EFFECTS OF CONSECUTIVE THIOURACIL EXPOSURES IN THE JUVENILE AND ADULTSINGLE COMB WHITE LEGHORN CHICKEN ON BODY-WEIGHT AND REPRODUCTIVE-PERFORMANCE

The effects of consecutive exposures to dietary thiouracil (TU) in juv enile and adult Single Comb White Leghorn chickens on plasma thyroxine (T-4) concentrations, egg production (EP), egg weight (EW), and eggsh ell quality were determined. Thiouracil was provided in the feed at le vels of 0, 0.1, or 0.2% (PTRT) from 0 to 6 wk of age and at levels of 0 or 0.1% (TRT) from 32 to 38 wk of age. Body weight gain was stimulat ed but T-4, EW, EP, and eggshell quality were generally reduced by 0.1 % TU TRT. However, TU PTRT alleviated a latent depressing effect of TU TRT on BW after 38 wk of age. Thiouracil PTRT, particularly at the 0. 2% level, induced significant decreases in EW but increased EP between 32 and 50 wk. The effects of early thyroid suppression in juveniles w ith TU PTRT on the subsequent reproduction of adults were primarily in response to a delay in the onset of sexual maturity, and not directly to prolonged responses in T-4 or BW that extended into lay.