Effects of days on feed and trenbolone acetate + estradiol 17-β implantation on empty body composition and retained energy of Charolais × Angus steers across serially slaughtered endpoints

ABSTRACT Objective The objective of this experiment was to evaluate the effects of feeding duration and trenbolone ac-etate + estradiol 17-β implantation on empty body com-position of Charolais × Angus steers. Materials and Methods Pairs of Charolais × Angus steers (n = 80; BW of 271 ± 45 kg) were randomly allo-cated to treatment (Revalor-XS [REV] on d 0 and 190 or nonimplanted control [CON]) and slaughter date (0, 42, 84, 126, 168, 210, 252, 294, 336, and 378 d on feed [DOF]) in a 2 × 10 factorial design. Empty body composition (percentages of moisture, CP, ether extractable fat, and ash) was determined via proximate analysis of blood, hide, internal cavity components, bone, and carcass soft tissue. Data were analyzed using mixed models with d 0 BW as a covariate and pair as random effect. Results and Discussion No treatment × DOF in-teraction was exhibited for any variable. Empty body moisture decreased in a quadratic trend at approximately 0.0373%/d beginning at 61.9% on d 0 and ending at 47.8% on d 378. Empty body CP decreased linearly by approxi-mately 0.0071%/d beginning at 18.7% on d 0 and ending at 16.0% on d 378. Empty body fat increased linearly dur-ing the first 252 d at a rate of approximately 0.0466%/d starting from 14.0% on d 0, before plateauing at approxi-mately 31.8% from d 252 through d 378. Empty body ash remained constant during the experimental period (5.6%). Empty body CP (16.7% CON vs. 17.4% REV), empty body ash (5.4% CON vs. 5.7% REV), and empty body moisture (50.7% CON vs. 51.6% REV) were greater for REV steers. In contrast, empty body fat (27.3% CON vs. 25.3% REV) was greater for CON steers. Implication and Applications These data indicate that REV altered the percentage of compositional gain as evidenced by altering the ratio of CP accretion to fat accretion of the empty body. Moreover, REV partitioned available energy more efficiently by increasing CP weight while fat weight remained similar to controls, resulting in improved CP:fat accretion of the empty body.