Suppression of reproductive behaviour and gonadal function in female horses-An update

Abstract

The behaviour of mares is often detrimental to their performance resulting in frequent demand for methods to suppress gonadal function. In addition, prevention of unintended reproduction especially in feral horse populations may require methods for suppression of gonadal function. Surgical ovariectomy is a safe method but not an acceptable approach in feral mares and undesired in mares where future breeding is considered. There are different approaches for artificial prolongation of the luteal phase resulting in transient inhibition of oestrus and ovulation. Among those, treatment with natural or synthetic progestogens is considered the most common and successful method. Whereas application of intrauterine devices may result in prolongation of luteal function in non-pregnant mares, intrauterine insertion of glass balls is no longer recommended because of complications in individual mares. There are several safer alternatives that may be of interest, especially for population control in free-roaming horses. Treatment with long-acting deslorelin implants inhibited ovulation and oestrus behaviour in mares for limited and variable time intervals in a dose-dependent manner. The effect of GnRH vaccines varies considerably among individual mares, is age dependent, and oestrus-like behaviour may still occur. Contraception via immunization against native porcine or recombinant zona pellucida antigen is successful, but immunocontraception is as much a result of ovarian inactivity as an antibody-based block to sperm-oocyte binding. In conclusion, several treatments for suppression of gonadal function in mares are available, but there are advantages and disadvantages associated that have to be considered. The treatment of choice will thus differ with regard to the demands.

Keywords: behaviour; female horse; oestrus suppression; ovulation.

FIGURE 1

Regulation of the oestrous cycle in female horses and targets for inhibition of oestrus: 1. prolongation of the luteal phase, 2. treatment with progestins, 3. application of intrauterine devices, 4. treatment with long‐acting GnRH analogues, 5. immunization against GnRH, 6. immunization against zona pellucida proteins. In green: prolongation/enhancement of the physiological mechanism; In red: inhibition/disruption of the physiological mechanism