Seasonality in mares
Introduction
Animals have developed many strategies for seasonal breeding that ensure that their offspring is born at the appropriate time of the year. In the horse, as in many other species, the circannual rhythm of reproduction is cued primarily by photoperiod changes. This environmental signal is translated to an endocrine signal in the pineal gland, which secretes melatonin during the phase of darkness. In the mare, short daylength is associated with a decrease in gonadotropin secretion and consequently a decrease in ovarian activity. The mechanism whereby gonadotropin and presumably GnRH secretion is decreased during the anestrous period is not well understood in mares. It has been proposed that the absence of cyclic activity is the result of a lack of positive signals, e.g. long daylength, favorable climatic and nutritional conditions, the presence of a stallion, that stimulate GnRH and gonadotropin secretion during the breeding season. Recent data in horses and other species suggest that seasonal reproductive inactivity may be the result of a direct/active inhibition induced by signals such as short daylength, adverse climate, poor nutrition. The mare, as other seasonal breeders, has an endogenous circannual reproductive rhythm and the main role of seasonal clues appears to be to synchronize the endogenous rhythm to winter and summer.
The horse is a seasonal polyestrous species with onset of the breeding season occurring in spring, associated with increase in daylight, temperature and availability of food. The natural breeding season occurs from April to September in the Northern Hemisphere (Hughes et al., 1975). Regulations in many breeding associations have established as the official birth date of foals, January 1 in the northern hemisphere, and an official breeding season, from February to June, resulting in the same official age for all foals born the same season (Ginther, 1992). This results in economic pressure for the horse breeders to breed mares as early as possible in the year to have an age advantage over foals born later in the year. Horses that are born early in the year have an advantage and perform better than horses born later (Langlois and Blouin, 1996). This arbitrary birth date has stimulated researchers to understand the mechanisms of reproductive seasonality in mares and develop methods for induction of an early onset of the breeding season in mares. Most of the work has focussed on photoperiod and it has been demonstrated that artificial photoperiod, simulating long days, can be used to advance the time of the first ovulation of the year in mares Burkhardt, 1947, Palmer and Guillaume, 1992. Even though this method has been widely used since at least 15 years it has remained the subject of active research. Recent findings have increased our understanding of the mechanisms and limitations of this important regulator in seasonal reproductive function and more effective methods using artificial photoperiod have been developed in recent years (Guillaume et al., 2000). Despite the efficacy of increased daylength for induction of cyclic ovarian activity in early spring, this method requires that artificial photoperiod treatment is started in December and there is considerable variation in the interval from start of treatment to first ovulation in individual mares. For these reasons, researchers have attempted to design pharmacological treatments that would at the same time decrease the treatment-to-first-ovulation interval and result in a more consistent and predictable response. In recent years, these studies have significantly increased our understanding of the hormonal (steroids and gonadotropins) and neuroendocrine (opioids, dopamine) control of seasonal reproduction in mares and have opened new avenues for the manipulation of the onset of reproductive activity in the spring.
Section snippets
Endogenous circannual rhythm of reproduction
The seasonal reproductive pattern is the result of a circannual endogenous rhythm that is entrained by external environmental factors such as photoperiod, temperature, nutrition and body condition. In ewes, Karsch et al. (1989) provided clear evidence for the existence of a circannual reproductive cycle. Ovariectomized, estrogen-implanted ewes maintained in constant lighting condition for 5 years maintain circannual changes in LH secretion. However, the period of elevated LH levels,
Photoperiod
It is well accepted that in the horse photoperiod is the most important external factor that influences the circannual endogenous reproductive rhythm (Ginther, 1992). Additional light exposure during winter and early spring stimulates ovarian activity in anestrous mares and is commonly used to advance the onset of the breeding season (Burkhardt, 1947). However, the correct response to this stimulatory photoperiod is subject to specific conditions and reproductive activity does not always follow
Role of melatonin
Over the last 20 years, melatonin has been the subject of many studies in horses and there is strong evidence that melatonin is one of the key elements in the control of seasonal reproduction. The role of melatonin in other species is beyond the scope of this review and has been reviewed elsewhere (Malpaux et al., 1999). It has been demonstrated that the pineal gland is implicated in the control mechanism of seasonal reproduction and translates the photoperiodic signals registered by the eye to
Practical problems associated with the management and study of the anestrous mare
The transition in and out of the anestrous period is a gradual process in mares and presents us with several challenges with regard to the clinical management and the study of anestrus in mares. These challenges include (1) prediction of the occurrence and onset of anestrus, (2) diagnosis of anestrus, (3) estimation of the stage/depth of anestrus, (4) prediction of the time of spontaneous return to cyclic ovarian activity and (5) the appropriate time to start a treatment to stimulate the return
Methods available to induce ovulation, cyclic ovarian activity early in the season
Owing to the economical pressure, there has always been an interest in treatment protocols for the induction of ovulation in anestrus mares. The most common and reliable method for induction of cyclic ovarian activity remains artificial photoperiod but other techniques are available and could also be useful in clinical practice especially when combined with artificial photoperiod.
Conclusion
In recent years our understanding of the regulation of seasonal regulation of reproduction in mares has increased significantly. This new insight will undoubtedly lead to better treatment strategies for the induction of cyclic ovarian activity in the early spring. Hopefully, in the coming years we will also have a better understanding of mechanisms controlling the development of anestrus. With the continued pressure of the equine industry to develop methods to extend the breeding season, it
Acknowledgements
Peter Nagy is grateful to INRA for the post-doctoral research position and to the Hungarian State Eötvös Fellowship for financial support during the preparation of the manuscript.
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