A cow normally produces only one egg per oestrous cycle (which lasts 21 days) and the gestation period is 40 weeks. On average, a cow produces only 2-3 calves in her lifetime. Thus, without intervention, the rate at which a particularly desirable cow can be used to improve the genetic status of a herd is slow.
However, recent advances in techniques for embryo transplantation are revolutionising the rate of genetic improvement. The essential stages are as follows:
Farmers may, of course, buy-in embryos from sources anywhere in the world to transfer to their own recipient cows.
Embryo transfer has the potential to bring about genetic improvement twice as fast as AI alone.
In vitro fertilisation (IVF)
Here, unfertilised eggs are fertilised in the laboratory and cultured for a few days until they have developed into early embryos. These are then transplanted, using a special long syringe, into the uteri of the recipient cows who are at the correct receptive stage of the oestrous cycle. The technique has recently been greatly improved. Obviously, it is possible to choose from high quality parents. The eggs to be used may be fully mature ones, recovered after ovulation, from the oviduct of a superovulated cow. However, this recovery of eggs from the oviduct requires surgery. (See the next Background box).
Two other sources of eggs are available and are offered routinely by some AI centres.
Two related topics not covered by the syllabus but which might be raised in discussion are controlling the sex of cattle offspring and cloning.
The ability to control the sex of offspring would bring significant advantages; it would mean, for example, that dairy farmers could choose to implant only female embryos. Two possible approaches exist, both are problematic. It is possible to distinguish and discriminate between X and Y carrying sperm. The process is tedious and unreliable but only tiny quantities are required for Al. The other approach is to sex embryos; at present this looks the more promising avenue.
Cloning has been achieved with cattle as well as with sheep (see page 19). The procedure involves removal of the nuclei from unfertilised eggs and replacing each nucleus with one taken from an undifferentiated cell of an embryo. As the donor nuclei are identical, the artificially produced zygotes have the potential to develop into identical offspring which together form a clone. The process has potential value in the cloning of offspring from matings of superior parents. However, the techniques are imperfect. There are many casualties and it is far from being a commercially viable proposition at present.