Effect of constant and of changing photoperiod on plasma LH and FSH concentrations and age at first egg in layer strains of domestic pullets

1. ISA Brown pullets were transferred from 8 to 14 h or from 14 to 8 h phot operiods at 35 or 56 d of age. Controls were maintained on constant 8 or 14 h photoperiods from day 1. 2. Blood samples were obtained immediately before each daylength change and subsequently at 7 d intervals until 1st egg in the treated groups and at 7 0 d of age and then at 14 d intervals until Ist egg in the constant photope riod controls. Plasma luteinising hormone (LH) and follicle stimulating hor mone (FSH) concentrations were determined using homologous radioimmunoassay s. 3. prior to 16 weeks, LH was consistently higher in birds on constant 14 h photoperiods than in those on constant 8 h, but was down-regulated as birds approached maturity so that LH concentrations in the 2 groups were similar during the final 10 d before the first egg was laid. FSH concentrations ro se steadily with age but with a tendency for concentrations to be higher in the 8 h than in the 1 1 h treatment. Birds on constant 8 h daylengths matu red 18.3 d later than those on constant 14 h photoperiods. 4. A 6 h increment in photoperiod given at 35 d or 56 d, resulted in an inc rease in LH within 7 d in both cases. FSH concentration did not respond to an increase in photoperiod at 35 d but rose following the same increase at 56 d. This was associated with a 3-week advance in sexual maturity, whilst age at Ist egg in birds photostimulated at 35 d was similar to the age with a constant 14 h photoperiod. 5. LH concentration fell when photoperiod was reduced from 14 to 8 h at eit her 35 or 56 d and remained below the constant 8 h controls for many weeks before rising to a concentration not significantly different from other gro ups in the final 10 d before 1st egg. FSH concentrations in birds exposed t o a decreased daylength at 35 d, although more oscillatory, were similar to the constant 8 h photoperiod controls. In birds exposed to the same decrea se at 56 d, FSH concentration initially tumbled but was similar in tie 2 gr oups during the latter stages of rearing; neither differed significantly fr om the constant daylength controls during the 60 d before 1st egg. Sexual m aturity in both groups given a reduction in photoperiod was delayed by abou t 2 weeks compared with constant 8 h controls. 6. Change in FSH concentration following an increase in daylength was a bet ter predictor of age at Ist egg than change in LH. However, I;SH concentrat ions after 14 weeks of age were rather similar in short day and long day co ntrols and in the 2 groups given reductions in photoperiod at 35 d and 56 d , despite differences of nearly 5 weeks in mean age at Ist egg amongst thes e 4 treatments.