Feeding higher Co amounts showed increased ruminal fiber digestion. Cobalt-lactate is a highly rumen soluble source of Co and has demonstrated increased ruminal molar acetate concentrations and decreased ruminal ammonia concentrations when lactating dairy cows were fed a high forage ration (Pretz et al., 2014). These results indicated increased fiber digestion and synthesis of microbial protein. Twenty-four late-lactation (µ = 238 DIM and 36.5 kg milk) Holstein dairy cows (10 primiparous and 14 multiparous), were block by milk yield, DIM, and parity and randomly assigned to 1 of 2 treatments being: 1) control (C): ration containing cobalt carbonate fed at 58 mg/cow/d and 2) (T): ration containing 5 g/cow/d of a 1% Co-lactate product (Co-Max). Total mixed rations (TMR) were 70% forage (60% alfalfa baleage and 40% corn silage) and 30% of the respective experimental grain mix on a DM basis. Lactational responses were previously reported. This study further evaluated the total-tract nutrient digestion by collecting TMR and manure samples for measuring nutrient digestibility via internal markers (acid insoluble ash and iron). In addition, manure samples were submitted for stereo microscopy and scanning electron microscopy evaluation. Manure samples were collected at 4 h intervals for 3 consecutive d from each cow during wk 3 and 4. Total-tract DM (53.7 and 56.8% for C and T, respectively), CP (63.8 and 64.0%), NDF (46.3 and 48.9%), and ADF (39.8 and 42.9%) digestibility coefficients were numerically greater, but not significantly (P > 0.10) different for cows fed T compared with cows fed C. Stereo microscopy demonstrated that manure samples from cows fed T were more transparent than manure samples from cows fed C. Scanning electron microscopy indicated small visible improvements in digestibility of fiber components observed as appearance of hollow pits in the fiber particles. Feeding additional Co as cobalt-lactate visually appeared to increase digestibility of fiber particles when evaluated by stereo and scanning electron microscopy.