Influence of photoperiod on puberty in the female pony

Thirteen 6-8 month-old fillies were assigned to 3 treatment groups: Group 1 had a 16-h fixed daily photoperiod (16L:8D, N = 4), Group 2 a daily photoperiod equivalent to ambient daylength (control, N = 5), and Group 3 a 9-h fixed daily photoperiod (9L:15D, N = 4). The light treatments extended from 17 December to 9 August. Hair shedding occurred first in Group 1 followed by Group 2, then Group 3. The proportions of fillies with 1 or more ovulations (puberty) by the end of the project were 2/4, 5/5, 2/4 in Groups 1, 2 and 3, respectively. The hypothesis that a fixed daily photoperiod which corresponded to maximum daylength at the summer solstice in temperate zones (16 h) would hasten puberty was not supported. Instead, the 16-h light treatment interfered with the attainment of puberty, as indicated by the reduced proportion of fillies ovulating, reduced numbers of ovulations, luteal bodies, and oestrous periods/filly, the extended intervals from the beginning of the project to the first ovulatory oestrus and the shorter period of oestrus. Hair coat changes were dissociated from changes in reproductive activity in Group 1. Puberty also appeared to be retarded in Group 3, but the results were less pronounced than those for Group 1.