ABSTRACT:A feedlot growth performance experiment and 2 metabolism experiments were conducted to evaluate dietary roughage concentration and calcium magnesium carbonate in steers fed a high-grain diet. In Exp. 1, one hundred ninety-two crossbred yearling steers (320 ± 10 kg of initial BW) were fed diets based on steam-flaked corn with 0, 0.75, or 1.5% CaMg(CO 3 ) 2 . There were no effects (P ≥ 0.13) on ADG, DMI, G:F, or total water intake due to CaMg(CO 3 ) 2 . In Exp. 2, five ruminally and duodenally fistulated steers (263 ± 9 kg of initial BW) were used in a 5 × 5 Latin square design, with 5 dietary treatments arranged in a 2 × 2 + 1 factorial: 1) 3.8% dietary roughage and no CaMg(CO 3 ) 2 ; 2) 7.6% dietary roughage and no CaMg(CO 3 ) 2 ; 3) 11.4% dietary roughage and no CaMg(CO 3 ) 2 ; 4) 3.8% dietary roughage and 1.5% CaMg(CO 3 ) 2 ; and 5) 7.6% dietary roughage and 1.5% CaMg(CO 3 ) 2 . Water consumption was less (quadratic, P = 0.003) when 7.6% dietary roughage was fed compared with 3.8 or 11.4% dietary roughage. Intake of DM was not affected (P ≥ 0.16) by dietary roughage or by CaMg(CO 3 ) 2 . Poststomach and total tract starch digestion decreased (linear, P < 0.01) as dietary roughage increased. Ruminal pH tended (P = 0.08) to increase as dietary roughage increased but was not affected (P = 0.60) by CaMg(CO 3 ) 2 . In Exp. 3, DMI and ruminal pH were continuously monitored in a 6 × 6 Latin square design using 6 ruminally and duodenally fistulated Holstein steers (229 ± 10 kg of initial BW). A 3 × 2 factorial treatment structure was utilized, with factors consisting of dietary roughage concentration (4.5, 9.0, or 13.5%) and CaMg(CO 3 ) 2 inclusion (0 or 1.0%) to replace MgO and partially replace limestone. A dietary roughage × CaMg(CO 3 ) 2 interaction (P = 0.01) occurred as steers consuming 13.5% roughage, 1.0% CaMg(CO 3 ) 2 had greater DMI per meal than those consuming 4.5% dietary roughage, no CaMg(CO 3 ) 2 and 9.0% dietary roughage, 1.0% CaMg(CO 3 ) 2 . Steers consuming 13.5% dietary roughage, 1.0% CaMg(CO 3 ) 2 and 9.0% dietary roughage, no CaMg(CO 3 ) 2 had greater meal length (min/meal; P = 0.01) than steers consuming 4.5% dietary roughage, no CaMg(CO 3 ) 2 . Total tract OM digestibility decreased linearly (P = 0.01), and ruminal pH increased linearly (P = 0.01) with increasing dietary roughage concentration. Inclusion of CaMg(CO 3 ) 2 can replace limestone and MgO but did not produce ruminal pH responses similar to those observed by increasing dietary roughage in high-concentrate diets.

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