Non-receptive or -resistant mare

Managing “Silent Heat” in the Mare

– Silent heat (estrus) in the mare can be annoying at best and disastrous to a breeding program at worst. This article offers some useful tips.

By Jos Mottershead

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” somewhere between 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, or even “show”. 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 regimen. Some mares may 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!

Uterine Edema

Cross-section of uterine horn showing pronounced edema per ultrasound

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. Once the mare is in physiological estrus – manifested by uterine edema per ultrasound, or somewhat less reliably by cervical relaxation if palpation per rectum or per vaginum is the only option, more frequent evaluations will be required to determine suitable timing for breeding. There are mares that will not display any signs of receptivity to the stallion 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 or ultrasounded regularly. If an impending ovulation is detected, and the mare will still not “stand”, then A.I. may be an option.

Some lactationally anestrus mares are genuinely “anestrous” – that is no follicular or hormonal activity. Often these mares will typically resume their regular estrous cycles once the foal is weaned if not too late in the season, or the following spring if a late foal.

Follicle and CL compared per ultrasound

Follicle (top) and CL (bottom) compared per ultrasound

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 and/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 developing follicle or an established CL (Corpus Luteum) and a subsequent palpation a few days later may be necessary. Ultrasound however will establish the status immediately and is therefore preferable if available. Another useful 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 rather than truly anestrous). 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 – presuming it to be 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 series may be called for.

There are various protocols available, typically using a GnRH such as Histrelin or Deslorelin. This may be coupled with P4 (progesterone) and/or Estrogen depending upon the circumstance. A useful list of protocols can be found on the BET Labs website

If a functional CL is present, then estrus may be promoted with the use of the hormone Prostaglandin F2α, or one of its analogues such as Cloprostenol. This will perhaps give a better “time line” on the ensuing estrus, although response variations may occur. Further palpations or ultrasound may be required in order to pinpoint impending ovulation and the correct time to breed. Serial palpations or ultrasounds every other day are useful if the possibility exists.

P&E bottles

Long-acting P&E from BET Pharm

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 (as long as the mare is cycling), and as it gives a predictable day of ovulation in a high percentage of uses (>80% when combined with an ovulation promoter), it is an ideal tool with which to manage the silent heat mare. In the US, biorelease P&E and biorelease altrenogest are available in a single-injection form from BET Pharm (Lexington, KY), and produce an almost equally reliable timing of estrus and ovulation as the daily P&E treatments with greater convenience.

With some mares once estrus has been confirmed per ultrasound as evidenced by uterine edema, if they still refuse to “stand” for a stallion and AI is not an option, treatment with exogenous estrogen (where legal) may be beneficial. It should not however be regarded as a “panacea to cure all ills”, and awareness that a mare’s attitude towards a stallion may change at the last moment is essential!

“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.

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