The Mare Cycle
The Normal Mare Cycle
Mares are seasonally polyoestrus, which means that they come into "season" multiple times during Spring, Summer and sometimes Autumn. This is influenced by daylight hours but about 15% of mares cycle all year round. Cycling can already start at an age of 2 and can keep going until the mares are in their '20s. However fertility begins to decline in their late teens. Early in the Breeding season mares may go through a transitional phase where their hormone levels are not strong enough to cause them to be truly in season but they may tease for many day or even weeks, this is called “Spring Heating”.
One mare cycle consists of a follicular phase, oestrous, and a luteal phase called dioestrous. This cycle is controlled by hormones produced in the brain and genital tract. Vets can use synthetic forms to influence the natural cycle.
Follicular phase/oestrous: lasts 4-7 days, the mare is receptive to the stallion and will "tease", the genital tract is getting ready for conception. During this phase a rectal scan will show one or more dominant follicles (>25mm), folds of the uterus wall and a softened cervix. Scanning only happens in a crush, to ensure the safety of the mare, vet and handler.
At the last 48hr of oestrous one or more follicles have matured to the stage where they are ready to undergo ovulation and release an egg. A rectal scan will show decreased uterine folds, the follicle will usually be >35mm and might change shape and get softer. At ovulation the follicle collapses and becomes known as a CL, corpus luteum.
The dioestrous is initiated when the CL forms after ovulation and begins to release progesterone. The luteal phase or dioestrous typically lasts 14-15 days, during which the mare is not receptive to the stallion. The cervix closes and the genital tract is ready to accept and nurture the possible embryo. A scan at this stage will show one or more CL's and sometimes there is fluid present in the uterus post ovulation/cover. This fluid needs to be flushed out and the mare is given oxytocin to help expel the fluid.
The aim is to have semen in the mare as close to ovulation as possible. The semen meets the egg in the oviduct and the resulting embryo stays there for around 5 days before travelling into and then throughout the uterus until it implants around day 16.
When there is no embryo, the uterine wall starts releasing Pg, prostaglandin, late during dioestrous. Pg breaks down the CL and stops progesterone formation so the body recognises it is not in foal and the next oestrous phase begins.