Chasteberry is an herbal supplement that is used to support mood balance and hormone health in female and male horses.
The Chastetree berry (Vitex agnus-castus) plant is a shrub that grows in the Mediterranean and parts of Asia. The fruits and leaves contain active ingredients that influence hormonal balance in horses.
This herb is purported to support hypothalamic and pituitary function in horses. Anecdotally, chasteberry is said to help make moody mares easier to handle and may have a calming effect on aggressive stallions or geldings.
Chasteberry is not recommended for pregnant and lactating mares or growing horses because of potential effects on the reproductive system which have not been evaluated in these animals. 
Mad Barn’s Chasteberry Powder is made of pure dried chastetree berry fruit without any additional ingredients. It should be fed at a rate of 5-20 grams per day, increasing the serving size slowly to limit avoidance.
Why Use Chasteberry in Horses?
Chasteberry is primarily used in horses to enhance mood and improve behaviour, by regulating some hormone levels.
In traditional medicine, Chasteberry extract (Vitex) was originally thought to promote chastity, hence its name. The dried fruit was used by monks to decrease sexual desire. You might also find it labelled under the name Monk’s pepper.
Today, it is commonly used to treat premenstrual syndrome (PMS) and menopause in women. It has been shown to lower prolactin levels which helps alleviate symptoms of reproductive cycling in women. 
Though the effects of this herb have not been well-researched in horses, it is growing in popularity due to a strong body of anecdotal evidence from veterinarians and horse owners who have seen positive transformations in their equine companions.
Chasteberry can support optimal levels of several reproductive hormones: luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen and progesterone. Therefore, chasteberry might help moody mares with behaviour issues that are related to being in season.
Chasteberry can alleviate some manifestations of Cushing’s Disease (PPID) but does not control hormonal output from the tumor.  There is also some evidence that it may interfere with the actions of pergolide, the drug commonly used to treat Cushing’s disease. 
Benefits of Chasteberry in Horses
1) Improves shedding
Excess hair growth or lack of shedding, known as hirsutism, is one of the most obvious features of Cushing’s disease/PPID.
Chasteberry was shown to stimulate shedding, and support a healthier, shinier coat. This also led to a reduction in excessive sweating, making horses more comfortable and heat tolerant.
2) Reduces fatigue and improves energy levels
Chasteberry was found to reduce fatigue, support better energy levels and improve regulation of thyroid activity. The thyroid gland is a key regulator for the body, controlling metabolism and many other functions.
3) Supports mood and attitude
Horses with depression and low mood show improvements with Chasteberry, making them easier to handle and more cooperative.
By supporting hypothalamus and pituitary function, Chasteberry can support mental well-being and improve handling.
4) Supports horses with laminitis
In horses with Cushing’s/PPID, chasteberry reduced incidence of laminitis and possibly improved laminitis pain.
The hoof changes associated with laminitis are painful for horses. Chasteberry was shown to reduce signs of pain associated with laminitis.
5) Normalizes urination and drinking behaviour
Chasteberry was found to reduce frequency of urination (polyuria) and drinking (polydipsia).
Additional Positive Effects
Research suggest that chasteberry works by nutritionally supporting the normal function of the pituitary gland and endocrine system.
By supporting healthy pituitary function, chasteberry could have numerous beneficial effects.
The hypothalamus and pituitary gland are key parts of the brain that receive signals from the body and produce hormones to regulate a wide array of metabolic processes.
Chasteberry also helps relieve the insulin resistance and ovarian abnormalities in PCOS – polycystic ovarian syndrome – in women.  Some mares with metabolic syndrome and hyperinsulinemia also exhibit ovarian and cycling abnormalities and therefore might benefit from chasteberry supplementation. 
Chasteberry activates opioid mu receptors in the brain, which may be partially responsible for the energizing and pain-relieving effects of chasteberry.