Are Ambient Short-Day Cues Necessary for Puberty in a Short-Day Breeder?1

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Abstract
Some exposure to long days (LD) is necessary for female sheep to achieve puberty during short days (SD). In this regard, a 5-wk block of long days in lambs otherwise raised in short days will result in puberty at the normal age. In the present study, Suffolk lambs were raised in various artificial photoperiods to assess the role of short days. An increase in circulating progesterone to luteal phase levels was used as the criterion for puberty. Controls exposed to short days except for 5 wk of long days between 17 and 22 wk (SD-LD-SD) began repetitive reproductive cycles at the expected age (34 .+-. 0.7 wk, mean .+-. SE). When the final block of short days was eliminated and exposure to long days was continued after 22 wk of age (SD-LD-LD), puberty was delayed (41 .+-. 0.8 wk of age). Removal of the first block of short days (LD-LD-SD) did not prevent normal puberty, and the initiation of cycles (29 .+-. 0.9 wk) occurred earlier than in controls. Lambs in which the pineal gland was denervated at 22 wk so that it was unable to transduce short-day cues after exposure to long days (SD-LD-X) exhibited puberty at an age (33 .+-. 1.2 wk) similar to that of controls. The results indicate that a) neonatal exposure to short days is not necessary for subsequent recognition of long days; b) exposure to long days during the final stages of sexual maturation, rather than short days, delays puberty, and c) long day lengths set events into motion for normal puberty many weeks later, which can occur in the absence of additional photoperiod information about season. These combined findings raise the possibility that the short days of spring and autumn are largely permissive to the onset of puberty and that it is the long days of summer followed by their disappearance which times the initiation of reproductive cycles in spring-born lambs.