The oestrous cycle of mares

By James Condron

One has to admire the durability and constitution of Midday as much as her undoubted star quality, in completing a hat trick of Group 1 victories for the season (and a fifth career Group 1) at Longchamp in the Qatar Prix Vermeille this month. The daughter of Oasis Dream will likely head to Santa Anita in an attempt to win consecutive Breeders’ Cup Filly and Mares Turf titles instead of taking on the colts with her male counterparts in the Arc at Longchamp next month. Midday has clearly improved from three to four under the care of her trainer, the legendary Henry Cecil, long recognised as a master trainer of fillies and mares with six 1,000 Guineas and eight Oaks victories on his British Classic résumé in addition to many Group 1 victories around the globe.

Fillies and mares are widely accepted as not being as strong as colts and geldings. This can be evidenced in the sales ring where unless a filly is very well bred (having long term potential as a broodmare), she will on average sell for a figure considerably less than if she were a colt. In addition, the current International Classifications for horses in training show only three fillies in the top 25 three-year-olds in training; Snow Fairy, Music Show and Sarafina and only two fillies/mares amongst the top 25 four-year-old and older horses; Goldikova and Midday. There are also a series of weight allowances for mares and fillies in Pattern races, both at home and internationally, ranging from 3 lb to 5 lb on the flat and 7 lb in Graded and Listed races over jumps, in an effort to create a level playing field for these races. In this context, the achievements of the Aga Khan’s Zarkava in winning the 2008 Arc and the victories of the great Ridgewood Pearl trained by John Oxx in the Prix Du Moulin and Breeders’ Cup Mile in 1995 cannot be underestimated.

Training fillies and mares can be challenging in a number of respects and is not simply a matter of training them in the same manner as you would train colts and geldings. While the mechanics of getting a filly/mare fit for the track remain the same, the trainer and his team have to map out a campaign for their horse, having to plan training and races around the mare’s oestrous cycle (essentially the equine equivalent of the menstrual cycle), while ensuring that she is at the peak of her condition for her target races.

The oestrous cycle begins in early spring when increasing light and warmth, the gaining of condition from more nutrient grass and increasing feed in the case of a thoroughbred in training, stimulates the onset of the cycle, which varies in duration from 18 to 22 days, separated into a period when the mare is in season, otherwise referred to as ‘in heat’ and is receptive to a stallion, lasting for four to six days and a inactive period known as dioestrus, which lasts 16 days. It appears to be quite difficult to get a mare to perform to her best when she is ‘in heat’ as very often mares undergo drastic character changes, with common symptoms including sluggishness, irritableness and tiredness. Roberta Baxter in a very interesting article on the Horse and Hound website (http://www.horseandhound.co.uk/horsecare/1370/57207.html) states that some mares suffer similar symptoms to women suffering from Post Menstrual Tension, such as pain from ovulation and changes in behaviour. Baxter states that increased oestrogen affects the behavioural centres of the brain to induce sexually responsive behaviour and may lead to greater sensitivity and aggression, all of which impacts upon the mares willingness to train and perform.

There a number of methods which can manipulate the oestrous cycle of mares; herbal treatments (more commonly used with sport horses), ‘equine pills’ which mimics progesterone (an anabolic hormone produced during pregnancy) thus preventing ovulation, injectable progestagen which also mimics progesterone and the practise used by studs of using artificial light, which brings forward the onset of the oestrous periods, essentially imitating spring time. While more and more advanced methods of manipulating these cycles are being developed, trainers and vets need to keep aware of the Jockey Club’s list of banned substances to ensure that no breaking of rules takes place and subsequent sanctions.

The ultimate manipulation of a mare’s oestrous cycle is of course pregnancy. Most mares perform much better on the track when pregnant; however as the rules of racing stipulate that a mare cannot race if she is more than 120 days pregnant, the window of opportunity to exploit this improvement is limited. This improvement is due primarily to the fact that the mare will not be in season, consequently easier to train and due to the presence of progesterone, in the horse’s system, which builds muscle and condition, while also increasing a mare’s appetite. An example of the improvement which can be made by a pregnant mare on the racecourse is the Joanna Morgan-trained, Miss Gorica, who is in foal to Dylan Thomas, who won the Listed Naas Sprint Stakes in the early stages of her pregnancy before gaining more ‘black type’ when third in the Group 3 Brownstown Stakes at Fairyhouse in June.

As the mare’s oestrous cycle will generally end by October of each year, national hunt trainers do not have to worry about a mare being in heat for much of the season, ironically until the spring time when the season comes to a climax with Cheltenham and Aintree in Britain and Fairyhouse and Punchestown in Ireland. This is worth considering for punting purposes, as a mare may not be guaranteed to run to her best once her oestrous cycle starts again.

Sir Francis Baron’s famous Latin phrase “scientia potentia est” translated and paraphrased as “knowledge is power” is in itself a useful motto for a punter to follow. The purpose of this article is not to pinpoint specific fillies, mares or trainers to follow but essentially to inform of the intricateness and complexities of the mare’s oestrous cycle and how it can impact upon a mare’s or filly’s behaviour and consequently performance on the racetrack. The minutest of details can sometimes make the difference between winning and losing bets and lays in horse racing.

By James Condron