A COMPARISON OF MONENSIN AND CHLORTETRACYCLINE IN HIGH-CONCENTRATE AND HIGH-FORAGE DIETS FOR IMPLANTED AND NONIMPLANTED FINISHING STEERS AND HEIFERS

Abstract

In a factorial design experiment, 452 steers and heifers were fed ground forage or rolled grain-based diets supplemented with chlortetracycline (CTC) or monensin (MON). The effects of implanting steers with either 200 mg progosterone and 20 mg estradiol benzoate (PEB) or 36 mg resorcylic acid lactone (RAL) and heifers with 200 mg testosterone propionate and 20 mg estradiol benzoate (TEB) were also determined. On average, grain-fed cattle gained faster (P < 0.01) than forage-fed cattle, requiring 13 days less to reach finished condition. They also required 13% less feed per unit gain (P < 0.01) and dressed out 1.29 percentage units higher than forage-fed cattle but "cut out" only 0.4 percentage units higher, returning $18.78 more per head to labor. Heifers, averaging across all other treatments, gained slower (P < 0.05), required 6.7% more feed per unit gain but dressed out and "cut out" as well as steers, to return $7.39 less to labor per head. Cattle fed diets supplemented with MON took 4 days longer to finish, ate less (P < 0.05) feed but did not gain significantly more efficiently than those fed CTC-supplemented rations. Steers fed MON also had lower (P < 0.05) dressing percentages and returned $10.09 less per head to labor. Implanted cattle gained (P < 0.01) faster and more efficiently but required the same number of days to reach finished condition as did unimplanted cattle, thus producing heavier carcasses and returning to labor, on average, about $17.93 and $12.26 more for steers and heifers, respectively. The acetic/propionic acid ratio in the rumen fluid was significantly lower for cattle fed diets containing MON than for those fed diets fed CTC. Cattle fed forage-based diets and those receiving implants yielded significantly leaner carcasses than those fed grain-based diets or those receiving no implant. Key words: Monensin, chlortetracycline, implants, forage, beef steers, beef heifers