Partitioning intake and outflow of nitrogen and water in cattle grazing tropical pastures

Abstract

The quantities of nitrogen and water consumed and excreted by cattle grazing tropical pastures were measured by means of a number of techniques made possible by surgical preparation of animals. The development of portable equipment permitted the continuous infusion of chemical markers and the continuous collection of body fluids. In the dry season the mean nitrogen intake of four steers grazing a native grass pasture was only 20 g/day. When the steers were transferred to a pasture improved with the legume Stylosanthes hamata, nitrogen intake increased to 48 g/day, a value in excess of their maintenance requirement c. 30 g/day. Water turnover values increased simultaneously. More detailed measurements of intake and output of nitrogen and water were made with two steers grazing a tropical pasture of low quality. The mean nitrogen intake of the animals, weighing 600 kg, was c. 90 g/day, most of which could be accounted for in the urine and faeces. The animals consumed some 45 l/day of water, of which approximately two-thirds came from drinking. More than a third was used in evaporation and a similar amount was egested as faeces. The significance of water and nitrogen use for nutritional and environmental reasons is discussed.