Benchmarks for Embryo Transfer Programs
contributed article by Dr. Vince Collison, Collison Embryo and Veterinary Services,
Rockwell City, Iowa
What are realistic expectations for an embryo transfer program? Most producers who utilize ET have a pretty good idea of what to expect. When new producers want to start an embryo transfer program, it can be a real let down when their first collection comes in with only one to three embryos or even a total bust.
More than once we’ve had new producers experience results that were less than what they expected and were ready to give up after the first attempt. The other scenario is the new producer that obtains a large collection of 20 or more grade one embryos that results in 80 percent or higher pregnancy rates. After such an awesome start, it can set expectations unrealistically high.
Numbers Game
Looking over results from 2023, there were 5.79 usable embryos produced per collection. This is all the good and the bad combined. One has to realize that basically ET is a numbers game and while results can be all over the board, with increased numbers of collections you will find a number that is average for your herd.
I was at a meeting where a practitioner was telling about one of his clients who used the number of embryos collected as a selection-criteria for his donors. Because of his selection for donors with larger flushes, his client had developed a herd that averaged 13 embryos per collection.
When I started ET practice, I was told that 30 percent of the cows will make 70 percent of the embryos and over the years I have found this statistic to be pretty true. So, if you are lucky enough to have your best donor fall into the 30 percent of higher producing donors, it may take less collections to meet your needs. But if you need to make 100 embryos to transfer in the spring, it will take over 17 collections to produce that number based on the overall average.
This is probably one of the reasons that IVF has become so popular. It is much easier to build up a bank with IVF because donors can be collected every two weeks versus conventional flushes which typically have an interval of five to eight weeks between collections.
Embryo Development Rates
With IVF we like to look at embryo development rates (EDR) of oocytes collected. For IVF reverse sort collections, we expect about a 33.6 percent embryo development rate from the oocytes collected. If they are not reverse sorted and non-sexed semen is used, we will see about a 1 percent improvement in embryo development rate at 34.6 percent.
We probably see the lowest EDR when we use frozen sexed semen for fertilizing oocytes. This usually averages about a 25 percent EDR for frozen sexed semen. There is also a lot of variability between bulls with the frozen sexed semen. Part of the reason for the lower EDR is that the semen is injured to some degree by the sorting process and then there is some damage due to the freezing on top of that.
If I have my choice of using frozen sexed semen or using the reverse sort process for IVF, I will prefer to use the reverse sorted semen unless there is some proven history of the frozen sexed semen working well in IVF. Part of the reason that reverse sort probably performs better is that the dead sperm can be cleaned up from the sample during the reverse sort process. With frozen sexed semen, there is no way to clean up the dead sperm from the sample post thaw.
Pregnancy Rates
For pregnancy rates there is some variability between IVF versus conventional embryos. I typically tell producers to expect about a 50 percent pregnancy rate on frozen IVF embryos. There tends to be more pregnancy loss with IVF embryos, so this average will often dip below 50 percent.
If you can coordinate to transfer IVF embryos fresh, you can typically help the pregnancy rates by 5 percent or more. It seems that results with IVF embryos can tend to be more all over the board versus conventionally collected embryos which tend to have more consistent results.
On frozen conventional embryos, pregnancy rates will often range between 50 to 75 percent with a 65 percent pregnancy rate being fairly typical. With fresh transferred conventional embryos, you can also see about a 5 percent or more increase in pregnancy rates. Fresh conventional transfers can even see pregnancy rates as high as 80 percent or more when everything is right with the recipient quality and nutrition.
Recipient Preparation
When setting up recipients for fresh transfers, I like to set up about six cows per donor collected as a guideline. This will give you about the right number of cows for fresh transfer if you reach the average number of embryos collected.
When setting up recipients with a 7-day CIDR program, I hope to see about 75 to 80 percent or more of the cows display standing heat by the end of the third day after pulling the CIDRS. That leaves about 20 percent or less that haven’t displayed heat. On those we will give GNRH to force ovulation so they can be used for transfers at the scheduled appointment.
At that percentage, a high percent of the non-responders will usually have good CL’s when given GNRH. Many of those were probably cows that were going to come in within the next day. If we don’t see at least 75 to 80 percent in by the end of day three, then I am less optimistic that a high percentage of the non-responders will work.
We will still use GNRH on these cows, but typically we will have more kick outs on this type of group. When it comes to optimizing recipient response, make sure that they are getting their nutritional needs met. Meeting the energy and protein needs of the recipient is critical for getting the best response to synchronization. Also, make sure the trace mineral needs are met as well, this is critical for formation of a good CL on the ovary as well as general metabolic needs.
Use these numbers as a guideline for reaching your targets when planning your needs for spring transfers. I am always satisfied when we have a collection of six embryos or more, as I know we are holding up the average. If you feel your average embryos per collection needs to improve, work with your practitioner to investigate ways to improve results.
learn more collisonembryoservices.com
Dr. Vince Collison is co-owner of Collison Embryo and Veterinary Services PAC
in Rockwell City, Iowa.
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