Effect of Synovex-S on growth rate and plasma thyroid hormone concentrations in beef cattle.

Abstract

Eighty beef steers (320 kg initial body weight) were equally divided among 4 lots ajid fed ad libitum an all-concentrate diet to determine if plasma thyroid hormone concentrations differed between nonimplanted (NI) and Synovex-S implanted (SI) steers. Ten steers in each lot were implanted at the start of a 120-day trial, and jugular blood was obtained at Days 60 and 120 for plasma analysis of thyroxine (T4), triiodothyronine (T3) and T3-resin uptake. Plasma T4 and T3 concentrations were determined by specific double antibody radioimmunoassay. Average daily weight gains for the NI and SI steers were 1.65 vs 1.96 kg (P<001) from 0 to 60 days and .89 vs 1.16 kg (P<001) from 61 to 120 days. Concentrations of T4 (μg/100 ml) for NI and SI steers were 6.6 vs 7.3 (P<.001) at 60 days and 6.1 vs 6.3 at 120 days. The respective values for the concentration of T3 (εg/100 ml) were 162 vs 160 and 151 vs 145; T3-resin uptake ratio, 1.02 vs .99 (P<05) and 1.00 vs .97 (P<.05) and calculated Free Thyroxine Index, 6.71 vs 7.21 (P<05) and 6.08 vs 6.23. These data suggest that free plasma T4 was greater in SI steers during the initial half of the trial and that throxine binding capacity was slightly increased throughout the trial.