Advancing the Breeding Season - Overview

Take Home Message: Due to the January 1^st universal birthdate set by many horse registries, breeders in the western hemisphere have sought means to produce foals early in the year before the natural physiological breeding season has begun. Through both pharmacological and/and physiological manipulation of the hypothalamic-pituitary-ovarian axis, success can be achieved with a reasonable degree of predictability.

I. INTRODUCTION

As daylight increases, nonpregnant mares gradually emerge from a relative state of ovarian quiescence to regular cyclicity. This seasonal onset of reproductive function lies under neuroendocrine control of the hypothalamic-pituitary-gonadal axis, which throughout the seasons, is influenced by a number of stimulatory and inhibitory factors. The transition appears to be tightly regulated by dopaminergic control, as well as paralleled by a decrease in melatonin production, although the precise pathways remain unclear. As mares are exposed to daylight and increasing temperatures,¹ there is an increased synthesis of hypothalamic gonadotropin releasing hormone (GnRH), which may also, at least in part, be regulated by a family of neuropeptides called kisspeptins.² Gn RH, in turn, begins to stimulate the release of follicle stimulating hormone (FSH) from the anterior pituitary gland. Luteinizing hormone (LH) concentrations do not parallel FSH release at this early stage of vernal transition. The mare’s ovaries respond to FSH with the gradual development of follicles and secretion of estrogen. These early follicles often grow slowly compared to the growth rate of follicles during the breeding season.³ Some follicles may eventually grow to 30+ mm in diameter, but they are often poorly vascularized and contain lower granulosa cell content than follicles that are recruited to ovulation.³ Mares begin to show clinical signs of a prolonged estrus period followed at last by a surge of LH and the first ovulation of the season.

II. EXOGENOUS REGULATION

Because the value of horses in many breeds is dependent on requirements set forth by their registries, hormonal regimes aimed at shortening the interval to this first ovulation of the season have been examined. Often the effectiveness of early intervention depends on how deeply a mare resides in anestrous at the beginning of treatment. To date, increasing a mare’s exposure to light by providing 16 h of light followed by 8 h of darkness has proved to be a potent and effective means of shortening the time to first ovulation of the season. McCue et al⁵ have also shown that a shorter duration of light exposure (1- 2 hours) following the onset of darkness is effective in inducing earlier cyclicity, presumably by disrupting the duration of melatonin secretion. It is also important to note that providing an increasing plane of nutrition, especially in aged mares,⁶ has been shown to hasten the onset of the first ovulation, possibly involving hormones associated with digestive metabolism.

In attempt to hasten the interval further, researchers have examined the efficacy of various hormonal preparations that include progestins (± estrogens), administered either through intramuscular injection, per os, or via a vaginal pessarie, as well as the powerful dopaminergic antagonists, domperidone and sulpiride. Administration of these preparations is labor intensive. Often more than one of these treatments, combined with photoperiod manipulation, is implemented to hasten the onset of seasonal cyclicity. A recent study , requiring far less labor management, has shown that the combination of a single dose of t he long-acting estrogen compound, est radiolcyprionate (ECP), followed 24 later with a single dose of a dopamine antagonist, given to anestrous mares significantly shortens the interval to first ovulation in treated mares compared to nontreated mares.⁷ In this experiment, an estrous mares received 100 mg ECP and either 1.5 mg domperidone in biodegradable microparticles or sulpiride (either 0.75g or 1.5g) in a non-particle liquid formulation on the following days. Mares in the sulpiride treatment group received subsequent injections of sulpiride at the same dose on days 6 and 11. The authors reported a significant effect of dopamine antagonist treatment on day of first ovulation with treated mares ovulating earlier than control mares. Furthermore, both of the dopamine antagonist treatment also had an effect on ovulation success (P = 0 .005), with mares treated with either domperidone or sulpiride ovulating within 32 days (25% and 77%, respectively) when compared to control mares (11%).

Other researchers have examined and compared the effect of eFSH and deslorelin, a GnRH analogue, on ovarian stimulation and embryo production during the vernal transition period. One group reported that twice daily injections of eFSH (12.5 mg IM) or deslorelin (63 ug IM) were equally effective in inducing ovulation in early transitional mares, within a predictable time of treatment.⁸ As well, administration of 10-125 ug of buserelin, another GnRH agonist, through twice daily injections has also been used to stimulate follicular development in an estrous mares.⁹

III. EFFICIENT MANAGEMENT

All of the treatments discussed above require extensive management of mares. For lighting regimens, mares must be housed in barns, stalls or paddocks. The cost for bedding, feed and electricity continues to rise. Hormonal management is also expensive both in labor costs associated with mare management, as well as the products themselves. The advent of a mobile blue light mask^a that shines a low intensity light into a single eye of a mare over the same time interval as programmed stable lighting offers breeders a cost-effective, low management means of advancing the breeding season. Researchers have found no difference in the interval to first seasonal ovulation between mares wearing the mask and artificial stable lighting.¹⁰ Furthermore, mares wearing the masks can remain on pasture, thereby reducing feed costs, as well as providing a healthier exercise environment for many horses. The only drawback to the mask is that the battery life currently only extends through a single breeding season.

IV. CONCLUSION

There are many effective ways of advancing the equine breeding season, but they are often costly and require extensive mare management. A plausible solution that both reduces costs associated with mare management, as well as provides an optimal environment for many horses may lie in the further development of the blue light mask and the life expectancy of its battery.

REFERENCES AND FOOTNOTE

1. Guerin MV, Wang XJ. Environmental temperature has an influence on timing of the first ovulation of seasonal estrus in the mare. Theriogenology 1994;42:1053–1060.
2. Decourt C, Caraty A, Brian t C, et al. Acute injection and chronic perfusion of kisspeptin elicit gonadotropins release but fail to trigger ovulation in the mare. Biol Reprod 2014;90(2)36:1-12;
3. Tucker KE, Cleaver BD, Solute ME, et al. Relationship between IG F-1, 17 ahydroxyprogesterone and progesterone on estradiol 17B content in the equine follicle. Biol Reprod 1992;46(suppl 1):155.
4. Tucker KE, Cleaver BD, Sharp DC. Does resumption of the follicular estradiol synthesis during vernal transition involve a shift in steriodogenic pathways? Biol Reprod 1993;48(suppl 1):188.
5. McCue PM, Logan NL, Magee C. Management of the transition period: physiology and artificial photoperiod. Equine Vet Educ 2007;146–150.
6. Carnevale E M, Hermenet MJ, Ginther OJ. Age and pasture effects on vernal transition in mares. Theriogenology 1997;47:1009 –1018.
7. Mitcham PB, Thompson DL, Burns PJ, et al. Recent advances in the use of an estradiol-dopamine an tagonist protocol to induce ovulation in seasonally anestrous mares. J Equine Vet Sci 2014;34(1):105-106.s
8. Raz T, Carley S, Card C. Comparison of the effects of eFSH and deslorelin treatment regimes on ovarian stimulation and embryo production of donor mares in early vernal transition. Theriogenology. 2009;71(9):1358-66.
9. McCuePM. Hormone therapy in the mare, in Proceedings. World Equine Veterinary Association Congress 2013; Budapest, Hungary.
10. Walsh CM, Woodward EM, Prendergast JP. Mobile blue light therapy is as effective as stable lighting at advancing seasonal reproductive activity in mares. J Equine Vet Sci 2014;34(1):103.

^a Equilume Ltd.,
Kildare, Ireland.