Embryo transfer basics

A cow normally produces one calf a year. With embryo transfer (ET), an outstanding cow can produce many calves a year carried by other females — and her embryos can be frozen, sold and shipped worldwide.

The two roles

• Donor — the genetically superior female whose embryos are collected. A good donor cycles regularly, conceives easily and is in good health and body condition.
• Recipient — a healthy, fertile female of lower genetic value that carries the calf. The calf's genetics come entirely from the donor and the sire; the recipient contributes the pregnancy and the mothering.

In vivo (conventional) embryos

1. The donor is given a series of hormone treatments to superovulate — release more eggs than usual.
2. She is inseminated, typically more than once around heat.
3. About 7 days later the embryos are flushed out of the uterus with a non-surgical procedure.
4. Each embryo is graded under a microscope.

Mississippi State University Extension reports that, on average, around eight transferable embryos per flush can be expected, with large variation between cows and flushes.

In vitro (IVF) embryos

Eggs (oocytes) are collected directly from the donor's ovaries with an ultrasound-guided needle (OPU — ovum pick-up), fertilized in the laboratory, and grown for about a week. IVF can be repeated frequently, can work in donors that respond poorly to superovulation, and uses very little semen — useful with rare or expensive sires. Some countries treat IVF embryos differently from in vivo embryos for import, so check before buying for export.

Grading

Under the International Embryo Transfer Society (IETS) system each embryo gets two numbers: a stage of development (1–9) and a quality grade (1 excellent or good, 2 fair, 3 poor, 4 dead or degenerating). Grade 1 and 2 embryos can be transferred fresh or frozen; grade 3 embryos freeze poorly and are normally transferred fresh.

Fresh or frozen

Embryos can be transferred fresh on the day of the flush or frozen in liquid nitrogen for later use or sale. Some freezing methods allow direct transfer — the straw is thawed and loaded straight into the transfer gun on farm. Ask the seller which method was used and how to thaw.

Recipient management: where most success is won or lost

• Recipients must be synchronized so that their cycle is at the same stage as the embryo — about day 7 after heat — on transfer day.
• They must be healthy, fertile, cycling and in good body condition, with good nutrition before and after transfer.
• Mississippi State University Extension reports pregnancy rates averaging approximately 65 percent for fresh embryos and 55 percent for frozen-thawed embryos. Embryo quality, recipient condition and synchrony, and technician skill all move that figure.

Planning

Book an experienced ET practitioner months ahead, plan enough synchronized recipients, and check with your breed association before you start: most require DNA testing of the donor and sire, and parent verification of the calves, for registration.

Key takeaways

• In vivo embryos come from a superovulated, flushed donor; IVF embryos are made in the lab from collected eggs.
• Embryo quality and well-synchronized, well-fed recipients drive results.
• Fresh embryos generally achieve higher pregnancy rates than frozen ones.
• Check registration and import rules before you buy or produce embryos.