Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Artificial breeding of dairy cattle was introduced into the United States from Denmark about 10 years ago, and into Iowa 6 years ago. Artificial insemination of dairy cattle is now practiced in 42 states. A survey by the United States Department of Agriculture, covering 39 of these 42 states, indicates that more than one million dairy cattle, in 225,000 herds, were bred artificially in the United States in 1947. As compared with 1946, this represents an increase of 100% in cows, and 60% in herds using the service.
Kilpatrick, Warren J. and Raps, Greg
"Progress In Artificial Breeding,"
Iowa State University Veterinarian: Vol. 10
, Article 5.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol10/iss3/5