PLASMA-CONCENTRATIONS OF CORTICOSTERONE AND THYROID-HORMONES IN BROILERS PROVIDED VARIOUS LIGHTING SCHEDULES

The purpose of this study was to measure plasma corticosterone and thy roid hormone concentrations in broilers exposed to various photoschedu les. Day-old male broilers were placed on litter floors in light-contr olled chambers. Four chambers were randomly assigned to each of four l ight treatments: 1) 23 h light (L):1 h dark (D) from 1 to 56 d of age (designated extended, E); 2) 1L:3D from 1 to 56 d (intermittent, I); 3 ) 6L:18D from 1 to 14 d and 1L:3D from 15 to 56 d (brief-L BI); and 4) 6L:18D from 1 to 14 d and 23L:1D from 15 to 56 d (brief-E, BE). Blood samples were collected 0, 4, and 20 h after lights-on (1200 h) at 13, 41, and 55 d of age. Corticosterone concentration did not differ among light treatments or collection times and was decreased at 41 d compar ed with 13 d (.65 vs 2.11 ng/mL). Triiodothyronine (T3) increased and thyroxine (T4) decreased with age. At 13 d, there were light treatment by sampling time interactions for T3 and T4. Plasma T3 was elevated i n Treatments BI (3.11 ng/mL) and BE (3.40 ng/mL) compared with Treatme nts E (2.39 ng/mL) and I (2.30 ng/mL) at 0 h; the former two treatment s showed decreased T3 concentrations at 4 and 20 h compared with 0 h. Plasma T4 showed reciprocal changes to T3. There were no differences i n T3 or T4 for light treatments or sampling times at 41 and 55 d. The light treatments did not cause severe stress, and elevated T3 concentr ations in Treatments BI and BE at 13 d were most likely associated wit h feeding during the dark period.