References
Reproductive management of the transitional mare
Abstract
The mare is a seasonally polyoestrous long-day breeder with a physiological breeding season lasting from April–October in the Northern Hemisphere. The hypothalamic-pituitary-gonadal axis in the mare is subject to a circannual endogenous rhythm that is primarily regulated by day length. Increasing ambient photoperiod in the spring alters the pattern of melatonin secretion. The resulting stimulation of hypothalamic gonadotropin-releasing hormone secretion triggers pituitary follicular stimulating hormone release and follicular growth. Exposure of mares in deep anoestrus to a stimulatory photoperiod remains the most successful method of advancing the first ovulation of the season. The most commonly used lighting regimen is providing a fixed length of 15–16 hours of light exposure and 8–9 hours of dark, with a minimum light intensity in a stable of 100-lux (100–200 watt incandescent bulb). Other methods include using an additional 2.5 hours of light beginning at sunset and a pulse lighting system, providing 1 hour of light, 9.5–10.5 hours after the onset of darkness, during the photosensitive phase. Alternatively, the EquilumeTM light masks provide a unilateral LED light source emitting 50 lux of blue-light directly to the eye during the hours after dusk (until 11 pm). Mares that have not been maintained under lights, or that have been exposed to ineffective light therapy, may require therapeutic hormonal intervention to advance the onset of the first ovulation of the season. Many hormone protocols involving progestins, GnRH, dopamine agonists and recombinant luteinising hormone/follicle stimulating hormone have been studied with variable results. Therapy is typically more effective when started either in late transitional mares or following a period of stimulatory artificial photoperiod.
The mare is a seasonally polyoestrous long-day breeder, with a physiological breeding season lasting from April–October in the Northern Hemisphere. Evolution has ensured that foals are born at times of favourable environmental and nutritional conditions, thus maximising their chances of survival.
The hypothalamic-pituitary-gonadal axis in the mare is subject to a circannual endogenous rhythm that is primarily regulated by day length (Ginther, 1992). Light signals detected by the retina are processed by photosensitive retinal ganglion cells. The retino-hypothalamic tract sends signals to the suprachiasmatic nucleus in the hypothalamus, which is connected to the pineal gland via the superior cervical ganglia (Murphy, 2019). Increasing ambient photoperiod in the spring inhibits the release of the neurotransmitter norepinephrine, which subsequently inhibits melatonin secretion from the pineal gland. Removal of the inhibitory action of melatonin on the mare's reproductive axis results in stimulation of hypothalamic gonadotropin releasing hormone (GnRH) secretion, triggering pituitary follicle stimulating hormone (FSH) release and follicular growth (Donadeu and Watson, 2007). Initially, follicles are steriodogenically incompetent, with positive feedback on luteinising hormone (LH) secretion only occurring when a dominant follicle releases sufficient oestrogen to cause an LH surge, resulting in the first ovulation of the year and the end of the transition period (Davis and Sharp, 1991). The first ovulation of the season is reported on average to be 7 April +/- 9.1 days in horses and 7 May +/- 21.1 days in pony mares (Sharp, 1983).
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