Effect of an inverse subtropical (19 degrees 13 ' N) photoperiod on ovarian activity, melatonin and prolactin secretion in Pelibuey ewes

Twenty-one Pelibuey ewes were used from December 21, 1996 to December 21, 1 998. Fourteen of them had never been exposed to artificial photoperiod, and they were maintained on natural photoperiod until March 21, 1997, when the y were assigned to natural photoperiod (control group, n = 8) or to inverse photoperiod (n = 6). The other seven animals had been kept on a long photo period (16L:8D) from October 21, 1996 to December 21, 1996, when they enter ed the present study and were subjected to a gradual decrease in photoperio d, so that they reached an equinox photoperiod (12L:12 D) on March 21, 1997 . At that time, they were assigned to natural photoperiod (n = 3) or to inv erse photoperiod (n = 4). Blood samples for progesterone determination were taken twice a week from all the animals. During the second year of the stu dy, prolactin was measured in the samples from five animals in inverse phot operiod and from five control ewes. Hourly samples were obtained to determi ne the 24-h melatonin profile of five animals from each group on September 21, 1997, December 21, 1997, March 21, 1997, and June 21, 1997. Exposure to inverse photoperiod resulted in a gradual shift on the annual reproductive cycle, so that the second ovulatory season was advanced by 5 months in the ewes kept on inverse photoperiod as compared to the control ewes (P < 0.05 ). There were wide variations in the dates for the onset and the end of the ovulatory season within the inverse photoperiod groups, and three animals in this groups maintained ovulatory activity for at least 18 consecutive mo nths. The duration of melatonin secretion was directly related to the lengt h of the dark period (P ( 0.05), and this response was not affected by the calendar date. Prolactin concentrations were directly related to daylength, however, they were also affected by calendar date, being lower in the inve rse group as compared to the corresponding time of the annual photoperiodic cycle of ewes on natural photoperiod. It is concluded that reproductive ac tivity, melatonin secretion and prolactin secretion of Pelibuey ewes respon d to the small variations in photoperiod that are present at 19 degrees 13' N, and that under natural conditions, photoperiod appears to be the main re gulator of ovarian activity at this latitude. However, other factors such a s temperature or humidity may act as modulators, and their relative importa nce could increase at more equatorial latitudes. (C) 2000 Elsevier Science B.V. All rights reserved.