Battery cage chinchilla and foxes in fur farming, and most recently for the Asian palm civet for kopi luwak production of coffee. An early reference to battery cages appears in Milton Arndt's 1931 book, "Battery Brooding", where he reports that his cage flock was healthier and had higher egg production than his conventional flock. At this early date, battery cages already had the sloped floor that allowed eggs to roll to the front of the cage, where they were easily collected by the farmer and out of the hens' reach. Arndt also mentions the use of conveyor belts under the cages to
Metabolic waste excreta. Some mammals including humans excrete uric acid as a component of their urine but it is only a small amount. These compounds form during the catabolism of carbohydrates and lipids in condensation reactions, and in some other metabolic reactions of the amino acids. Oxygen is produced by plants and some bacteria in photosynthesis, while CO is a waste product of all animals and plants. Nitrogen gases are produced by denitrifying bacteria and as a waste product, and bacteria for decaying yield ammonia, as do most invertebrates and vertebrates. Water is the only liquid waste from animals and photosynthesizing plants.