Effect of essential oils, tylosin, and monensin on finishing steer performance, carcass characteristics, liver abscesses, ruminal fermentation, and digestibility1

A feedlot (Exp. 1) experiment was conducted to evaluate the effects of an essential oil mixture (EOM), experimental essential oil mixture (EXP), tylosin, and monensin (MON) on performance, carcass characteristics, and liver abscesses. A metabolism experiment (Exp. 2) was conducted to evaluate the effects of EOM. EXP. and MON on ruminal fermentation arid digestibility in finishing steers. In Exp. 1, 468 yearling steers (398 ± 34 kg initial BW) were used in 50 pens (10 pens/treatment) and received their respective dietary treatments for 115 d. Five dietary treatments were compared in Exp. 1: 1) control, no additives (CON): 2) EOM. 1.0 g/steer daily: 3) EX.P. 1.0 g/steer daily; .1) EOM. 1.0 g/steer daily plus tylosin, 90 mg/ steer daily (EOM+T): and 5) monensin, 300 mg/steel' daily plus tylosin, 90 mg/steer daily (MON+T). Compared with CON, steers fed MON+T had decreased DM1 (P < 0.01), and steers fed EOM+T and MON+T had improved G:F (P < 0.02). Average daily gain was not different among treatiiieiits (P > 0.5S). There was a trend (P = 0.09) for a treatment effect on 12th-rib fat thickness, which resulted in a significant increase in calculated yield grade for the EOM+T treatment. No at lieu carcass characteristics were affected by treatment (P »= 0.10). Prevalence of total liver abscesses was reduced for steers fed tvlosin compared with no tylosin (P < 0.05). In Exp. 2, 8 ruminally fistulated steers (399 + 49 kg initial BW) were assigned randomly to 1 of 1 treatments in a replicated 4 x 4 Latin square designed experiment. Treatments were 1) CON, 2) EOM. 3) EXP. and 4) MON with feeding rates similar to Exp. 1. There were no differences in DM1, OM intake, and aI)pal'ent total tract DI'vl or OM digestibilities among treatments (P > 0.30). Feed intake patterns were similar among feed additive treatments (P > 0.13). Total VFA (P = 0.10) and acetate (P = 0.06) concentrations tended to be affected by treatment with EOM numerically greater than CON. Average ruminal pH ranged from 5.59 to 5.72 and did not differ among treatments. Addition of a EOM or monensin to a diet containing tylosin improves G:F. 1 nit little difference was observed in metabolism or digestibility. [PUBLICATION ABSTRACT]