The present study was performed during the natural equine breeding season, from July to October.
Fifteen normal estrous cycling mares of light horse breeds, between the ages of 6 and 16 years
old, weighing 460–520 kg were exposed to natural photoperiod. These animals were maintained on
alfalfa and oat hay and were provided water ad libitum for the entire experimental period. Mares were
housed in outdoor paddocks at the Animal Science Horse Barn of the University of California, Davis.
Experiments were approved by the Animal Care and Use Committee at the University of California,
Davis.