Overview
The mare's ovaries (ovaria) are the largest among domestic animals and are bean-shaped. They hang from the mesovarium in the sublumbar region, caudal to the kidneys and far from the pelvic inlet, so they are reached per rectum with the arm stretched cranially. Each relates to the tip of the uterine horn of its side.
Structure and cycle
- Inverted organisation: the vascular zone (medulla) forms the surface, covered by peritoneum, and the parenchymatous zone (cortex), with the follicles and corpora lutea, lies inside.
- Ovulation fossa (fossa ovarii): a depression on the free border, the only part of the surface not covered by peritoneum; all follicles ovulate through it.
- Corpus luteum: forms within the ovary and barely protrudes, so it is hard to feel on palpation; it is identified by ultrasound.
- Seasonality: the mare is a seasonally polyoestrous long-day breeder. In winter the ovaries are small and inactive (anoestrus); in the spring transition many follicles grow without ovulating, and in the breeding season the preovulatory follicle grows large and softens before ovulation.
Clinical relevance
- Cycle monitoring: rectal palpation and ultrasound of follicle size, shape and consistency, together with endometrial oedema, are used to time mating or insemination and the induction of ovulation (hCG, deslorelin).
- Lighting programmes: artificially extending daylight from winter brings forward the start of the breeding season.
- Granulosa cell tumour: the most common ovarian tumour in the mare. It is usually unilateral and multicystic; it causes anoestrus, persistent oestrus or stallion-like behaviour, and the contralateral ovary becomes small and inactive. Raised blood inhibin and anti-Müllerian hormone support the diagnosis; treatment is ovariectomy.
- Ovarian haematoma and haemorrhagic anovulatory follicles: the ovary enlarges, but these are benign and resolve on their own.