The hormonal cycle of the mare is much more complex than we often give it credit for. Regularly described as ‘moody’ or ‘difficult’, mares’ bodies go through a lot of changes during oestrus. So, how can this affect their body, and how can we help?
The mare’s oestrus cycle
The mare’s cycle generally responds to increased daylight hours – this is known as a photoperiod. As daylength increases, mares begin to cycle. This means that births can occur in the most suitable season for the survival of young, such as spring.
There are two main phases in the cycle, determined by the internal changes to sexual organs and the glandular system:
- Oestrus: the time in which the mare is in heat
- Dioestrus: the time in which the mare is not in heat
During winter, when mares do not experience ovulation, they are in a phase called anestrus – no hormonal or physical changes related to ovulation occur during the body at this time.
Hormonal activity
Oestrus is also known as the follicular phase, due to the presence of follicles. When the pituitary gland in the brain is stimulated by increased daylight, follicle-stimulating hormones (FSH) are produced and released into the bloodstream. The follicle contains an ovum – the female part of an egg. The follicles produce a hormone called estrogen, which is responsible for behavioural changes, such as the mare being more receptive to stallions. Other behaviours associated with this include tail swishing, decreased performance, hyperexcitability, and discomfort. The release of estrogen can also cause physical changes, such as ovarian discomfort and ligament laxity.
So, how can we support our mares?
The benefits of agnus castus
Vitex agnus castus, colloquially known as Chasteberry, is a perennial shrub tree commonly found in regions such as the Mediterranean and parts of Asia. The fruits and leaves have been used to aid menstrual and menopausal disorders in humans for centuries, because they contain active ingredients that help influence hormonal balance. In women, it has been shown to reduce prolactin levels, which helps alleviate the symptoms of the menstrual cycle.
Although research in horses is more limited, several studies have found that it can be beneficial for supporting optimal levels of several reproductive hormones, such as FSH, estrogen and progesterone.
Studies have also found that agnus castus can benefit horses with PPID (pituitary pars intermedia dysfunction), formerly known as Cushing’s disease. It has been suggested that it can:
- Improve shedding
- Reduce fatigue and improve energy levels
- Support mood and attitude
- Help to reduce the likelihood of laminits
- Help with hydration
These benefits, along with those that support our mares, are thought be a result of the effect that agnus castus has on the normal function of the pituitary gland and endocrine system. By helping to regulate hormone levels, the level and frequency of intense fluctuations in mood and behaviour can be reduced.
How does it actually work?
The fruits of agnus castus contain active compounds that are believed to influence dopamine activity and sex hormone regulation. These compounds include flavonoids and essential oils.
The extract has been shown to stimulate dopamine receptors, which in turn decreases the release of hormones involved in the oestrus cycle. This can help reduce overproduction of hormones such as prolactin or FSH, and stabilise irregular cycles and associated behaviours.
Would my mare benefit from it?
Whether your mare would benefit from agnus castus depends on the signs she is showing. It is most commonly used for horses with hormone-related behaviours such as irritability and sensitivity and may help to take the edge off by supporting more balanced hormones. While some mares respond well, others may show little change, so it is best used as part of a broader management approach, and with guidance from your vet or nutritionist.
Take away
Overall, while the mare’s hormonal cycle is complex and can influence both behaviour and performance, there are supportive options available. Agnus castus may offer a natural way to help promote hormonal balance and reduce some of the more challenging symptoms associated with oestrus, although responses can vary between individuals. As with any nutritional support, it is important to consider the whole horse and seek professional guidance where needed to ensure the most appropriate approach for your mare.
Written by Anna Jones BSc (Hons) Equine Science - Customer Care & Marketing Associate
References
Aurich, C., & Kaps, M. (2022). Suppression of reproductive behaviour and gonadal function in female horses—An update. Reproduction in Domestic Animals. https://doi.org/10.1111/rda.14129
Claes, A., Ball, B. A., Scoggin, K. E., Roser, J. F., Woodward, E. M., Davolli, G. M., Squires, E. L., & Troedsson, M. H. T. (2017). The influence of age, antral follicle count and diestrous ovulations on estrous cycle characteristics of mares. Theriogenology, 97, 34–40. https://doi.org/10.1016/j.theriogenology.2017.04.019
Curry, M. R., Eady, P. E., & Mills, D. S. (2007). Reflections on mare behavior: Social and sexual perspectives. Journal of Veterinary Behavior, 2(5), 149–157. https://doi.org/10.1016/j.jveb.2007.07.007
Hedberg, Y., Dalin, A-M., Ohagen, P., Holm, K., & Kindahl, H. (2005). Effect of Oestrous-cycle Stage on the Response of Mares in a Novel Object Test and Isolation Test. Reproduction in Domestic Animals, 40(5), 480–488.https://doi.org/10.1111/j.1439-0531.2005.00611.x