Effect of calcium oxide (CaO) treatment of barley straw and salt on rumen fermentation, microbiota, digestibility, growth, and carcass quality of cattle was assessed. A replicated 4 × 4 Latin square metabolism experiment was conducted using eight heifers fed a wheat finishing diet with barley silage (B-SIL), barley straw (B-S), or 5.0% CaO-treated barley straw (CaOS) with or without NaCl (CaOS-NS). Growth over 115 d was assessed using 75 individually housed steers fed the above diets and an additional diet (I-CaOS), where CaO was added to straw just before feeding. There was no effect (P ≥ 0.08) of diet on rumen fermentation, digestibility, or carcass quality, although CaO decreased (P < 0.001) maximum pH and retained Na was decreased (P < 0.05) by CaOS-NS. Rumen bacterial abundance was altered (P ≤ 0.05) by diet. The average daily gain (ADG) of B-SIL and CaOS-NS steers was 14.1% greater (P ≤ 0.05) than BS and CaOS steers, whereas the gain:feed of CaOS-NS steers was 14.2% greater (P ≤ 0.05) than B-S and CaOS steers. Steers fed I-CaO had similar ADG and gain:feed to other treatments. CaO-treated straw without NaCl could replace barley silage in wheat diets, without compromising digestibility or growth in steers.