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Veterinarian View | October 2023

Strategies for Transferring Embryos

contributed article by Dr. Vince Collison, Collison Embryo and Veterinary Services,

Rockwell City, Iowa

Last month I wrote about using the universal ABC form for recording embryo information. This article will go more in depth on how to use some of this information on transfer day, specifically when to transfer the stages listed on the form. Every embryo will be listed with a specific stage and grade. Stages range from 8 to 4, with the 4 being the least advanced embryo and the 8 as the most advanced embryo. We consider the stage to be the age of the embryo based on the following:

• Stage 4 to be a 6.5-day-old embryo

• Stage 5 to be a 7-day-old embryo

• Stage 6 to be a 7.5-day-old embryo

• Stage 7 to be an 8.0-day-old embryo

• Stage 8 to be an 8.5-day-old embryo

Timing for Transfer

This is the information we need when trying to determine the timing of when to transfer the embryo to the recipient. Also, we consider the day the recipient cow is in heat to be day 0. So, if you are wanting to put a stage 5 into a cow that is in heat on Monday AM, you would target the transfer for the next Monday AM. If transferring a stage 7 into the same recipient that was in heat on Monday AM, you would target for the next Tuesday AM. These are the guidelines for transfer timing in the strictest sense. In the real world, we have to plan transfers to fit into tight schedules and to also work with the recipient heats that are available on transfer day.

Transferring Conventional Embryos

If working with conventionally collected embryos, you will likely see mostly stage 4 and 5 embryos. There can also be stage 6 and 7 embryos, but they will be less likely. We typically try to schedule conventional embryo transfer 7 days after the heat. With conventional embryos there seems to be more flexibility in transferring embryos to an asynchronized recipient.

If we are transferring one embryo, it’s pretty easy to line her up for the transfer 7 days later. We will transfer frozen conventional embryos plus or minus 24 hours from day 7. On fresh conventional embryo transfers we feel you can go up to 36 hours out of synch and still see good results.

This information is based off experience, and I recall one flush we performed where all the recipients had heats 24 to 36 hours after the donor. These recipients had been set up to transfer a group of frozen embryos a couple days later and I convinced the owner to transfer this nice group of 10 fresh embryos to the recipients so they wouldn’t have to freeze them. In this case we ran 9 out of 10 pregnant on cows that came in 24 to 36 hours behind the donor.

So, if using conventional embryos, we will synchronize recipients so they will be in heat 6 to 8 days prior to the transfer date with the bulk of the recipients in heat 7 days prior. Conventional embryos are typically flexible enough that we don’t spend a lot of time trying to be perfect in the matching of embryo stage to recipient synchrony. Mostly we try to avoid getting any extreme asynchrony such as putting a stage 7 embryo into day 6 recipients.

Transferring IVF Embryos

If using IVF produced embryos, you will see predominately stage 7 and 6 embryos. There will even be embryos as advanced as stage 8 and some lesser advanced stage 5 embryos. It is very rare to see a stage 4 IVF embryo that has been frozen. In the case of IVF embryos, we feel it is best to be stricter about staging the embryo to the appropriate age recipient heat.

When synchronizing recipients, we typically use a 7 day CIDR program and schedule the transfer date 10 days after removing CIDRS. We also like to give GNRH to any non-responders 72 to 84 hours after pulling CIDRS to force ovulation and prevent recipients from coming into heat late, eliminating day 6 recipient transfers. Therefore, on the transfer day we will be dealing with only day 8 and day 7 recipients which will align better with the stage 8 – 5 embryos we see with IVF embryos.

So basically, if we pull CIDRS on a Monday, you will see heats on Wednesday (will be day 8 cows) and Thursday (will be day 7 cows) and set the transfer date for the following Thursday. We will have given GNRH later in the day on the Thursday after pulling CIDRS to the recipients that haven’t shown heat yet. We will try to transfer stage 8, 7 and 6 IVF embryos to the day 8 cows and use the stage 6 and 5 embryos in the day 7 cows as much as possible.

GNRH Recipients

For the non-responders that received GNRH, we will use stage 5 or even stage 6 IVF embryos. If conventional embryos are available, we will often use those on the GNRH cows since they will typically be stage 4 or 5. When it comes time to transfer, we will often run the day 8 recipients through first. It allows us to transfer all of the more advanced embryos (stage 8-7) first and keeps the day 8 cows from advancing to far from the time they were in heat as the day progresses.

If this is not possible, we will work to match the appropriate stage embryo to when the recipient was in heat as they come through in random order. As we get into the afternoon, this probably is less of an issue as all of the recipients have become more advanced, make precise staging less of an issue. On days where we have large numbers of recipients to run through the chute for IVF transfers, we will have recipients get to 8.5 to even 9 days post estrus at the time of transfer and we will still see good pregnancy rates.

IVF Guidelines

We feel it’s important to use these guidelines for staging when using frozen IVF embryos. There have been cases where we did not have specific recipient heats recorded for staging and we still will get along very well. In these cases, we usually did not start transferring until afternoon, which puts the majority of the recipients at a point farther from their heat. This causes more cows to automatically match with more of the advanced stages typically seen with IVF embryos. When using fresh IVF embryos, we feel there is more flexibility in staging embryos to recipients similar to fresh conventional embryos. We try to narrow this down to plus or minus 12 hours in the case of fresh IVF.

Dr. Vince Collison is co-owner of Collison Embryo and Veterinary Services PAC

in Rockwell City, Iowa.

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