Day-old tom poults were exposed to 1 of the following light treatments: continuous incandescent light (24L[light]L), 23L:1D [dark] incandescent light, 12L:12D incandescent light or natural day lengths only. Mortality was recorded daily. At wk 1, 2, 4, 6 and 8, after poult placement, body weights and feed consumption were measured. At each of these times, 10 poults were sacrificed from each treatment group for evaluation of plasma corticosterone. Day lengths utilized in this study had no consistent effect on poult-performance parameters except from 0-2 wk of age. During this time, poults in the 24LL and 23L:1D treatments consumed less feed but were similar in body weight to poults in the other treatments. Adrenal function was also not consistently affected by day length. In all treatment groups, plasma corticosterone was lowest during the 1st 2 wk posthatching, when compared to hormone levels at 4, 6 and 8 wk. The lower corticosterone that occurred during the 1st 2 wk posthatching may have been involved in early poult mortality.
