This paper deals with practical systems where growing pigs or poultry are allowed to self select from two or more feeds. Most commonly a selection between just two feeds has been studied. Only systems are discussed that gave a choice of two feeds where a combination of these feeds gave a combined diet composition that approximately met the nutrient requirements of the pigs or poultry.

Data from singly housed choice-fed animals is most valuable in evaluating their ability to select a suitable nutrient intake. A number of experiments have been published indicating that growing pigs have a poor ability to select different proportions of two feeds. Pigs can discriminate between feeds on the basis of their concentration of crude protein or amino acid. However, the majority of pigs eat predominantly one feed even though a mixture of two feeds would be a more appropriate choice.