Introduction
In nature, horses have evolved to be seasonally polyoestrous in their reproductive cycles. Having more than one oestrous cycles per breeding season means that mares continue to ovulate regularly, approximately every 21 days throughout the breeding season (Brinsko 2011). Without intervention, the breeding season of a mare generally occurs during the summer, which in New Zealand is equivalent to November through March. Foaling during this time maximizes the foal’s chance of survival during winter. The breeding season is composed of two parts, the follicular phase (Oestrus) and the luteal phase (Diestrus). The oestrus phase is when the mare will be sexually receptive and prepared to accept and transport sperm to the oviducts for fertilization; this phase generally lasts only for 5-7 days (Brinsko 2011). Ovulation, the release of the eggs from the ovaries, is thought to occur at the end of oestrus. The Diestrus phase is when the mare is not receptive to the stallion. If the mare is left to reproduce without intervention, she will generally foal 11 months later anywhere from October to February.
In 1791 the first equine studbook was published for Thoroughbred horses (The Editors of Encyclopaedia Britannica, 2011). This became an official record of the pedigree of purebred horses. As thoroughbred and standardbred horses evolved from utility animals to a form of income, organizations dedicated to regulating the breed lines decided that all horses have the same designated birthday, August 1st in the southern hemisphere (Brinsko 2011). In correlation to this date, the official breeding season also starts on August 1st (Brinsko 2011). Considering this breeding is highly financially motivated, it is desired commercially to have foals born as early as possible within the official breeding season. The motivation behind this is rooted in the notion that the older the foal is, the more likely it will have developmental advantages over other horses born that season; Older foals will generally be larger/stronger and will likely command a higher price at the time of sale (Meyers-Brown 2017).
Each mare has a gestation period of approximately 11 months, therefore in order to foal at the beginning of August, the mare needs to conceive as close to September 1st as possible. This date was decided without regard to the natural cycles of the mare and therefore we are asking the mares to foal in a season they would generally be trying to avoid. Therefore, the only way to alter the equine reproduction cycle is to induce human intervention in a manner referred to as photoperiod manipulation.
Discussion
Under human’s desire to breed according to standardbred and thoroughbred studbooks, the equine reproductive physiology lends itself to human control, and the mare’s reproductive cycle can be manipulated to suit manmade demands. Generally, equine breeders will stimulate spring transition and oestrus earlier than it would naturally occur. In theory, the horse’s reproductive efficiency is improved with elongated exposure to sunlight. The photoperiod regulates the activity of GnRH secretion, which in turn modulates reproductive activity (Brinsko 2011). To do this, many owners will use artificial lights to trick the mare’s physiology into moving into spring transition to promote earlier breeding.
Since mares are polyoestrous, naturally their breeding season is in summer and consequently, they would be in anoestrous, the period where the mare is not cycling, during the winter. As the day length or photoperiod increases, spring transition, the period of time when the body is preparing to cycle, begins. The oestrus cycle is signaled to begin when the retina is exposed to longer periods of light. During the winter, when the exposure to light is at a minimum, the pineal gland of the brain signals to release the hormone melatonin. Melatonin, in turn, suppresses the production of gonadotrophin-releasing hormone (GnRH). GnRH is the hormone that catalyses the hormonal cascade controlling equine reproduction. GnRH is responsible for the release of the hormones gonadotrophins which stimulates the growth of the reproductive organs, or gonads. Two types of gonadotrophins that are vital for ovulation to occur are follicle-stimulating hormone (FSH) and Luteinising hormone (LH). As the day lengthens, the retina is exposed to light for longer periods of time, which causes the pineal gland to signal a reduction in melatonin production. The decrease of melatonin allows for more GnRH to be produced and in turn increases the production of FSH and LH. FSH is used for follicular recruitment in the ovaries, while LH is used for follicular maturation as well as production of estrogen, ovulation, and luteinisation. If a corpus luteum (CL) is present it will produce progesterone which will have a negative feedback effect on the release of LH, and the estrogen it produces will have a negative feedback effect on the release of FSH. Oppositely, if a dominant follicle is present in the ovaries it will stimulate a positive feedback mechanism promoting the further release of LH. It will also secrete inhibin and estrogen which in turn decreases the amount of FSH secreted. From here the follicular phase of the oestrus cycle is reached and the dominant follicle grows with estrogen production and pregnancy is initiated.
The physiological breeding season can be manipulated to fit into the commercial breeding season by artificially manipulating the photoperiod. The change in daylight hours at various times of the year is a potent environmental factor that alters GnRH production indirectly through the pineal gland (Albrecht de David, n.d.). It is believed that the minimum amount of light necessary for a mare’s physiology to recognize the transition period is approximately 14 to 16 hours of light stimulus per day (Brinsko 2011). The most common equine breeding practice to promote oestrus is to use artificial lights starting on July 1st; it can take 60-70 days for this lighting regimen to induce a fertile ovulation, allowing for a mare to foal anywhere from the beginning of August to September (Geraldine, 2017). The average duration from the onset of artificial lighting exposure to ovulation is approximately 60-70 days (Meyers-Brown 2017). Therefore, in order to sync into the commercial calendar of reproduction a mare should be exposed to this artificial lighting by July 1st. Methods of reproductive manipulation include elongated artificial light exposure, light masks or hormonal treatment.
There are two commonly accepted practices to alter the amount of light a horse is exposed to per day: elongated artificial light exposure and Equilume light masks. Artificial lighting programs consist of exposing horses to the natural sunlight throughout the day and then, when the sun sets, keep the mares exposed to light, whether that be keeping the lights on in the stalls or to turn lights on in the fields. Ideally, horses should be exposed 16 hours of light per day for approximately 8 weeks in order to inhibit melatonin and trigger an early release of GnRH to stimulate the ovaries (Albrecht de David, n.d). Type of lights use vary from halogen, incandescent, and fluorescent, as long as there are no shadows in the stall/paddock they generally perform to the same result. Under this program, a few concerns may be to ensure that horses are not exposed to 24 hours of light and to ensure that this regimen is not stopped too early (Albrecht de David, n.d). The second method of artificial lighting regimen is the use of Equilume light masks. This technique uses the same ideology, of elongating the number of hours the retina is exposed to light, although it is conducted by having a horse wear a mask that shines a blue light into their eye (Murphy, n.d). Through research conducted by the Equilume team, the blue light is proven to be almost as effective at advancing the onset of oestrus in mares as traditional artificial lighting methods (Murphy, n.d). This regimen is through to have the same concern, there should be no shadows, the horse should not be exposed to more than ~16 hours of light and the program should be continuous. Any lapses may cause the mare to revert back to an anoestrus state.
Conclusion
In order to manipulate a mare’s reproductive cycle to foal as early as possible within the official breeding season, a breeder will have to use photoperiod manipulation or drugs to bring the mare out of anoestrus. Methods of photoperiod manipulation include elongated exposure to light or light masks. Both of these methods use the light to make the body believe it is going through spring transition when the ovaries start to function after long periods of rest (anoestrus). The physiology mimics the natural cycle; the retina is exposed prolonged periods of light which initiates the melatonin production in the pineal gland to be suppressed, and without melatonin production, GnRH stimulates the synthesis and secretion of gonadotrophins, FSH and LH.
Not many side effects of manipulating the photoperiod of mare have been noted. Although, Dr. Barbara Murphy and her research team at Equilume declared some side effects of using imposed breeding seasons versus following physiologic breeding seasons, claiming that this could cause longer gestation periods and smaller foals (Murphy, n.d). On average, mares that foal during the winter have longer gestation lengths by an average of 10 days (Murphy, n.d). Consequently, if a mare foals late she represents an economic loss to the breeder by possibly missing the foal heat for the next season.
Man has altered equine reproduction cycles in order to profit in the long run. A system perfected over millions of years through evolution became redirected by man’s willpower. These animals have been referred to as “inefficient breeders” in comparison to other domesticated animals, although this inefficiency is due to the restructuring of the equine breeding system. These breeding inefficiencies are emphasised when horses are bred outside of their natural breeding season.