Matching of embryo stages and grades with recipient oestrous synchrony in bovine embryo transfer

The efficacy of matching embryos with recipients on the basis of embryo stage and grade and donor-recipient oestrous synchrony was investigated using the records of 13,663 embryos that were collected and transferred at a commercial embryo transfer centre. The selection of early blastocysts for exact oestrous synchrony cows was effective and resulted in the highest pregnancy rates. Selection of early morulae was effective for recipients in oestrus after the donor but not when transferred into exact and negative recipients. The matching of late morulae with recipients in oestrus after the donor was not effective and had no influence on pregnancy rates. The selection of late, hatched and collapsed blastocysts for transfer into recipients in oestrus before the donor was ineffective and pregnancy rates were higher in exact and +12 hour recipients. Pregnancy rates declined 23.6 per cent in quality grades 1 to 4 whereas the range between stages was 13.3 per cent. Higher quality embryos of all stages gave the highest pregnancy rates. Examination of pregnancy rates of grades within stages suggested that the more developed the embryo the more difficult it is to grade. The difference in pregnancy rates between exact and -24 (6.9 per cent) and +24 (4.8 per cent) hour recipients was small and declined a further 4.7 per cent and 9.8 per cent in -36 and +36 hour recipients. Grade 3 and 4 embryos tolerated asynchrony better than grade 1 and 2, and early morulae tolerated asynchrony better than the other stages. It was concluded that the matching of certain embryo stages with the donor-recipient oestrous synchrony is advantageous but not always possible.