Effect of direct-fed microbials on utilization of degradable intake protein in receiving steers

One hundred ninety-two crossbred beef steers (280 ± 25 kg) were assigned to a 5 × 2 factorial; degradable intake protein (DIP; 80, 90, 100, 110, 120% of requirement) with and without a direct-fed microbial (DFM) primarily containing Lactobacillus acidophilus and Enterococcus faecium (10⁹ CFU steer⁻¹ d⁻¹). Dry matter intake, morbidity, and immune response were not affected (P ≥ 0.11). The first 28 d, average daily gain (ADG) did not differ with DIP in control, but increased in a cubic fashion with DFM (DIP × DFM; P = 0.05). No differences (P ≥ 0.25) in ADG occurred from days 29 to 56; however, there was a tendency (P = 0.08) for a cubic increase in ADG with increasing DIP with DFM over 56 d. The first 28 d, growth efficiency did not differ across DIP levels in control but increased linearly with DFM (DIP × DFM; P = 0.05). No differences (P ≥ 0.21) in efficiency were observed from days 29 to 56 or overall. Without DFM, fecal pH decreased between days 7 and 14; however, with DFM there was no change in pH (DFM × time; P < 0.05). Performance response to DFM is dependent on DIP; however, DFM does not impact morbidity or humoral immune response.