Materials and Methods
One hundred seven steers (initial BW = 602 ± 6.2 lb) were stratified by BW and randomly allotted to 12 pens with eight or nine steers per pen and four pens per treatment. Diets are presented in Table 1. Treatments were: 1) ad libitum consumption of a 92% concentrate diet throughout the experiment (AL); 2) ad libitum consumption of an 85% corn silage-based diet for the first 205 lb of gain followed by a 50% corn silage-based diet for the next 298 lb of gain and a 92% concentrate diet for the final 139 lb of gain before harvest at an average of 1244 lb BW (CS); and 3) prescription intake (PI) of a 92% concentrate diet to achieve stepwise increases in ADG. Steers in the PI regimen were fed to achieve a predicted gain of 2.5 lb/day (NRC, 1984; net energy gain equation for large-framed steer calves) for the first 205 lb of gain. Steers were then fed to achieve a predicted gain of 3.0 lb/day for the next 298 lb of gain. Feed was offered ad libitum for the final 139 lb of gain before harvest at an average of 1244 lb BW. Concentrations of CP, vitamins, minerals, Rumensin, and Tylan (provided courtesy of Elanco, Greenfield, IN) were increased in diets fed to steers in the PI regimens in Periods 1 and 2 to ensure equal intake of these components as steers fed the AL diet. Steers were implanted with Synovex-S on day 0 (provided courtesy of Fort Dodge Animal Health, Overland Park, KS), and Revalor-S on day 84 (provided courtesy of Hoechst-Roussel Agri. Vet. Co., Overland Park, KS).
Steers were fed once daily beginning at 0800 hours. Feed refusals were weighed, recorded, and discarded daily. Feed samples were analyzed weekly for DM to adjust feed intake for dietary moisture content. Composite feed samples were dried in a forced-air oven at 55oC, ground to pass a 1-mm screen, and analyzed for DM (Goering and Van Soest, 1970) and N by a combustion type autoanalyzer (Leco FP-2000, Leco Corp., St. Joseph, MI). Pens were constructed of metal gates and cable, had concrete slatted floors, and were located in an open-sided barn.
Initial and final BW were determined using the average weight from two consecutive days. Steers were weighed every two weeks, and DMI of steers in the PI regimen was adjusted to meet maintenance requirements as BW increased. All steers in a pen were harvested when the average BW of steers within a pen was approximately 1244 lb. Carcass data were collected by trained university personnel.
Data were analyzed using GLM procedures of SAS (1998) for a completely randomized design. Treatment means were compared using the PDIFF statement of SAS when protected by a significant (P < 0.10) F value. Pen served as the experimental unit for all analyses.
Research animal care protocols followed guidelines recommended in the Guide for the Care and Use of Agricultural Animals in Agriculture Research and Teaching (Consortium, 1988).