Determination of corpora lutea numbers in Booroola--Texel ewes using transrectal ultrasound

Ovulation rates in Booroola--Texel ewes are currently determined by laparoscopic examination of the ovaries during the luteal phase allowing for carrier status of the ewe to be diagnosed. However, this is an invasive procedure and the aim of this work was to determine whether or not transrectal ultrasound would be an acceptable and accurate alternative to laparoscopy for this purpose in ewes. Transrectal ultrasound was performed using a 5.0 MHz transducer in 18 ewes restrained in the standing position. They were then euthanized and laparoscopy was performed, following which the reproductive tracts were recovered. An additional 22 ewes were subjected to scanning in dorsal recumbency, using a 7.5 MHz transducer, at various stages of the luteal phase before being euthanized and the reproductive tracts recovered. The ovarian structures identified using each method were then compared. The results demonstrated that laparoscopy is an accurate method of determining ovulation rates in ewes. Transrectal ultrasound examination appeared to be an acceptable and non-invasive procedure. The use of a 7.5 MHz transducer with the ewes in dorsal recumbency produced the best results and clearest images of the ovaries, which allowed for the determination of the number of corpora lutea (CL) and the presence of other structures. Accuracy improved with experience in the technique, the stage of the luteal phase at which the examination was performed, the number of CL present in the ovary and the age of the ewe. Further work is needed to confirm this finding due to the small numbers of animals in the present study. This ultrasound technique, however, has the potential to provide an alternative to laparoscopy for the determination of numbers of CL in ewes.