Influences of season and artificial photoperiod on stallions: testicular size, seminal characteristics and sexual behavior.

This study investigates how the length of daylight hours affects the sexual reproductive cycle in stallions. The researchers found that artificial manipulation of daylength significantly impacts aspects like testicular size, semen quality and sexual behavior.

Study Methodology

• The study involved 21 stallions, which were assigned to three different groups. Each group underwent different treatments related to photoperiod or the length of daylight.
• The first group was the control group, which was subjected to a natural, ambient photoperiod.
• The second group, named S-L, experienced 8 hours of light and 16 hours of darkness for 20 weeks from 16th July 1982, then switched to 16 hours of light and 8 hours of darkness from 2nd December 1982 to 5th March 1984.
• The third group, S-S, experienced 8 hours of light and 16 hours of darkness from 16th July 1982 until March 1984.

Measurements and Findings

• All stallions had their total scrotal width (TSW) measured every four weeks during the experiment.
• At ten distinct periods throughout the experiment, semen samples were collected from each stallion and evaluated, while their sexual behavior was also noted.
• It was found that the S-L stallions, who were exposed to 16 hours of light in December, had double the sperm production in February as compared to November.
• Notably, this increased sperm production in the S-L group was found to be significantly higher than that of the control or the S-S group.
• The artificial increase in daylight hours also resulted in larger testes and reduced time to ejaculation for the S-L stallions as compared to the other two groups.

Conclusions

• Despite continuous exposure to prolonged light periods, there was eventually a decline in TSW and sperm output in the S-L group, which the researchers suggest could be due to photorefractoriness – the diminished response to prolonged exposure to light stimuli.
• The S-S stallions were found to have seasonal cycles that coincided with those of the control group.
• Overall, it was concluded that all three groups exhibited significant seasonal cycles based on changes in TSW and sperm output, suggesting that stallion reproductive behavior and biology is influenced by changes in daylength.