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
During a time period when shortages, ecology, and conservation are routinely a concern to government, business, and individuals, there are renewed efforts to make food production more efficient. Recycling of animal wastes involves the refeeding of animal excreta to other or the same species of food producing animals usually a a fraction of the total diet. For centuries animal wastes have been recycled by allowing hogs to follow cattle and chickens to roam the barnyard. Animals will consume portions of their own feces when fed a diet deficient in certain nutrients.
Moellers, Ken C. and Vetter, R. L.
"Recycling Animal Wastes,"
Iowa State University Veterinarian: Vol. 36
, Article 1.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol36/iss3/1