THE EFFECT OF GROWTH-PROMOTING IMPLANTS AND FEEDING DURATION ON LIVE PERFORMANCE AND BEHAVIORAL CHARACTERISTICS, BIOMETRIC MEASUREMENTS, EMPTY BODY COMPOSITION, AND ENERGY RETENTION OF SERIALLY-HARVESTED BEEF STEERS

The current study strives to understand the effects of an extended release trenbolone acetate + estradiol-17β growth promoting implant upon live growth performance, feeding behavior, skeletal growth, and empty body composition of crossbred beef steers over a feeding period of 378 d compared to non-implanted controls. In experiment 1, Charolais x Angus steers {n = 80; start of trial body weight (BW) 271 ± 99 kg} were randomly allocated to implant treatment and harvest date in a 2 x 10 factorial experiment. Steers were paired within genetic group according to initial BW, frame score, and adjusted final body weight (AFBW). Within each pair, a steer was randomly allocated to one of two treatments; implanted with Revalor-XS (REV; 200 mg TBA + 40 mg E2 containing 4 uncoated pellets and 6 pellets with a proprietary time release coating technology) on d 0 and d 190 or non-implanted control (CON) to represent a non-hormone treated cattle (NHTC) marketing strategy. Eight steers comprised of 4 pairs were randomly assigned to 1 of 10 harvest dates at d 0, 42, 84, 126, 168, 210, 252, 294, 336, and 378 DOF. Steers were fed in open lot dirt surface pens (20 animals/pen) equipped with 4 GrowSafeTM nodes per pen. Body weight was recorded at each harvest period and individual consumption data were recorded multiple times per day via electronic identification (EID) reads every second. Average daily gain (ADG) was 9.4% greater (P < 0.01) for REV steers (1.42 kg REV vs 1.30 kg CON) during the 378d trial. Dry matter intake (DMI) did not differ (P = 0.15) between treatments but averaged 8.7 kg during the trial and decreased (P < 0.01) in a quadratic manner from 3.2% of BW during the initial period to 1.03% of BW after 378 DOF. Implanting with REV improved (P < 0.05) gain to feed ratio (G:F) by 8.2% during the trial, however a TRT x DOF interaction (P < 0.05) occurred after 293 DOF whereby REV steers became less efficient. Consumption visit frequency was 48.8 events/d during the initial period and decreased (P < 0.01) during the trial ending at 16.1 events/d. Daily consumption time began at 111 min/d during the initial period and decreased (P < 0.01) by 0.12 and 0.15 min/d for CON and REV, respectively. Implanted steers spent approximately 8 and 7.5 min/d less (P < 0.01) visiting and consuming feed, respectively. Singular consumption visits lasted 2.80 min/event during the initial period and increased (P < 0.01) 0.1 min/event for each 42 d period. Dry matter intake per consumption event began at 2.5 kg/event and increased (P < 0.01) 0.3 kg/event for each 42 d period. Implanted steers consumed 0.05 kg DM more (P < 0.01) per consumption event than CON. Consumption rate for each consumption event was greater (P < 0.01) for REV (290 g/min CON vs. 422 g/min REV). These data indicate live growth performance and feeding behavior were impacted by both growth enhancement technology and duration of finishing. In experiment 2, 24 h prior to each of the 10 harvest dates, all steers were measured for hip height, rump length, hip width, shoulder height, 2/3 body length (BL), body depth, body width, and frame score. Forty-eight h after harvest, a digital image was obtained of the lateral aspect of the right side of each carcass. Carcass area, maximum length and width were digitally measured against a common standard. Body width was 1.17 cm greater (P < 0.01) in REV steers and 2/3 BL tended (P = 0.06) to be greater in CON steers. Hip height increased (P < 0.01) 0.05 cm each d during the 378 d feeding period. Rump length increased (P < 0.01) 0.03 cm/d. Hip width increased (P < 0.01) 0.06 cm/d. Shoulder height increased (P < 0.01) 0.06 cm/d. Two-thirds BL increased (P < 0.01) 0.11 cm/d. Body depth increased (P < 0.01) 0.07 cm/d. Body width increased (P < 0.01) 0.04 cm/d. Steers administered REV yielded 516 cm2 greater (P < 0.01) surface area on ½ of the carcass, or one side, than CON; moreover surface area increased 21.0 cm2 /day. No TRT effect (P = 0.57) was observed for maximal carcass length, however maximal carcass width was 3.9 cm greater (P < 0.01) for REV steers. Steer carcasses increased 0.16 cm/day in length and 0.07 cm/day in width. These data indicate biometric measurements and carcass dimensions were impacted by both growth enhancement technology and duration of finishing. In experiment 3, during harvest and fabrication, samples were collected to determine empty body composition via proximate analysis of blood, hide, internal cavity components, bone, and carcass soft tissue. Proximate analysis of each tissue was multiplied by mass to assimilate empty body percentages of moisture (EBM), crude protein (EBP), ether extractable fat (EBF), and ash (EBA). Retained energy (RE) was calculated as the difference between the baseline steers (n = 8; initial steers harvest on d 0) and those representing each treatment across harvest endpoints, as was empty body weight gain (EBWG), hot carcass weight gain (HCWG), and ratio of HCWG to EBWG. Metabolic ratio (MR) was calculated as the ratio of RE to daily megacalorie consumption (DMC). Using individual laboratory results for each animal, the difference in total energy was calculated and averaged by treatment and harvest day by subtracting values for each animal from the mean of the baseline steers. Moisture, protein, fat, and ash accretion (MA, PA, FA, AA, g/d, respectively) was calculated as the difference measured in grams per day on trial amongst each treatment and harvest endpoint with the baseline steers representing d 0. Empty body moisture decreased (P < 0.01) 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 protein decreased (P < 0.01) linearly by approximately 0.0071%/d beginning at 18.7% on d 0 and ending at 16.0% on d 378. Empty body fat increased (P < 0.01) in a quadratic trend at approximately 0.0450%/d beginning at 14.0% on d 0, until d 294 and plateaued at approximately 32.0% through d 378. Empty body ash remained constant (P = 0.29) over the feeding period (5.5% - 5.8%). Empty body protein (16.7% CON vs 17.4% REV) and EBA (5.4% CON vs 5.7% REV) were greater (P < 0.01) for REV steers, and EBM (50.7% CON vs 51.6% REV) tended to be greater (P = 0.07) for REV steers. In contrast, EBF (27.3% CON vs 25.3% REV) was greater (P < 0.01) for CON steers. Ratio of EBP to EBF was less (P < 0.05) for CON steers (0.692:1) compared to 0.752:1 for REV steers and decreased (P < 0.01) approximately 0.002/d from at 1.378 on d 0 to 0.542 on d 378. Empty body weight gain was 2129 g/d during the initial 42 d period and decreased in a linear fashion (P < 0.01) to 1185 g/d at d 378, decreasing in gain 2.5 g/d. Hot carcass weight gain was similar as it decreased from 1479 to 962 g/d in a linear fashion (P < 0.01) across the feeding period, slowing by 1.4 g/d. Empty body moisture weight, EBPW, EBFW, and EBAW increased (P < 0.01) 520, 190, 490, and 80 g/d from d 0-378, all increasing (P ≤ 0.01) in a quadratic fashion. Implanted steers exhibited an increase (P < 0.01) in EBWG and HCWG (1695.5 and 1359 g/d) compared to CON (1457 and 1169 g/d), or a 239 and 190 g/d increase. Implanted steers also contained more (P < 0.01) EBMW, EBPW, and EBAW (+20.7, +8.5, and +3.1 kg) than CON. These results reflect PA, MA, and AA, as REV steers exhibited increased (P < 0.01) tissue accrual by +67.3, +162.3, and +19.1 g/d, respectively, compared to CON. Fat accretion did not differ over time (P = 0.42) and between treatments (P = 0.76). Protein to fat accretion ratio was greater (P < 0.01) in REV (0.48) compared to CON (0.35). Protein to fat accretion ratio also decreased (P < 0.01) across DOF starting at 0.54 on d 0 and ending at 0.39 on d 378. These data indicate that growth-promoting implants alter composition of gain during the finishing period and protein accretion is increasing at a decreasing rate while fat is increasing at an increasing rate.