The effect of dietary protein on reproduction in the mare. IV. Serum progestagen, FSH, LH and melatonin concentrations during the anovulatory, transitional and ovulatory periods in the non-pregnant mare

INTRODUCTION I t is economical ly important to Thoroughbred breeders that barren and maiden mares exhibit normal, ovulatory oestrous cycles and also conceive as early as possible during the breeding season. In winter, most but not all non-pregnant mares pass through a period of anoestrus, or sexual and ovarian inactivity (anovulatory period), when neither developing follicles nor corpora lutea are found in the ovaries. The phases that follow in the seasonal cyclical reproductive rhythm of the mare are the transitional period (early spring) and the active breeding phase (ovulatory period: late spring and summer). These changes are controlled by the reproductive hormones, which in turn are influenced by environmental factors, particularly daylight length, temperature and nutrition. Studies involving ovariectomised mares indicated a definite seasonal pattern in the secretion of LH and FSH, where low baseline LH and FSH concentrations similar to those found in mid-dioestrus were found during the winter months, in contrast to higher LH and FSH concentrations during summer. As these mares were ovariectomised, it can be assumed that external factors, particularly the photoperiod, played an important regulatory role in the absence of ovarian hormones. Furthermore, LH concentrations are lower during the 1st ovulation of the active breeding season compared to subsequent ovulations, which implies that LH secretion is affected by season. The concentration of LH was also found to be higher during the penultimate ovulation than during the last ovulation during the ovulatory season. A slight increase in the plasma LH concentration found 15–18 days after the last ovulation of the active breeding season indicates that failure of ovulation at the onset of the anovulatory season is due to a lack of an adequate ovulatory LH surge and final growth of a preovulatory follicle. No correlation was found between FSH concentrations and the number of large follicles from the time of the penultimate ovulation until 31 days after the last ovulation. The decreased LH and FSH concentrations during the non-breeding period indicate that the hypothalamushypophysis axis is relatively inactive during the anovulatory period but that this activity increases during the transitional period. The exteroreceptive stimulus of daylight length results in direct stimulation of the nervous system and pineal gland and consequently the secretion of melatonin. Melatonin may have a suppressive effect on the release of GnRH. There is also an inverse correlation between daylight length and melatonin secretion. This means that an increase in daylight length decreases the total secretion of melatonin, which in turn decreases the suppressive effect on GnRH release. Although it is accepted that daylight length plays an important role on the secretion of GnRH, it is not the only factor involved, as nutrition, ambient temperature and other forms of stress also affect ovarian activity. It is not known whether the diet, specifically a deficiency of essential amino-acids, affects the production of GnRH or the protein hormones, LH and FSH, in the horse. The available essential amino-acids will probably be utilised initially for essential body functions and not for production of gonadotrophic hormones. The purpose of this study was to investigate the influence of the dietary protein intake on the concentration of certain reproductive hormones during the anovulatory and transitional periods in the mare and their effect on the onset of the breeding season (ovulatory period).