Ultrasonic evaluation for the time of ovulation in ewes treated with norgestomet and norgestomet followed by pregnant mare’s serum gonadotropin.

This research studied how long it takes for ovulation to occur in ewes (female sheep) after treatments with certain hormones, looking at the time difference between estrus onset and ovulation. They found that ovulation typically happens between 70 to 80 hours after treatment and the hormone PMSG is found to reduce this time interval.

Research Procedure

• The subjects of the study were 40 Dorset and Rambouillet x Dorset ewes. The observations were carried out in spring and fall where each season had 20 ewes (total 40) undergo the treatment.
• The ewes were randomly divided into two groups for different treatments. In one group, the ewes received only a norgestomet implant for 10 days. The other group received a norgestomet implant, but also an injection of 500 IU of PMSG at the time of implant removal.
• A technology known as the HeatWatch Estrus Detection System was used to determine the time of estrus onset (the period when the female is fertile and ready to breed). Then, with the help of rectal ultrasonography, the researchers monitored the ovaries every six hours to determine the time of ovulation.

Results and Findings

• The study encountered some issues where four ewes from each group lost their implants before the scheduled removal on the tenth day and had to be removed from the study.
• Within 108 hours post implant removal, estrus was detected in 81% of the implant-only group and 88% of the implant + PMSG group.
• The time from implant removal to the onset of estrus and to ovulation, was not affected by whether the experiment was conducted in spring or fall.
• The time difference between the onset of estrus and ovulation (INT), was similar in both groups. However, the mean time to ovulation (OVUL) and to the onset of estrus (EST) was significantly shorter in the group that also received the PMSG hormone.
• Ovulation was found to occur, on average, between 70 to 80 hours after the removal of the norgestomet implant. Furthermore, the inclusion of the PMSG hormone reduced the time from implant removal to ovulation.