Influence of oral antibiotics on digestive function in Holstein steers fed a 71% concentrate diet

Four Holstein steers (164 kg) with cannulas in the rumen and proximal duodenum were used to evaluate the effects of oral antibiotic administration on characteristics of digestion of a 71% concentrate diet. Treatments were as follows: 1) no antibiotic; 2) 350 mg.steer-1.d-1 of chlortetracycline (CTC) provided in the feed (A350); 3) 350 mg.steer-1.d-1 of CTC plus 350 mg.steer-1.d-1 of sulfamethazine provided in the feed (AS700); 4) 22 mg of CTC/kg BW (3.6 g.steer-1.d-1) provided in the drinking water (A3600). Drinking water intake averaged 23.5 L/d and was not affected (P > .10) by treatments. Antibiotic treatments increased passage (P < .05) of OM to the small intestine. This effect was largely due to decreased (27%, P < .05) ruminal ADF digestion. Passage of nonammonia N to the small intestine was greater (10.4%, P < .01) for A3600 vs A350 and AS700; the latter was similar to the control. This effect was due to an increased (23.4%, P < .05) synthesis of microbial N. Total tract digestion of OM (P < .05), starch (P < .05), N (P < .10), and DE (P < .10) were slightly lower (2.7, 1.6, 2.8, and 2.7%, respectively) for antibiotic-supplemented diets. Total tract starch (P < .10) and N (P < .01) digestion were further decreased (1.2 and 3.5%, respectively) for A3600 vs A350 and AS700. Postruminal digestion of ADF was greater (131%, P < .10) for antibiotic-supplemented diets, compensating for the lower ruminal digestion. There were no treatment effects (P > .10) on ruminal pH.(ABSTRACT TRUNCATED AT 250 WORDS)