The substantial damage and pain inflicted by conventional milking machines causes mastitis (bacterial infections) and literally tears the soft tissue flesh of the teat canal lining, documented in study published in the Irish Veterinary Journal. The result is a loss of the natural defense mechanisms of the teat canal to help keep bacteria out and an inability of the cow to milk properly. This damage causes uneven udders and eventually the complete loss of function of the associated quarter of the udder. The milk quality of the cow diminishes and the production drops. The end result is that a young animal that has produced milk for only a year or two is sent to be slaughtered. Dairy beef represents twenty percent of the U.S. beef supply.

The increased use of modern milking equipment combined with less personal attention given to the resulting cows not milking well has substantially increased the US cull rate. The combined efforts of universities and the National Mastitis Council in setting protocols for milking cows has done nothing to improve the troublesome trends as shown in the graph below.

From: http://www.nass.usda.gov/index.asp, Quick Stats section