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To determine the effects of finishing system (conventional vs. natural), dietary adaptation length (14 vs. 28 d), and direct-fed microbial (DFM) supplementation (no DFM vs. DFM) on growth performance and feeding behavior, 120 yearling steers (390 ± 2.8 kg) were used in a completely randomized design with a 2 × 2 × 2 factorial arrangement of treatments. Feed intake was monitored using the Insentec feeding system. Blood samples were collected every 28 d. After 140 d on feed, steers were slaughtered and carcass characteristics collected. Conventionally fed steers had greater (P ≤ 0.001) final BW, carcass weight, and dressing %. Dry matter intake was not influenced (P ≥ 0.31) by treatment. Length of dietary adaptation period did not influence (P ≥ 016) final BW, ADG, and G:F. There was a feeding system × DFM interaction (P ≤ 0.02) for ADG and G:F with conventionally fed steers fed DFM having the greatest (P ≤ 0.05) and naturally fed steers supplemented with DFM having the least (P ≤ 0.05) ADG and G:F. Number of visits to the feed bunk and number of meals per day did not differ (P > 0.05) among treatments. Time eating per visit and per meal was greater (P = 0.05) in steers supplemented with DFM than in steers not supplemented with DFM. On day 56 and 140, plasma glucose concentration was greater (P ≤ 0.03) in steers adapted in 14 d than in steers adapted in 28 d. On day 84, plasma glucose concentration was greater (P = 0.02) in naturally fed compared to conventionally fed steers. On day 112, there was a dietary adaptation period × DFM interaction (P = 0.004) for plasma glucose concentration with glucose concentration greatest (P ≤ 0.05) in steers adapted in 14 d supplemented with DFM and in steers adapted in 28 d not supplemented with DFM, least for steers adapted in 28 d supplemented with DFM, with steers adapted in 14 d not supplemented with DFM intermediate (P ≤ 0.05). On day 112 and 140, plasma urea N concentration was greater (P ≤ 0.05) in steers adapted in 28 d than in steers adapted in 14 d. These data indicate that conventionally fed steers generally had improved growth performance compared to naturally fed steers. Length of dietary adaptation and DFM supplementation had minimal effects on growth performance but did interact with feeding system to influence feeding behavior and blood metabolite concentrations.