The pituitary and testicular responses to GnRH challenge between 4 and 14 months of age in thoroughbred colts born in spring and autumn.

The research article investigates the impact of birth season and body weight on the onset of puberty in Thoroughbred colts, using the hormone buserelin to trace changes in luteinizing hormone (LH) and testosterone levels over time.

Research Methodology

• The researchers performed regular Gonadotropin releasing-hormone analogue challenges using buserelin in Thoroughbred colts from 4 to 14 months of age. The colts were divided into two groups according to their birth seasons- spring (n = 6) and autumn (n = 5).
• The challenges were carried out every 8 weeks, with the main goal to establish the onset of puberty in the colts.

Findings on Luteinizing Hormone (LH) Levels

• In all colts, the secretion of luteinizing hormone followed a seasonal pattern.
• High baseline and maximal concentrations were observed during the spring and summer seasons.
• Low concentrations were observed during the winter season.

Findings on Testosterone Levels

• The colts had undetectable testosterone levels before the spring season.
• Thus, the colts born in the autumn were younger than the spring-born colts when a testosterone response to buserelin was first observed.
• The average weight of the autumn-born colts at the time of the first detectable testosterone response in the following spring (in October) was 300 kg.
• The spring-born colts had reached this weight in the winter, specifically in May and June, but did not exhibit a significant testosterone response until the following spring at an average weight of 352 kg.

Conclusions

• The researchers propose that in order for the onset of puberty to occur in these Thoroughbred colts, they must reach a certain threshold body weight.
• This must occur concurrently with a stimulatory photoperiod (when day length is increasing, i.e., during spring season).