When it comes to reproduction, the terminology is unique and sometimes downright strange! I’ve been asked about “dummy eggs”, “speckled eggs”, “soft eggs” and “the egg up egg down injection”. It’s enough to make you wonder how the equine species managed before the advent of modern veterinary medicine!

However the question on everybody’s lips around covering time is: “Is she gone”? On busy stud farms, where stallions are covering large numbers of mares in an artificially brief season, there is usually only one chance for mares to be covered on any one cycle, and whether or not that mare has ovulated within 48-72 hours of her cover is vital to her chances of conceiving.

To this end, the use of ovulation induction agents is commonplace. This article explains how they work and how best to use them.

The ovulation induction agents most commonly used in Ireland are hCG (Chorulon, Intervet) and deslorelin (Ovuplant, Dechra).

hCG (Chorulon) has an LH (Luteinising Hormone) like activity which mimics the natural surge in a mares’ own LH levels as she nears ovulation. It is typically administered at a dose of 1500 IU (one vial) and more than 80% of mares will ovulate within 48 hours with the majority expected to ovulate at 36 hours +/- 4 hours after administration. Ideally Chorulon should be administered when an oedema pattern is present on scan and the dominant follicle is approaching 40mm. Chorulon can sometimes fail in mares and this is most likely to occur when it is administered either too early or too late in oestrus, when it is used repeatedly during one breeding season, and in older mares. Anecdotally, mares that fail to respond to Chorulon often do so repeatedly, so it is worth taking note if this happens and switching to an alternate ovulation induction agent thereafter. In our practice, we aim to give the Chorulon 12-24 hours before cover so that it has already started to work prior to cover. This maximises the chance of the mare ovulating soon after cover and also leaves time to try other hormones if the Chorulon fails and still have the mare ovulate within 72 hours.

Deslorelin implant (Ovuplant). This is a long acting GnRH (Gonadotrophin Releasing Hormone) analogue and works on the pituitary gland of the brain rather than directly on the ovary. The effect of this is to cause an increase in the release of FSH and LH from the ovary which stimulates follicle growth and ovulation. Ovuplant is useful early in the season, particularly if the mare is having her first cycle of the year when ovulation can be difficult to predict. It is also useful in mares that have failed to respond to Chorulon, in older mares and in mares where the cover is being given earlier than desired due to stallion availability. With Ovuplant, ovulation is expected 41 hours +/-3 hours after administration. While deslorelin is considered to be more reliable and to yield a more predictable time of ovulation than hCG, it is not fail safe and some mares do not respond. Similarly to Chorulon, in our practice we aim to give the Ovuplant implant approximately 12-24 hours prior to cover. Deslorelin has the effect of temporarily suppressing follicular development so we routinely remove the implant after ovulation to prevent a delay in return to oestrus should the mare fail to go in foal. Also, because the use of deslorelin results in a surge of both FSH and LH, it is common for mares to ovulate smaller follicles that would not naturally have ovulated, as well as the dominant follicle, and so multiple pregnancies can occur, particularly in stallions with good sperm longevity.

CROSS-COVER

In the event of a mare failing to ovulate within 48 hours of administration of either Chorulon or Ovuplant, a short acting GnRH analogue can be tried. Buserelin (Receptal) can be given twice daily to promote ovulation. Alternatively, many mares ovulate within 48 hours of prostaglandin administration if given late in oestrous and a ‘PG’ injection can also be tried if other drugs have failed.

Of course there is always the option of a cross-cover, if a mare fails to ovulate on time but these days with busy stallion schedules... it would seem easier to get her through the eye of a needle!!

Andrea Ryan MVB Cert EP Cert EM (Stud Med) is a member of the Equine Group of Veterinary Ireland and is in practice in Kilcash Equine Centre in Clonmel

Email: hq@vetireland.ie

Telephone: 01-4577976