Document Type

Thesis - Open Access

Award Date

2026

Degree Name

Master of Science (MS)

Department / School

Animal Science

First Advisor

Zachary Smith

Abstract

The receiving phase is a stressful transition period in beef production. Newly weaned cattle are offered low-energy, high-forage diets promoting lean tissue and bone development prior to the finishing phase. Steroidal implants containing trenbolone acetate (TBA) and estradiol benzoate (EB) enhance lean gain. Aggressive early implant exposure has been shown to negatively impact intramuscular fat deposition. This two-part study evaluated the initial and carryover effects of increasing doses of combined TBA and EB weaning implants on growth performance, serum metabolites, and carcass traits. In Exp. 1, newly weaned Charolais × Red Angus crossbred beef steers [n = 240 initially; body weight (BW) = 251 ± 21.9 kg], were allocated by BW into twenty-four pens (10 steers per pen; 8 pens per treatment), and assigned to one of three treatments: no implant (NI), 50 mg TBA + 7 mg EB (Synovex Primer, Zoetis, Parsippany, NJ; PR), or 100 mg T BA + 1 4 m g EB ( Synovex C hoice, Zoetis; CH), with implants administered subcutaneously in the middle of the ear on within 48 hours of weaning. Steers were fed a low-energy, high-forage dry-lot diet (1.08 Mcal/kg NEg) for 117 days. Following this backgrounding phase, steers were reallocated into two finishing studies (Experiment 2.a and 2.b). Steers received two finishing implants (100 mg TBA + 14 mg EB followed by 200 mg TBA + 28 mg EB), and were fed for a total of 299 to 304 days, respectively. In Exp 1, increasing doses of TBA and EB linearly increased (P < 0 .01) BW a nd average daily gain (ADG). Dry matter intake (DMI) had no effect from treatment (P = 0.67), but a treatment × day interaction (P = 0.03); thus, G:F linearly increased (P < 0.01) with higher TBA and EB doses. There was no treatment × day interaction for serum urea nitrogen (SUN) (P = 0.26) and non-esterified fatty acid (NEFA) concentrations (P = 0.90); however, there was a day effect for NEFA concentrations (P = 0.01). Cumulatively from weaning to harvest, final BW and ADG increased linearly (P < 0.01) with increasing weaning implant dose. Linear increases (P < 0 .01) w ere a lso observed in hot carcass weight (HCW) and adjusted final body weight (AFBW). Notably, weaning implant did not affect marbling score (P = 0.69) and tended to increase dressing percentage linearly (P = 0.10). Retail yield and calculated yield grade both showed quadratic effects (P = 0.03). A quadratic increase (P ≤ 0.01) was observed in 12th rib fat thickness and empty body fat percentage. Administering a higher-potency implant at weaning to steers gaining ~1.09 kg/d on a low-energy diet significantly improves backgrounding growth. This initial weight advantage carries over into the finishing phase.

Publisher

South Dakota State University

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Rights Statement

In Copyright