Management of Spring Transition

Introduction

The transition in and out of the anestrous period is a gradual process and presents us with several challenges with regard to the clinical management: estimation of the stage/depth of anestrus, estimation of the time of first ovulation and choice of treatment to hasten the return to cyclic activity.

Physiology

The annual rhythm of reproductive activity - In the horse the circannual rhythm of reproduction is primarily regulated 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 day length 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 anestrus 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 day length, favourable climatic and nutritional conditions, the presence of a stallion. Recent data suggest that seasonal anestrus should be regarded rather as a direct/active inhibition induced by signals such as short day length, temperature, and nutrition.

Mares have an endogenous circannual reproductive rhythm and the main role of seasonal clues is to synchronize the endogenous rhythm to winter and summer. The endogenous rhythm can be displaced in time (shift to the left in Figure 1) but its shape cannot be modified as such. Photoperiod is likely the only factor able to displace this sinusoidal curve. By exposing the mare to artificial long daylight before the winter solstice, the curve is displaced to an earlier date (to the left on the graph in Figure 1).[...]