DIETARY ENERGY-SOURCE AND DENSITY - EFFECTS OF ROUGHAGE SOURCE, ROUGHAGE EQUIVALENT, TALLOW LEVEL, AND STEER TYPE ON FEEDLOT PERFORMANCE AND CARCASS CHARACTERISTICS

The effects of roughage level (10, 20, or 30% roughage equivalent [RE] ), roughage source (alfalfa vs cottonseed hulls), roughage regimen (co nstant RE vs 2% RE during the mid-finishing period), tallow level(1.2 vs 4.6%), and steer type (British crossbred [BRITX] vs Bos indicus cro sses [BRX]) were evaluated in three experiments with a common allotmen t and several overlapping treatments. Steers (n = 432; initial weight = 326 +/- 26 kg) were divided into three BW blocks and allotted random ly to 72 pens and 24 treatments. Steers were fed steam-flaked, sorghum grain-based finishing diets for 124 to 166 d. Diets with 20% RE decre ased gain efficiency and 30% RE diets decreased both gain (linear, P < .07) and efficiency (linear, P <.001) compared with 10% RE diets. Redu cing roughage level during the mid-finishing period improved overall g ain efficiency 2, 7, and 24% (P >.2, <.05, and <.001, respectively) fo r the 10, 20, and 30% RE diets, respectively. Steers fed cottonseed hu lls consumed more feed (9.6 sis 8.8 kg/d, P <.001) but tended to gain less (1.53 vs 1.58 kg/d, P =.11) than steers fed alfalfa, were leaner, and had fewer carcasses grading Choice (62 vs 77%, P <.05). Feeding 4 .6% tallow decreased DMI(P <.05) and improved gain efficiency (P <.05) compared with 1.2% tallow. The BRITX steers consumed more feed (6%, P <.001) but were somewhat less efficient (3.5%, P <.05) than BRX steer s. Carcasses from BRITX steers tended to be fatter than carcasses from BRX steers and more of them graded Choice (62 vs 37%, P <.01). Commer cial BRX steers did not perform as well as BRITX steers on higher-ener gy-density diets (4.6% tallow or variable roughage regimen). Knowledge of the genetic background of feeder cattle can be important in the se lection of dietary energy density and marketing expectations.