Answered by, Holly Mason, DVM, MS, Utah State Veterinary, Courtesy of AAEP
Question: Can pasturing one gelding with a group of mares stimulate them to come into heat?
When addressing heat (AKA estrus) in mares there are two broad categories: 1) estrus induction (AKA manipulation) and 2) estrus detection.
Answer: To answer your question specifically, exposure to another horse, be it a gelding, stallion or mare, does not necessarily induce estrus. A mare will cycle on her own depending on her geographic location when the day length is long enough to stimulate her natural cycle. In the northern hemisphere under natural circumstances mares typically begin cycling in the early spring and continue cycling throughout the summer into early fall. There are a small number of mares that will cycle all year.
When referring to estrus detection, this describes observing signs of estrus in the mare such as frequent urination, squatting and posturing in a receptive fashion. A mare can show overt signs of estrus while alone or when exposed to another horse. More reliably they will show estrus when exposed to a stallion. There seems to be a misconception among mare owners that geldings can have an effect on mares similar to the effect that stallions have. I have this impression because it is common for clients to request appointments for breeding examinations based on how their mare(s) are interacting with a gelding. Many clients report that their mare is showing signs of estrus when exposed to a gelding. My examinations of most of these mares show an unreliable correlation to where they are in their cycle or specifically how close they are to ovulation.
A stallion has the full compliment of hormones and behavioral traits to elicit a far more reliable response in a mare. It is for this reason that most breeding operations will use a stallion as part of the management protocol. Stallions are useful not only as a source of semen, but also to interact with the mares as a means to accurately detect estrus. That being said, there is a lot of individual variation among mares and some will accurately show estrus to geldings or even other mares.
If you do not have access to a stallion, there are other methods available to manipulate the estrous cycle if your goal is to get your mare(s) in foal. These methods typically involve administering hormonal therapy to manipulate the estrous cycle and are used quite commonly in the industry.
There are several advantages to utilizing hormonal therapy to manipulate the cycle including fewer exams required by your veterinarian to determine where she is in her cycle and more accurately being able to predict when to order semen if you are using frozen or cool shipped or when to send her to the stallion for a live cover. Hormonal therapy can significantly reduce time and labor costs when you do not have a stallion available for teasing to accurately detect estrus. Your veterinarian can assist you in determining what methods are most suitable for you and your mare. I hope this helps.
* Reprinted with permission of the AAEP. To view the entire article please visit http://www.aaep.org then click Horse owners, Ask the Vet