The effect of photoperiodic history on egg laying in dwarf broiler hens.

An experiment was conducted in dwarf broiler breeders to determine whether the decrease in egg production towards the end of a laying year is due to the development of photorefractoriness or to age per se. The birds were reared on short days (8 h light/day) and subjected to dietary restriction to prevent BW exceeding the minimum required to support egg production. They were transferred at 22 wk of age to long days (20 h light/day) while controls were maintained on short days for a further 2, 14, or 30 wk. At the end of these periods, dietary restriction was relaxed to stimulate egg production, and the control short-day birds were transferred to long days. The number of days to the onset of egg laying after lifting dietary restriction was less in short-day controls than in hens exposed to long days for 14 wk [19.5 +/- .5 (SE) versus 24.0 +/- 1.1 days, P < .01] or 30 wk (26.4 +/- 1.3 versus 41.2 +/- 1.1 days, P < .001). The number of eggs laid during the 12 wk after first oviposition in hens exposed to long days for 2, 14, or 30 wk before relaxing dietary restriction was not significantly different from that of the short-day controls. The numbers of eggs laid during the 12 wk after first oviposition decreased (P < .001) as the hens became older irrespective of photoperiodic history. These observations indicate that in dwarf broiler breeders, photoperiodic history, and therefore the development of photorefractoriness, is less important than age per se in maintaining intensity of egg laying in the 1st laying yr.