What to do about mare-ish behavior!


There are lots of ways to help a mare be more comfortable as she cycles through estrus. Understanding the estrus cycle of a mare is the place to start.


  • Typically, a mare will cycle every 19-26 days. This means she will end a heat cycle, have a few days in between, then go through another heat every 19-26 days. Heats usually last a week or so.


  • Most mares cycle from spring to fall as the days are longer. These mares typically have a longer and more pronounced heat in the spring and fall, and cycle in between. Some mares cycle all year long. This might be because of artificial lighting or living year-round with longer days, as is the case the closer you are to the equator.


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We often attribute “bad behavior” to a mare being mare-ish.


  • Mares can be known for crankiness, soreness, reluctance to work, a generally bad attitude when they are in their 5-7 day heat.


  • You might notice aggressive behavior, or lots of tail lifting and urinating towards other horses.


  • Some of these changes could be related to the estrus cycle, or it could be something else. Being overworked, having ulcers, a bad saddle fit, or even an injury can certainly cause some horses to exhibit “mare-ish” behavior.


two mares in a pasture

A more natural horse lifestyle can help mares be less mare-ish. These ladies are turned out together and stabled next to each other.


If you and your veterinarian determine the estrus cycle is a prime cause of mare-ish behavior, it’s time for you to consider options.


  • You can go to the Regu-mate route. This drug (altrenogest) needs to be given daily and it actually suppresses estrus in mares. You need to avoid getting this drug on your skin, the hormones can create chaos in your body – regardless of your gender. In stallions and geldings, aggressive behavior can be modified with the use of Regu-mate.



  • It’s starting to look like Depo in horses actually works the same way Valium does – on the brain as a calming or anti-anxiety drug.


  • Because Depo is actually a human drug, there are no safe or reliable reports of the actual correct dosage in horses. There’s a ton more info on this here in this article.


  • The FEI does not allow altrenogest in any male horse for competition, and no horses may compete on medroxyprogesterone. The USEF is considering this for medroxyprogesterone and currently, you must declare this drug with the USEF before you compete.




  • The USEF has now listed medroxyprogesterone as a prohibited substance as of December 2019. Depo takes a long time to clear from a horse’s body, so between December of 2019 and June of 2020, there is no penalty if your horse tests positive. However, penalties start happening on June 1, 2020. Time to plan ahead for next year’s show season if you use depo.


  • Depo has been linked to 23 cases of horse fatalities over three years. There’s documentation about this that you can read here.



bottle of horse medications

Lots of options besides this one.


You also have other options if your mare needs some support to be more comfortable.


  • There are other hormone options, like adding estrogen or oxytocin to your mare’s routine. These must be expertly timed and your vet can help you with that.


  • There are some un-scientific reports of supplements helping also.


  • Have you heard about the marbles? In some mares, a glass marble placed in her uterus at the exact perfect timing suppresses estrus for about three cycles. It then needs to be removed. More on that here.


  • If you and your mare are at your wit’s end, she can be surgically spayed.



Working with your vet, you can learn to notice her cycle and pay attention to her behaviors. You might find that her mare-ish behavior is actually not because of estrus at all, in which case you have some other avenues to explore. Like saddle fit, muscle soreness, mystery lamenesses, and so on.


Is your mare a bit mare-ish?


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