http://www.ciwf.org.uk/includes/documents/cm_docs/2009/e/early_weaning_and_maternal_deprivation_2009.pdf

http://www.animalsaustralia.org/factsheets/dairy_cows.php

http://www.cattletoday.com/archive/2004/November/CT368.shtml

http://elmu.umm.ac.id/file.php/1/jurnal/A/Applied%20Animal%20Behaviour%20Science/Vol69.Issue3.Oct2000/1684.pdf