Embryo Transfer In Beef Cattle
by Dr. Roger L. Davis DVM
Embryo Transfer is an ever changing and expanding field. The first calf produced experimentally was in 1951, but it took until the 1970's for the first calf to be commercially produced (in England). The commercial embryo transfer industry in North America developed in the early 1970's with the introduction of exotic European breeds of cattle. Embryo transfer is basically used to exploit the genetics of the female in the same way that frozen semen was used to exploit the genetics of the male. As ET technology developed and improved, embryo transplant became more commonplace and affordable. It was used for genetic improvement rather than for increasing numbers of a particular breed or phenotype. Today it is the main method of moving genetic matings around the world.
Embryo Donor Selection:
There is no way of pre-determining how a donor will respond to super-ovulation or how many useable embryos she will produce. To improve the odds of a successful flush a donor that has calved 50-90 days and has a good reproductive history and relative youth will be your best bet. Donors should be on an increasing plane of nutrition with no specific nutritional deficiencies. Cows with condition scores from 2.5-3.5 (moderate flesh) out of 5 are preferred. Overfat cows generally create problems. Virgin heifers and cows that "come up open" are less predictable. The most often asked question is "how many embryos can I expect from my flush?" My pat answer is "from zero to eighty". The average is six to eight, but averages are made up of any combination of numbers, including zeros. Averages are also meaningful when there are at least 50 flushes included.
ET should be limited those cows with superior genetic merit. Just because the letters "ET" are in an animals registered name, it doesn't make them genetically better or worth more money. ET is a great way to disseminate genetics of the "elite" cows. It will also disseminate undesirable genetics, so make sure your donor is free of known genetic defects.
When purchasing a proven donor cow, check her history of embryo production and calving history.
Donor Sire Selection:
Today frozen semen is available from a large number of sires. Choose a sire that meets your genetic and phenotypic criteria. High quality semen is a must. If there is any question about the semen quality have it tested preferably at a bull stud by qualified personnel.
To ensure that the embryos are exportable to most countries of the world, the semen must meet Canadian standards for export to USA or have been legally imported into Canada. Owner's Use Only and Canada Use Only semen will limit export opportunities if this is a concern. Natural service works well providing the bull to be used has been tested and has high quality semen. The ultimate when using fresh semen is to collect it and inseminate raw fresh semen. This is also an excellent treatment for donors who consistently give large numbers of unfertilized ova for no reason.
Embryo Recipient Selection:
Recipients must be selected for reproductive performance, ability to calve, disposition and milking ability. Virgin beef heifers should be cycling and be a minimum of 13 - 14 months old. Pregnancy rates are good and handling of recipients when synchronizing them tends to be easier because of the lack of calves at side. Calving and milking ability tend to be the downside. If heifers are used the genetics for milk should be known and use should be limited to embryos from low birth weight matings.
Cows with calves at side that have calved at least 50 days and are on an increasing plane of nutrition with access (preferably force fed) to vitamins and minerals work well. Attention should be paid to disposition and udder. First calf heifers work well providing they are in good condition and are at least 70 days post partum. Thin first calf heifers should be fed well and given more time post partum or not used. Open cows are not recommended unless they have a known non-reproductive reason for being open (exposed to a problem breeding bull).
Dairy heifers work well from a pregnancy standpoint, but can be a problem at calving and require high level of care. Lactating dairy cows that are managed like beef cows tend to be difficult to re-use as recipients when nursing an ET calf. Dairy cattle tend to have a higher incidence of leucosis titers which will show up in your ET calf and cause problems with future health testing for entry to bull studs. I don't recommend using dairy heifers and cows for beef ET and if they are used insist on negative leucosis test.
Leasing of recipients works well providing the management in the lease herd is excellent. In such cases a bonus (10-20%) above the price for commercial calves is usually paid based on the weight of the weaned ET calf. It is best to pay a higher bonus for good weights, but build in a penalty clause for frozen ears, swollen joints, frozen feet (things that make the calf not marketable as a purebred). The owner of the embryo pays implanting and synchronization costs.
Super Ovulation and Synchronization:
The greatest breakthrough in ET in the past 7 years has been in the areas of super-ovulation and recipient synchronization. The use of CIDRs in donors has increased average embryo production and decreased the number of non responsive donors. CIDRs make it possible to be more efficient at flushing large numbers of donors at the same time and lining them up with large numbers of recipients. Donors can be super-ovulated if they are a minimum of 50 days post partum and cycling. The exact day of cycle is not important. I suggest you consult your ET practitioner about this method.
Recipients can be synchronized with CIDRs in small or large groups with or without heat detection. More efficient use of recipients (large groups or small) is the greatest benefit compare to using natural heats or Prostaglandins (Estrumate or Lutalyse). Pregnancy rates may be slightly lower with CIDRs, but more pregnancies will result from a group of recipients due to a lower rejection rates and tighter synchronization. This method is also excellent for the producer that has a small number of recipients to line up with a donor. I recommend that details of the synchronization method that suits you should be discussed with your ET practitioner.
The advent of Direct Transfer technology has made it possible to efficiently implant embryos into recipients with natural heats 6, 7 or 8 days previous. This works well if the ET practitioner is located in close proximity and if the recipient pool is large enough. Pregnancy rates with natural heats are generally slightly higher than synchronized heats.
Expected Embryo Transplant Results:
Embryo production varies greatly from donor to donor and flush to flush. Average production is approximately 6 freezable (Excellent & Good) and 8 transferable (Excellent, good, fair & poor) embryos per super-ovulation.
Pregnancy rates vary from flush to flush with fresh averages 60-70% and frozen 50-60%. Many factors affect pregnancy rate such as embryo quality, recipients, technical ability, and donor. Some donors consistently produce embryos with higher pregnancy rates that others with embryos of similar grade. This last factor seems to be uncontrollable and unpredictable.
CETA Certified Embryo Transfer Practitioners:
Veterinarians that are certified by the Canadian Embryo Transfer Association (CETA) must meet high standards to maintain certification. The certification program is comprehensive and required by the Canadian Food Inspection Agency (CFIA) as part of the accreditation process. Practitioners must be CETA Certified and accredited by CFIA in order to produce embryos for export.
Embryos are an efficient, cost effective and safe way to move genetics throughout the world. Thanks to cutting edge research, embryos that are processed according to the International Embryo Transfer Society (IETS) standards by certified and accredited embryo transfer practitioners can be exported to most countries around the world. IETS washed embryos pose zero risk for the transmission of any known disease, including BSE and foot and mouth. The washing of the embryos with outer shells (zona pellucida) that are intact with an enzyme called trypsin, removes any potential viral and / or bacterial agents. Only embryos that are processed in approved laboratories (mobile or fixed) by ET practitioners that are CETA certified and approved by CFIA can be exported. I recommend that all embryos be washed by approved ET teams to ensure that they will qualify for future export in case markets become available in the future. To maintain export status, the embryos must be stored under the direct supervision of an approved ET practitioner. Embryos stored on farm with your semen will lose their qualification.
If embryos are produced for a specific market, you should have your ET practitioner check on specific requirements such as donor testing and special semen qualification. Although most countries accept washed embryos, some such as China, Russia, New Zealand, Australia and Japan have additional requirements.
Marketing features of exporting embryos to foreign countries compared to live cattle include:
Advanced ET Technology:
Embryo sexing requires that a small biopsy be removed from the embryo and analyzed using DNA technology to determine the sex. This technology is considered cost effective and is utilized quite commonly in dairy cattle. On average, the pregnancy rate with frozen-sexed embryos is slightly lower that non- manipulated embryos. The procedure is quite tedious, time consuming and adds cost to frozen embryos. It's use questionable as to it being cost effective in beef herds. The biopsy also penetrates the zona pellucida (shell) which yields the embryos non exportable to some countries.
Ultrasonography is used commonly to evaluate ovaries, detect early pregnancy (27 days), and determine the sex of the fetus (55-70 days gestation). Ultrasound is a very useful tool and can help utilize recipients more efficiently by early pregnancy testing and re-use of open recipients. Examining the ovaries of Donor cows and recipients can also be useful in determining if the ovaries are functioning properly. Ultrasound is an excellent tool to use in general reproductive examinations as well.
Embryo Splitting has been used more commonly in the past. It is used to produce identical twins. The number of calves from a given number of embryos can be increased, however twice as many recipients are needed and the pregnancy rate id decreased. The high cost of recipients makes this technology questionable from an economics standpoint.
IVF or In-Vitro Fertilization is basically "test-tube calves". This procedure is quite effective in producing embryos, however the pregnancy rates can be disappointing and the abortion rates are high with high incidence of giant calves which leads to low numbers of healthy live calves. This technique is practical for very valuable cows that will not reproduce using conventional ET. Frozen IVF embryos yield varying and mostly disappointing pregnancy results.
Cloning uses ET as part of the process to produce pregnancies. This technology has greatly improved and is restricted to research and very valuable animals such as transgenic animals like Dolly the sheep that are used to produce rare pharmaceuticals. In Canada there are presently restrictions on selling production (semen) from cloned or genetically modified animals.
Embryo transfer has come a long way in the past 30 years with the technological advances. The thing that hasn't changed is the fact that you should still only flush the genetically superior donors with a track record. Economics still rules the decisions. Now more than ever ET is used for movement of genetics worldwide. More now than ever, the good ones are bringing a premium and are easy to market. Embryo transfer is a tool to help get more of those "good ones".