Reimplant strategies improve rate of weight gain in growing beef steers and heifers fed in confinement for slaughter for at least 200 days

Objective

Our objective was to evaluate growth performance of feedlot steers and heifers administered a sham terminal implant or Synovex Choice, Synovex Plus, or Synovex ONE Feedlot terminal implant 60 or 120 d after initial implantation with Synovex Choice.

Materials and Methods

Two studies (steer and heifer) were conducted across 4 locations using 2,400 crossbred steers (n = 1,200; BW = 300.5 ± 31.1 kg) and heifers (n = 1,200; BW = 296.5 ± 27.5 kg). Within sex, cattle were blocked by BW and pen and randomized to 1 of 4 treatments: (1) Choice-Sham, initial implant of Synovex Choice (Zoetis Inc., Parsippany, NJ; 100 mg of trenbolone acetate [TBA] and 14 mg of estradiol benzoate [EB]) fol- lowed by terminal reimplantation of an empty needle; (2) Choice-Choice, initial implant of Synovex Choice followed by terminal reimplantation with Synovex Choice; (3) Choice-Plus, initial implant of Synovex Choice followed by terminal reimplantation with Synovex Plus (Zoetis Inc.; 200 mg of TBA and 28 mg of EB); and (4) Choice-One, initial implant of Synovex Choice followed by terminal reimplantation with Synovex ONE Feedlot (Zoetis Inc.; 200 mg of TBA and 28 mg of EB). Within sex, each site was assigned randomly to reimplant on either d 60 or 120. Data were pooled across reimplant interval by sex. Within sex, animal was the experimental unit and treatments were commingled within pen (n = 3 or 4 pens per site per sex). Body weights were recorded on d 0 and 200.

Results and Discussion

Overall ADG from d 0 to 200 was greater for steers (P < 0.01) and heifers (P ≤ 0.02) in the Choice-Choice, Choice-Plus, and Choice-One treatments compared with Choice-Sham, with no differences (P ≥ 0.15) between reimplant treatments. Minimal differences were noted in implant site retention and reaction rates between treatments within sex. There were no differences (steers, P = 0.67; heifers, P = 0.51) between treatments for the percentage of deads and removals nor the incidence of bullers (P = 0.80).

Implications and Applications

Synovex Choice as an initial implant followed by terminal implantation with Synovex Choice, Synovex Plus, or Synovex ONE Feedlot 60 to 120 d later improves ADG in feedlot steers and heifers for at least 200 d.