Thus, TDN really is a ME based system. Many problems are associated with TDN system. Most serious one is it does not consider different efficiencies for maintenance and production. One lb of TDN from forage does not equal to 1 lb of TDN from a concentrate for ruminants are another problem. Double corrections for digestibility and inaccuracies contributed by proximate analysis are also problems.
Net energy based systems are the best.
You find the animal requirement based on their body weight, physiological condition and their production level. Feed table provides the NE values for maintenance and production. Plug in the numbers and you are ready to feed the animals. Note that maintenance requirement represents a big portion of the total requirement.