Mares are “moody” because their reproductive cycle creates genuine hormonal shifts every 21 days during breeding season, and those shifts directly change how they feel and behave. Unlike geldings, whose hormone levels stay relatively flat, mares experience rising and falling waves of estrogen and progesterone that influence everything from their tolerance of other horses to their sensitivity under saddle. Some mares barely show it. Others become almost unrecognizable for a few days each cycle.
The 21-Day Hormonal Rollercoaster
A mare’s estrous cycle has two distinct phases: estrus (heat) and diestrus (the quiet period between heats). The full cycle runs roughly 21 days. During estrus, which lasts about 5 to 7 days, estrogen levels peak. This hormone drives the mare to be receptive to a stallion, and it produces the classic signs owners notice: frequent urination, standing with hind legs apart, lifting the tail to one side, and clitoral winking. Some mares become distracted, hypersensitive to leg aids, or reluctant to work during this window.
After ovulation, the cycle flips. The ovary forms a structure called the corpus luteum, which pumps out progesterone for the next 14 days or so. Progesterone has a calming effect on behavior. Mares in diestrus are typically more focused, more tolerant, and easier to ride. They actively reject stallions during this phase, sometimes with squealing, striking, or kicking. Around day 13, progesterone drops, follicle-stimulating hormone rises again, and the whole process restarts. That transition from progesterone dominance back to rising estrogen is when many owners notice the sharpest mood swing.
It Can Actually Hurt
Hormones aren’t the whole story. The physical process of growing a follicle on the ovary can cause real discomfort. Back pain related to increased ovarian sensitivity is well documented in mares. As the follicle swells before ovulation, some mares experience abdominal pressure and inflammation that makes them irritable, girthy, or unwilling to engage their hindquarters. Large anovulatory follicles (ones that grow but never release an egg) and ovarian cysts can make this worse because the ovary stays enlarged for longer than normal.
This is an important distinction for owners: a mare who pins her ears when you tighten the girth, flinches under saddle, or resists collection might not just be “hormonal.” She could be in genuine pain from ovarian activity. Anti-inflammatory medication can help identify whether the behavior stems from a pain source or a hormonal one. If the crankiness disappears with pain relief, the ovaries themselves are likely causing physical discomfort rather than just mood changes.
Breeding Season Makes It Worse
Mares are seasonal breeders. In the Northern Hemisphere, the natural breeding season runs from roughly April through September. Daylight length is the trigger. As days get longer in spring, the pineal gland produces less melatonin during the shorter nights, which signals the brain to ramp up reproductive hormones and restart ovarian cycling. During the short days of winter, many mares enter a quiet period called anestrus, where ovarian activity drops significantly and the mood swings essentially stop.
This is why owners often notice their mare is a dream to ride in December and a nightmare in June. The winter calm isn’t better training or maturity. It’s the absence of cycling. Between winter anestrus and full summer cycling, there’s a messy transitional period in early spring when hormones fluctuate unpredictably, follicles grow and regress without ovulating, and behavior can be especially erratic. This transitional phase can last weeks and often catches owners off guard.
Signs That Cycle With the Hormones
Normal estrus behavior varies enormously between mares. Some show almost no outward signs. Others become so distracted or reactive that riding becomes difficult. Common behaviors during heat include:
- Tail lifting and clitoral winking when near other horses
- Frequent posturing and urination, sometimes in small amounts
- Hypersensitivity to leg and spur aids, particularly around the flanks
- Distraction and calling out, especially if geldings or stallions are nearby
- Resistance to collection or reluctance to engage the hindquarters
During diestrus, the same mare may become more focused but also more defensive. Squealing, kicking out at neighboring horses, and general irritability toward herd mates are typical progesterone-phase behaviors. So even the “off” phase of the cycle isn’t necessarily easygoing.
When Moodiness Signals a Medical Problem
Persistent, extreme, or out-of-pattern moodiness sometimes points to an underlying condition rather than normal cycling. Ovarian tumors, particularly granulosa-theca cell tumors, are the most significant concern. These tumors produce abnormal levels of hormones and can cause behavior ranging from constant heat signs to stallion-like aggression, depending on which hormones the tumor secretes. The affected ovary typically becomes enlarged while the opposite ovary shrinks.
Other conditions that mimic hormonal moodiness include gastric ulcers, urinary tract problems, and low-grade musculoskeletal pain. Gastric ulcers are especially easy to confuse with estrus-related behavior because they share overlapping signs: girthiness, irritability during tacking up, picky eating, and post-meal discomfort. The key difference is timing. Hormonal behavior follows a roughly three-week pattern and worsens during breeding season. Ulcer-related behavior tends to be constant regardless of where the mare is in her cycle, and it often worsens with feeding schedules or stress rather than with proximity to other horses.
Managing Hormonal Behavior
The most common pharmaceutical approach is a synthetic form of progesterone given orally each day. It essentially tells the mare’s body she’s in diestrus all the time, suppressing estrus behavior and the associated mood swings. The international equestrian federation permits its use in competition mares. When the medication stops, normal cycling resumes within a few days.
Chasteberry (Vitex agnus-castus) is widely marketed as a natural alternative for hormonal mares. It has a long history of use in human medicine for menstrual cycle issues, and it works by binding to certain receptors in the pituitary gland. However, the evidence in horses is weak. A study presented to the American Association of Equine Practitioners found that chasteberry extract did not produce meaningful clinical improvement in horses with pituitary dysfunction, and in some cases, the relevant hormone levels actually worsened during treatment. Despite its popularity in equine supplement aisles, there’s little controlled data showing it reliably smooths out estrus-related behavior in otherwise healthy mares.
Beyond medication, practical management strategies can make a real difference. Keeping mares away from geldings during peak estrus reduces stimulation. Adjusting training expectations around known cycle timing helps avoid frustrating sessions. Some owners track their mare’s cycle on a calendar and plan competitions during diestrus when focus and tolerance tend to be highest. For mares whose behavior is severe enough to affect quality of life or safety, a veterinary workup including an ultrasound of the ovaries can rule out cysts, anovulatory follicles, or tumors that might be amplifying normal cycling into something more extreme.