Using Estrous Behavior to Time Initiation of Oxytocin Administration to Prolong Luteal Function in Mares.

The research investigated whether using particular patterns in mare’s estrous behavior could identify the ideal time to initiate oxytocin treatment to extend the luteal phase in mares. The researchers concluded that oxytocin administration starting 8 days after estrus onset effectively elongates the luteal phase for a majority of mares.

Research Methodology

• The study employed 23 light breed mares, aged between 4 to 20 years. These mares were exposed to a stallion, with researchers observing signs of sexual receptivity. Mares that did not show signs were administered a dose of 250 μg cloprostenol intramuscularly before being tested again after 3-4 days.
• On the first day of observed estrous behavior (Day 0), mares were divided into two groups. The control group received no treatment while the oxytocin group was given a once-daily intramuscular dose of 60 IU oxytocin from Day 8 to 17.
• Blood was collected every 4 days throughout Day 17, and weekly afterward until Day 45. The blood samples were used to measure the level of progesterone, an indicator of functionality of the corpus luteum. The period between Day 0 and the day when the progesterone level dropped below 1.0 ng/mL was defined as the interestrus interval.

Research Findings

• The average interestrus interval was longer in the oxytocin-treated mares compared to the control mares, with a significant statistical difference (32.4 ± 4.2 days versus 21.8 ± 1.5 days).
• Within the oxytocin group, more than half of the mares (54.5%) had an interestrus interval longer than 31 days, with nearly half (45.5%) showing an interestrus period of up to 45 days.

Conclusion

• The researchers concluded that luteal phase, the duration after ovulation and before the next period of estrus, can be extended beyond 30 days in most mares by administering oxytocin once daily starting at day 8 of the observed behavioral estrus.
• This finding suggests that monitoring estrous behavior can help time the administration of oxytocin treatment to suppress estrus and prolong the luteal phase in mares.