Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Although we find a wide range of estrous cycling patterns, the mare is usually considered seasonally polyestrous - cycling only during certain seasons of the year. In the natural state. the seasonal breeding was advantageous to the survival of the foals as they were born during the spring and summer with moderate temperatures and adequate food supply.
"The Effect of Artificial Lighting on the Estrous Cycle of the Mare,"
Iowa State University Veterinarian: Vol. 37
, Article 6.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol37/iss3/6