Cranky mares

Is this the look you get in the spring from your cranky mare?

Spring is in the air as the buds and blossoms poke their way out of the thaw and the sun lingers in the afternoon sky. For many, spring is a time of jubilance and appreciation for outdoor recreation – especially with horses! Winter coats have all but shed, and it’s time to hit the trail or head to a show. Download a PDF of this article here.

However, for some mare owners, spring marks the beginning of a cyclical battle to achieve harmony amid a hormonal roller coaster. In this article, we’ll discuss ways to manage the mare in heat , so you can enjoy a smooth ride.

What’s going on in there?

The mare’s reproductive system can be active all year round, but predominantly will be active when days are long. So, between March and September, most mares are in full-swing heat cycles. Because mares gestate for about 335 days (11 to 12 months), it makes sense that they would be most sexually active during the warmer seasons – not only so that their offspring will fare well in mild climate, but also because food is most plentiful in the spring and summer when the mare needs to be producing milk.

Unlike humans who experience a menstrual cycle, mares function on an estrous cycle that is dominated by alternating waves of estrogen and progesterone, the two sex hormones instrumental to reproduction. The average equine estrous cycle is 21 days, with 5 to 7 days of estrogen dominance called estrus (“in heat”) and 14 to 17 days of progesterone dominance termed diestrus (“out of heat”). The two major phases of the cycle are separated by ovulation, when the egg erupts from the ovary to begin a migration through the uterine tract.

During the time of the migration, the tract environment changes so that conditions are optimal for fertilization and subsequent conception. The chart here depicts the rise and fall of the predominant hormones involved in the mare heat cycle. Kind of looks like that roller-coaster, doesn’t it?

Photo courtesy of

Signs of Heat

Estrus behaviors can vary from subtle to embarrassingly blatant. These are the more typical behaviors associated with being in heat:

Subtle Signs of Heat:

  • Slight lifting of tail at rest and/or when horses walk by
  • Turning haunches towards other horses or people
  • Increased frequency of urination
  • Occasional vocalization (squealing) when touched in perianal or perivulvar area
  • Throws head and moves away when touched in flanks/ belly
  • Herd-bound more than usual.

Subtle Signs of Heat:

  • Squatting and urinating frequently
  • Vulvar “winking” in which the distal portion of the vulva opens and closes
  • Squealing and/or threatening to kick when touched
  • Arching back and hopping when brushed over lower back or perianal region
  • Aggressively backing up to try to kick other horses or people
  • General agitation and inability to focus.

The most important thing to remember when you’re enduring a mare in heat is that this is natural behavior for her. While it’s never OK for a horse to kick or be aggressive toward people, owners should keep in mind that the mare is driven by a strong instinct to reproduce and that the behaviors that she exhibits naturally entice stallions. Therefore, we can hardly blame them for the cyclical predicament that they’re in! 

Physiology behind Behavior

Many of us who have to endure cranky mares in heat find ourselves asking them, “Why all the fuss?!” Believe it or not, there is a method to the madness. A heat cycle will reach its pinnacle at the point of ovulation. From that point, there’s a short, finite window of time that the egg must be fertilized by a spermatozoa in order for an embryo to be conceived. If the mare is bred too early or too late, conception is far less likely. Therefore, her behavior will change accordingly. This table explains:

Bottom Line

No two mares are exactly alike. Some mares show no adverse behaviors during their heat cycles, while others become fire-breathing dragons. The average mare may show signs for about 1 week each month during the spring and summer, while a smaller subset of mares are reported to have heat issues up to 10 months out of the year!

Many owners also report that mares seem to have the worst heats as they near the end of their annual cycle. Clearly, no two mares go through it the same. Therefore, management plans must be custom formulated to suit the needs of each horse and owner.

If you’re lucky enough to be able to throw the mare in the back 40 for one week per month, so be it. But if the mare is in work and handled daily, additional measures will likely have to be taken to calm her. 

Natural supplements are a great place to start, and we’ve included our favorites in the chart. They have a proven track record and in general are affordable, especially if you only give them during the week that the mare shows signs of heat. Remember, no two mares are the same, so you may have to experiment a bit with supplements to see which gives you improvement.

Click here for a downloadable PDF.

Any medication must be prescribed, which means that your veterinarian will be involved with the decisions. Medical management is effective in most cases, provided that it is affordable. 

Download the PDF of surgical options here.

But, any surgery carries some risk. Post-operative complications such as colic and infection are reported with spaying procedures and if the mare undergoes a procedure involving general anesthesia, there is a small additional risk there. However, for those who want to be done riding the hormonal roller-coaster once and for all, surgery is the only way.

Article by Contributing Veterinary Editor Grant Miller, DVM.

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