British breed beef steers (n = 48; 370 kg) were assigned to pasture or feedlot diets and one of two doses of 25-hydroxyvitamin D 3 (VITD; 0 or 500 mg) to evaluate the effects of VITD on performance, carcass traits, and meat quality. All steers were implanted with a combination trenbolone acetate/estradiol implant prior to diet initiation. Pasture-finished cattle received 6.8 kg/hd per day of pelleted distillers' grains, wheat midds, and soy hulls while continuously grazing predominantly bromegrass pasture. The feedlot diet contained 10% wet distillers' grains, corn, corn silage, and chopped hay. Steers from both diets also received monensin. Steers were harvested after 112, 133, or 154 d on feed (DOF) to minimize 12th rib fat differences. Twelve steers from each dietary treatment received 25-hydroxyvitamin D3 boluses orally 7 d prior to assigned harvest date. Longissimus, semimembranosus, and gracilis muscles were collected, cut into steaks, aged 3, 7, or 14 d, and evaluated for instrumental color and Warner-Bratzler shear force (WBSF). Calpastatin activity, vitamin and mineral content, and sensory traits were analyzed on steaks aged 3 d postmortem. At harvest, feedlot steers were heavier (P = 0.0370; 584 kg; 132 DOF) than pasture-fed steers (563 kg, 130 DOF) and had greater ADG (P < 0.0001; 1.74 vs. 1.51 kg/d). Pasture-fed steers had less 12th rib fat (P < 0.0001; 6.1 vs 8.7 mm) and kidney, pelvic, and heart fat (P = 0.0108; 1.64 vs. 1.83%); however, neither diet nor VITD significantly affected final yield grade (P = 0.1014). In addition, pasture-finished steers had lower (P = 0.0141) marbling scores than did feedlot steers (Slight45 vs. Slight90). Lipid percentage differed by muscle (P < 0.0001) as the gracilis had the least lipid followed by semimembranosus and logissimus (1.54, 1.94, and 2.54% of wet tissue, respectively). Pasture-fed steers had greater longissimus C18:2 cis-9, trans-11 (CLA) and C18:3n3 concentrations (P < 0.0001) than did feedlot steers (0.95 and 0.63 vs. 0.19 and 0.26 mg/100 mg lipid, respectively). Feedlot steers had greater a* values compared with pasture-fed steers (22.67 vs. 22.24; P = 0.0318), indicating increased redness. The a* values differed by muscle (P < 0.0001) as the gracilis had the greatest a* values followed by the semimembranosus and longissimus (23.22, 22.25, and 21.90), respectively). L* values increased with increased aging time. Sensory analysis showed that the gracilis muscle from steers supplemented with 25-hydroxyvitamin D3 was more tender than that from the unsupplemented control steers. Calpastatin activity on d 3 postmortem differed by muscle (P < 0.0001) with the gracilis having the greatest activity followed by the longissimus and semimembranosus (2.65, 2.08, and 1.82 units of activity/g tissue, respectively), but was not affected by VITD or diet. WBSF decreased with increased aging times for the longissimus and gracilis, whereas the semimembranosus was most tender on d 7. Neither VITD nor diet affected WBSF. Whereas VITD supplementation increased plasma calcium in treated steers at harvest, muscle calcium concentrations were not increased, and thus increased calpain activation was not achieved. Nonetheless, data indicate it is possible to finish steers on pasture by supplementing with distillers' co-products without substantially increasing time needed to reach market weights when compared with feedlot finishing or negatively impacting sensory traits.;Keywords. Beef, distillers' grains, pasture-finished, tenderness