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Supplemental Protein Withdrawal and Monensin in Corn Silage Diets of Finishing Steers¹

University of Kentucky², Lexington 40506

Abstract

Ninety-six Hereford and Angus steers averaging 278.6 kg in weight initially were utilized in a 168-day trial to study the effects of monensin and supplemental protein withdrawal on growth, feed efficiency and carcass characteristics. The steers were randomly allotted to four treatments. Each treatment consisted of three replicate pens containing eight steers each. For the first 84 days of the experiment, steers receiving treatments 1 and 2 were fed the control diet of corn silage, ground shelled corn and soybean meal. During this same time period, steers receiving treatments 3 and 4 were fed the same diet plus 200 mg monensin per steer daily. On day 84 supplemental soybean meal was withdrawn from steers in treatments 2 and 4 and replaced with ground shelled corn. The resulting four treatments for the 168-day trial were: C-control diet; CSR-control diet for the first 84 days and soybean meal removed from the diet for the last 84 days of the trial; M-control diet plus 200 mg monensin per steer daily; and MSR-200 mg monensin per steer daily with soybean meal fed for the first 84 days and removed from the diet for the last 84 days of the trial. Overall average daily gains for the 168-day trial were: C-.98 kg, CSR-.78 kg, M-1.07 kg and MSR-.92 kilogram. Removal of supplemental protein from the control diet resulted in a decreased (P<.05) gain for days 85 to 168 of the trial (C-.81 and CSR-.36 kg/day). Average daily gains for steers fed M were .87 kg and those in the MSR treatment averaged .61 kg for the same time period. Efficiency of gain (TDN/kg gain) of steers for the total trial were: (C) 7.02, (CSR) 8.28, (M) 5.83 and (MSR) 6.14. Total digestible nutrients per kilogram gain for M and MSR were lower (P<.05) than C, and CSR was higher (P<.05) than all treatment groups. All treatment groups had similar dressing percentages. CSR steers had smaller (P<.05) ribeye areas and less fat over the rib. Marbling scores and carcass grades did not differ significantly among treatment groups. These data suggest tliat monensin had an apparent protein-sparing effect in steers fed a corn silage diet during the supplemental protein withdrawal phase of the study.

¹ This paper (77-5-33) is published with the approval of the Director of the Kentucky Agricultural Experiment Station.

² Department of Animal Sciences.