The objective of this study was to evaluate the efficacy of layer manure ash (LMA) byproduct in lactating dairy cow diets as a replacement for buffering capacity of sodium bicarbonate. Layer manure ash is derived from egg-layer manure through a controlled thermochemical reaction. Rumen pH, feed intake, milk yield and composition were measured in 16 ruminally cannulated Holstein cows averaging 183 (SD = 64) DIM. A replicated 4 × 4 Latin square design with 2-wk adaption periods and 1-wk collection periods was used to evaluate cows fed a common diet with the following treatments: 1) 0.86% calcium carbonate (Negative Control), 2) 0.86% sodium bicarbonate + 0.86% calcium carbonate (Positive Control), 3) 0.5% LMA + 0.43% calcium carbonate (0.5% LMA), and 4) 1.0% LMA (1.0% LMA). Starch and neutral detergent fiber content of the base diet were 24 and 30% of DM. Data were analyzed using MIXED procedure in SAS with cow as experimental unit. Daily rumen pH was not different across all treatment diets (6.01 ± 0.04, P > 0.10), but the negative control diet had the lowest (P < 0.05) minimum rumen pH over a 24-h period with greatest variability (5.96 ± 0.28 SD). Area under the curve (AUC) < 5.8 as a measure for sub-acute rumen acidosis tended to be more severe (1.32 (0.70 to 2.51 95% CI), pH units x h/d, P < 0.08) for cows fed the negative control diet and had the most (P < 0.001) amount of time spent under pH 5.8 compared with the other treatments. The cows receiving 1.0% LMA in their diet consumed less (P < 0.001) DM (26.1 vs 27.6 and 27.4, kg/d) than cows fed the positive control and 0.5% LMA diets. Yields of solids-corrected milk and fat did not differ (39.43 ± 2.3 and 1.55 ± 0.10, kg/d, P > 0.10) among treatments. Milk fat % tended (P < 0.10) to be higher when cows received the positive control diet compared with the 0.5% LMA diet (3.90 vs 3.76 ± 0.13). These results reveal that the 0.5% LMA diet does not differ from positive control diet containing sodium bicarbonate in rumen buffering ability, and may be a suitable replacement for sodium bicarbonate in rations for high producing dairy cows.