cows by feeding TRT. Under commercial conditions, the negative DCAD diet resulted in increased milk yield in early lactation, better reproductive performance, and reduced culling rate in multiparous cows. In primiparous cows, there were no treatment effects on milk production or culling, but treatment was detrimental for pregnancy rate. The meter assessment revealed that when used with fresh plasma, which requires additional sample preparation, the device showed some potential to classify cows as hypocalcemic, but overall its diagnostic accuracy was fair to low. Our results suggest that a negative DCAD diet in the close-up dry period should be targeted to multiparous cows, where we observed health and performance benefits.