VARIATION IN THE ABILITY OF A LONG-DAY FOLLOWED BY A SHORT-DAY PHOTOPERIOD SIGNAL TO INITIATE REPRODUCTIVE ACTIVITY IN EWES AT DIFFERENT TIMES OF THE YEAR

Three experiments examined the importance of the time and duration of exposure to a long day followed by a short day photoperiod signal in i nitiating reproductive activity in ewes. In Expt I, ewes were maintain ed on short days (8.5 h light:15.5 h dark) from 21 December interrupte d with either 105 long days (18 h light:6 h dark; LD) from 9 February or 35 LD from 9 February, 16 March or 20 April. Exposure to long days followed by short days advanced the onset of reproductive activity in comparison to control ewes maintained on simulated natural photoperiod . Exposure to long days for 105 days delayed the onset of reproductive activity (August 2 +/- 3 days; P < 0.05) compared with 35 days beginn ing on the same date Bury 13 +/- 5 days). The interval from the end of the long day signal to the onset of reproductive activity was shorter (P < 0.001) however, after 105 LD than after 35 LD. In Expt 2, contro l ewes were moved from natural photoperiod to simulated natural photop eriod on I November and subsequently exposed to short days from 21 Dec ember. Four other groups were also exposed to this basic photoperiodic signal sequence but it was interrupted with either 70 LD from 16 Nove mber, or 35 LD from 16 November, 21 December or 20 April. More ewes (P < 0.05) initiated reproductive activity after exposure to 70 LD from 16 November and 35 LD from 21 December or 20 April compared with contr ol ewes maintained on short days or ewes given 35 LD from 16 November. The interval from the end of long days to the onset of reproductive a ctivity was less (P < 0.01) in ewes given 70 LD than in ewes given 35 LD. In Expt 3, ewes on natural photoperiod were given either 90 LD fro m 21 September, 35 LD from 21 September, 26 October, 30 November, 4 Ja nuary or 8 February followed by short days. The majority of ewes that received long followed by short days after the winter solstice resumed reproductive activity. However, all photoperiod signals given between the autumn equinox and the winter solstice failed to initiate reprodu ctive activity in ewes during the experiment. Thus we conclude that, i n ewes, the reproductive neuroendocrine axis is insensitive to long da ys followed by short days between the autumn equinox and the winter so lstice. The reproductive axis of ewes regains sensitivity to the induc tive effects of long days followed by short days at a time close to th e winter solstice. Between the winter and summer solstices, long days followed by short days maintain the anoestrous state and provide the c ue for initiation of reproductive activity.