The artificial insemination (AI) of cattle

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.

AI catheter
Advantages of AI over natural breeding


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


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

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About 500 cows inseminated

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9 months gestation

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Calves have own offspring at 2 years old; begin lactation

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1 year of lactation

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Performance result of calves

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Bull possibly in use for AI


Thus, by the time the performance results of a bull's offspring are known, he will be up to 7 years old. During this time, a great deal of semen can have been collected and stored.

Semen collection

The bull is induced to ejaculate into an artificial vagina. This contains water at about 45C 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.

Semen collection apparatus
Background

Semen is usually collected from a bull as it mounts a suitably restrained live 'teaser' animal. such as another bull, or a cow. Bulls can also be trained to mount a mechanical dummy. The teasing process may involve inducing the bull to participate in one or more false mounts, (allowing him to mount but not ejaculate) sometimes with different teaser animals, before finally being allowed to ejaculate. Such procedures enhance both quantity and sperm count of the ejaculate. The operator holds the artificial vagina close to the teaser, parallel to the anticipated path of the penis. When mounting occurs, the operator directs the penis into the artificial vagina then ejaculation will take place. Semen is usually collected from a bull approximately four times a week.

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.

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