Historically, the process of embryo transfer involved the selection of genetically superior cows followed by a five-day treatment with follicle-stimulating hormone to cause the release of multiple ova (instead of one) at the time of ovulation. This donor female was artificially inseminated with semen from a genetically elite bull, and after 7 or 8 days the embryos were recovered and transferred to recipient females whose estrous cycles were synchronized with that of the donor female.
Embryo transplantation has evolved from a few centralized transplant stations to many smaller clinics offering on-farm services. Although actual methodology has changed little since the 1970s, two major improvements are the use of less follicle-stimulating agent over fewer days in donor cows and the need for fewer sperm for donor insemination. With experienced professionals, the embryo recovery rates are expected to be >75%, with four to eight good-quality embryos per donor collection.
In recent years, a single-embryo on-the-farm collection approach has become popular with progressive dairymen. This approach uses no follicle stimulatory hormones to treat donor cows. Single embryos are collected from the highest producing cows in the herd and are then transferred individually to cows in the lowest producing group of the herd. Also, frozen embryos can be purchased commercially by producers for transplantation on a year-round basis. Pregnancy rates following transfer of excellent-quality frozen-thawed embryos are similar to those obtained after transfer of fresh embryos. The use of cryopreserved embryos is predicted to be the market growth area, with the potential for the use of frozen embryos in cattle herds being virtually unlimited.
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