The objective was to determine the optimal dietary cation-anion difference (DCAD) required to maximize 3.5% fat-corrected milk (FCM) and feed efficiency (FE; FCM per dry matter intake (DMI)) in lactating dairy cows. When potassium carbonate was added resulting in four dietary treatments: 250, 300, 350, and 400 meq/kg DCAD, increasing DCAD linearly increased FCM and FE suggesting an optimal DCAD of at least 400 meq/kg. In a subsequent study comparing the relative effectiveness of potassium versus sodium, cation source had no effect on DMI or FCM. However, milk fat percentage and FE were highest when sodium was used as the sole cation source. Finally, surface response equations developed from literature data showed that DMI, FCM, milk fat %, rumen pH, and fiber digestibility increased linearly with DCAD. This suggested that improved FE was a function of improved rumen function, energy availability, and partitioning of absorbed energy toward milk energy.