Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 17, 1998
Publication Date: N/A
Interpretive Summary: Chlortetracycline has been used in cattle diets at sub-therapeutic levels since the 1950's. Growth promotion to varying degrees has been attributed to this practice, but the literature is void of studies that investigate the direct effects of CTC-feeding on endocrine mechanisms in ruminants. This study showed that sub-therapeutic feeding of CTC to young growing beef fsteers reduced the sensitivity of the pituitary to a releasing hormone challenge. This may be a factor that helps explain the results of earlier studies that showed an effect of CTC on performance, carcass composition, and(or) quality, and may promote improved energy efficiency of tissue deposition but not necessarily promote muscle deposition.
Technical Abstract: The effect of a sub-therapeutic level of chlortetracycline (CTC) fed to growing beef steers under conditions of limited and adequate dietary protein was determined on plasma concentrations of growth hormone (GH) and thyroid hormones before and after a challenge with thyrotropin releasing hormone (TRH) + GH releasing hormone (GHRH). Thirty-two young beef steers (Avg BW = 285 kg) were assigned to a 2 x 2 factorial arrangement of treatments of either a 10% or 13% crude protein diet (70% concentrate, 15% wheat straw, and 15% cottonseed hulls) and either a corn meal carrier or carrier + 350 mg CTC daily. Steers were fed ad libitum amounts of diet for 56 d and then each steer was injected via the jugular cannula with 1.0 ug TRH + .10 ug GHRH/kg BW in 10 mL saline) at 0800. Blood samples were collected at -30, -15, 0, 5, 10, 15, 20, 30, 45, 60, 120, 240, and 360 min after releasing hormone injection. After 84 d on trial, the pituitary and samples of liver were collected and analyzed for 5'-deiodinase activity. CTC attenuated the response to releasing hormone challenge of GH by 26% for both area under the response curve (P < 0.03) and peak response (P < 0.10). CTC attenuated the response to releasing hormone challenge of T4 for area under the curve by 12% (P < 0.08) and peak response by 14% (P < 0.04). Type II deiodinase activity in the pituitary was 36% less (P < 0.02) in CTC-fed steers compared to steers not fed CTC. Feeding sub-therapeutic levels of CTC to young growing beef cattle may influence tissue growth regulation via the pituitary.