A study of seasonal changes in serum levels of reproductive hormones, semen production and libido in stallions located in Canterbury, New Zealand

Abstract

In a preliminary study of seasonal reproductive changes in stallions, four blood samples per week were collected on three separate days from each of four standardbred and two pony stallions for ten consecutive months. Each serum sample was analysed for testosterone, LH and FSH using radio-immunoassay techniques. During the same period paired ejaculates were collected one hour apart one day per week and volumes of both gel-free semen and gel, motility, spermatozoa/ml, total spermatozoa/ejaculate and pH were measured for each semen sample. Libido was monitored by recording reaction time and number of mounts/ejaculate for each collection. On the days of semen collection two blood samples were taken, one before sexual stimulation and one immediately after the first ejaculate. There was a marked effect of season on serum testosterone, LH and FSH levels, however the extent of change and timing of peak levels varies between stallions. Serum FSH levels started rising in late July-early August and generally reached a sharp peak during September, followed by a gradual decline thereafter. In several cases, testosterone showed a pattern of change similar to the of FSH, with peak levels occurring 2-4 weeks after FSH. This apparent association between these two hormones suggested that FSH may modify testosterone production possibly by altering the responsiveness to the testes to LH stimulation. In contrast, although LH levels began to rise at about the same time as FSH levels, peak values were not reached until November, two months after FSH peaked. The seasonal pattern of circulating LH levels more closely resembled the annual changes in daily photoperiod. These results, along with seasonal changes in seminal parameters, were consistent with the view that FSH is involved in initiating early seasonal changes in spermatogenic activity. There was some evidence for an inverse relationship between testicular function and FSH levels which supported the concept of a negative feedback on FSH secretion by a principle of testicular origin. Only LH was significantly (P<0.05) altered by sexual stimulation which produced a slight increase in the LH levels recorded from post-ejaculate blood samples. This response was most marked during the breeding seasons. Marked seasonal effects were noted for seminal characteristics, with seminal volumes, total spermatozoa per ejaculate and libido all increasing, and pH decreasing slightly, during the spring and summer months. However, no consistent effect of season was found for spematozoal motility. Maximum libido occurred during the period of rapidly increasing testosterone levels. Also peak spermatozoal output followed peak testosterone levels by approximately two months thereby demonstrating the importance of testosterone in the regulation of spermatogenesis as well as sexual behaviour. Collectively the data obtained presented a clear picture of the marked seasonal pattern of reproductive activity in stallions located in Canterbury, New Zealand. Whilst the small number of animals studies did not allow definite conclusions to be reached about the specific contributions of each hormone to this phenomenon, the results obtained supported the view that stallion fertility varies throughout the year and that this variation is related to changes in photoperiod.