Cobalt (Co) is a metallic element that stimulates the production of erythropoietin (EPO). Post-race urine and plasma samples from horses on the same team consistently showed significantly elevated Co concentrations, with some exceeding international thresholds. To investigate the source, all feed and supplements from the team's stable were collected and analyzed for Co content using ICP-MS. As suspected, a particular mixed feed, which was rarely used at other stables, contained the highest Co concentration (5.14 mg/kg) among all samples. This product was subsequently chosen for a feeding experiment. Three healthy Thoroughbred mares with no prior exposure to this feed were fed 4 kg daily for 29 days. Urine and plasma samples were collected every 3–4 days and analyzed by ICP-MS. Plasma Co concentrations steadily increased in all horses but remained below the 25 ng/mL threshold. In contrast, urinary Co concentrations exceeded the 100 ng/mL threshold within 10 days, peaking at 144.54–218.00 ng/mL before eventually falling below the threshold. After a three-month ban on the problematic feed, subsequent samples confirmed that Co concentrations returned to normal. These findings demonstrate that Co-positive cases can result solely from the administration of commercially available feed, underscoring the importance of monitoring feed composition as a part of doping control strategies.