SELECTION FOR WEIGHT-GAIN OR FOOD CONVERSION IN BROILERS AFFECTS THE PROGESTERONE PRODUCTION CAPACITY OF LARGE FOLLICLES IN THE REPRODUCTIVE ADULT BREEDERS

1. Selection for leanness by selecting for food conversion (FC-line) o r 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 progestero ne (P4), but not oestradiol concentrations were significantly differen t, 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 decr eased P4 secretion was investigated in vitro using granulosa cells of Fl follicles in a primary cell culture system. Basal P4 secretion tend ed to be lower in the GL line, whereas LH stimulation resulted in a di minished difference between the two lines. This was mainly caused by a higher relative P4 increase in the GL-line. 3. Pre-incubation with FS H increased the LH stimulated P4 secretion equally in both lines, prob ably through an increase of LH receptors. 4. The results demonstrate t hat selection for lean or fast growing, fat birds can also change proc esses apparently unrelated to the selection criteria, but with importa nt effects on reproduction characteristics.