Abstract There is limited information on the dose-dependent responses to trenbolone acetate (TBA) and estradiol benzoate (EB), in newly-weaned steers fed a low-energy (NEg = 1.06 Mcal/kg) diet during an initial dry-lot phase. This experiment evaluated the influence of TBA and EB dose on growth performance and feed efficiency in newly-weaned beef steers during a 91-day dry-lot phase. Charolais-cross steers (n = 240; initial shrunk BW = 251 ± 21.9 kg) were allotted to one of 24 uncovered concrete pens (n = 10 steers/pen) in a randomized complete block design. Each pen was assigned to 1 of 3 treatments: Control (no implant; NI), SYNOVEX Primer 50 mg TBA + 7 mg EB; Zoetis, Parsippany, NJ; PR, and SYNOVEX Choice 100 mg TBA + 14 mg EB; Zoetis; CH. Steers were individually weighed, processed (respiratory and clostridial vaccination and parasiticide application), then allocated to their study pen on day -1. The following morning (day 1), steers were again weighed and implanted (if applicable to the treatment) in the left ear, and an implant retention check was conducted 28 days later. The initial BW was the average of the BW measures collected on day -1 and day 1. Steers were fed twice daily in equal proportions, a forage-based diet containing 14.7% crude protein. For the first 14 days while calves were transitioning to milled feed, diet deliveries were managed using a prescribed feed delivery plan. From day 15 to 91, feed delivery followed a slick bunk management strategy to allow for ad libitum access to feed. Data were analyzed as a randomized complete block design with treatment as a fixed effect and block as a random effect. Linear and quadratic contrasts were used to assess treatment effects. Additionally, the analysis was conducted using implant dose as a continuous variable (0, 50, and 100 mg TBA; 0, 7, and 14 mg EB, equivalent to 0, 5, and 10 mg estradiol-17β) to obtain the dose-dependent rate of change in growth responses. Increasing doses of TBA and EB linearly increased (P = 0.01) BW (2.1% and 3.5%) and ADG (6.4% and 11.6%) for PR and CH compared to NI. Implant did not alter DMI (P = 0.57), which may be due to the controlled intake during the initial 14 d as cattle adapted to ad libitum intake; thus, G:F increased linearly (P = 0.01) by 7.5% and 13.8% with increasing TBA and EB dose. These results indicate that increasing implant potency during the dry-lot phase increases BW and ADG without altering DMI, resulting in improvements in G:F.
Peschel et al. (Wed,) studied this question.