Antimicrobial supplementation alters digestibility and ruminal fermentation in a continuous culture model

A dual-flow continuous culture system was used to evaluate the effects of laidlomycin propionate and bacitracin zinc on fermentation, nutrient digestibility, and microbial efficiency. Factors were laidlomycin propionate (LP; 2 mg/L of culture volume) and bacitracin zinc (BAC; 1.4 mg/L), and treatments were as follows: (1) no BAC or LP (CON), (2) LP without BAC (LP), (3) BAC without LP (BAC), and (4) LP and BAC (LP/BAC). A fifth treatment was supplemented with monensin sodium (MON; 6 mg/L) to act as a positive control. Both LP and LP/BAC had significantly greater pH than MON (P < 0.05). Antibiotic treatment did not affect NH 3-N concentration (P = 0.62), but did influence total VFA production (P = 0.02). Monensin fermenters had a greater proportion of total VFA than did LP fermenters (P < 0.05), and improved VFA production by 7.2% compared to CON. Laidlomycin suppressed the production of both acetate (P < 0.01) and butyrate (P = 0.05), and acetate was further reduced when LP was fed in combination with BAC (P = 0.01). Laidlomycin numerically reduced the acetate:propionate ratio (P = 0.12). These results suggest that bacitracin may produce ionophore-like effects on pH and VFA production in vitro.