Cobalt (Co) is essential for rumen microbial metabolism to synthesize methane, acetate and methionine. It also serves as a structural component of vitamin B12 (cobalamin), which functions as a coenzyme in energy metabolism. A study was conducted to determine if Co form (carbonate v. glucoheptonate) supplemented above the National Research Council requirements would improve digestibility of a low-quality forage diet and change serum cobalamin concentrations. Nineteen ruminally cannulated cows (577 ± 13 kg) were fed individually in a completely randomized experimental design. Cows were fed a grass hay diet that contained (79·2 g/kg crude protein, 565 g/kg total digestible nutrients, 633·2 g/kg neutral detergent fibre (NDF), 874·2 g/kg dry matter) at a rate of 0·02% of body weight on a as fed basis for a 62-day study, which consisted of three periods; acclimation (AC), treatment (TR) and residual (RE). Measurements taken in the AC period were used as covariates for analysis in the TR and RE periods. Cows were stratified by age (5 ± 0·4 years) and lactational history, and assigned to receive 12·5 mg supplemental Co in one of two forms: (1) 27·2 mg of Co carbonate (CC, n = 11 cows) or (2) 50 mg of Co glucoheptonate (CGH, n = 8 cows). Supplement was administered daily via a gelatin capsule placed directly into the rumen 2 h after feeding. During the last 96 h of each period, forage digestibility was measured using an in situ nylon bag technique. Blood samples were collected 4 and 6 h following feeding, and 24 h before the end of each period. A treatment × period interaction was detected for in situ organic matter (OM) disappearance at 96 h; (TR period: 684 and 708 ± 81 g/kg; RE period: 676 and 668 ± 75 g/kg, for CC and CGH, respectively). Once inclusion of Co in the CGH group was removed, OM disappearance was reduced by 4·01% compared with 0·82% in the CC cows. The NDF disappearance (OM basis) was less for the TR compared with the RE at 48 h (629 and 652 ± 39 g/kg, respectively). However, by 96 h NDF disappearance was greater for TR than the RE (704 and 689 ± 44 g/kg; respectively). No differences were detected for cobalamin serum concentrations or rate of fibre fermentation. The outcomes of the current research signify that there may be a slight residual effect of Co supplementation on fermentation; there was also an indication that Co source may enhance the overall extent of fermentation.