Effects of exogenous progestogens on reproductive phenomena in mares.

The research study investigates the impact of externally administered progestogens on various reproductive processes in mares. It specifically highlights the efficacy of progesterone dosage in blocking estrus and ovulation at different cycle stages, and how postpartum estrus and ovulation are affected. It also notes the inefficacy of melengestrol acetate.

Study Procedure and Findings

The paper presents several findings:

• When injecting mares with progesterone in an oil solution mid-cycle, the researchers found that estrus and ovulation were blocked at a dosage level of 100 mg per day or higher. A daily dose of 50 mg, on the other hand, only prevented estrus and not ovulation.
• One observation from the study was that the length of time between the end of treatment and estrus seemed to be dose-dependent. However, the seasonal variations had an undisclosed impact on the cyclic patterns following the treatment.
• In the early phase of estrus (day 1), the administration of neither 50 mg nor 100 mg could stop estrus or block ovulation.
• In terms of postpartum reproductive activity, 100 mg per day of progesterone was found to delay estrus and ovulation. Notably, this effect was not achieved with half the dose (50 mg daily). Moreover, a single dosage of 500 mg of crystalline progesterone did not consistently delay postpartum estrus and ovulation.

Inefficacy of MAP & Melengestrol Acetate

The researchers also tested other progestogens but did not observe any effects on the reproductive activity:

• Methyl acetoxyprogesterone (MAP) demonstrated little to no impact on estrus or ovulation regardless of the dose (ranging from 400 to 1782 mg per day), whether it was administered orally in the middle of the cycle or to mares which exhibited erratic estrus early in the breeding season.
• Moreover, dosages of 10 or 20 mg per day of orally administered Melengestrol acetate, a type of progestogen used in birth control, did not show any effects when given to mares in between their cycles.
• Finally, the researchers noted that neither a direct injection of 500 mg of crystalline MAP nor injecting 100 mg per day in an oil solution could block postpartum reproductive activity.

The study suggests a complex interplay between progestogen dosage, cycle stage, and other potentially influential variables such as the mare’s seasonality.