A practical guide to estimate the age of the early CL by ultrasonography in mares examined for the first time to be used as recipients in a commercial embryo transfer program.

The echogenicity of the early CL undergoes obvious changes during the first 5 days of luteal development. This phenomenon could potentially be used to estimate the age of the CL in mares which have not been checked frequently for the diagnosis of ovulation. The objective of this retrospective study was to determine the accuracy of estimating the age of the early CL (Day 0 to Day 4; Day 0 = Day of ovulation) in recipient mares examined for the first time in the breeding season to be used in a commercial embryo transfer (ET) program. The post-transfer pregnancy rate and embryo loss of 28 recipient mares whose CL age was estimated at the first scan of the breeding season (at least 2 months gap from the previous examination) were compared to that of 41 recipients in which the Day of ovulation had been determined by daily examinations and that received an embryo on the respective day. There was no difference (P > 0.1) in pregnancy (85.7 and 85.3%) or early embryonic loss (12.5% and 11.4%) between groups, respectively.