Cobalt source and folic acid (FA) may affect vitamin B₁₂ (B₁₂) synthesis and milk production. A study was conducted to determine effects of Co source (Zinpro Corp.) and metabolizable folic acid on milk production and serum vitamins. Lactating cows on 7 dairies were fed standard lactation diets and received 3 treatment (Trt) premixes in a switchback design (4 60 d periods: 30 d adaptation, 30 d data collection). Trace minerals (TM) Zn, Mn, Cu (40, 20, 3.5 ppm, respectively) were from Availa (AV) or ProPath (PP) amino acid complexes (Zinpro Corp.). Treatments were: 1) CON: AV TM, 1 ppm Co from Co glucoheptonate (COPRO) and 0 ppm FA; 2) ACF: AV TM, 1 ppm Co from Co pectin (CoPectin) and 3 ppm FA; 3) PPD (ProPath Dairy, Zinpro Corp.): PP TM 1 ppm Co from CoPectin and 3 ppm FA. Data were analyzed using Proc MIXED of SAS. Experimental unit was herd (milk data), or cow (transition cow index (TCI), serum). Effects were Trt, temperature humidity index, days since start (fixed), and herd (random). Milk yield was greatest (P < 0.01) for PPD (40.2 kg), intermediate for ACF (39.6 g) and lowest for cows fed CON (38.8 kg). ECM was greater (P < 0.05) for PPD (42.2 kg) cows than CON or ACF (41.8 or 41.6 kg). Milk fat percent decreased (P < 0.01) across Trt (3.99, 3.87, and 3.81% for CON, ACF and PPD, respectively). Milk fat yield was similar across Trt (1.54, 1.54, and 1.53 kg/d for CON, ACF, and PPD respectively). Milk protein % was greater (P < 0.01) in cows fed PPD and CON (3.19%) than ACF (3.16%). Milk true protein yield increased (P < 0.01) across Trt with PPD > ACF > CON (1.28 > 1.25 > 1.23 kg/d, respectively). The SCC was variable (P < 0.05) across Trt at 115, 107 and 154 (1000/mL) for CON, ACF and PPD, respectively. The TCI was greatest (P < 0.01) for PPD (7.0 kg/d) compared with ACF and CON (2.7 and 1.4 kg/d). Serum FA was similar among Trt (13.2, 13.8, and 13.1 ng/mL for CON, ACF and PPD respectively). Serum B₁₂ was greatest (P < 0.05) in PPD (184 pg/mL) compared with CON and ACF (162 and 157 pg/mL). Compared with CON and ACF, feeding PPD increased milk, ECM, and milk protein yield, improved TCI, and increased serum B₁₂ status in lactating cows on commercial dairies.