Reproducao equina

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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 Animalshave 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, favorableclimatic 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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Correspondingauthor.

0378-4320r00r$ - see front matter q 2000 Published By Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 3 2 0 Ž 0 0 . 0 0 1 3 3 - 0

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P. Nagy et al.r Animal Reproduction Science 60–61 (2000) 245–262

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 aseasonal 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 ŽHughes 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 ŽGinther, 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 horsesborn later ŽLanglois 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 ofthe first ovulation of the year in mares ŽBurkhardt, 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...
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