Question

We live in Portugal and are having problems with lack of estrus in our mares. Could you suggest a change in management or a supplement that can help us stimulate heat in these mares?

Answer

Here are some reasons why the mares in your area are not coming into estrus.

1.  Because of your proximity to the equator and little change in daylight hours, mares will often cycle year-round. However, reproductive specialists have found that mares near the equator will sometimes shut down for 3 or 4 months to reset their reproductive tracts, much like what mares in the temperate areas experience. If everything else appears normal with the reproductive tract, then perhaps what you are seeing is a temporary shutdown. If this is the case, then the best plan of action is to just wait and let them come back when they are ready.

2. If mares are in poor body condition or are losing weight, they are less likely to show signs of estrus. Also, mares that are protein-deficient may not ovulate. This is Mother Nature’s way of helping mares so they do not get pregnant at a time when they cannot physically support a growing fetus or nursing foal.

3. When mares have been given anabolic steroids (commonly used on the racetrack and with halter show horses), it wreaks havoc on their estrous cycles. After ceasing use of the steroids, it may take up to a year to get the mares’ hormonal cycles straightened out.

4. Physical problems or states will keep mares from coming into estrus: pregnancy, cystic or damaged ovaries, uterine infection, endometritis, pseudopregnancy, etc.  All of these should be diagnosed and treated by a veterinarian.

Suggestions:

1.  Make sure the mares have a balanced diet so there are no deficiencies or excesses.  The diet should also provide plenty of energy so that mares are gaining weight if they are too thin. For obese mares the diet should maintain weight, but it is not the time to try to slim down the mares. The particular nutrients to pay attention to are vitamin A and vitamin E. Omega-3 supplementation may also be useful.

2. In the United States, it is very common to use artificial lighting to lengthen the daylight hours so that the mares start cycling in December and January. The goal is to have the mares spend 16 hours in light and 8 hours in dark. At the equator, the day and night are equal in length–12 hours light and 12 hours dark. I am not sure how much your daylight differs from this. Perhaps putting the mares under lights for 16 hours may have an effect. The light needs to be strong enough to have an effect; a 200-watt bulb is sufficient for an average stall.

3. Hormones can be used to regulate estrus in mares. If mares are not showing signs of estrus, progesterone (injectible, implant, or the oral product Regumate) can be given for 10 to 14 days and then followed with prostaglandin. Mares should come into heat in 3-5 days. This method is used commonly to synchronize herds that are scheduled for artificial insemination (AI).  This may not work if mares are in anestrus, but it is useful if mares are in the transitional phase (moving from anestrus to estrus).

4. The presence of a stallion on the farm, especially if he is in close proximity to the mares, can help to bring mares into estrus. Even if there is no breeding stallion, a teaser stallion or a hypermasculine gelding can be useful, particularly if he is vocal and will call to the mares. Bringing the stallion or teaser up to the mares on a daily basis has been shown to increase signs and detection of estrus.

5.  I am not aware of any herbal remedy or specific nutrient that would help jumpstart mares into cycling. Tryptophan was tested as a supplement for reversing winter anestrus and was not successful in the Northern Hemisphere. Perhaps it may have a different effect on mares that are already in sufficient daylight hours, but I am not aware of any studies on mares in the equatorial region.