Mareish

Introduction

When it comes to mare health, “Estrus” can be an intimidating topic to approach. But there’s nothing to fear, it really is so much simpler than it seems. 

What do you think of when you think of Estrus? Have you heard the term before? You might have heard the term: “In Heat” used to refer to the mare’s reproductive cycle, and this is a great place to start. 

When people say a mare is “In Heat” this means she is in the Estrus Phase of her cycle. The mare’s reproductive cycle is composed of two phases: 1) the Estrus Phase, and 2) the Diestrus Phase. During the Estrus Phase the mare is sexually receptive and ovulation typically occurs towards the end of the period. Conversely, in Diestrus the mare is “Out of Heat,” meaning ovulation will not occur and the mare typically will not show signs of sexual receptivity. 

All mares are different, just like people, and different mares will show vastly different behaviours and symptoms across their cycles. For some mares it can be very obvious when they are in their Estrus Phase, they will show obvious changes in temperament such as increased ear pinning, squealing, kicking, or biting (just to name a few examples), or may present as more “difficult” to ride during sessions. Some people call this being “Mareish,” and it often comes up as a complaint among riders and owners. Whereas some mares might not show any signs at all. 

Whether a mare shows significant changes or minor ones, being “Mareish” doesn’t have to be a bad thing. It’s all a part of a natural biological process and is a perfectly normal aspect of mare health. 

When Does Estrus Occur? 

You might have also heard the term: “In Season.” This can sometimes be used interchangeably with “In Heat” or more specifically it can refer to the mare’s Estrus Phase during breeding season.

Most mares (about 80%) are seasonally polyestrous, meaning they will go into heat (Estrus Phase) only at certain points in the year. We call this breeding season and it generally takes place between late April through to the end of August. There is a seasonal element in which evidence has shown that the increase in daylight hours (coinciding with warmer temperatures and better nutrition) influences the start of the mare’s regular cycling. 

Some mares (the remaining 20%) are polyestrous, meaning they cycle normally throughout the entire year. This has been observed mainly in warmer climates and is linked to differences in weather and temperature. In North America, very few mares will ovulate outside of breeding season. That being said, it’s not entirely uncommon to observe “Estrus-like” symptoms year round. Hormonal changes may appear erratic and not as easy to track as during the breeding season (April-August). Particularly during the early spring transition, March, April, May, and the fall as the breeding season is wrapping up. Many owners also report significant changes in their mares’ temperament around sudden weather and environmental changes.  

The period of the year where mares are not cycling normally is called Anestrus. Meaning, no Estrus cycling is occurring (as seen with seasonally polyestrus mares). This term is not to be confused with Diestrus, which as we discussed earlier refers to the “Out of Heat” phase of the mare’s regular cycling. But it’s okay if you get them mixed up, lots of people do! 

How Long Does Estrus Last

During regular cycling (breeding season), the total length of the cycle is 21 days. This means that the mare will ovulate once every 21 days during the season. Typically, the Estrus Phase will last 6 days, while the Diestrus Phase lasts 15 days. Though some variance is normal, with Estrus ranging from 4-10 days and Diestrus 12-18 days. Mares may ovulate at any point during the Estrus Phase, but most commonly towards the end within the last 24-48 hours. 

There is a whole bunch of really cool stuff about hormones and anatomy and the formation of the follicle that goes along with it, but I think we’ll leave it here for today. It’s definitely a lot to unpack, but when you break it down it’s really quite easy to wrap your head around. Just like with human biology it can feel a bit awkward to talk about sometimes, but the more you talk about it the more you learn, and the less daunting the topic becomes.