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Feedlot heifers (n = 3,778; initial BW = 310 kg; SD = 28 kg) were utilized in a 3 x 2 factorial arrangement (9 pens/treatment) to investigate increasing implant dose and terminal window (TW) timing on growth and carcass merit. Component with Tylan® implant dose treatments were 1. Initial TE-IH [80 mg trenbolone acetate (TBA), 8 mg estradiol (E2)] and terminal TE-H (140 mg TBA, 14 mg E2; IH/H), 2. Initial TE-IH and terminal TE-200 (200 mg TBA, 20 mg E2; IH/200), or 3. Initial TE-200 and terminal TE-200 (200/200). Heifers were fed 171 d with terminal implant administered 100 or 60 d prior to slaughter; initial implant window was inverse (71 or 111 d) to TW. Data were analyzed as a mixed model with block included as a random effect. No significant dose x TW interactions occurred for growth or carcass characteristics (P ≥ 0.06) therefore, main effects are presented. Terminal window treatments resulted in similar (P &amp;gt; 0.25) live performance. Heifers reimplanted 60 d prior to slaughter had improved (P &amp;lt; 0.05) marbling score and lower YG2 carcasses. Final weight, gain, HCW, yield, and LM area increased (linear; P &amp;lt; 0.05), while marbling score and fat thickness decreased (linear; P &amp;lt; 0.05) with increasing heifer implant dose. Heifers implanted with IH/200 had lower (P &amp;lt; 0.05) DMI compared to IH/H. IH/200 and 200/200 had improved (+2.1%; P &amp;lt; 0.05) efficiency compared to IH/H. HCW for 200/200 was +3.2 and 5.5 kg greater than IH/200 and IH/H, respectively. 200/200 were leaner with +7.4% YG 1&amp;2 and -5.4% YG 4&amp;5, but -8.5% Prime &amp; Choice carcass compared to IH/H. Shortening TW from 100 to 60 d prior to slaughter did not alter growth performance. Increased implant dose in heifers improved gain, efficiency, HCW and yield with some quality grade considerations.

273 Effect of Feedlot Implant Dose and Terminal Window Timing on Heifer Performance and Carcass Characteristics