Artificial insemination and semen processing
How artificial insemination works: collecting and evaluating semen, extenders, freezing, insemination technique, sexed semen, and a doses sum.
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Through artificial insemination (AI), a single outstanding bull can sire hundreds of thousands of calves across many countries, long after his death. AI has been one of the most powerful tools for genetic improvement of livestock, especially dairy cattle, and it also limits the spread of venereal diseases and removes the cost and danger of keeping bulls on every farm. It depends on a chain of careful steps, from collecting semen to placing it in the right place at the right time.
Artificial insemination and semen technology are standard topics in animal reproduction and veterinary courses.
A short history
- 1780: Lazzaro Spallanzani obtained puppies by artificially inseminating a dog.
- Early 1900s: Ilya Ivanov in Russia developed practical AI for horses, cattle and sheep.
- 1930s and 1940s: commercial cattle AI spread in Denmark, the United States and elsewhere; egg-yolk extenders and antibiotics improved semen survival.
- 1949: C. Polge, A. U. Smith and A. S. Parkes discovered that glycerol protects sperm during freezing, leading to the first calf from frozen semen in 1951.
- 1980s–2000s: sexed semen, sorted by flow cytometry to give mainly female or male offspring, became commercially available.
Today most dairy cows in many countries are bred by AI, while AI use in beef cattle, buffalo, pigs, sheep and goats varies widely.
Producing frozen semen
Collection
Usually with an artificial vagina, a warm, lubricated tube that the bull mounts on a teaser animal or dummy; electro-ejaculation is used for some animals. Bulls are tested free of diseases such as brucellosis, tuberculosis, bovine viral diarrhoea and venereal infections.
Evaluation
- Volume (bulls about 4 to 8 mL per ejaculate) and colour and appearance.
- Concentration, by haemocytometer, photometer or computer-assisted analysis (bulls often about 1 to 1.5 billion sperm per mL).
- Motility: the proportion moving progressively, under a warm-stage microscope.
- Morphology: the proportion of normal sperm; abnormal heads and tails reduce fertility.
Ejaculates below standard are discarded.
Extension
Semen is diluted with an extender that:
- provides an energy source (fructose, glucose) and buffers;
- protects sperm membranes from cold shock (egg yolk, milk, or plant-based alternatives);
- contains a cryoprotectant, usually glycerol, for freezing;
- includes antibiotics to control bacteria.
Packaging and freezing
Extended semen is filled into straws, usually 0.25 mL "French mini-straws", printed with the bull's identity. After slow cooling and equilibration at about 4 °C, straws are frozen in liquid nitrogen vapour and then plunged into liquid nitrogen at −196 °C, where they can be stored for decades.
Thawing
A straw is thawed in water at 35 to 37 °C for about 30 to 45 seconds, wiped dry, loaded into an insemination gun and used promptly, protected from cold and sunlight.
Insemination
In cattle, the recto-vaginal technique is used: the inseminator guides the cervix with a hand in the rectum and passes the gun through the cervix to deposit semen in the uterine body, just beyond the cervix. Timing follows the AM–PM rule or fixed-time AI protocols. In pigs, a catheter deposits semen in the cervix (or deeper with post-cervical catheters); fresh, liquid semen is used more often than frozen. Sheep often need laparoscopic intrauterine insemination for good results with frozen semen.
Beyond AI: embryo technologies
AI multiplies the genes of superior males. Embryo transfer does the same for females: multiple ovulation and embryo transfer (MOET) uses hormones to make a donor cow release many eggs, which are fertilised by AI, flushed out and transferred to recipient cows. Ovum pick-up and in vitro fertilisation (OPU-IVF) collects eggs directly from the ovaries for fertilisation in the laboratory. These methods are now widely used in elite dairy and beef herds, often combined with genomic testing of embryos.
Benefits and limitations
Benefits: rapid spread of superior genetics; progeny testing and genomic selection of bulls; lower costs than keeping bulls; control of venereal disease; safer farms; access to breeds worldwide.
Limitations: needs trained technicians, a liquid nitrogen supply chain and good heat detection; poor technique or timing lowers conception; widespread use of a few sires can reduce genetic diversity and increase inbreeding.
How many doses from one ejaculate?
A bull's ejaculate has 6 mL at 1.2 billion sperm per mL, with 70 per cent progressive motility.
- Total sperm = 6 × 1.2 × 109 = 7.2 × 109.
- Progressively motile sperm = 0.70 × 7.2 × 109 ≈ 5.0 × 109.
The semen centre aims for 20 million progressively motile sperm per straw before freezing (about half survive freezing and thawing):
- Doses = 5.0 × 109 ÷ (20 × 106) ≈ 252 straws.
- At 0.25 mL per straw, total extended volume ≈ 252 × 0.25 = 63 mL, so about 57 mL of extender is added to the 6 mL of semen.
With two collections a week, one bull could supply over 25,000 doses a year.
Key points
- AI spreads superior genetics, controls venereal disease and removes the need for farm bulls.
- Glycerol cryoprotection (1949) made frozen semen possible; sexed semen gives mainly one sex.
- Steps: collect (artificial vagina), evaluate (volume, concentration, motility, morphology), extend, fill 0.25 mL straws, freeze in liquid nitrogen at −196 °C.
- Thaw at 35–37 °C for about 30–45 s and inseminate promptly into the uterine body (cattle, recto-vaginal method).
- Success depends on heat detection, timing, technique and semen handling.
What the exams ask
- School-leaving and entrance papers: what AI is and its advantages.
- Degree and research-fellowship entrances: semen collection and evaluation, extenders and cryoprotectants, freezing and storage, insemination technique, sexed semen.
- Veterinary and professional examinations: semen quality standards, doses per ejaculate, AI programmes, conception rates and troubleshooting.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is semen stored in liquid nitrogen?
At −196 °C, all biological activity stops, so sperm can be kept for decades without losing fertility, as long as straws never warm up during storage or handling.
Is sexed semen as fertile as conventional semen?
Conception rates are usually somewhat lower, because sorting damages some sperm and doses contain fewer sperm. It is mostly used on heifers, which conceive more easily.
Can a farmer thaw semen in their pocket?
No. Uncontrolled thawing gives variable temperatures that kill sperm. Use a thermometer-controlled water bath at the recommended temperature and time.
Sources
- Senger, P. L. Pathways to Pregnancy and Parturition, 3rd edition. Current Conceptions, 2012.
- Polge, C., Smith, A. U. and Parkes, A. S. "Revival of spermatozoa after vitrification and dehydration at low temperatures." Nature 164, 666 (1949).
- Foote, R. H. "The history of artificial insemination: selected notes and notables." Journal of Animal Science 80 (E. Suppl. 2), 1–10 (2002).
- Vishwanath, R. and Moreno, J. F. "Review: semen sexing – current state of the art with emphasis on bovine species." Animal 12 (S1), s85–s96 (2018).
- Hafez, E. S. E. and Hafez, B. (eds) Reproduction in Farm Animals, 7th edition. Lippincott Williams and Wilkins, 2000.
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