Q: My warmblood mare, who is six, seems to cycle all year around, and she’s really tricky to deal with when she is in season. She is grouchy towards me and other horses, doesn’t like to be groomed or have rugs taken on/off (threatens to bite and/or kick), and under saddle she is tight in the back to the point that her canter feels four-beat, and cranky off the leg. When she is not in season, she is a delight.
I can’t afford to put her on Regu-mate. A friend has suggested a chaste berry supplement. Do you have any advice?
Vet Dave van Zwanenberg replies:
Chasteberry supplement may well help her, and it is certainly worth a try. However, in my experience horses that are as hormonal as you describe when in oestrus do not tend to behave like they are in dioestrus unless their hormones can be maintained in a dioestrus-like state.
Sadly, this usually means that you need to intervene in one of these different ways. The most commonly-used method that does not involve pregnancy and is very reliable is once daily oral supplementation of progesterone (Regumate or similar trade name). This works in a similar manner to the human pill; the oral progesterone tells the body that it is in dioestrus and maintains it in this state.
A less commonly-used alternative, but just as effective, is the use of an anti GnRH vaccine. The vaccine works by immunising the mare against one of her own hormones; the body recognises this hormone as foreign, and mops it up like any other non-self matter in the body.
This disrupts the signalling involved in the process, and stops her from coming into season. The vaccine is given in two injections four weeks apart, and then a yearly booster. Cost-wise this works out at a similar to daily administration of progesterone for a year.
One option you do have for oral progesterone is to use it until you know there is a period you are not wanting to work her, stop the medication at this point and let her come into season. Once the season is over, you will have a two-week period where you don’t need to supplement her with progesterone, as she will naturally be producing her own. By this method you could have a season every six weeks but only use Regumate for three weeks, one week being her in season and the other two her producing her own progesterone.
Getting your mare pregnant is one option to give you respite for a year, and will work well for this period. However, you will loose the ability to work your mare in the later stages of pregnancy and also whilst she is nursing the foal.
There is the option of getting her pregnant and then aborting the pregnancy after day 35, once the placenta has formed progesterone-secreting tissue; however, this is very much of more dubious ethics than the other three options discussed so far.
You may read about using a marble in the uterus to confuse the mare into thinking she has a 16-day pregnancy. This is not a good option, as it carries a low rate of success, and it can be difficult to remove the marble afterwards.
There are several other methods that you may come across in your search of the literature, such as repeated injections of oxytocin or deslorelin implants. The reason these are not used routinely is that they suppress oestrus for an uncertain range of intervals (a few extra days up to a few extra weeks) and have varying levels of success at each attempt.