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Angus and Angus cross Limousin steers (n = 182; initial BW = 681 ± 61.2 lb) were used to evaluate the influence of an estradiol-trenbolone acetate implant (24mg/120mg) on production efficiency and carcass traits when administered at specific stages of growth. Treatments (TRT) were as follows: No Implant, control (NI), Early Implant, d 1, BW = 681 lb (EI); or Delayed Implant, d 57, BW = 845 lb (DI). Comparisons were also made between the NI and implanted treatments (I; EI + DI). Steers were procured at weaning and were backgrounded (47 d) prior to the initiation of the experiment. Initial predicted carcass composition was 14.9% protein, 13.3% fat, 54.6% moisture, and 17.2% bone. Days on feed was constant across TRT. After 56 d, ADG and gain efficiency (G/F) were improved (P < 0.01) by implants, (NI vs. EI; 3.70 vs 4.19 lb and 0.227 vs. 0.257). At d 57 predicted carcass composition was not different among treatments. From 57 to 112 d, DI caused higher ADG than NI or EI (NI 3.64, EI 3.46, and DI 3.92 lb; P < 0.05) and higher G/F (NI 0.155, EI 0.150, and DI 0.173; P < 0.01). Cumulative ADG (3.64 vs 3.81 lb; P < 0.05) and G/F (0.175 vs. 0.186; P < 0.01) were improved by implants for NI vs. I, respectively with no differences between treatments that involved implants. Cumulative DMI was similar for all TRT. Implants increased dressing percent (63.5 vs. 64.1%; P < 0.05), hot carcass weight (752 vs. 778 lb; P < 0.01), and LM area (11.9 vs 12.6 in2; P < 0.010) for NI vs. I, respectively. Ribfat and kidney, pelvic, and heart fat were unaffected by TRT. Treatment had no effect on the whole carcass proportions of fat, protein, or water. Implants advanced maturity scores (NI A51 vs. I A59; P < 0.01). Marbling scores were reduced (P < 0.05) by EI but not by DI (NI Small65, EI Small20, DI Small36). The percent intramuscular fat content of the LM was reduced (P < 0.10) by EI and was unaffected by DI (NI 5.1, EI 4.0, DI 4.8%). Treatment affected (P < 0.10) the proportion of carcasses with marbling scores greater than Modest0 (NI 23.6, EI 7.8, DI 22.6%). The results of this study suggest that growth of intramuscular fat is sensitive to anabolic growth promotants administered during early periods of growth.

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