SILAGE OR LIMIT-FED GRAIN GROWING DIETS FOR STEERS .2. EMPTY BODY ANDCARCASS COMPOSITION

The influence of energy source (silage- or grain-based) on empty body and carcass composition and adipocyte cellularity independent of rate of gain was tested. Sixty-four Angus steers were allotted to either a forage (ad libitum) or grain (limit-fed) diet for a growing phase (145 d) followed by 45, 75, or 105 d of ad libitum access to a grain-based diet. Eight steers were slaughtered initially and eight from each tre atment were slaughtered at the end of the growing phase, and at each o f the termination dates. The silage growing diet consisted (DM basis) of 55% sorghum silage (approximately 24% dry matter), 22% alfalfa hay, 11% ground shelled corn, and 11% soybean meal. The grain-based growin g diet was composed of 77% ground shelled corn, 5% soybean meal, 14% c ottonseed hulls, 3% molasses, and 1% salt and mineral; it was limit-fe d to produce the same rate of gain as the silage diet. No implants or ionophores were used. At the end of the growing phase, the steers fed grain were heavier and had a higher percentage of fat in the empty bod y (24 vs 19% fat) and the carcass (26 vs 21% fat) than did steers fed forage. Rate of gain during the growth phase was related positively to percentage of carcass fat; when corrected for fill, data for both die ts fit one regression line for fat vs rate of gain. When adjusted for gain during the growing phase, fat content was not different in empty body or carcass, but internal fat was higher (P < .10) for steers fed grain. After 45 d on the finishing diet, carcass fat remained low (23% ), but after 75 and 105 d, fat content reached 27%. Source of energy d id not detectably affect carcass composition independent of rate of ga in. Cell size of adipocytes from four adipose depots increased with ti me on feed but were not affected by diet during the growing phase. Lea n Choice beef can be produced in only 45 d in the feedlot with medium- framed Angus cattle.