RESUMO
Auction-derived feeder calves (n = 1,601; initial BW = 273.5 ± 4.7 kg) were used to examine effects of delayed administration of the initial steroid implant on health, performance, and carcass characteristics of feedlot cattle. Steers were procured from multiple auction markets in the southeastern United States and shipped to a central Kansas feedyard over a 6-wk period from December 2009 to January 2010. Steers were rested overnight before processing, then were randomly assigned, within arrival block, to 1 of 2 treatments: 1) implanted with Revalor-XS (40 mg estradiol and 200 mg trenbolone acetate) immediately upon arrival or 2) implanted with Revalor-XS 45 d postarrival. Cattle were weighed by pen using a group scale immediately after processing of each block. Feed deliveries were measured using load cells on feed trucks and recorded daily. Cattle were evaluated daily for morbidity and mortality by feedyard health personnel. Sick or injured cattle were removed from the home pen for further diagnosis and treatment. Individual animal health data were obtained and recorded daily. Final BW was calculated by dividing HCW by average dressing percent of the pen. Carcass data (quality grade and yield grade) were obtained by USDA personnel; presence of lung lesions, pleural adhesions, and liver abscesses were evaluated by trained university personnel. Delaying the initial implant tended to reduce morbidity (24.7 vs. 28.5%; P = 0.13) and reduced the percentage of animals requiring early salvage harvest due to chronic, nonresponsive respiratory disease (1.8 vs. 3.3%; P = 0.02). However, there were no effects of timing of implant administration (P ≥ 0.31) on rates of retreatment, mortality, lung lesions, or pleural adhesions. Implanting immediately upon feedlot arrival resulted in no effect on ADG or feed conversion (P ≥ 0.30). Cattle implanted upon arrival had numerically greater HCW and yield grade vs. cattle implanted on d 45; however, these differences were not significant (P ≥ 0.16). Delaying the initial implant 45 d did not greatly influence animal health, performance parameters, or carcass characteristics in feeder calves at high risk of developing bovine respiratory disease.