Enormous numbers of different breeds have developed during many millenia, most very well acclimatized to local conditions. It is estimated that 5,000-6,000 breeds exist today. Four thousand belong to the so-called "big nine" (cattle, horse, donkey, pig, sheep, goat, buffalo, domestic fowl, duck). However, the trend has moved to renewed selection efforts during the latest decades, for example, the present specialized daily milk production capacity of a Holstein Friesian averages 10.6 gal (40 l) of milk compared to the African N'Dama, produces 1.1 gal (4 l). These highly productive but very vulnerable breeds are now found all over the world, and in many places they have totally replaced the original breeds or have been crossbred with them. In this way, the original breeds disappear, and with them go extremly important genetic variations. In 1993, the Food and Agriculture Organization (FAO), started a project called Global Strategy for the Management of Farm Animal Genetic Resources, which is responsible for the preservation of livestock of no economic value. The categories "extinct" or "critically threatened breed" have been introduced, as has been done for wild animals.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.