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Abstract
1. This paper reviews evidence from 15 experiments, reported over a span of 44 years, in which pullets were reared from hatching to sexual maturity on 2 or more constant photoperiods. 2. The evidence strongly indicates that earliest age at first egg (AFE) was observed when pullets were held on constant 10 h days (though earlier maturity is easily induced by increasing the photoperiod during rearing). The pair of equations which best describe the relationship between AFE (y,d) and photoperiod (x,h) are for x < 10 h, y = 175.8- 1.731x; for x > 10 h, y=155.5 + 0.301x. 3. This 2-straight-line model, hinged at 10 h, should be used in preference to curvilinear models published earlier, which wrongly predict that pullets reared on long days (14h to 17h) mature faster than birds reared on constant 10 h.