The function of all domestic animals is to utilize the coarse rough reeds and transform them into something useful to mankind, either food, clothing, or energy. The animal thus acts as a concentrator for low grade ores, so to speak. The class of animals which will produce most human food, clothing, or energy for man from a given amount of feel--other cost being equal--is the most economical and the one which will remain longest with us as population becomes more dense and the question of food supply becomes keener.
Experiments have shown the dairy cow to be probably the most economical "reducer" of coarse feeds. That is, from a given amount of feed she will return more human food than any other class of animals.
Carroll, W. E., "Bulletin No. 127 - Report of the Richmond-Lewiston Cow Testing Association" (1913). UAES Bulletins. Paper 78.