Maximizing Energy Retention in Grazing Ruminants

SUMMARY (1) Costs of locomotion are frequently ignored in models determining the optimal diet of free-ranging ungulates. In order to determine whether such omissions are justified, the relationship between bite size and distance travelled per day is investigated in several wild species of ruminant grazing natural pastures in the Serengeti National Park, Tanzania. (2) The distance travelled to obtain a gram of food rises exponentially as bite size declines. Animals feeding on short swards have small bite sizes, but it is suggested that animals feeding selectively on course grasslands would also have small bite sizes. It is shown that the heat production in locomotion of selective animals can form a large fraction of the daily intake of metabolizable energy. (3) A simple model of the energy exchange in free-ranging ruminants is developed incorporating parameters of diet quality and heat production. The model applies to pastures which do not set a limit to the daily intake of dry matter. The optimum level of selection for high quality constituents in the grass sward is found by determining the conditions in which energy retention (net energy) is at a maximum. (4) For any particular pasture, the optimum level of selection is found to be strongly dependent on the cost of locomotion per day and on the difference between the metabolizable energy concentration of high and low quality constituents, weakly dependent on the average quality of forage, and independent of the ruminant's resting metabolic rate. (5) The model predicts that the optimum level of selection by large grazing herbivores will be lower than that of smaller animals. This is because large herbivores gain more from digesting low quality constituents due to their ability to digest cell walls, and also because the foraging costs of large animals are higher.