Forage contributes between 70 and 80% of the metabolizable energy (ME) and in excess of 70% of the protein consumed by ruminant livestock in the U.K. (Baker and Wilkins, 1975). The contribution from forage to the energy and protein consumed is highest for sheep and lowest for dairy cattle. For all classes of stock in the U.K., a major proportion of the nitrogen (N) or crude protein (CP) consumed is obtained from fresh or grazed forage.
Research with fresh forage, both nutritional and production work, has been faced with the difficulty of measuring or estimating food intake by the animal while grazing. The measurement of protein supply from fresh or grazed forage is further made difficult as flow measurements throughout the alimentary tract are required. For forage, as with other foods given to ruminants, estimates of the supply of N or protein available to the animal are obtained from measurements of flow from the stomach or into the proximal duodenum and at the terminal ileum. The difference between these two flow rates, the disappearance of N, NAN (non-ammonia N) or AAN (amino acid N), is taken as the apparent absorption or supply of protein from the food. Values may be obtained for individual amino acids or for total amino acid N (TAAN).