A large majority of dairy cows in the US, Canada and European countries are artificially inseminated. Artificial insemination of beef cows is also popular, particularly in purebred herds. The advantage of AI over natural service is that it facilitates rapid genetic improvement by allowing use of only the top bulls. Some bulls have sired more than 100,000 offspring via AI. Semen is most commonly collected from bulls in bull studs using an artificial vagina, as described below. Electroejaculation is an alternative method used with bulls that cannot mount or are too fractious for easy handling (e.g. range bulls). Finally, semen can be collected by massage of the seminal vesicles and ampullae per rectum. Semen Collection via Artificial Vagina Semen collection from bulls using an AV requires three people: one to handle the teaser animal, one to control the bull and one to collect the semen. It is important that the collection area have non-slip flooring to avoid injuries and because ejaculation may be inhibited if the bull is nervous about his footing. Bulls are heavy and should have regular hoof care. Poor hoof condition can inhibit the bull from mounting or cause pain when dismounting. A steer is most commonly used as a teaser and mount animal. Female teasers are not recommended because of the potential risk of intromission and spreading of venereal disease. The teaser should be a calm animal of the appropriate size for the bull being collected. The back and rear quarters of the teaser are washed with a disinfectant every collection day. It is also common for the rear quarters to be clipped routinely. The bull's preputial hair should be clipped in preparation for using the AV. These sanitary precautions are intended to minimize microbial contamination of the semen being collected. For collection, the teaser is positioned straight in front of the bull for mounting. Oftentimes, it helps to arouse the bull if the steer is led around the collection arena with the bull behind, then stopped abruptly, similar to the behavior of a cow in estrus. The artificial vaginal (pictured at right) uses thermal and mechanical stimulation to stimulate ejaculation. The liner of the AV is filled with water at 42-48 degrees Celsius, and the inner surface is lubricated with something like K-Y jelly. An insulating cone is placed over the end from which the collection tube protrudes to avoid subjecting the semen to temperature shock. False mounting is an effective way to sexually stimulate the bull. Providing two false mounts with two minutes of active restraint and one additional false mount maximizes sperm cell numbers. Final preparations are made to the AV between the second and third false mounts. Proper construction of the AV is important to avoid damaging the bull's penis and to avoid stressing sperm cells. Successful semen collection with an AV depends on the bull being comfortable around people, and they need to be trained to use the AV. However, bulls are big and dangerous, and personnel safety should be emphasized constantly. Most bulls in artificial insemination centers have a nose ring installed as a valuable and humane means of physical control. Bulls undergoing semen collection should be haltered and one should never tie a bull up by their nose ring - if startled, then can rip it out which is not only quite traumatic but yields an animal that may be exceptionally difficult to control. During collection, the person handling the AV must remain aware of where their feet are relative to the bull. As the animal ejaculates, it is common for him to jump forward. To avoid foot injury, collectors should wear boots with steel toes. In artificial insemination centers, bulls are typically collected 2 or 3 times per week, with 2 or 3 ejaculates per collection day. The following images depict routine semen collection from a bull.