Endocrine and testicular changes associated with season, artificial photoperiod, and the peri-pubertal period in stallions.

The research article investigates the reproductive cycle of stallions, its correlation with seasons and artificial photoperiod, and characteristics that shape the peri-pubertal period in this animal species.

Seasonal Reproductive Cycle

• The study begins with an examination of a stallion’s annual reproductive cycle. The reproductive capacity of stallions hits its peak during the longer daylight hours of spring and summer. This coincides with an annual increase in the concentrations of Luteinizing Hormone (LH), Follicle-Stimulating Hormone (FSH), and testosterone in the stallion’s serum, followed by a decline (‘regression’) and subsequent growth or ‘recrudescence’ in a stallion’s testicular function.
• The annual cycle in LH secretion likely indicates a seasonal replenishment and reduction in the pituitary gland’s LH content, which is sensitive to changes in daylight (or photosensitive).

Long-day Photoperiod and Reproductive Function

• This study posits that stallions, like many other seasonal breeders, may have an innate annual rhythm in their reproductive function. A significant finding is that this rhythm can be influenced by changing the balance between light and dark hours—referred to here as the light:dark ratio or photoperiod.
• For instance, introducing a ‘long-day’ photoperiod (16 hours of light compared to 8 hours of dark), after 20 weeks of ‘short day’ (8 hours light compared to 16 hours dark) in December, was found to hasten the renewal (‘recrudescence’) of the stallion’s seasonal sexual reproduction. On the other hand, the study found that this method was not effective in lengthening the time during which the stallion displays peak reproductive capacity.

The Peri-pubertal Period in Stallions

• The research identified that the infantile period in colts, characterized by low gonadotropin concentrations and minimal gonadal activity, lasts approximately 32 weeks.
• The pre-pubertal period is signaled by changes in the hypothalamic-pituitary axis, which result in an increased secretion of LH and FSH between the ages of 32 to 40 weeks.
• Testosterone concentrations in serum show a significant increase around the age of 75 to 80 weeks. Puberty, which is defined in this instance as the age when the first ejaculate containing a minimum of 50 million sperm with more than or equal to 10% progressive motility was obtained, was observed to occur at 83 weeks of age.