Differential sensitivity of ovarian follicles to gonadotrophin stimulation in broiler and layer lines of domestic fowl

Differences in the reproductive potential of female broiler parent and layer fowl are related to differences in the recruitment of follicles to the preovulatory hierarchy. Broiler and laying hens were fed ad libitum or a restricted diet during rearing and ad libitum after sexual maturity. The sensitivity of thecal tissue, from ovarian follicles of different sizes, from layer and broiler fowl to stimulation by pregnant mares' serum gonadotrophin (PMSG) was determined to test the hypothesis that differences in egg production are associated with changes in the sensitivity of ovarian follicles to stimulation by gonadotrophin. The activity of ornithine decarboxylase 0.5-4 h after injection of 200 iu PMSG kg-1 body mass was assessed in thecal tissue from small white follicles 2-3 and 5-6 mm in diameter and in the smallest preovulatory yellow follicle. Basal activity was similar in layers and broiler breeders. PMSG-stimulated ornithine decarboxylase activity in follicles of 5-6 mm diameter was greater in broiler breeders than in layers. There was no difference between the lines in PMSG-stimulated ornithine decarboxylase activity in follicles of 2-3 mm diameter or in small yellow follicles. It was concluded that the low egg production and high prevalence of eggs with defective shells in broiler parents compared with layers may be related to the increased sensitivity of follicles to endogenous gonadotrophins at the critical stage of recruitment to the preovulatory hierarchy.