Effects of chlortetracycline and Synovex-SReg. on growth rate and on plasma growth hormone and thyroid hormone concentrations following administration of thyrotropin-releasing hormone and GH-releasing hormone in beef steers.

摘要

Twenty-four Angus steers (365 kg +or-4) were assigned randomly to a 2x2 factorial arrangement of treatments of either 0 or 350 mg chlortetracycline d-1, with or without Synovex-S to test the effects of chlortetracycline (CTC) and estrogenic implant on release of growth hormone (GH) and thyroid hormones. Steers received ad libitum a concentrate-forage diet over a 112-d feeding study, and growth and carcass characteristics were determined. On days 30, 56, and 106, steers received an injection, via jugular catheter, of 1.0 micro g kg-1 BW thyrotropin-releasing hormone (TRH)+0.1 micro g kg-1 BW GH-releasing hormone (GHRH) and blood was collected from -30 to 360 min post-injection. Overall, compared with non-implanted steers, Synovex-S increased (P<=0.009) rate and efficiency of gain, decreased (P=0.05) time to peak for GH concentrations, whereas baseline concentrations of thyroid-stimulating hormone (TSH) were increased (P=0.03). Additionally, the TSH response curve (P<=0.05) and magnitude of triiodothyronine (T< sub>3; P=0.01) response were greater following TRH+GHRH challenge for implanted versus non-implanted steers. There were no main effects of CTC; however, CTC attenuated (P<=0.05) the effects of implant on T< sub>3 release, slaughter weight, and carcass composition. In conclusion, Synovex-S alone increased BW gain and enhanced the response of GH, TSH, and T< sub>3 to a TRH+GHRH challenge, whereas CTC alone did not affect rate and composition of gain or release of pituitary and thyroid hormones. However, CTC appeared to mitigate the effects of implant on T< sub>3 release and carcass composition.