Seasonal variation in the egg production of fowls: effect of temperature and change of day length

Abstract

1. Two groups of pullets were reared on a 12 and a 23½ hr. day length respectively and at an environmental temperature of 64–65° F. Two months after the first bird came into lay, the day length of the second group was reduced to 12 hr. over a period of about 8 weeks (phase I).2. Both groups came into production at the same time, but the group reared on a 23½ hr. day grew more rapidly.3. The reduction in day length caused moulting and low production for several months, whereas the group which had been reared in a 12 hr. day continued to lay steadily.4. Soon after the reduction of day length half of each group (subgroups B and D) was exposed to an artificial thermal autumn and winter, which had no detectable effect on production (phase II). After this the temperature was allowed to rise to 64–65° F.5. When the day length of the remaining birds (subgroups A and C), still kept at 64–65° F., was increased to 23½ hr., there was no significant increase in production (phase III).6. When their day length was then reduced to 12 hr., they moulted heavily and went out of production (phase IV).7. Simultaneously the day length of subgroups B and D was increased to 16J hr. (phase IV). This had no significant effect on production.8. The yield of the birds which had their day length reduced at the beginning of the experiment was twenty-three ovulations less than that of the others at the end of the artificial winter, and nineteen less at the end of the laying year. The laying year was concluded before phase IV. There was no difference in yield between the birds which had an artificial autumn and winter and those which lived in a warm environment.9. Data are given on the frequency distribution of the yield after a reduction in day length, over the remainder of the year, and also on the correlation between the yields in these two periods.10. The eggs of the group reared in a 23½ hr. day length tended to be larger than those of the other group in conformity with their larger body weight. Apart from this the various treatments had no effect on egg weight.11. There was no relation between the egg yields and the variations in absolute and relative humidity.12. In a subsidiary experiment it was found that, if the reduction in day length was completed a month before the birds began to lay, there was no significant effect on subsequent production.13. In the light of these experiments it seems that the essential cause of winter pause and the annual rest, in birds given supplementary lighting, is the decline in day length in late summer and autumn.