Control of the oestrous cycle in mares with altrenogest.

The research examines the potency of altrenogest, a synthetic progestagen, in controlling the estrous cycle in mares for successful breeding by assessing its effects in field trials using 449 mares in the 1980 breeding season.

Research approach and procedure

• The study process involved two field trials. Altrenogest was administered to the mares at a daily oral dosage of 27mg for 15 consecutive days.
• The first trial aimed to compare the treated mares with controls to assess the clinical effectiveness of altrenogest.
• The second trial was designed to evaluate the treatment’s effectiveness during prolonged or erratic spring estrus.

Findings of the first trial

• The first trial showed estrus suppression in 94% of the treated mares.
• The post-treatment response appeared to be linked to both the timing within the transition from winter anestrous to the spring breeding season and the degree of ovarian activity when the mares were treated.
• When treatment was applied during the early transition phase (before 15 March), the average diameter of the largest follicle measured approximately 1.4cm.
• No significant differences were observed in the duration of estrus or the interval to conception between treated and control mares.

Comparison of early and late transition periods

• On the other hand, mares treated after 15 March (considered the late transition period) had a larger average diameter follicle (~2.1cm) and exhibited a reduction in the duration of estrus and the interval to conception as compared to control mares.
• For late transition period treatments, more mares exhibited regular post-treatment estrous cycles compared to controls or those treated during the earlier transition.
• However, there were no differences observed in pregnancy rates among all the groups.

Findings of the second trial

• The second trial centered on mares that had displayed estrus for 9-30 days before treatment.
• All mares returned to estrus within 10 days after altrenogest treatment, and a large percentage of them conceived within 18 to 45 days.

Conclusion

• In summary, the research presented the clinical effectiveness of altrenogest in controlling the oestrous cycle in mares, which could be valuable for managing individual mare breeding schedules and potentially improving conception rates.