Silent Estrus (Behavioural Anestrus) in Mares

In silent estrus, mares either do not show estrus, or are slow to show detectable signs using standard teasing methods despite the fact that ovulation occurs.

The degree of reduced expression of estrus varies from partial (sub estrus) to complete (anestrus).

The incidence of silent estrus is higher in maiden mares early in the breeding season and in mares with a young foal `at foot’.

Other factors that affect estrus behaviour include being at grass with very dominant mares, and stallion preference. Fillies that are in training and have been treated with anabolic steroids may be more likely to suffer from the condition due to `androgenization’.

In many cases, it is a failure of the estrus detection system rather than a true reproductive disorder of individual mares. However, it has been associated with reduced estradiol concentrations in the peripheral circulation and a shorter interval from luteolysis to ovulation.

Diagnosis

Rectal and vaginal examinations confirm that the mare is in estrus and has follicles of an ovulatory size. It is essential to distinguish the condition from a prolonged luteal phase in which there is also follicular development.

Treatment

The treatment is based on thorough and careful teasing. Frequent and persistent teasing may persuade the mare to show estrus.Alternatively, placing the mare in a stable next to a stallion may be helpful.

If permissible, artificial insemination can be used. To breed mares naturally during a silent estrus, some form of restraint may be necessary; many mares approaching ovulation accept the stallion when twitched and hobbled.

An intramuscular injection of estradiol benzoate (10-20 mg) 6 hours before breeding can be tried as a last resort.

The veterinary surgeon must ensure that the mare is physiologically ready to be bred. In some cases when the mare is not psychologically prepared for breeding, estrogens are of little value, and tranquilizers may be more appropriate.