SOME MECHANISMS INVOLVED IN THE RESPONSE OF MARES TO PHOTOPERIODIC STIMULATION OF REPRODUCTIVE ACTIVITY

The horse is a long-day breeder. In stallions, the annual variation of the reproductive ability is small and it is mainly expressed through changes in hormonal levels and consequently through sexual behaviour a nd accessory glands secretions. On the other hand, the reproductive st atus of mares changes considerably around the year. An anovulatory per iod is systematically observed during autumn and winter in young mares 2 or 3 years old and in adult mares that have suckled a foal during t he previous summer (Palmer and Driancourt 1983). Only half of non lact ating adult mares, older than 3 years, however, exhibit winter inactiv ity. These differences indicate that the physiological mechanism that drives the initiation of winter inactivity differs between the three t ypes of mares. Moreover, in adult mares, parturition and lactation pla y an important role in the initiation of inactivity, even though mares cycle normally during lactation. The annual variation of the reproduc tive function is illustrated by plasma progesterone profiles in normal mares or by the variation of circulating LH in ovariectomised mares ( fig 1). The annual variation of LH in ovariectomised mares clearly dem onstrates the existence of an annual rhythm for the reproduction in ma res. This rhythm parallels the changes in the daylength with a delay o f approximately 2 months, which suggests that light may influence this reproductive rhythm.