Embryo transfer in cattle has recently gained considerable popularity with seedstock dairy and beef producers. While most modern applicable embryo transfer technology was developed in the 1970s and 1980s, the history of the procedure goes back considerably farther. Walter Heape performed the first embryo transfer in Angora rabbits in 1890. Embryo transfer in food animals began in the 1930s with sheep and goats, but it was not until the 1950s that successful embryo transfers were reported in cattle and pigs by Jim Rowson at Cambridge, England.
The first commercial embryo transfers in the United States were done in the early 1970s. Initially, embryos were recovered from valuable donors and transferred to recipient animals using surgical procedures. It was not until current non-surgical methods were developed in the late 1970s, the process became more affordable and the practice in general has grown in popularity. Current technologies have taken this a step further with successful cloning of embryos creating genetically identical animals. This article will discuss some of the basics of embryo transfer and its current applications in the beef cattle industry.
Justification for use of ET in the Beef Industry
The reproductive potential of each normal newborn calf is enormous. There are an estimated 150,000 potential “eggs” or ova in the female and countless billions of sperm produced by each male. By natural breeding, only a fraction of the reproductive potential of an outstanding individual can be realized. The average herd bull will sire anywhere from 15 to 50 calves per year and the average cow will have one calf per year. With artificial insemination (AI), it is possible to exploit the vast numbers of sperm produced by a genetically superior bull, however the reproductive potential of the female has been largely unutilized. She will produce an average of eight to 10 calves in her entire lifetime under normal management programs. Like artificial insemination has done for the bull, embryo transfer is a technique that can greatly increase the number of offspring that a genetically important cow can produce. It has been said that to dramatically improve the genetic base of a given cow herd it will take 10 to 20 years to accomplish using only natural service. By incorporating AI, these improvements can be seen in seven to eight years. Through the use of an aggressive ET program this change is accelerated to four to five years.
The use of ET in the beef industry has been implemented largely by purebred breeders (beef and dairy) with some growing use by show calf breeders. The breeders who have utilized ET have been pursuing these basic goals: to improve genetic selection by increasing the number of progeny from females that are either proven or perceived to be superior under any number of criteria; or to multiply the number of cattle in a program in order to expand the herd or to meet market demands.
There have been additional reasons given to rationalize the use of ET. Supply and demand will always result in semen with increased value. ET will allow a breeder to generate more offspring from rare and valuable semen. Current technologies even allow for the application of sexed semen and embryos to give further control over the selection process and specificity.
It has also been stated that ET will increase the accuracy of selection traits. Caution should be used before putting too much faith in this argument. Even with the use of ET, cows will have fewer progeny than bulls which results in breeding values with lower accuracies. The breeder must keep in mind that the performance of ET calves is partially reflected by the milking ability of the recipient female. Therefore, performance data on ET calves is not directly credited to the performance evaluation of the donor female. However, the natural progeny from ET calves will eventually contribute to the original donor's Expected Progeny Differences (EPDs) at a later date.
Whatever the stated reason, ET has had a significant impact on the beef industry. Take for instance the Angus breed. Registrations of Angus cattle over the past 15 years indicate a substantial increase in the use of ET. In 1987, 3.6 percent (5,105) of all calves registered in the Angus breed were a result of embryo transfer. In 2002, 25,093 calves resulting from ET were registered. This was 8.9 percent of all calves registered. While Angus is one of the largest breeds in the United States, similar numbers (percentages) have been documented in other breeds as well thus we know that the popularity is across the industry and has been recognized as an effective means for retaining exceptional genetics.
ET is a very structured process that requires good in order to obtain successful results. Considerable amounts of time and significant capital outlay can be involved in an ET program and thus the producer will want to take every step to get it right the first time. Any ET vet or technician will tell you that the “devil is in the details,” as the old saying goes. A successful program will require close attention to ALL the management components because just a small deviation can cause an entire collection to go awry. The following outlines the basics of a sound ET program.
The first and possibly the most important step in the process is the selection of the donor cow. She will be an animal that possesses genetic traits that a given operation finds highly desirable and wants to recreate as rapidly as possible. She will be a female that is known to be free of reproductive abnormalities or genetic defects. However, this does not necessarily mean she is a deserving donor candidate. This is typically a highly subjective decision but should be based on ultimately how the multiplication of this animal's genetics will affect the efficiency and profitability of the owner.
Regardless of selection criteria, the value of the calves from a donor should be high enough to justify the added expense. Selection criteria can be based on actual performance, EPD's, phenotype, relationship to other outstanding individuals, or some combination of these factors. As noted, consideration must be given to the marketability of the calves.
The purchase of a potential donor female can be an expensive proposition. The breed, selection criteria, and marketing opportunities will eventually determine the value of a donor female. Prices may range from a few thousand dollars to tens of thousands of dollars to an extreme of several hundred thousands of dollars. The individual breeder will have to determine the purchase price that is economically feasible for their operation.
Once the donor cow is selected the first step is to superovulate or produce multiple ova (eggs) for simultaneous fertilization and subsequent collection. Initially the donor female is treated with the gonadotropin (hormone) called follicle stimulating hormone (FSH). This hormone is administered twice daily for four days in the range of eight to 14 days following estrus while a functional corpus luteum (CL) is present on the ovary. As a result of the treatment, multiple follicles should develop on the ovaries of the donor. Multiple numbers of eggs will be released at estrus, one from each follicle
In order to bring the cow in estrus, a prostaglandin (hormone generated from lipid or fat source in the body) such as LutalyseTM is injected on the fourth day of FSH treatment schedule. The prostaglandin will cause the CL to regress and estrus to occur approximately 48 hours later.
The amount of FSH given to a donor will vary based on the transfer record of the donor (egg numbers and quality). The response of the donor to FSH is highly variable from cow to cow and can be a source of great frustration for the breeder. Most females will respond to the superovulation treatment with an average of five to seven transferable embryos but as indicated this is highly variable and is believed to be dependent on a number of factors. Results can range from zero to several dozen eggs per flush. In isolated cases, some cows simply will not respond to FSH treatment. Dr. Don Coover with SEK Genetics indicated a belief shared with other ET vets that “normal dosages” of FSH may, in fact, be over-stimulating the female resulting in a depression yield of follicles. Experimentation in his facility has shown acceptable results with as little as 40 percent of normal dosage rates.
Proper insemination of the donor female is an important and critical step in the Embryo Transfer process. The superovulation step produces multiple eggs which are commonly released over the course of several hours. Thus, high quality semen must be delivered with proper timing in order to yield the maximum number of fertilized eggs.
Because of the variability in the number of eggs and the timing of their release, females will be inseminated several times during and after estrus. The common practice would be to inseminate a superovulated cow at 12 and 24 hours after the onset of standing heat. The cost of the semen could very well affect the number of inseminations. As with normal artificial insemination, semen should be placed in the body of the uterus or at the entrance into each uterine horn.
Current ET procedures for embryo recovery or flushing is generally accomplished through non-surgical techniques at approximately seven days after breeding. The recovery process is relatively simple and can be completed in well under an hour. This process requires specific instrumentation and training.
Initially, the donor is given an epidural block at the tailhead to prevent straining. A flexible rubber tube catheter is passed through the cervix and into the body of the uterus. The cuff is inflated with saline solution to hold the catheter in place and to prevent backflow of fluids. Saline solution is flushed into the uterine horns through holes at the tip of the catheter that precede the cuff. The solution-filled uterine horn is gently massaged and the fluid containing the embryos is drawn back out through the catheter. This solution is collected through a filter and into a cylinder or dish. Embryos are then located retrieved by examination under a microscope.
Embryo Evaluation and Processing
Upon collection, embryos are evaluated under a microscope for stage of development and quality of the embryo. Embryos are collected on day six to eight after breeding and are usually in the morula through blastocyst stage. It should be noted that the visual evaluation of embryos is a subjective evaluation and is not an exact science. The following standardized coding system (Table 1) is recognized by the International ET Society, Savoy, Illinois.