Cropper (1987) considered the influence of the ‘bulkiness’ of foods upon the diet selections made by growing lambs. The food pairs offered differed in both crude protein (CP) concentration and digestibility. The lambs offered such choices did not completely avoid the more bulky food, which would have been expected from an optimal foraging point of view (Krebs & McCleery, 1984). This led to the suggestion that the consumption of small quantities of the more bulky food was beneficial to the lamb, perhaps to maintain rumen function. The foods used in Cropper's (1987) experiment had different metabolisable energy concentrations and so the ratio of protein:energy would have varied between foods, thus these lambs may have attempted to select diets in order to achieve a specific ratio of such nutrients.

The objective of this experiment was to test whether nutrient density (concentrations of protein and energy) has an influence upon diet selection. This was achieved by offering pairs of foods with different nutrient densities but a constant ratio of metabolisable protein (MP): metabolisable energy.