AI is a routine procedure on dairy farms and the vast majority of dairy cattle are produced -in this way. A network of AI centres was established by the (now defunct) Milk Marketing Board but all commercial AI is now in private hands. The address of one of the largest private concerns, which has centres throughout the UK, is provided later in the booklet.

AI can be carried out by technicians from approved AI centres, by qualified vets or, increasingly, on a DIY basis by licensed farmers on their own cows. Semen is stored in plastic straws, holding 0.25cm³ of semen. Filling and labelling of straws is mechanised. For insemination, a straw is fitted into a special insemination gun (catheter) which is inserted into the cow's vagina and positioned so that the semen is deposited beyond the cervix. Insemination is carried out 12-24 hours after the beginning of oestrus, which ensures that sperm arrive at the site of fertilisation a few hours before ovulation. As the timing must be precise, recognition of oestrus by the herdsmen is critical.

• Allows the choice of using the best possible bulls of proven quality in improving the genetic make-up of the cattle population. Farmers have access to genes from bulls of a quality which they may not individually afford. Frozen semen can be transported globally.
  

• Disease control. Many potentially devastating diseases are spread by sexual contact. Because of the extremely tight controls exerted over both the health of donor bulls and the technical procedures themselves, these risks are vastly reduced.
  

• Cost effectiveness. The cost of an AI straw is around £10, this is as nothing compared with the costs of a Holstein bull (possibly £10,000 to buy). A bull is expensive to rear, is relatively unproductive, vulnerable to disease or accident and may even prove to be infertile.
  

• Flexibility. For a variety of reasons, a herdsman may not wish all calves to be sired by a single bull with the same characteristics. It may well be impracticable to keep sufficient bulls to cover all possible requirements.
  

• Safety. Although there are differences between breeds, any bull can be aggressive and is potentially dangerous. This was a major stimulus to the initial setting up of AI services.

Students should be aware of the principle of progeny testing in order to prove the quality of a bull. They should know that:

• semen can be collected from bulls over a period of years and stored.
  

• samples of stored semen can be used to inseminate high quality cows and the performance of the progeny can be measured. In this way, the quality of semen from individual bulls is assessed before it is made available on a wide scale.
  

• the semen from quality bulls is used for large scale Al. Bulls of lesser quality are culled. A bull can sire offspring over a period long exceeding its natural life. Sometimes, when a bull has donated sufficient semen, it is culled to avoid maintenance costs.

Observations by students

Opportunities will be limited by practical and organisational considerations, but it might be possible for some students to visit a farm to coincide with the AI person's visit, to observe insemination and perhaps oestrus behaviour; alternatively there may be a local AI centre whose staff are willing to show small groups round. Arrangements would need to be explored via local vets or dairy farm managers.

Background

Flow chart illustrating progeny testing programme

1-2 year old bull provides semen

About 500 cows inseminated

9 months gestation

Calves have own offspring at 2½ years old; begin lactation

1 year of lactation

Performance result of calves

Bull possibly in use for AI

Semen collection

The bull is induced to ejaculate into an artificial vagina. This contains water at about 45°C held between a stout external rubber casing and an inner lubricated rubber sleeve. The object is to simulate the feel of a cow's vagina. The semen is collected in an insulated tube. After collection, the semen is checked for contaminants such as blood, pus or faeces. It is also examined microscopically for concentration and normality of spermatozoa.

Dilution and storage of semen

Semen is diluted in an 'extender'. This provides an appropriate concentration of spermatozoa, allowing more inseminations from each sample. A dilution of around 50 times is usual. The extender also nourishes and protects the spermatozoa during storage and distribution. Typically, the extender contains:

1. milk or egg yolk to protect against cold shock (the initial cooling below body temperature);
   

2. glycerol as a cryoprotectant (to protect damage due to the formation otherwise of ice crystals during freezing);
   

3. a buffer (usually citrate) to prevent pH changes due to, for example, lactic acid produced during sperm metabolism;
   

4. glucose (and/or other sugars) to provide an energy source for the spermatozoa, as well as the correct overall water potential for their survival;
   

5. antibiotics, to kill pathogens.

Candidates will be expected to be aware of the functions of these components, as described above.

Semen is packed into the plastic straws and stored in liquid nitrogen at -196° C. Each straw contains around 20 million spermatozoa. There is slow deterioration of the effectiveness of semen with time.

For use, the straws are thawed in warm water for a few seconds before insemination to reactivate the spermatozoa.