Reproducao equina

Animal Reproduction Science 60–61 Ž2000. 245–262 www.elsevier.comrlocateranireprosci

Seasonality in mares
P. Nagy ) , D. Guillaume, P. Daels
Department of Physiology of Reproduction in Domestic Mammals, National Institute of Agricultural Research (INRA), 37380 Nouzilly, France

Abstract In this review, we have attempted to summarize, based on recent data obtained in our laboratory and elsewhere, our current understanding of the regulatory mechanisms of seasonality and discuss the implications with regard to treatment strategies to advance the onset of cyclic reproductive activity in the early spring. q 2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Mare; Seasonality; Anestrus; Photoperiod; Melatonin; Neurotransmitters; Thyroid hormones

1. 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 directractive inhibition induced by signals such as short daylength, adverse climate, poor nutrition. The mare, as other seasonal breeders,
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