There are various reasons for "silent" heat in the mare - some not understood completely, as we haven't got inside the heads of horses yet!
Lactational anestrus is one situation where silent heats are seen, and this (obviously) occurs following foaling and for about 3 months after, or sometimes until after the foal is weaned. Sometimes the mare will show estrus on the "foal heat" 5 to 15 or so days after foaling, and not at all from there on, and sometimes she will only show estrus in the absence of the foal and under heavy teasing. Many of these mares are having endocrinological estrus - in other words, the hormones are doing what they should be doing, and indeed, the mare is producing a follicle and ovulating, but she just won't "stand" to be bred. Some of these mares will "break down" in the face of heavy teasing - one of the leading reasons for poor cycle/conception rates in a live cover program (that is pregnancies achieved per estrus cycle) is a bad teasing regime. Some mares require teasing for up to 15 minutes - a cursory pass by the stallion's stall eliciting no response is not sufficient to pronounce a mare "out" - and that teasing must be done regularly, a hit and miss "oh we should tease Bessie today" doesn't cut it!
For mares that are undergoing estrus but not showing it, monitoring ovarian activity by rectal palpation on an every-other-day basis, or ultrasound 2 to 3 times per week is essential. There are mares that will not shown any signs of estrus except for the day they are ovulating, and even then only after heavy teasing. The breeding of these mares would probably be missed if they were not palpated. If an impending ovulation is detected, and the mare will still not "stand", then A.I. may be an option.
Some lactationally anestrus mares genuinely are "anestrus" - that is no hormonal activity. Often these mares will resume their regular estrous cycles once the foal is weaned.
For mares that do not have foals at foot, but are apparently anestrus, probably the first line of approach (assuming that regular effective teasing has been carried out) is to have the mare palpated or ultrasounded. This will allow an evaluation of the ovarian status - i.e. whether there are "structures" present. A single palpation will not necessarily determine whether that "structure" is a forming follicle or a CL (Corpus Luteum), and a subsequent palpation may be necessary, but ultrasound will establish the status immediately, and is therefore preferable if available. Another valuable diagnostic with these mares is the blood-progesterone assay. If the mare has progesterone levels >1 ng/ml, that is indicative of her having ovulated (and therefore to be a behaviourally anestrus mare). It may be necessary to take a sample once a week for three consecutive weeks to accurately determine her status, as one could be sampling during estrus (with the obvious absence therefore of a functional CL).
If there does not appear to be follicular activity present - assuming it is the middle of the breeding season and winter anestrus or "transitional phase" is not a possibility, then re-assessment in 5 to 7 days, or the use of the blood-progesterone assay may be called for.
If a CL is present, then estrus can be induced with the use of the hormone Prostaglandin, or one of it's analogues. This will perhaps give a better "time line" on the ensuing estrus. Further palpations or ultrasound may be demanded in order to pinpoint impending ovulation, and the correct time to breed. "Serial palpations" every other day are useful if the possibility exists.
A more useful hormonal manipulation tool with the behaviourally anestrus mare is progesterone and estradiol. As "P&E" can be started at any point in her cycle, and as it gives a predictable day of ovulation in a high percentage of uses (>80%), it is an ideal tool with which to manage the "silent heat" mare. Biorelease P&E and biorelease altrenogest are now also available in a single-injection form, and produce an almost equally reliable timing of estrus and ovulation to the daily P&E treatments.
"Silent heat" is one of the more annoying habits of a mare, at least from a breeder's perspective, but with proper monitoring in many, if not most cases - as long as the mare is merely behaviourally anestrus - it is manageable.
© 1999, 2007, 2013 Jos Mottershead and Equine-Reproduction.com
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