The Effects of Ahemeral Light and Dark Cycles on Egg Production in the Fowl

This review deals first with theoretical aspects of the subject and discusses a model which attempts to predict rates of lay for hens kept under non-24-hour (ahemeral) light-dark cycles. It is concluded that the model is unsatisfactory, chiefly because the intra-clutch interval for an individual hen does not remain stable under different light-dark cycles. It is suggested that the average interval between all ovipositions may be a more characteristic feature of a hen than the mean intra-clutch interval. New evidence is presented concerning the effect of ahemeral cycles on rate of lay, egg weight, yolk weight, albumen weight, shell weight, time of ovulation and time of peak luteinising hormone release. Hens exposed to 27 h. and 30 h. light-dark cycles lay larger eggs but fewer eggs than 24 h. controls. These effects are achieved rapidly and are readily reversible, so that ahemeral cycles may be used to manipulate egg-weight in practical situations. Eggs formed by hens under a 27 h. light-dark cycle remain, on average, 1 h. longer in the shell gland than those of 24 h. controls. There is also a 10% improvement in shell thickness associated with a 27 h. cycle but this may arise partly because eggs are laid during the dark period and only partly because of the longer oviducal term. The mechanism responsible for the timing of ovulation, which results in eggs being laid during darkness, is considered in detail. It is shown that oviposition times can be controlled by alternating bright light with dim light and the critical intensities needed to achieve phase-setting are described. Finally, it is argued that the mechanisms controlling rate of egg output and timing of oviposition are separate and, though both are influenced by light, the systems are clearly distinguishable so that the photoperiodic and the phase-setting properties of a light pattern should always be considered separately. Egg production can be modified by employing either the photoperiodic properties or the phase-setting properties of light, but the end results are not the same.