Abstract: Three experiments were conducted: in Experiment 1, 96 steers (334 kg) were used in a 126 days finishing trial to compare ground oyster shell and limestone as supplemental Ca sources at dietary Ca levels of 0.70 vs. 1.40%, in a 2x2 factorial arrangement. In Experiment 2, 96 heifers (354 kg) were used in a 149 days finishing trial to evaluate oyster shell and limestone as Ca sources at dietary Ca levels of 0.50 vs. 0.9%, in a 2x2 factorial arrangement. In Experiment 3, 4 steers (399 kg) with cannulas in the rumen and proximal duodenum were used in a 4x4 Latin square design to evaluate treatment effects on characteristics of digestion. The calcium content of limestone and oyster shell was 33.3 and 34.3%, respectively. Ca reactivity was 17.9 and 5.87 min, respectively for limestone and oyster shell. In Experiment 1, there were no treatment effects (p>0.20) on DMI, ADG, gain efficiency, dietary NE, dressing percentage, KPH, LM area and marbling score. Increasing dietary Ca level from 0.7-1.4% tended to slightly increase (1.2%, p<0.10) estimated carcass retail yield and there was a tendency (p<0.10) for an interaction between Ca level and source on fat thickness. Fat thickness was similar for oyster shell at the 2 levels of supplementation. However, with the limestone, fat thickness was 29% greater for diets supplemented to contain 0.7% Ca than for diets containing 1.4% Ca. There were no treatment effects (p>0.20) on fecal pH. As expected, increasing dietary Ca level increased (p<0.01) fecal Ca concentration. In Experiment 2, there were no treatment effects (p>0.20) on ADG, DMI, gain efficiency and dietary NE, dressing percentage or LM area. In contrast with Experiment 1, there were no treatment effects on fat thickness and retail yield. However, KPH was greater (8.9%, p<0.1) for oyster shell than for limestone supplemented diets. In Experiment 3, Ca source did not affect (p>0.20) ruminal microbial efficiency. However, ruminal OM digestion was greater (8.3%, p<0.05) for oyster shell than for limestone supplemented diets. The increase in OM digestion was associated with numerical increases (8.6 and 4.6%, respectively) in ruminal NDF and starch digestion. There was an interaction (p<0.01) between Ca source and level on postruminal OM digestion. Increasing dietary Ca level using oyster shell depressed (7.4%) postruminal OM digestion compared to that of the other treatments. Otherwise, there were n effects (p>0.20) of Ca level and source on apparent total tract digestion of OM, NDF, starch and N. There were no treatment effects (p>0.20) on ruminal pH, VFA molar proportions and estimated methane production. As expected, increasing dietary Ca level from 0.5-0.9% increased (p<0.01) Ca flow to the duodenum (32.3%) and fecal excretion (40.4%). Apparent ruminal digestion of Ca was low (1.2%) across treatments, being slightly negative (-10.7%) for the 0.5 levels of dietary Ca and slightly positive (13.2%), for the 0.9% level of dietary Ca (p<0.05). Conversely, apparent post-ruminal Ca absorption was greater (34.6%, p<0.05) for diets supplemented with 0.5 vs. 0.9% Ca. There were no treatment effects (p>0.20) on apparent total tract Ca digestion. We conclude that increasing dietary Ca levels beyond standard requirements for maintenance and tissue growth may not enhance performance of feedlot steers and heifers fed steam-flaked corn-based high concentrate finishing diets. Notwithstanding the greater reactivity of oyster shell vs. limestone, difference between sources in terms of growth performance and ruminal pH and digestive function are small.