The objective of the experiment was to compare the effect of dietary inclusion of a prebiotic fermentation product of Lactobacillus acidophilus (LaP, RumaCell; 5 mL•animal -1•d -1) or monensin on performance of replacement beef heifers. Heifers received a total mixed ration containing either LaP (n =77) or monensin (MON; Rumensin; 200 mg•animal -1•d -1; n = 79). Heifers were fed for 71 days in a GrowSafe unit so individual feed intake could be measured. Heifers were weighed every two weeks and feed efficiency calculated by residual feed intake (RFI). At the end of the RFI trial, heifers remained on their diets for an additional 27 days and were estrus synchronized using the 14-d CIDR + PG protocol and bred by artificial insemination (AI) followed by natural service. Prior to estrous synchronization reproductive tract scores (RTS; 1 = infantile to 5 = cycling/presence of corpus luteum) were measured. Continuous variables were analyzed using generalized mixed models whereas categorical data were analyzed by logistic regression. Body weights, average daily gain, feed intake and RFI value were similar (P > 0.30) among MON and LaP supplemented heifers. Across treatments heifers gained 0.9 ± 0.1 kg/d while consuming 9.3 ± 0.5 kg of diets daily. Reproductive development as indicated by RTS was similar (P > 0.28) between treatments. However, estrus response increased (P < 0.01) and AI pregnancy rates tended to be greater (P < 0.07) for MON compared to LaP heifers. In contrast, the percentage of heifers pregnant by 60 d and 100 d (80.4% and 90.5%, respectively) were not different (P > 0.33) for MON and LaP heifers. In conclusion, addition of LaP to replacement heifer diets may result in growth and reproductive performance similar to an ionophore, if dietary energy is adequate for normal heifer growth.