Equine in vitro produced blastocysts: relationship of embryo morphology, stage and speed of development to foaling rate.

Context: Information on factors associated with developmental competence of equine in vitro -produced (IVP) blastocysts is lacking.

Aims: To determine the relationships of stage, grade, day of development, and specific morphological parameters of equine IVP blastocysts, to pregnancy and foaling rates.

Methods: Photomicrographs of 316 IVP embryos with known pregnancy outcomes were scrutinised individually by four observers. Inter-observer variation was assessed, and pregnancy outcome evaluated in relation to day of blastocyst development and assigned grade and stage. Individual component analysis was performed to determine the association of specific morphological parameters with foaling rate.

Key results: Overall pregnancy rate was 76.9% and foaling rate was 56.3%. The day of embryo development did not affect pregnancy rate but significantly affected foaling rate. Embryo stage did not affect foaling rate. Embryo grade affected foaling rate only for Day-9 embryos. Some morphological features in the bovine grading system did not predict outcome in equine IVP embryos. Significant individual parameters differed between Stage 5 and Stage 6 equine blastocysts.

Conclusions: Day of blastocyst development is the major factor related to foaling rate for equine IVP embryos. Notably, there was no effect of embryo stage on foaling rate and no evidence that prolonging culture until embryos advance in stage increases foaling rate. The standard bovine grading system is not directly applicable to equine IVP embryos; equine-specific staging and grading systems are proposed.

Implications: This information will allow laboratories to identify embryos with the highest developmental competence. Use of the proposed systems will increase consistency in embryo assessment among laboratories.