EFFECTS OF FEEDING MONENSIN TO HOLSTEIN STEERS

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Abstract
Three experiments were conducted to determine the effects of monensin on feed intake, rate of gain and feed efficiency of a total of 198 Holstein steers. In experiment I, 72 steers were fed ground ear corn and corn silage diets containing 0 (treatment 1), 22 (treatment 2) and 33 (treatment 3) g/tonne DM monensin for 112 days. Treatments 2 and 3 received 11 g/tonne DM monensin for the first 28 days. Daily gains were not significantly different (P > 0.05) throughout the trial and intake was slightly reduced (P > 0.05) with monensin addition. Feed efficiency in treatments 2 and 3 were improved 5.5 and 8.8% respectively, but only treatment 3 was significantly (P < 0.05) better than treatment 1. Rumen fluid from steers fed treatment 3 contained a higher proportion (P < 0.05) of propionic acid and a lower (P < 0.05) A:P ratio than did rumen fluid from treatment 1. Levels of acetic and butyric acid were reduced (P > 0.05) by monensin inclusion. In experiment II, 46 steers were fed hay crop silage and shelled corn diets containing 0 (treatment 1) and 33 g/tonne DM (treatment 2) monensin for 112 days. Gains were slightly (P > 0.05) higher and intake slightly (P > 0.05) lower on treatment 2. Monensin improved (P > 0.05) feed efficiency by 8.5%. In experiment III, 40 purchased and 40 station-reared steers were assigned by origin, in pairs, to 40 Calan electronic headgates. A 23 factorial experimental design was used to evaluate the effect of diet with or without monensin used separately and in combination with progesterone-estradiol implants. Steers were fed a high moisture ear corn and corn silage diet for 84 days. No 3-factor interactions (P > 0.05) were detected. Implant treatments gained significantly (P < 0.05) faster than either control or monensin alone. Intake was significantly (P < 0.05) lower in treatments fed monensin. Feed efficiency was improved (P < 0.05) 16% with both monensin and implant, 10% (P < 0.05) with monensin alone, and 6% (P > 0.05) with implant alone over control.