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A novel method of preventing behavioural problems associated with oestrus in the mare has been described.

Some mares become unruly and difficult to manage when in season and perform poorly as a result. Various methods of controlling this behaviour have been tried. Most are based on suppressing the oestrus cycle.

The oestrus cycle in the mare consists of two phases. In the follicular phase the predominant hormone is oestrogen. It makes the mare receptive to the stallion as ovulation approaches. It is during this phase that some mares become difficult to manage.

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Suppressing oestrus

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Report by Mark Andrews.

The luteal phase follows ovulation. A "corpus luteum" forms at the site of the recently ovulated ovarian follicle. The corpus luteum secretes progesterone that prepares the uterus for pregnancy. If the mare does not become pregnant, the uterus releases hormones ("prostaglandins") which break down the corpus luteum, allowing the cycle to be repeated. In the pregnant mare, the uterus recognises the presence of the embryo and does not release prostaglandins. The corpus luteum persists to maintain the pregnancy.

To prevent behavioural signs associated with the mare coming in season it is necessary to maintain adequate levels of progesterone. This can be achieved by getting the mare pregnant. But pregnancy itself may interfere with the mare's ability to perform. Other methods include progesterone by injection or the daily administration of synthetic progesterone-like drugs ("progestagens") in the food.

A new method of suppressing oestrus in mares has been investigated recently in America. Dr Gary Nie and his colleagues at Auburn University College of Veterinary Medicine, have demonstrated that a sterile 35mm glass ball (UGB) placed in the uterus prevents oestrus behaviour in about 40 of mares. They described the technique at the recent Annual Convention of the American Association of Equine Practitioners.

The optimum time to insert the ball is 24 hours after the mare has ovulated. After cleaning the area around the vulva, they insert the sterilised glass ball through the cervix into the uterus. Then the operator moves the ball to the junction between the uterine horns and body by manipulating it through the wall of the rectum. They usually introduce antibiotic into the uterus to prevent infection. Oxytocin may be given to remove any fluid that accumulates in the uterus after the procedure.

They remove the glass balls, either at the first season after a prolonged luteal phase, or after four consecutive ovulations if the treatment has not been successful.

The aim of the treatment is to produce a prolonged luteal phase. This was achieved in five of the twelve mares (41.7) studied. They had a prolonged luteal phase that lasted on average 88.8 days. The other seven mares were not affected by the UGB and continued to cycle normally.

The researchers did not notice any significant adverse effects in the mares in the study. Samples of the lining of the uterus before and after implantation of the UGB showed the treatment had no deleterious effects. Two mares had a very small amount of fluid in the uterus around the UGB a day or so after inserting it. The fluid did not progress. However, Dr Nie suggests that an ultrasound scan be carried out a few days after insertion of the UGB to check for signs ofendometritis. Spontaneous loss of the UGB did not occur.

"The technique offers the advantage of using the mare's own progesterone production to suppress behavioural oestrus" explains Dr Nie. "The disadvantage is that it does not work in all mares. " However in those mares in which it does work, it removes the need for the administration of exogenous progesterone.

For more details see: Use of a glass ball to suppress behavioral estrus in mares. Gary J Nie, Kristina E Johnson, Timothy D Braden James G W Wenzel . Proc AAEP (2001) 47, 246 - 248.