Effect of Long Photoperiod (L:D 18:6) on Organ Growth Kinetics, Histology and Serum Profiles of T3, T4, Corticosterone and Progesterone during Post-Hatched Development

The pullets of Indian Rhode Island Red (RIR) breed of domestic fowl were reared under a step-down photoperiod of LD 18:6 (long photoperiod; LP) from day 1 to day 90 post-hatch, and then shifted to LD 12:12 (NLD), to assess its effect on growth kinetics of lymphoid and reproductive organs, and serum hormone profiles, the observations were made at 30,60 and 90 days and the results were compared with pullets reared under NLD alone. The weights of thyroid and ovary in LP chicks showed significant increment, whereas the weights of liver and lymphoid organs were identical to that of NLD chicks. The weights of adrenal and oviduct decreased significantly. In general, serum levels of T₃, T₄, corticosterone and progesterone tended to show a gradual decrement with age in NLD chicks, but the LP chicks showed a differential change marked by high T₃ and T₄ levels throughout and, increased corticosterone and decreased progesterone levels at 30 and 60 days. Long photoperiod seems to have an initial depressive, but later stimulatory effect on growth of thyroid, while it has an Inhibitory effect on adrenal. The histometric data of ovary suggest a stimulatory effect of light, indicated by increased number of follicles and decreased degree of follicular atresia. It is concluded from the present observations that exposure of chicks to LP has differential effects on growth kinetics and, a favourable influence on reproductive hormones suggesting inherent photosensitivity and, no setting in of photorefractoriness during this period.