Estrus, Ovulation, And Serum Progesterone, Estradiol, And LH Concentrations In Mares After An Increased Photoperiod During Winter

The study conducted by W D Oxender et al. in 1977 investigated the effects of increased photoperiod on estrus, ovulation, and hormone concentrations in 15 seasonally anestrous mares. The mares were divided into three groups: outdoor-control, indoor-control, and indoor light-treated with a 16-hour photoperiod. The light-treated group ovulated significantly earlier than the control groups, with ovulation occurring 59.0±6.9 days after treatment initiation, which was 74 days earlier than the outdoor-controls and 50 days earlier than the indoor-controls. The light-treated mares also experienced more estrous cycles before April 21, averaging 4.2 cycles compared to 1.4 in the indoor-controls. Hormone levels associated with estrous cycles, such as estradiol and luteinizing hormone, were observed to increase with the onset of estrus. Additionally, hair loss was noted earlier in the light-treated mares. The study concluded that a 16-hour photoperiod initiated in early December could induce normal estrous cycles in anestrous mares within 2 months, potentially allowing for earlier breeding and foaling.