On a single feed an animal can increase its intake of a nutrient (eg. protein), as its ratio to energy is reduced, only by increasing its rate of energy intake. When given, as a choice, two feeds of a different protein but equal energy contents, it can vary its protein intake independently of Its energy intake by varying the proportion of each feed in its diet.

The experiment described here was designed to investigate the effect of feed protein content on the feed intake of young pigs and to test the proposition that young pigs, when given a choice between two feeds of different protein contents, a combination of which is non-limting, will select a diet which meets their protein requirements. In addition an investigation of the rules which govern the diet selection was carried out.

Four feeds (L, A, B and H) with similar energy contents (16.5 MJ DE per kg fresh feed) but different levels of crude protein (CP) were formulated and made into pellets. All feeds were intended to be non-limiting in vitamins and minerals.