Four dual-flow continuous culture fermenter studies were summarized to determine effects of Co source, (1 ppm Co) on nutrient digestibility, microbial protein production and efficiency and vitamin B₁₂ synthesis. Three Co sources, CoSO₄, cobalt glucoheptonate (COPRO, Zinpro Corp., Eden Prairie, MN) and Co pectin (CoPectin, Zinpro Corp.), were evaluated in 3 to 4 studies. Rumen inoculum was obtained and pooled from 2 ruminally cannulated Holstein cows consuming a lactation diet absent of Co. In each completely randomized study, treatments were run in triplicate over a 17 d fermentation period, sampled d 15–17. A standard lactation diet (100 g DM/d) was fed in 4 meals/d. Artificial saliva was continuously infused into the fermenters (1162 mL volume), for a 13%/h liquid dilution rate and 22 h solid retention time. Select data were analyzed using linear mixed effect regression model of R. Random effects were included in the model to account for different repeated measures intercepts. The 3 Co sources had similar (P ≥ 0.15) total VFA production (376 mmol/d), and DM, OM, ADF, NDF, and NSC digestibilities (65, 57, 42, 41 and 81% respectively). Vitamin B₁₂ production was similar (P ≥ 0.15) among the Co sources but numerically greatest for CoPectin (10870, 10557, 10281 pmol/d; CoPectin, COPRO and CoSO₄ respectively). CoPectin tended to have greater CP digestibility and microbial N production than CoSO₄, with COPRO intermediate (P ≤ 0.15; 78.6, 72.4 and 76.5% and 2.11, 1.91 and 2.04 g/d, respectively). Greater (P ≤ 0.05) amounts of feed N were converted to microbial N with CoPectin than CoSO₄, with COPRO intermediate (93.8, 92.4 and 93.5 respectively). CoPectin and COPRO produced greater amounts of microbial N/kg OM digested than CoSO₄ (P ≤ 0.05; 38.9, 37.8 and 35.1 respectively). Replacing 1 ppm Co from CoSO₄ with 1 ppm Co from CoPectin or COPRO increased microbial protein production and efficiency and numerically increased vitamin B₁₂ production in continuous culture. CoPectin improved microbial protein production and efficiency over COPRO.