Although a small proportion of mares will cycle all year round, most will stay in anoestrus throughout winter. They begin transitioning in spring with normal cycles occurring by summer.

This year, with poor weather conditions prevailing, thoroughbred mares who have been conditioned to cycle earlier are taking longer to come out of transition into normal cyclicity. We envisage similar issues for sports horse breeders, especially for those who wish to breed early in the season. So, it is timely to consider the options to help kick-start your mare out of transition into normal cyclicity.

Artificial light

The most common method to help make mares cycle normally is the use of artificial lighting. The technical term for this approach is photoperiodic manipulation. Providing artificial light from the start of December for 16 hours daily can advance the onset of ovulatory cycles in mares.

Traditionally, indoor barn lighting has been used to provide artificial light from dusk until 11pm. Timers can be used to make this system more convenient.

The brightness level is an important factor and it has often been said that you need to be able to comfortably read a newspaper in all corners of the barn when using this technique.

Another technique involves the use of the Equilume mask. The mask produces biologically effective low wave blue light for 15 hours per day into one eye. Both methods can be effective in bringing forward cyclicity in mares.

It is also important to remember the pregnant mares. Using artificial light in pregnant mares will reduce the risk of the mare falling into seasonal anoestrus after foaling.

Equilume masks have also been shown to increase the live birth weight of foals. This is beneficial for early foals who can have reduced birth weight in comparison to foals born later in the season.

Progesterone

Progesterone can be used in mid-late transitional mares to speed up the onset of normal cyclicity. Progesterone can be given orally or in PRID formation. This hormone usually has no effect on anoestrus or early transition mares. A mare must be in mid-to-late transition with follicles preferably of 20mm or larger for progesterone treatment to be effective.

The normal protocol involves giving the mare altrenogest (regumate) orally or applying a PRID for 10 consecutive days. This is followed by the cessation of treatment (stopping the oral altrenogest or removing the PRID) and seeing the mare two days later when she should be in season with a large follicle. Shorter and longer variations of this protocol exist and can be tailored for the individual mare.

Intravaginal progesterone via the PRID may be preferred by some breeders due to the reduced labour required. However, caution is advised on their use in maidens who may find the device uncomfortable and display signs of colic.

It must also be noted that a vaginitis discharge is not uncommon when using a PRID. However, research has shown this vaginitis is transient and inconsequential.

Dopamine antagonists

Dopamine antagonists, such as domperidone or sulpiride can hasten the onset of normal cyclicity while allowing the mare to continue displaying estrous signs. If an initial response is seen, treatment is continued until ovulation has occurred.

Both come in oral formations. If there is a suspicion of lactational anoestrous playing a role in delayed cyclicity, then domperidone is the drug of choice.

GnRH Agonists

GnRH agonists are drugs which stimulate the production of hormones which can hasten the onset of ovulation. Buserelin, deslorelin and goserelin are shown to be effective GnRH agonists. Repeat injections are required to fulfill the protocols involving deslorelin and buserelin.

One study has shown that the use of buserelin injection twice daily can stimulate follicular development in both aneostrus and transitional mares who have undergone a 16-hour light programme for a minimum of two weeks.

This protocol is the only one shown to hasten the onset of normal cyclicity in anoestrous mares.

Goserelin is available in a long acting implant formation called Zoladex. It is an expensive method and is usually only used as a last resort or in situations where daily treatment is not an option.

Other factors

Overall, general health and nutrition obviously play a role in the onset of normal cyclicity in the mare.

Focusing on your mare’s health and having a general clinical examination performed by your vet early in the year before the start of the breeding season may also be advantageous.

Dr Grainne Cunningham is an associate veterinarian at SES Equine Veterinary Services.