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

    February 1977 in “ American Journal of Veterinary Research ”
    W. D. Oxender, P. A. Noden, H. D. Hafs
    TLDR Increasing daylight to 16 hours in winter can make mares ovulate and cycle earlier.
    The study conducted in 1974-1975 involved 15 seasonally anestrous mares divided into three groups: outdoor-control, indoor-control, and indoor light-treated with a 16-hour photoperiod. The indoor light-treated mares ovulated significantly earlier, by 74 days compared to outdoor-controls and 50 days compared to indoor-controls. These 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, such as estradiol and luteinizing hormone, were observed to increase near ovulation in all mares. The study concluded that increasing the photoperiod to 16 hours in early December induced normal estrous cycles within 2 months, potentially allowing for earlier breeding and foaling. Hair loss was noted earlier in the light-treated group.
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