Rumen microbes convert Co to vitamin B₁₂, a coenzyme in energy metabolism and important for milk production; but B₁₂ synthesis and milk production outcomes vary by dietary Co source and level. Two studies were conducted on commercial dairies in the Midwest to determine the optimum source and level of Co in diets of lactating dairy cows. In study 1 and 2, Holstein cows (n = 46, 50 respectively), were blocked at calving by parity and calving date and randomly assigned to one of 2 treatments fed through 49 and 50 DIM, respectively. Cows were fed a basal lactating diet supplemented with 80 mg/d folic acid. Robotic milking systems individually fed cows the pelleted treatments and recorded milk production, milk composition and BW daily. Data were analyzed using MIXED procedures of SAS, with repeated-measures ANOVA for milk, components, and BW. Treatment, week, and interactions were fixed effects. Cow within treatment was a random effect. Study 1, cows were fed 1 ppm Co from COPRO (Co glucoheptonate, Zinpro Corp., Eden Prairie, MN) or a new Co source (CoPectin, Zinpro Corp.). There were no treatment differences (P ≥ 0.24) for fat yield (1.45, 1.51 kg), fat percent (3.61, 3.48%), BW (696, 700 kg) or BW change (−0.9, −0.1 kg) for cows fed COPRO or CoPectin, respectively. Cows fed CoPectin tended (P = 0.07) to produce more ECM than COPRO (43.5, 41.1 kg, respectively). Milk production (44.1, 40.9 kg) and protein percent (3.09, 3.01%) were greater (P ≤ 0.05) for cows fed CoPectin than COPRO, respectively. Study 2, cows were fed 1.0 or 1.5 ppm Co (DM Basis) from CoPectin. As prescribed by farm protocol, multiparous cows received a 20 mL B vitamin injection, d 2 postpartum. Milk (42.6, 41.6 kg), ECM (43.7, 42.8 kg), fat (1.57, 1.54 kg) and protein (1.31, 1.29 kg) were similar between Co levels (P ≥ 0.4), but numerically greater for cows fed 1.0 vs. 1.5 ppm Co, respectively. Concentrations of fat (3.78, 3.79%) and protein (3.12, 3.15%) and cow BW (653, 667 lb) were similar (P ≥ 0.10) for cows fed 1.0 or 1.5 ppm Co from CoPectin, respectively. Results demonstrate CoPectin is an improved Co source and can be effectively supplemented in lactation diets at 1.0 ppm DM.