Effect of Exogenous Progesterone on Luteinizing Hormone Secretion in Domestic Turkey Hens at Different Reproductive States

Abstract

To determine effects of progesterone (P4) treatment on luteinizing hormone (LH) secretion in turkey hens, two trials were conducted. Trial 1 was to determine changes in LH, P4, and testosterone (T) during photostimulation. Photosensitive turkey hens were maintained under short days (SD) of 6 h light and 18 h dark. At the beginning of Trial 1, blood samples were taken daily for 4 days, then one-half of the hens were switched to long days (LD) of 14 h light and 10 h dark, and daily blood samples were continued until 5 days after eggs were laid by all the hens switched to LD. Concentrations of LH, P4, and T increased significantly 1 day after switching hens from SD to LD, but the increase in P4 was initially low with a further increase occurring by 3 days prior to first eggs. In Trial 2, turkey hens were injected with exogenous P4 to determine if P4 is an initiator of the preovulatory surge of LH. P4 or vehicle were injected im in hens at three different reproductive states: (1) while hens were maintained under SD, (2) on the 5th day after hens were switched from SD to LD, and (3) after hens were laying for 1 week. The hens were serially bled at 10-min intervals for 8 h to monitor changes in LH and P4. After 2 h of serial bleeding, P4 or vehicle was injected and bleeding was continued for an additional 6 h. After P4 injection, its concentration increased rapidly from a base level of 0.25–1.20 ng/ml to a postinjection high level of 4.42–6.10 ng/ml within 20 min. The high level of P4 was then maintained throughout the remaining 6 h. No increases of LH secretion were observed after P4 or vehicle injection in hens at either State 1 or State 2. Small increases of LH secretion were detected about 2 h after P4 injection in hens at State 3, but these increases were not significantly above vehicle-injected controls. Thus, there was no positive feedback effect of P4 injection on LH secretion in this trial. These results suggest that P4 might not induce LH secretion in immature or mature turkey hens and might not be the factor which induces the preovulatory surge of LH in laying turkey hens. Nonsteroidal factors of ovarian origin might be involved in regulating the preovulatory surge of LH in turkey hens.