The production of high yielding dairy cows is often impacted by low rumen pH. The objective of this experiment was to evaluate different dietary buffers and their influence on dairy cow performance. The supplements included were; calcareous marine algae (Lithothamnion calcareum), with or without marine magnesium oxide (precipitated magnesia derived from seawater), and sodium bicarbonate. Fifty-two multiparous and 4 primiparous cows (62.7 ± 3.4 d in milk) were assigned to 4 experimental treatments based on parity, calving body condition score (3.1 ± 0.03), pre-experimental milk yield (34.7 ± 0.79) kg/d and previous 305 d milk yield (7073 ± 198). The diets were based on a forage: concentrate ratio of 46:54. Dietary treatments consisted of the control (283 g starch and sugar, and 230 g neutral detergent fiber (NDF) from forage per kg dry matter (DM) including no dietary buffer (CON); the control plus 3.5 g/kg DM calcareous marine algae (CMA); the control plus 3.5 g/kg DM calcareous marine algae and 0.9 g/kg DM marine magnesium oxide (CMA+MM); the control plus 7 g/kg DM sodium bicarbonate (SB). The experiment lasted for 80 d, including 7 d acclimatisation and 73 d of data collection. The CMA treatment increased the production of milk solids (fat and protein kg/d) compared with CON (+ 0.16 kg, P < 0.01), CMA+MM (+ 0.09 kg, P < 0.01) and SB (+ 0.10 kg, P < 0.01). Both CMA and CMA+MM increased milk fat yield (P < 0.05) compared with CON but were not different to each other or SB. All 3 buffer treatments increased milk fat concentration (P < 0.01) compared with the control but did not differ from each other. The SB treatment reduced milk protein concentration (P < 0.01) compared with all other treatments. The SB treatment reduced the efficiency of milk production, energy-corrected milk (ECM) per kg of DMI, (P < 0.01) compared with all other treatments. Results indicate that the addition of rumen buffering products can benefit milk solids production when included in lactating dairy cow diets. The use of calcareous marine algae when compared with sodium bicarbonate, in such diets, can increase milk production efficiency.