This study investigated the effects of a Bacillus-based direct-fed microbial (DFM; Strateris ECL, United Animal Health Inc., Sheridan, IN) supplementation on production, health, and reproduction of lactating dairy cows. The DFM was fed at 15 g/hd/d (7.35 × 10⁹ cfu/cow/d) to a total of 18,724 dairy cows in 5 different farms located across major dairy-producing regions of the US (WI, OH, MN, MI and ID). A longitudinal design was used with monthly milk production, components, health, and reproductive metrics monitored via Dairy Comp 305 or PCDART. Reproductive metrics, when available, were evaluated during the same monthly time period over consecutive years. A meta-analysis was then performed on the entire data set comparing 3 mo pre- and post-DFM supplementation. Production, components and reproduction data were analyzed by the PROC MIXED of SAS, with fixed effects of treatment*farm and treatment. Health metrics were evaluated by the PROC GLIMMIX of SAS with the interaction of treatment*farm and treatment as fixed effects. DFM supplementation reduced (P < 0.05) the incidence of ketosis (18 ± 1.3% vs. 12.5 ± 1.4%), retained placentas (13.5 ± 1% vs. 9.7 ± 0.9%) and decreased (P = 0.0342) somatic cell counts (SCC, 438 ± 10.1 vs. 404 ± 10.7*1000 cells/mL). DFM increased (P < 0.05) both milk fat (4.05 ± 0.03 vs. 4.30 ± 0.03) and protein (3.2 ± 0.01 vs. 3.3 ± 0.01) percentages, resulting in 1.1 kg higher (P = 0.0387) energy corrected milk (ECM, 35.3 ± 0.4 vs. 36.4 ± 0.4 kg/hd/d). DFM supplementation resulted in increased (P < 0.05) overall heat detection (52.6% vs. 57.3%), conception rate (36% vs. 43.8%) and 21-d pregnancy rate (17.9% vs. 23.9%) and reduced (P < 0.05) services per conception (2.8 vs. 2.3) and abortions (4.1 ± 0.3% vs. 3.1 ± 0.2%). Feeding a Bacillus-based DFM to dairy cattle for 3 mo improved transition cow health, reduced SCC, increased milk fat and protein, ECM, and enhanced reproductive performance. The impact on health and production are probably the secondary and tertiary effects of improved gastrointestinal health.