Synovex ONE Grower improves growth performance for at least 200 days in growing beef steers and heifers fed in confinement for slaughter.

Growth-promoting implants are broadly used in the feedlot industry to improve growth performance and to increase production efficiencies. With cattle being fed longer and to heavier weights, there is demand for extended-release implants that payout for at least 200 d. Our objective was to evaluate feedlot growth of Synovex ONE Grower, a moderate potency (150 mg trenbolone acetate [TBA] and 21 mg estradiol benzoate [EB]), extended-release, growth-promoting implant for 200 d. At four locations (Texas, Idaho, California, and Nebraska), 200 steers ( = 800; d 0 body weight [BW] = 320.2 ± 9.5 kg) and 200 heifers ( = 800; d 0 BW = 311.5 ± 9.5 kg) were blocked by BW and randomized to 1 of 2 treatments: 1) , empty subcutaneous needle inserted and extracted from the middle third of one ear; 2) , 150 mg TBA and 21 mg EB extended-release implant administered in middle third of one ear. Treatments were commingled within pen of the same sex ( = 4/site; 2/sex/site) in a split plot design replicated across four sites. Cattle were fed finishing ration ad libitum common to each geographical region at least once daily and were observed for any abnormal health events twice daily. Treatments were administered on d 0. Mid-study implant site evaluations were performed on d 35 or 41. Initial BW was recorded on d 0 and final BW was recorded on d 200 to 204. Cattle were harvested from d 201 to 231; however, carcass data were not collected due to slaughter facility complications brought on by the COVID-19 pandemic. Data were analyzed using the PROC MIXED and PROC GLIMMIX procedures of SAS (Version 9.4, SAS Institute, Cary, NC; < 0.05), and animal was the experimental unit. There were no treatment × sex interactions ( ≥ 0.052) for any variable. Final BW on d 200 was greater ( < 0.01) for steers and heifers implanted with ONE Grower compared to Control; ONE Grower improved final BW by 5.7% for steers and 3.9% for heifers. Overall average daily gain (ADG) from d 0 to 200 was greater ( < 0.01) for ONE Grower steers and heifers compared to Control with an increase in ADG of 13.1% for steers and 8.9% for heifers. For cattle implanted with ONE Grower, implant retention rates at d 35 or 41 were 95.7% and 96.3% for steers and heifers, respectively. There was no difference ( ≥ 0.32) in percentage deads, removals, or bullers (steers) between treatments. Synovex ONE Grower improved final BW and ADG in feedlot steers and heifers fed for at least 200 d.