Effects of Feeding Monensin in Combination with Zeranol and Testosterone-Estradiol Implants for Growing and Finishing Heifers

Abstract

Thirty-six crossbred heifer calves were fed during an 84-day growing trial and an 84-day finishing trial to evaluate monensin as a feed additive when fed alone or in combination with zeranol or testosterone-estradiol implants. No synergistic effect (P>.05) between additive and implant compounds and no additive × implant interactions (P>.05) were detected during the feeding trials. Heifers fed 200 mg of monensin per day during the growing trial gained .76 kg/day compared with .68 kg/day (P<.05) for heifers fed the control diet. Gains of heifers implanted with zeranol (.74 kg/day) or testosterone-estradiol (.81 kg/day) were not different (P>.05) but were greater (P<.05) than gains (.60 kg/day) of the non-implanted heifers. During a subsequent 84-day finishing trial, average daily gains were 1.17 kg/day for heifers fed monensin (33 g/MT of diet) and 1.13 for the control treatment (P>.05). Average daily feed intakes and feed required per unit of gain were 6.8 and 5.8 kg for heifers fed monensin compared with 7.8 and 6.9 kg for controls (P<.05). Live weight gains and feed utilization values were similar (P>.05) for the implant treatments. Rumen fluid characteristics were similar (P>.05) for heifers within the implant treatments throughout the study. However, rumen fluid from heifers fed monensin contained less (P<.05) acetic acid, more (P<.05) propionic acid and less (P<.05) butyric acid than did rumen fluid from control animals.