Selection for weight gain or food conversion in broilers affects the progesterone production capacity of large folucles in the reproductive adult breeders

1. Selection for leanness by selecting for food conversion (FC‐line) or selection for rapid growth and, thus, a concomitant fat increase (GL line) has also resulted in a lower egg laying rate of the fat birds. In birds of both lines that were effectively laying, plasma progesterone (P4), but not oestradiol concentrations were significantly different, with the lower concentration found in the GL line. This difference could not be attributed to differences in follicle population because this was similar in both groups of selected birds. 2. The in vivo decreased P4 secretion was investigated in vitro using granulosa cells of Fl follicles in a primary cell culture system. Basal P4 secretion tended to be lower in the GL line, whereas LH stimulation resulted in a diminished difference between the two lines. This was mainly caused by a higher relative P4 increase in the GL‐line. 3. Pre‐incubation with FSH increased the LH stimulated P4 secretion equally in both lines, probably through an increase of LH ...