Bulk tank milk (BTM) provides a practical, non-invasive matrix for repeated herd-level surveillance in dairy farms. This study evaluated a four-season BTM framework for bovine brucellosis, influenza A virus, and Q fever in Korean dairy farms. In 2025, 602 BTM samples were collected from 167 farms in spring and summer and 134 farms in autumn and winter. Seasonal comparisons were interpreted cautiously because farm composition changed after summer. Bovine brucellosis was tested by milk ring test, influenza A virus by M-gene real-time RT-PCR, and Q fever antibodies by ELISA. An exploratory avian influenza virus (AIV) antibody ELISA was also applied to whey. Coxiella burnetii PCR testing was restricted to nine participating farms in Daegu Metropolitan City, and one BTM real-time PCR-positive farm was further investigated using sera from 105 lactating cows. All BTM samples were negative for bovine brucellosis, no influenza A virus M-gene RNA was detected, and no samples exceeded the positivity threshold in the exploratory AIV antibody ELISA. Q fever antibodies were detected in 343 of 602 BTM samples (57.0%). C. burnetii DNA was detected by real-time PCR only in spring and summer BTM samples from one of the nine Daegu farms. In this farm, all serum samples were PCR-negative, whereas 46 of 105 cows were antibody-positive (43.8%). These findings support repeated BTM testing as a complementary herd-level screening approach, particularly for Q fever monitoring.