Double correction for digestibility. Long term study a. In general, TDN values for forages overestimate the usable energy of such feeds by ruminants. This serves as the basis for the CNES in which requirements for maintenance and production are calculated independently. Note relative amounts of feed used for maintenance versus production.
Maintenance is a BIG expense of animal production.
Nonetheless, we seldom directly measure NE system due to the cost and difficulty of determining the NE values. The NE values in feed tables are derived from TDN, DE, body weights and regression equations based on experiments depending on the species.