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
The fact that the horse is seasonally polyestrous makes this species a reproductive challenge for horsemen and equine practitioners. Mares will show several estrous cycles during the breeding season assuming pregnancy does not terminate estrus. The receptive season is limited to such a length oftime that parturition occurs in the spring ofthe year. The challenge arises when trying to devise methodsthat would allovl parturition to take place earlier in the year. This is desirable only because most breed registries have imposed an arbitrary birthdate ofJanuary 1st to all foals born in a single year. Each yearling is considered one year of age on January 1 of each year regardless of their actual age. Concern among horseowners regarding this stipulation lies in the eligibility oftheir horses for age limited races, shows, or events. In order to be competitive, actual birthdates as close to January 1 are desired.
Smith, J. A. and Redding, W. R.
"Methods of Shortening the Anestrous Period in Mares,"
Iowa State University Veterinarian: Vol. 51
, Article 14.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol51/iss2/14