The effects of equine somatotropin (eST) on follicular development and circulating plasma hormone profiles in cyclic mares treated during different stages of the estrous cycle.

This research investigated the impact of equine somatotropin (eST) on the ovarian development and hormone production in horses and ponies. The study found that administering eST to mares had a marked impact on certain hormone levels and follicular development during the estrous cycle.

Study Goals and Methods

• The main objective of this study was to observe the impact of equine Somatotropin (eST), a growth hormone, on follicular development and various hormones in horse and pony mares.
• The researchers administered eST to sixteen mares on different days post-ovulation and observed their hormonal responses and follicular growth.
• The mares were given injections of eST at a concentration of 25 micrograms per kg of body weight. Injections were given daily during two stages of the estrous cycle, on either days 6-12 (Treatment A) or days 13-19 (Treatment B). Control groups were also maintained where the mares were given the carrier vehicle without eST (Treatments C and D).
• Blood samples were collected every 24 hours from the mares for hormonal analysis, and ultrasonographic scans of the mares’ ovaries were conducted every 48 hours to observe follicular development.

Research Findings

• The study found that circulating levels of insulin-like growth factor-I (IGF-I), a hormone that promotes growth and development, significantly increased in the mares by the third day after treatment with eST.
• Mares from Treatment B, those treated with eST on days 13-19 post-ovulation, showed a decrease in plasma concentrations of estradiol, a form of estrogen, when compared with control groups during the first five days after ovulation.
• The level of luteinizing hormone, which plays a crucial part in reproductive cycles, was found to be different in mares from Treatment A compared to control mares on days -8 to -1 before ovulation.
• There were no noted differences in the number of follicles (a component of the ovaries where eggs develop) per size category larger than or equal to 8mm in diameter or the average size of follicles within each size category among the treated and control groups.
• However, the treatment with eST significantly increased the number of smaller follicles (less than or equal to 7mm in diameter) on the ovaries during the early stages of the estrous cycle as compared to control mares.
• The increase in small follicles was specifically observed on the third and seventh day post-treatment, and at the onset of standing estrus.

Implications of the Findings

• The findings suggest that eST administration influences hormone levels and follicular development in mares, potentially influencing their reproductive cycles and fertility.
• The changes in follicle size distribution suggest that eST might have a role in the early stages of ovarian follicular maturation.
• These findings can provide insights into potential interventions for managing reproductive health in equines.