Estrus, Ovulation, and Serum Progesterone, Estradiol, and LH Concentrations in Mares After an Increased Photoperiod During Winter

In this study from 1974-1975, 15 seasonally anestrous mares were divided into three groups to assess the impact of increased photoperiod on estrus and ovulation. The indoor light-treated group, exposed to a 16-hour photoperiod, ovulated significantly earlier than both the indoor-control and outdoor-control groups. The light-treated mares ovulated 59 days after the start of the experiment, which was 74 days earlier than most outdoor-controls and 50 days earlier than indoor-controls. The light-treated mares also experienced more estrous cycles before April 21, averaging 4.2 cycles compared to 1.4 in indoor-controls. Hormone levels associated with estrous cycles were observed to change normally in all groups. Additionally, hair loss occurred earlier in the light-treated mares. The study concluded that increasing the photoperiod to 16 hours in early December could induce normal estrous cycles in anestrous mares within 2 months, potentially allowing for earlier breeding and foaling.