Selectively bred milking cows
These cows were bred to do one thing only to have a calve and produce up to 20 litres of milk in stead of just 2
http://www.ciwf.org.uk/includes/documents/cm_docs/2008/s/science_worksheets_selective_breeding.pdf
They do this by artifical insemination, which is injecting the cow itself with genetically modifidy sperm so that calf produces more milk them usual.
This takes place on many dairy farms and labs across the globe.
the people that do this are the farmers that need to meet the worlds high demand for milk/diary products.
this process started in russia in the later 19th century and in the early 20th century.
http://www.biotopics.co.uk/edexcel/biotechnol/artins.html
http://www.ehow.com/about_5052929_history-artificial-insemination-cattle.html