Calibrin-Z (CZ), a thermally processed calcium montmorillonite, has been shown to enhance dairy productivity due to its barrier function in the digestive tract. Two classic dairy farms from different locations in Yamagata Prefecture, Japan, were used in this on-farm feeding trial. The objective was to investigate the effects of CZ feeding on lactating cow performance under a common dairy feeding regimen in Japan. Concentrate feed was purchased from local feed companies, corn and grass silage was produced on-farm, and CZ was added to the Total Mixed Ration (TMR) with 20 g/cow/d for 3 mo. The TMR was formulated to meet Japanese nutrients requirements standard, and collected for mycotoxin analyses. Milk yield and quality and fecal firmness were recorded daily. Blood was drawn before and 30 d after CZ was fed, and albumin, BUN, GGT, hematocrit, and white blood cell counts were determined. Disease incidence and general health were recorded. Analysis of variance was used to determine the effects of CZ. Mycotoxin concentrations, such as aflatoxin, DON, fumonisin, and zearalenone were detectable in the TMR, but were below Japanese regulations. Manure firmness was numerically increased on one farm, but no changes were found on the other farm over the 3 mo feeding period. There were no differences in general blood chemistry; however, CD4+ and CD14+ were higher (P < 0.05) after CZ was fed for 30 d. In addition, CD3+ and CD335+ were numerically higher (P > 0.05) when CZ was included in the ration. Interestingly, a 47% (Farm A) or 40% (Farm B) reduction of disease incidences was observed as compared with the same period of the prior year. Among them, mastitis decreased 60% on Farm A, and enteritis decreased 100% on Farm B. Milk production was increased (P < 0.05) on both farms (Farm A, 25.0 vs. 26.4 kg; Farm B, 27.0 vs. 28.4 kg/cow/d) after CZ was fed; while milk fat, protein, lactose, bacteria count, urea nitrogen, and somatic cell count remained unchanged. In conclusion, addition of Calibrin-Z to Japanese non-medicated rations may enhance general health in lactating cows; consequently, increased milk production and a reduction of disease occurrence were observed on the studied farms.