Over the years, this has led to the development of different
distinctive varieties (also called breeds or strains,
etc) of various species. This is particularly noticeable in animals and
plants used for food.
Give some examples of different breeds in the following categories - add more categories of your own if you can:
Milk cattle, e.g. > Friesians, Holsteins, Jerseys, Guernseys
Beef cattle, e.g. > Herefords, Charolais etc
It is also noticeable in animals kept as pets, and in garden flowers.
Give some examples of varieties of pets together with different varieties bred by Man.
> Dogs e.g. poodles - Alsatians (German shepherds) - Doberman Pinscher etc
However, a more rapid increase has been brought about through more
systematic application of the principles of genetics and reproductive
technology. This is especially useful in economically important
species, e.g. cattle, sheep, pigs, etc.
It is perhaps interesting to note that different levels of
technology can be applied to the different sexes!
It is expected that you have a reasonable knowledge of the
background to cattle breeding; the role of bull (male) and cow
(female), and the events leading to the production of milk.
What is the difference between semen and sperms?
> semen is fluid containing sperms
Why do you think AI is especially useful for dairy cattle?
> problems dealing with (stroppy!) bulls
>economic rewards - milk production
>the process of milk production relies on regular pregnancy of cows, so lost opportunities for pregnancy result in loss of revenue
What stage/event during the oestrous cycle should AI be
> ovulation (egg release) - oestrus ("heat"): time when female will accept male
|best bulls - of proven
take best (?) available on the farm
use semen from several different
bulls on different cows in the herd
farms are not likely to support
semen can be transported
|bull must be in same field as cow
be checked for possibilities of
|disease may be spread by sexual contact
|about £10 per straw (dose of semen)
|Up to £10 000 per bull
are relatively straightforward to
deal with by AI
can be dangerous
- to farm workers
- to public when loose in fields
|semen can be stored for several years
|bulls have limited life
are stored in insulated flasks
containing liquid nitrogen which needs
are expensive to rear and maintain,
and may become sterile due to
The following factors are checked:
- concentration of sperms (number per ml)
- absence of contaminating material (blood, pus, etc)
- normality and motility of sperms (some sperms have more than one head or tail, or do not swim consistently in a straight line)
against shock due to the first stages of
|albumen (white of egg) or milk
protection against damage due to
ice crystals formed during freezing
|pH buffer - against lactic acid
|citrate buffer solution
against osmotic damage
and as energy source for respiration
|as insurance/ to kill any bacteria present
How might lactic acid be produced?
> as a result of anaerobic respiration (by sperms?)
What is meant by osmotic damage?
> water entering or leaving sperm cells due to differences in concentration on either side of cell membrane (partially permeable)
The straws of semen may be kept for several years - even after the
death of the "donor" bull - but the sperms become gradually less and
less effective. Comprehensive records are kept.
Why do you think these records are necessary?
> so that a bull's semen is not used on closely related cows
> for pedigree purposes
A straw containing semen is quickly thawed in warm water, then
inserted into a syringe-like "insemination gun" or catheter, and the
semen is deposited into the cervix at the far end of the cow's vagina.
It is hoped that sperms will be in the oviduct when eggs are released.
1) Young candidate bull (1-2 years old) provides semen.
2) This semen used to impregnate about 500 cows
3) These 500 cows (pregnant for 9 months) give birth to (500?) calves.
4) Some of these calves are raised to maturity (2½ years) - impregnated and have own offspring (after another 9 months) - then they commence lactation (1 year).
5) Lactation figures are collated to give performance result for progeny, used to determine worth of bull for use in the programme.
Total up the time taken between stages 1 and 5.
How old is a bull likely to be by the time he is accepted for large scale use?
> 6-7 years
Bulls whose progeny do not produce satisfactory results are likely
to be culled.
The same fate is likely to befall a bull who has contributed a
reasonable amount of semen, "in order to reduce maintenance costs"!
Interestingly, artificial insemination is not approved for the
development of thoroughbred horses!
The cow is then artificially inseminated using semen from a proven
bull "in the usual way".
Each egg will be fertilised and start to develop into an embryo,
which takes some time to implant.
At an appropriate stage (6-7 days after insemination), the embryos
(still not implanted) are flushed using a catheter, i.e. pipe placed
into the uterus. Normally 4-7 embryos are recovered.
Meanwhile, (presumably 4-7) other cows have had their oestrous cycle
manipulated to be receptive and the embryos are placed into their
uteri, so they act as surrogate mothers to the embryos.
Alternatively, embryos can be frozen and stored - in a similar but
more closely controlled version of the techniques used in sperm
freezing. In this frozen state, they may be shipped to any part of the
world. In the past, embryos have even been implanted into completely
different species, e.g. rabbits, and re-implanted at their destination
into surrogate mothers.
Other simpler versions include:
- "non-surgical" removal of "ripe" eggs from a normal healthy cow as the follicles mature at the surface of the ovary, monitored by ultrasonic equipment
- removal of immature eggs from the ovaries of cows slaughtered at the abattoir (for whatever reason). These may need laboratory treatment to mature them before in vitro fertilisation, and eventually implantation into surrogate mothers.
It has been said that the techniques described above have the
potential to double the rate of genetic improvement achievable with AI
used on its own.
There are many ethical issues associated with biological
developments such as these, and you should be able to express an
informed opinion about the implications of these procedures. The
relationship between science and technology needs to be underpinned by
wider public knowledge of the facts involved, and a higher quality of
- The technology behind the production of Dolly the sheep has been applied to other species, but there are indications that there are some problems still to be solved. The frequency of success is not as high as it should be, and not all offspring develop as normally as can be expected.
Choosing the sex of offspring
- Male offspring are an inconvenience in many situations! Although it is theoretically possible to separate sperms carrying Y chromosomes from those carrying X chromosomes, the best approach may be to test the sex of embryos. only implanting females.
It is to be hoped that these techniques will not be applied to the
human species in the foreseeable future!