Photoperiodic requirements for LH release in juvenile broiler and egg-laying strains of domestic chickens fed ad libitum or restricted diets

Photoperiod response curves for LH release were obtained for juvenile female domestic chickens at 8 weeks of age by measuring changes in plasma LH concentrations after increasing the daily photoperiod from 8 to 10.5, 12.75, 15.25, 17.75 or 20 h. The birds were bred either for meat production (broiler) or commercial egg-laying and were fed ad libitum or a restricted diet, similar to that used under commercial conditions. Ovarian and oviduct growth was stimulated by 2 weeks after transfer to 20 h light/day in the dwarf broiler strain, irrespective of the dietary treatment, but not in birds of the egg-laying strain. Baseline concentrations of plasma LH were higher in the egg-laying than in the dwarf broiler strain birds. A significant effect of dietary treatment was observed on the changes in concentration of plasma LH in the non-photostimulated dwarf broiler, but not in the egg-laying bird. There was no significant interaction between dietary treatment and photoinduced LH release in birds of either strain. The shortest photoperiod needed to stimulate LH release (critical daylength) was < 10.5 h in the dwarf broilers and between 10.5 and 12.75 in the egg-laying birds. The shortest photoperiod needed to stimulate the maximum release of LH (saturation daylength) was between 10.5 and 12.75 in the dwarf broiler strain. The saturation daylength in birds of the egg-laying strain was longer, being between 12.75 and 15.25 h. It is concluded that there are differences in the photoperiodic response between chickens of different breeds and that they are not modified by restriction of food intake to 50-60% of that in control birds fed ad libitum.