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Thursday, April 29, 2010

Embryo Transfer in Livestock

            Embryo transfer (ET) in livestock is a concept that has been used since the 1930s to increase the reproductive potential of genetically superior animals. Much like artificial insemination (AI) has increased the number of offspring males may produce; embryo transfer has increased the number of offspring females can produce. There are approximately 150,000 eggs in the ovaries of the females and under normal production systems only a few of these will be fertilized and produce offspring. Until the 1970s, ET was only performed by surgical methods, so the practice was limited in use. When non-surgical methods for ET were developed in the late 1970s, popularity of the practice began to grow among livestock producers and popularity has continued to present day. ET allows livestock owners to produce rapidly offspring from their females, often with planned matings to superior males. Because of the low success rates and high costs of synchronization of donor and recipient females, ET is generally limited to animals with elite genetic potential.
            Use of ET is not limited to increase productivity of selected females. The practice can also be used to identify the AI potential of selected males, control the spread of disease, and international trade of livestock. ET is a valuable tool to overcome certain reproductive problems in females. Certain females may not be physically capable of carrying a fetus to term or milk production in donor females may be limited.
            ET is often used in the cattle industry to replicate genetics of genetically superior females with planned mating to selected bulls. Donor and recipient females must first be synchronized in their estrous cycles. Ideally, ovulation of the two would be on the same day to ensure a similar uterine environment of the recipient when the embryo is transferred. Synchronization can be completed with the use of supplemental hormones that will affect the estrous cycle in the cow. These include follicle stimulating hormone (FSH), equine chorionic gonadotropins (eCG), and prostaglandins (PGF2a). There are several methods for synchronization and producers should choose the methods that best fit their production schedule.
            Females are superovulated prior to AI to increase the number of viable ovum ovulated for fertilization. In cows, the number of females that respond to superovulatory treatments is close to 90%. Sows are generally not superovulated because the ovulation rate is normally higher and viability of collected embryos tends to be lower when superovulation treatment is used. Recent work with mares has shown an ovulation rate of 3-4 ovulations per mare and an increased number of embryos recovered when equine FSH is used to stimulate superovulation. The response typically varies between production systems.
            Embryos are collected on day 4-6 after ovulation by surgical methods or non-surgical flushes.  Uterine flushes are common in cattle because rectal palpation of the reproductive tract allows ET technicians to guide the collection pipette to the uterine horns for easier collection compared to a full uterine flush. The smaller size of ewes and does does not allow for rectal palpation of the reproductive tract, so more often than in cattle surgical methods are used for embryo collection and transfer. In does, transcervical collection of embryos has been successful using a full uterine flush. Embryo transfer and collection is most commonly performed through surgical methods in the sow, however, some embryo transfer via the cervix has been successful.
            The cost of ET is the largest obstacle for many producers. Technicians may have facilities to house both donor and recipient females for synchronization, but this varies between locations. Recently technology for embryo freezing and embryo bisection (splitting) has been developed, increasing the potential of viable embryos. Producers should consider the costs of embryo collection and transfer along with the low success rate of current ET technology before applying the practice in their livestock operation.
Literature Cited
Selk, Glenn. Embryo Transfer in Cattle. Oklahoma Cooperative Extension Fact Sheet. ANSI-3158. Accessed April 27, 2010.
Merck Veterinary Manual. 2008. Embryo Transfer in Farm Animals: Introduction. Accessed April 27, 2010.
Seidel, Jr., G.E. and S.M. Sidel. 1991. Training Manual for Embryo Transfer in Cattle. Food and Agriculture Organization of the United Nations. Rome.

--Ryan Goodman