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
There has always been a great deal of interest in efficient reproductive practices in equine breeding. Proper breeding time is best determined by assessing hormonal levels in the mare but as these are not readily measurable in most equine breeding operations the outward signs of estrus are utilized as practical substitutes. It is therefore necessary to understand the hormonal levels which characterize estrus if one desires to increase breeding efficiency by altering the mare's reproductive cycle.
Eckles, Sandra V.
"Progesterone Levels in the Cycling Mare and the Luteolytic Effect of Prostaglandins,"
Iowa State University Veterinarian: Vol. 37
, Article 8.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol37/iss3/8