Hormonal contraception of feral mares with Silastic rods.

The research investigates the effectiveness of hormonal contraception in feral horses using Silastic rods containing different dosages of hormones. The effectiveness of the contraception was primarily gauged by the rate of ovulation and pregnancy over a period of two years.

Research Methodology

• The research involved implantation of homogeneous Silastic rods containing either ethinylestradiol (EE), estradiol-17 beta (E), or progesterone (P) into feral mares within the age range of 4 to 10 years.
• The mares were separated into six treatment groups: P+E, P+HEE, LEE, MEE, HEE and Control-implanted mares (CI). The control group received implants containing no steroid.
• Two distinct groups of pregnant mares received P+HEE or HEE.
• Stallions were placed with the mares 15 to 26 months after implanting the rods. Blood samples were collected bi-weekly for serum analysis up to 28 months after implantation.

Results and Findings

• Both progesterone and ethinylestradiol were used as a measure of ovulation and pregnancy, with ovulation indicated by a single progesterone value greater than or equal to 2.5 ng/ml and pregnancy by two consecutive values greater than or equal to 2.5 ng/ml.
• In both groups that received EE and progesterone together (P+HEE and HEE), contraceptive efficacy reached 100% over two breeding seasons.
• Suppression of ovulation was found to be inversely related to the concentration of ethinylestradiol used in the implant, indicating that higher amounts of contraceptive hormone resulted in fewer incidences of ovulation.
• Regarding pregnancy rate: the control group had a 100% pregnancy rate, P+E had 78%, and LEE and MEE groups both had 25%. The HEE and P+HEE groups showed a 0% pregnancy rate, again underlying the effectiveness of EE in suppressing ovulation and thereby preventing pregnancy.

Conclusion

• The study demonstrates that utilizing Silastic rods to deliver hormones can be an effective method of contraception in feral mares. Specifically, rods containing higher concentrations of ethinylestradiol resulted in reduced ovulation rates and, as a consequence, low or non-existent pregnancy rates during the experiment duration.