Superovulation of the donor cow
The superovulation of the donor cow is the next step in the embryo-transfer process. “Superovulation” is when more than the normal number of ova is released from the ovary during a single oestrus (heat period). About 85% of all donors will respond to the administration of the super ovulatory hormone. An average of 6-8 transferable embryos are normally obtained. Some donor cows can be flushed every 60 days.
The basic principle of superovulation is to stimulate follicle development by administering a hormone, FSH, which has a follicle-stimulating activity. The FSH is administered twice a day for a period of 4.5 days, 8 to 14 days after oestrus, in the presence of a corpus luteum on the ovary. A prostaglandin (Estrumate or Lutalysis) is then administered on the 3rd day of the treatment schedule which will cause degradation of the corpus luteum. The donor will then come to heat 48 to 60 hours later.
Insemination of the cow
During ovulation, ova are released from a large number of follicles. This process takes place over a number of hours and it is therefore essential to make sure that enough sperm reaches the egg cells in the fallopian tubes. Most embryo transfer technicians will prefer to inseminate a donor cow a number of times over her heat period. Good quality semen with a high percentage of motile sperm is a critical factor in any embryo-transfer program. Semen should preferably be deposited at the entrance of the uterine horns or in the body of the uterus.
Flushing of the embryos
Embryos are collected by inserting a silicone catheter through the cervix into the last third of the uterine horn. The two uterine horns are then flushed separately with a special medium. The flushing takes place seven days after the oestrus of the cow and the embryos are collected in a petri dish, after which it is selected under a microscope and placed in a special medium.
Evaluation of embryos
Embryos are searched for under a microscope and classified according to a number of criteria:
- Shape and uniformity of the zona pellucida (sheath)
- Variation in cell size
- Compaction of the blastomeres
- Colour and texture of the cytoplasm
- Diameter of the embryo
- Presence of loose cells
- Presence of vesicles
These criteria are used by the embryo veterinarian to classify embryos into grades 1 to 3 and developmental stages from 1 to 9. The classification is essential to determine how embryos will be transferred into recipients.
Selection and preparation of recipients
Proper management of the recipient herd is a critical factor in the success of any embryo program. Cows or heifers should be selected for healthy reproductive systems, calving ease, good milking characteristics and maternal instincts. Rising nutritional levels and general herd health are also crucial in the preparation process of recipients. Vaccination and deworming programs should preferably be carried out a month before the synchronization program. Normally, 10 to 12 recipients are prepared for each donor cow.
An average of 6 to 8 transferable embryos are obtained with each flush. With a 60% conception and 3 flushes per cow, a cow can produce 12 calves per year. Normally this number will be achieved during the lifetime of a cow.
The preparation process of the recipients involves the implantation of an intravaginal CIDR / Prid which with withdrawal +/- 8 days later, a group of recipient animals to come on heat in synchronization with the donor animals. The environment of the donor animal and recipients’ wombs will match up when the embryo is transferred 7 days later. Heat observation is very important. Recipients should be properly identified, monitored 2 to 3 times a day, and the date and time of the heat period should be recorded.
Transfer of embryos
Before transfer, the recipient is examined by an embryo veterinarian to determine if she is fit to receive an embryo. The ovaries are palpated to determine which ovary has ovulated and the uterus’ tone and symmetry are assessed. The transfer of embryos involves loading an embryo into a 0.25 ml straw that is placed in an embryo pistolette. The embryo is carefully aspirated under a microscope using a 1ml syringe. The transfer is done carefully under the influence of an epidural anaestetic. The vaginal area is cleaned and opened by an assistant so that the transferor can carry the pistolette to the cervix. The pistolette is carefully manipulated through the cervix and the embryo is deposited in the front third of the uterine horn. It is important not to damage the inner wall of the uterus, as this will drastically affect the chances of a successful pregnancy.
Embryos can be transferred within a few hours after recovery or can be stored in liquid nitrogen for later use. The freezing process can lead to a reduction of up to 10% in the conception rate.
Benefits of embryo transfers are:
- Economical
- Fast method of genetic multiplication of a herd
- More offspring from selected cows
- Short generation intervals
- Valuable genetic lines may be increased