The effects of stage of production and implant exposure on feedlot performance, carcass characteristics, and relative mRNA gene expression

Abstract Text: Black-hided heifers (n = 187; 362 kg) were used in a 122 d finishing study to determine the effects of a trenbolone acetate-estradiol implant [Revalor^® 200 (200 mg of trenbolone acetate and 20 mg of estradiol)] on feedlot performance, carcass characteristics, and relative mRNA gene expression when administered at specific stages of production.  Treatments included 1) no implant (CON); 2) implantation on d 0 (EARLY); or 3) implantation on d 56 (LATE).  A subset of heifers from each treatment were harvested at d 28 and 84 to collect LM samples that were utilized to measure relative gene expression involving myogenesis and intracellular signaling mechanisms.  After d 55, ADG (1.68 vs. 1.31 kg) and G:F (0.172 vs. 0.134) were improved for EARLY vs. non-implanted heifers (P <0.05).  From d 56 to 122, ADG improved with implantation and was greatest for LATE (P <0.05), while G:F was only improved by LATE (P <0.01).  Overall, implantation improved ADG (1.24 vs. 1.04 kg) and G:F (0.136 vs. 0.114) compared to CON heifers (P <0.01), regardless of timing.  Dry matter intake was not affected (P =0.41) by implantation protocol.  Implantation increased HCW (340 vs. 320 kg), dressing percentage (65.9 vs. 65%), and LM area (92.3 vs. 85.8 cm²) vs. non-implanted cattle (P <0.05), regardless of timing.  Back-fat, marbling score, and REA/HCW ratio were unaffected by treatment (P >0.18), as well as quality and yield grade distributions (P>0.21).  The mRNA expression of myostatin and insulin-like growth factor-1 were not affected by treatment at d 28 (P >0.18). At d 84, myostatin was significantly reduced in heifers that had been implanted compared to CON (5.01 vs. 8.06; P =0.02), regardless of timing.  Insulin-like growth factor-1 was not affected by treatment (P >0.18), but mRNA expression levels were decreased at d 84 compared to d 28 (31.8 vs. 50.2; P <0.05). The expression of paired box 7 was increased in EARLY cattle compared to CON cattle at d 28 (0.682 vs. 0.467; P =0.05).  Paired box 7 mRNA gene expression of EARLY heifers at d 28 was greater than LATE heifers at d 84 (0.682 vs 0.371; P <0.05).  The results of this study suggest that anabolic growth promotants improve cattle performance and production efficiency without altering carcass quality, independent of exposure time.  This study also indicates that stage of production has the greatest effect on relative mRNA gene expression.

Keywords: feedlot and carcass performance, gene expression, implantation