When "Bad Behavior" Is Actually Estrus: A Practical Equine Perspective

When “Bad Behavior” Is Actually Estrus: A Practical Equine Perspective 

A mare that suddenly resists collection, swishes her tail excessively, squeals at neighboring horses, refuses to engage her hindlimbs, or becomes unpredictable under saddle is often quickly labeled as difficult¹. But in many cases, the reproductive cycle is playing a bigger role than owners and trainers realize. 

Estrus-related behavioral issues are extremely common in performance mares. In fact, according to an AAEP survey, nearly 90% of respondents believed estrus negatively affected athletic performance, while only 1% felt ovarian activity had no impact at all¹. Similar concerns have also been documented in mollies, where estrus-associated signs included excessive tail whipping, frequent urination, unwillingness to work, and performance inconsistency². 

In practice, the challenge is not simply recognizing estrus—it is determining whether the ovary is truly the primary cause of the behavioral complaint. 

Start with a Structured History 

Many cases are approached emotionally by owners and trainers rather than systematically. Before initiating hormonal therapy or considering surgery, a detailed history is critical. 

Key questions include¹: 

• When did the behavior first appear? 

• Is it cyclical or constant? 

• Does it worsen during estrus? 

• How severe and frequent are the episodes? 

• Has the mare responded to hormonal treatments or NSAIDs previously? 

• Are there rider-related or training-related triggers? 

These details often help differentiate hormonal causes from orthopedic, behavioral, or rider-associated issues. 

Not Every “Hormonal Mare” Has an Ovarian Problem 

Physiological hormonal fluctuations alone can alter behavior and performance. Estrogen has been associated with increased motor activity, excitability, and muscle sensitivity in several species¹. Many mares also exhibit back discomfort or reduced willingness to work during estrus. 

However, pathological ovarian conditions must not be overlooked. 

Non-neoplastic ovarian disorders such as: 

• Ovarian abscesses 

• Hematomas 

can cause abdominal discomfort and secondary behavioral changes. 

More importantly, ovarian neoplasms—especially granulosa cell tumors (GCTs) and granulosa-theca cell tumors (GTCTs)—are well-recognized causes of abnormal behavior in mares³. 

Affected mares may show: 

• Stallion-like behavior 

• Aggression 

• Persistent estrus 

• Infertility 

• Poor rideability 

• Unpredictable temperament changes 

These tumors account for approximately 2.5% of equine neoplasms and are usually unilateral and benign^1,4. 

Rule Out the “Look-Alikes” 

One of the biggest clinical mistakes is attributing every behavioral complaint to estrus¹. 

Conditions that commonly mimic reproductive discomfort include¹ : 

• Pneumovagina 

• Vaginitis 

• Cystitis 

• Urolithiasis 

• Sacroiliac pain 

• Low-grade lameness 

• Back pain 

Even rider imbalance or inappropriate training pressure can present as “mare attitude.” 

A simple but useful clinical tip is the use of NSAID trials. Improvement following anti-inflammatory therapy may indicate pain-associated behavior rather than purely hormonal causes. 

Medical Management: Helpful but Often Temporary 

Medical suppression of ovarian activity can improve behavior in selected mares. 

Options include¹: 

• Oral altrenogest 

• Intramuscular progesterone 

• Progesterone-estrogen implants 

• GnRH vaccination 

• False pregnancy induction protocols 

However, results are variable, and many treatments only temporarily suppress clinical signs. 

When Surgery Becomes the Best Option 

For mares with confirmed ovarian neoplasia or severe, treatment-resistant behavioral issues, ovariectomy remains the definitive treatment. 

Today, standing laparoscopic ovariectomy is considered the preferred surgical approach in equids⁵. 

Compared with open surgery, laparoscopy offers: 

• Lower hemorrhage risk 

• Reduced infection rates 

• Minimal adhesions 

• Faster recovery 

• Earlier return to athletic function 

• Better cosmetic outcomes¹ 

Importantly, the standing approach also avoids the risks associated with general anesthesia. 

That said, bilateral ovariectomy may not be ideal in genetically valuable mares intended for breeding, making case selection extremely important. 

Clinical Take-Home Message 

Behavioral complaints in mares should never be dismissed as “just estrus” without investigation. A structured diagnostic approach—combining reproductive examination, pain assessment, history taking, and exclusion of non-reproductive causes—is essential before initiating hormonal or surgical intervention. 

For the practicing veterinarian, the goal is not simply suppressing estrus, but identifying whether the ovary is truly the source of the problem. 

References 

1. Straticò P, Hattab J, Guerri G, Carluccio A, Bandera L, Celani G, Marruchella G, Varasano V, Petrizzi L. Behavioral disorders in mares with ovarian disorders, outcome after laparoscopic ovariectomy: a case series. Veterinary Sciences. 2023 Jul 25;10(8):483. https://doi.org/10.3390/vetsci10080483  

2. Petrizzi L, Guerri G, Stratico P, Cuomo A, Vullo C, De Amicis I, Robbe D, Varasano V. Laparoscopic ovariectomy in standing mule mares. Journal of Equine Veterinary Science. 2020 Jan 1;84:102857. https://pubblicazioni.unicam.it/bitstream/11581/433923/1/Journal%20Equine%20Vet%20Sci%202020.pdf  

3. Derar D, Ali A, Al-Sobayil F, Refaai W. Granulosa cell tumor in Arabian mares: prevalence, risk factors, clinical and histopathological findings and outcome of surgical removal. Frontiers in Veterinary Science. 2025 Nov 13;12:1689782. https://doi.org/10.3389/fvets.2025.1689782  

4. Goto A, Tagami M, Kato F, Suzuki T, Yamaga T, Murase H, Sato F, Tsogtgerel M, Niikura T, Moriyama T, Chiba A. Equine nonneoplastic abnormal ovary in a draft mare with high serum anti-Müllerian hormone: a case study. Journal of Equine Science. 2021;32(4):147-51. https://pubblicazioni.unicam.it/bitstream/11581/433923/1/Journal%20Equine%20Vet%20Sci%202020.pdf  
5. Straticò P, Varasano V, Guerri G, Celani G, Palozzo A, Petrizzi L. A retrospective study of cryptorchidectomy in horses: Diagnosis, treatment, outcome and complications in 70 cases. Animals. 2020 Dec 21;10(12):2446. https://www.mdpi.com/2076-2615/10/12/2446