Technical Abstract: Cobalt (Co) is essential for rumen microbial metabolism to synthesize methane, acetate and methionine. It also serves as a structural component of vitamin B12, which functions as a coenzyme in energy metabolism. A study was conducted to determine if Co form (cobalt carbonate vs cobalt glucoheptonate) supplemented above NRC requirements would improve fermentability of a low quality forage diet and change serum cobalamin concentrations. Twenty ruminally-cannulated cows (577 ± 13 kg) were individually fed in a completely randomized experimental design. Cows were fed a grass hay diet (7.9% CP, 56% TDN, 63% NDF, 87% DM) at 2.25% of BW for a 62 d study, which consisted of 3 periods (acclimation (AC), treatment (TR), and residual (RE)). Cows were stratified by age (5 ± 0.37 yr) and lactational history, and assigned to receive 12.5 mg supplemental Co in 1 of 2 forms: (1) 27.2 mg of cobalt carbonate (CC, n =11 cows) or (2) 50 mg of cobalt glucoheptonate (CGH, n = 9 cows). Supplementation was administered daily via a gelatin capsule placed directly into the rumen 2 h after daily feeding. During the last 96 h of each period, forage fermentability was measured using an in situ nylon bag technique. Serum samples were collected 4 and 6 h following feeding, 24 h before the end of each period. Measurements taken in the AC period were used as covariates for analysis in the TR and RE periods. A treatment × period interaction (P = 0.03) was exhibited for in situ OM fermentability at 96 h; (TR period, 68.44 and 70.83 ± 0.81 %, and RE period, 67.61 and 66.82 ± 0.75 %, for CC and CGH; respectively). Once inclusion of Co in the CGH group was removed, fermentability was reduced by 4.01 % compared to 0.82 % in the CC cows. The NDF disappearance (OM basis) was lower for the TR period compared to the RE period at 48 h (P < 0.001; 62.95 and 65.18 ± 0.39 %; respectively). However, by 96 h the NDF disappearance was higher for TR period than the RE period (P = 0.02; 70.44 and 68.89 ± 0.44 %; respectively). No differences were detected for cobalamin serum levels or rate of fiber fermentation. The outcomes of this research signify that while there are no residual effects of Co supplementation on fermentation, there is an indication that Co glucoheptonate supplementation does enhance the overall extent of fermentation. The extent of fiber disappearance is also improved with Co supplementation regardless of form.