Effects of hormone implants on estrus and ovulation in feral mares.

The research article investigates the impacts of various hormone implants on estrus and ovulation in feral mares. The key finding is that the hormone levels can have influence on these reproductive processes, with estrogen especially having a significant impact on incidences of estrus.

Experiment Setup

• The study involved five groups of thirty captive wild mares each.
• Each of the groups was implanted with silastic rods containing estradiol (E) and/or progesterone (P) in varying amounts: E only with 8g, P only with 24g, P+HE with 8g P + 8g E, HP+E with 12g P + 4g E, HP+LE with 12g P + 2g E. The groups were differentiated using high (H) and low (L) amounts of the steroid.
• A control group which comprised of thirty mares received rod implants that didn’t contain hormones.

Hormone Monitoring

• Five mares from each group had their blood tested every two weeks for a period of four months, then monthly over the following five months.
• Serum P levels were seen to increase from 0.3 +/- 0.1 to 1.8 +/- 0.1 ng/ml in the P only group within the first three weeks post-implantation.
• The raised levels remained stable for around two weeks, after which a gradual decrease began.

Testing for Estrus

• All mares were tested for estrus by placing them in an alley between two pens of stallions and visually monitoring the mares’ response to the stallions.
• Significantly fewer mares in the E only and P+HE groups exhibited estrus compared to the control animals (10 out of 23 and 13 out of 26, respectively, verses 22 out of 25 in the control group).
• Animals receiving 24g of P (P only) exhibited similar incidences of estrus to the control mares.

Ideal Hormone Implant Composition

• The serum E levels saw a minor increase in the groups receiving 8g of E (E only and P+HE groups).
• Overall, the results indicated that the levels and type of hormones implanted can influence the occurrence of estrus and possibly ovulation in feral mares.
• Interestingly, the study suggests that estrogen has a strong impact on the occurrence of estrus, thus being potentially critical for reproductive management in feral mares.