RUMEN METABOLISM AND FEEDLOT RESPONSES BY STEERS FED TYLOSIN AND MONENSIN

Abstract

Ninety-six Hereford steers with an average weight of 336 kg were fed tylosin (Tylan, Elanco Division, Eli Lilly & Co., Indianapolis, In., U.S.A.) (11 mg/kg of diet), monensin (Rumensin, Elanco Division, Eli Lilly & Co., Indianapolis, In., U.S.A.) (33 mg/kg of diet), tylosin plus monensin or a nonmedicated control diet for 105 days. The diets contained 60% barley, 36% alfalfa meal, 2% tallow, 1% Co + I salt and 1% vitamin premix. Organic matter and crude protein digestibilities tended to be higher (P < 0.10) in steers fed monensin. Both tylosin and monensin increased (P < 0.05) acid detergent fiber digestibility. Tylosin did not affect molar proportions of ruminal VFA or ammonia. Monensin increased (P < 0.05) propionic acid and reduced (P < 0.05) acetic and butyric acids. There were no treatment effects on ruminal dehydrogenase activity or plasma urea nitrogen. Ruminal free amino acids were significantly (P < 0.05) lower in steers fed tylosin, both alone and with monensin. There were no significant treatment effects on feedlot performance or carcass measurements and there was no evidence of a complementary interaction of tylosin and monensin. The incidence of liver abscesses was reduced (P < 0.05) by tylosin. Steers with normal livers gained 5% faster (P > 0.05) than steers with abscessed livers.