Materials and Methods
One hundred six crossbred steers (initial BW = 604 ± 4 lb) were stratified by BW and randomly allotted to 12 pens with eight or nine steers per pen and three pens per treatment. Four feeding regimens were investigated comparing diets consisting of high (H), intermediate (I), or low (L) proportions of corn silage. Steers in the first feeding regimen were offered a 92% concentrate diet (Table 1) ad libitum throughout the experiment (AL). The remaining steers were fed different diets in each of the following three periods of growth: Period 1 (604 to 811 lb BW), Period 2 (811 to 1107 lb BW), and Period 3 (1107 to 1265 lb BW). Steers in the second feeding regimen were fed diets containing a low (15%) proportion of corn silage throughout the experiment, but a prescription intake (PI) feeding strategy was used. In Period 1, DMI was restricted to achieve a predicted gain of 2.5 lb/day (NRC, 1984; net energy gain equation for large-framed steer calves). In Period 2, DMI was restricted to achieve a predicted gain of 3.0 lb/day. Feed was offered ad libitum in Period 3. Steers in the third feeding regimen (CS-HLL) were fed a diet containing a high (83.5%) proportion of corn silage in Period 1, and DMI was restricted to achieve a predicted gain of 2.5 lb/day. In Period 2, DMI of a diet containing a low (15%) proportion of corn silage was restricted to achieve a predicted gain of 3.0 lb/day. In Period 3, a diet containing a low (15%) proportion of corn silage was offered ad libitum. Steers in the fourth feeding regimen (CS-HIL) were fed a diet containing a high (83.5%) proportion of corn silage in Period 1 and DMI was restricted to achieve a predicted gain of 2.5 lb/day. In Period 2, DMI of a diet containing an intermediate (50%) proportion of corn silage was restricted to achieve a predicted gain of 3.0 lb/day. In Period 3, a diet containing a low (15%) proportion of corn silage was offered ad libitum. The diet for steers in the AL feeding regimen was formulated to contain 13% CP, 27 g Rumensin/ton, and 10 g Tylan/ton (Provided courtesy of Elanco, Greenfield, IN). Concentrations of CP, vitamins, minerals, Rumensin, and Tylan were increased in Periods 1 and 2 in the diets fed to steers in the PI, CS-HLL, and CS-HIL regimens to provide the same daily intakes of these components as steers fed the AL diet. Steers were implanted with Synovex-S on day 0 and Synovex Plus on day 84 (Provided courtesy of Fort Dodge Animal Health, 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 was determined using the average weight from two consecutive days. Steers were weighed every two weeks, and DMI of steers in the limit-fed regimens 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 1265 lb. Carcass data were collected by trained university personnel.